Chapter 44: Agents Affecting Mineral Ion Homeostasis and Bone Turnover

Calcium Stores. The skeleton contains 99% of total body calcium in a crystalline form resembling the mineral hydroxyapatite [Ca₁₀(PO₄)₆(OH)₂]; other ions, including Na⁺, K⁺, Mg²⁺, and F^-, also are present in the crystal lattice. The steady-state content of calcium in bone reflects the net effect of bone resorption and bone formation, coupled with aspects of bone remodeling. In addition, a labile pool of bone Ca²⁺ exchanges readily with interstitial fluid. This exchange is modulated by hormones, vitamins, drugs, and other factors that directly alter bone turnover or that influence the Ca²⁺ level in interstitial fluid.

Calcium Absorption and Excretion. In the U.S., ~75% of dietary calcium is obtained from milk and dairy products. The adequate intake value for calcium is 1300 mg/day in adolescents and 1000 mg/day in adults. After age 50, the adequate intake is 1200 mg/day. This contrasts with median intakes of calcium for boys and girls ≥9 years of age of 865 and 625 mg, respectively, and a median daily calcium intake of 517 mg for women >50 years of age.

Figure 44–2 illustrates the components of whole-body daily calcium turnover. Ca²⁺ enters the body only through the intestine. Active vitamin D–dependent transport occurs in the proximal duodenum, whereas facilitated diffusion throughout the small intestine accounts for most total Ca²⁺ uptake. This uptake is counterbalanced by an obligatory daily intestinal calcium loss of ~150 mg/day that reflects the calcium content of mucosal and biliary secretions and in sloughed intestinal cells.

The efficiency of intestinal Ca²⁺ absorption is inversely related to calcium intake. Thus, a diet low in calcium leads to a compensatory increase in fractional absorption owing partly to activation of vitamin D. In older persons, this response is considerably less robust. Disease states associated with steatorrhea, diarrhea, or chronic malabsorption promote fecal loss of calcium, whereas drugs such as glucocorticoids and phenytoin depress intestinal Ca²⁺ transport.

Urinary Ca²⁺ excretion is the net difference between the quantity filtered at the glomerulus and the amount reabsorbed. About 9 g of Ca²⁺ are filtered each day. Tubular reabsorption is very efficient, with >98% of filtered Ca²⁺ returned to the circulation. The efficiency of reabsorption is highly regulated by parathyroid hormone (PTH) but also is influenced by filtered Na⁺, the presence of nonreabsorbed anions, and diuretic agents (Chapter 25). Sodium intake, and therefore sodium excretion, is directly related to urinary calcium excretion. Diuretics that act on the ascending limb of the loop of Henle (e.g., furosemide) increase calcium excretion. By contrast, thiazide diuretics uncouple the relationship between Na⁺ and Ca²⁺ excretion, increasing sodium excretion but diminishing calcium excretion (Friedman and Bushinsky, 1999). Urine Ca²⁺ excretion is a direct function of dietary protein intake, presumably owing to the effect of sulfur-containing amino acids on renal tubular function.

Absorption, Distribution, and Excretion. Phosphate is absorbed from and, to a limited extent, secreted into the GI tract. Phosphate is a ubiquitous component of ordinary foods; thus even an inadequate diet rarely causes phosphate depletion. Transport of phosphate from the intestinal lumen is an active, energy-dependent process that is regulated by several factors, primarily vitamin D, which stimulates absorption. In adults, about two-thirds of ingested phosphate is absorbed and is excreted almost entirely into the urine. In growing children, phosphate balance is positive, and plasma concentrations of phosphate are higher than in adults.

Phosphate excretion in the urine represents the difference between the amount filtered and that reabsorbed. More than 90% of plasma phosphate is freely filtered at the glomerulus, and 80% is actively reabsorbed, predominantly in the initial segment of the proximal convoluted tubule but also in the proximal straight tubule (pars recta). Renal phosphate absorption is regulated by a variety of hormones and other factors; the most important are PTH and dietary phosphate, with extracellular volume and acid–base status playing lesser roles. Dietary phosphate deficiency upregulates renal phosphate transporters and decreases excretion, whereas a high-phosphate diet increases phosphate excretion; these changes are independent of any effect on plasma P_i, Ca²⁺, or PTH. PTH increases urinary phosphate excretion by blocking phosphate absorption. Expansion of plasma volume increases urinary phosphate excretion. Effects of vitamin D and its metabolites on proximal tubular phosphate are modest at best.

Role of Phosphate in Urine Acidification. Despite the fact that the concentration and buffering capacity of phosphate in extracellular fluid are low, phosphate is concentrated progressively in the renal tubule and becomes the most abundant buffer system in the distal tubule and terminal nephron. The exchange of H⁺ and Na⁺ in the tubular urine converts disodium hydrogen phosphate (Na₂HPO₄) to sodium dihydrogen phosphate (NaH₂PO₄), permitting the excretion of large amounts of acid without lowering the urine pH to a degree that would block H⁺ transport.

Actions of Phosphate. If large amounts of phosphate are introduced into the GI tract by oral administration or enema, a cathartic action will result. Thus, phosphate salts are employed as mild laxatives (Chapter 46). If excessive phosphate salts are introduced either intravenously or orally, they may reduce the concentration of Ca²⁺ in the circulation and induce precipitation of calcium phosphate in soft tissues.

History. Sir Richard Owen, the curator of the British Museum of Natural History, discovered the parathyroid glands in 1852 while dissecting a rhinoceros that had died in the London Zoo. Credit for discovery of the human parathyroid glands usually is given to Sandstrom, a Swedish medical student who published an anatomical report in 1890. In 1891, von Recklinghausen reported a new bone disease, which he termed "osteitis fibrosa cystica," which Askanazy subsequently described in a patient with a parathyroid tumor in 1904. The glands were rediscovered a decade later by Gley, who determined the effects of their extirpation with the thyroid. Vassale and Generali then successfully removed only the parathyroids and noted that tetany, convulsions, and death quickly followed unless calcium was given postoperatively.

MacCallum and Voegtlin first noted the effect of parathyroidectomy on plasma Ca²⁺. The relation of low plasma Ca²⁺ concentration to symptoms was quickly appreciated, and a comprehensive picture of parathyroid function began to form. Active glandular extracts alleviated hypocalcemic tetany in parathyroidectomized animals and raised the level of plasma Ca²⁺ in normal animals. For the first time, the relation of clinical abnormalities to parathyroid hyperfunction was appreciated.

Whereas American and British investigators used physiological approaches to explore the function of the parathyroid glands, German and Austrian pathologists related the skeletal changes of osteitis fibrosa cystica to the presence of parathyroid tumors; these two investigational approaches arrived at the same conclusions, as has been recounted by Carney (1997).

Chemistry. PTH molecules of all species are all single polypeptide chains of 84 amino acids with molecular masses of ~9500 Da. Biological activity is associated with the N-terminal portion of the peptide; residues 1–27 are required for optimal binding to the PTH receptor and hormone activity. Derivatives lacking the first and second residue bind to PTH receptors but do not activate the cyclic AMP or IP₃–Ca²⁺ signaling pathways. The PTH fragment lacking the first six amino acids inhibits PTH action.

Synthesis, Secretion, and Immunoassay. PTH is synthesized as a 115-amino-acid translation product called preproparathyroid hormone. This single-chain peptide is converted to proparathyroid hormone by cleavage of 25 amino-terminal residues as the peptide is transferred to the intracisternal space of the endoplasmic reticulum. Proparathyroid hormone then moves to the Golgi complex, where it is converted to PTH by cleavage of six amino acids. PTH(1-84) resides within secretory granules until it is discharged into the circulation. Neither preproparathyroid hormone nor proparathyroid hormone appears in plasma. The synthesis and processing of PTH have been reviewed (Jüppner et al., 2001).

A major proteolytic product of PTH is PTH(7–84). PTH(7–84) and other amino-truncated PTH fragments accumulate significantly during renal failure in part because they are normally cleared from the circulation predominantly by the kidneys, whereas intact PTH is also removed by extrarenal mechanisms. Rather than competing with PTH(1–84) binding at the receptor, PTH(7–84) may cause the PTH receptor to internalize from the plasma membrane in a cell-specific manner (Sneddon et al., 2003).

During periods of hypocalcemia, more PTH is secreted and less is hydrolyzed. In this setting, PTH(7–84) release is augmented. In prolonged hypocalcemia, PTH synthesis also increases, and the gland hypertrophies.

PTH(1–84) has a t_1/2 in plasma of ~4 minutes; removal by the liver and kidney accounts for ~90% of its clearance. Proteolysis of PTH (in the storage granule and in plasma) generates smaller fragments (e.g., a 33–36 amino acid N-terminal fragment that is fully active, a larger C-terminal peptide, and PTH[7–84]). Much of what circulates in the blood and is measured by older RIAs is the inactive C-terminal fragment that has a longer t_1/2 than the active N-terminal fragment or the intact PTH(1–84). Second-generation enzyme-linked immunosorbent assays for PTH, by contrast, can distinguish among these forms, but the clinical value of such information and whether it should affect therapeutic intervention remains controversial (D'Amour, 2008; Herberth et al., 2009).

Physiological Functions. The primary function of PTH is to maintain a constant concentration of Ca²⁺ and P_i in the extracellular fluid. The principal processes regulated are renal Ca²⁺ and P_i absorption, and mobilization of bone Ca²⁺ (Figure 44–3). PTH also affects a variety of tissues not involved in mineral ion homeostasis that include cartilage, vascular smooth muscle, placenta, liver, pancreatic islets, brain, dermal fibroblasts, and lymphocytes. The actions of PTH on its target tissues are mediated by at least two receptors. The PTH₁ receptor (PTH1R or PTH/PTHrP receptor) also binds PTH-related protein (PTHrP); the PTH₂ receptor, found in vascular tissues, brain, pancreas, and placenta, binds only PTH. Both of these are GPCRs that can couple with G_s and G_q in cell-type specific manners; thus cells may show one, the other, or both types of responses. There is also evidence that PTH can activate phospholipase D through a G_12/13–RhoA pathway (Singh et al., 2005). A third receptor, designated the CPTH receptor, interacts with forms of PTH that are truncated in the amino-terminal region, contain most of the carboxy terminus, and are inactive at the PTH₁ receptor; these CPTH receptors reportedly are expressed on osteocytes (Selim et al., 2006).

Regulation of Secretion. Plasma Ca²⁺ is the major factor regulating PTH secretion. As the concentration of Ca²⁺ diminishes, PTH secretion increases. Sustained hypocalcemia induces parathyroid hypertrophy and hyperplasia. Conversely, if the concentration of Ca²⁺ is high, PTH secretion decreases. Studies of parathyroid cells in culture show that amino acid transport, nucleic acid and protein synthesis, cytoplasmic growth, and PTH secretion are all stimulated by low concentrations of Ca²⁺ and suppressed by high concentrations. Thus, Ca²⁺ itself appears to regulate parathyroid gland growth as well as hormone synthesis and secretion.

Changes in plasma Ca²⁺ regulate PTH secretion by the plasma membrane–associated calcium-sensing receptor (CaSR) on parathyroid cells. The CaSR is a GPCR that couples with G_q and G_i. Occupancy of the CaSR by Ca²⁺ thus stimulates the G_q-PLC-IP₃-Ca²⁺ pathway leading to activation of PKC; this results in inhibition of PTH secretion, an unusual case in which elevation of cellular Ca²⁺ inhibits secretion (another being the granular cells in the juxtaglomerular complex of the kidney, where elevation of cellular Ca²⁺ inhibits renin secretion). Simultaneous activation of the G_i pathway by Ca²⁺ reduces cAMP synthesis and lowers the activity of PKA, also a negative signal for PTH secretion. Conversely, reduced occupancy of CaSR by Ca²⁺ reduces signaling through G_i and G_q, lessening inhibition of adenylyl cyclase and lowering activation of the G_q pathway, thereby promoting PTH secretion. Thus, the extracellular concentration of Ca²⁺ is controlled by a classical negative-feedback system, the afferent limb of which is sensitive to the ambient activity of Ca²⁺ and the efferent limb of which releases PTH. Acting via the CaSR, hypercalcemia reduces intracellular cyclic AMP content and activates PKC, whereas hypocalcemia does the reverse. The precise links between these changes and alterations in PTH secretion remain to be defined. Other agents that increase parathyroid cell cyclic AMP levels, such as β adrenergic receptor agonists and dopamine, also increase PTH secretion, but the magnitude of response is far less than that seen with hypocalcemia. The active vitamin D metabolite, 1,25-dihydroxyvitamin D (calcitriol), directly suppresses PTH gene expression. There appears to be no relation between physiological concentrations of extracellular phosphate and PTH secretion, except insofar as changes in phosphate concentration alter circulating Ca²⁺. Severe hypermagnesemia or hypomagnesemia can inhibit PTH secretion.

Effects on Bone. PTH exerts both catabolic and anabolic effects on bone. Normally, these processes are tightly coupled. Chronically elevated PTH enhances bone resorption and thereby increases Ca²⁺ delivery to the extracellular fluid, whereas intermittent exposure to PTH promotes anabolic actions. The primary skeletal target cell for PTH is the osteoblast, although evidence points to the presence of functional PTH receptors on osteocytes (O'Brien et al., 2008). PTH also recruits osteoclast precursor cells to form new bone remodeling units. Sustained increases in circulating PTH cause characteristic histological changes in bone that include an increase in the prevalence of osteoclastic resorption sites and in the proportion of bone surface that is covered with unmineralized matrix (Martin and Ng, 1994).

Direct effects of PTH on osteoblasts in vitro generally are inhibitory and include reduced formation of type I collagen, alkaline phosphatase, and osteocalcin. However, the response to PTH in vivo reflects not only hormone action on individual cells but also the increased total number of active osteoblasts, owing to initiation of new remodeling units. Thus, plasma levels of osteocalcin and alkaline phosphatase activity actually may be increased. No simple model can fully explain the molecular basis of PTH effects on bone. PTH stimulates cyclic AMP production in osteoblasts, but there also is evidence that intracellular Ca²⁺ mediates some PTH actions.

Effects on Kidney. In the kidney, PTH enhances the efficiency of Ca²⁺ reabsorption, inhibits tubular reabsorption of phosphate, and stimulates conversion of vitamin D to its biologically active form, calcitriol (Figure 44–3). As a result, filtered Ca²⁺ is avidly retained, and its concentration increases in plasma, whereas phosphate is excreted, and its plasma concentration falls. Newly synthesized calcitriol interacts with specific high-affinity receptors in the intestine to increase the efficiency of intestinal Ca²⁺ absorption, thereby contributing to the increase in plasma (Ca²⁺).

Calcium. PTH increases tubular reabsorption of Ca²⁺ with concomitant decreases in urinary Ca²⁺ excretion. The effect occurs at distal nephron sites. This action, along with mobilization of calcium from bone and increased absorption from the intestine, increases the concentration of Ca²⁺ in plasma. Eventually, the increased glomerular filtration of Ca²⁺ overwhelms the stimulatory effect of PTH on tubular reabsorption, and hypercalciuria ensues. Conversely, reduction of serum PTH depresses tubular reabsorption of Ca²⁺ and thereby increases urinary Ca²⁺ excretion. When the plasma Ca²⁺ concentration falls below 7 mg/dL (1.75 mM), Ca²⁺ excretion decreases as the filtered load of Ca²⁺ reaches the point where the cation is almost completely reabsorbed despite reduced tubular capacity.

Phosphate. PTH increases renal excretion of inorganic phosphate by decreasing its reabsorption by proximal tubules. This action is mediated by retrieval of the luminal membrane Na–P_i cotransport protein, NPT2a, rather than an effect on its activity. Patients with primary hyperparathyroidism therefore typically have low tubular phosphate reabsorption.

Cyclic AMP apparently mediates the renal effects of PTH on proximal tubular phosphate reabsorption. PTH-sensitive adenylyl cyclase is located in the renal cortex, and cyclic AMP synthesized in response to the hormone affects tubular transport mechanisms. A portion of the cyclic AMP synthesized at this site, so-called nephrogenous cyclic AMP, escapes into the urine. Measurement of urinary cyclic AMP is used as a surrogate for parathyroid activity and renal responsiveness.

Ions. PTH reduces renal Mg²⁺ excretion. This effect reflects the net result of enhanced renal Mg²⁺ reabsorption and increased mobilization of the ion from bone (Quamme, 2010). PTH increases excretion of water, amino acids, citrate, K⁺, bicarbonate, Na⁺, Cl^–, and SO₄^2–, whereas it decreases the excretion of H⁺. These effects are minor and generally can be seen only under tightly controlled circumstances.

Calcitriol Synthesis. The final step in the activation of vitamin D to calcitriol occurs in kidney proximal tubule cells. Three primary regulators govern the enzymatic activity of the 25-hydroxyvitamin D₃-1α-hydroxylase that catalyzes this step: P_i, PTH, and Ca²⁺ (see later for further discussion). Reduced circulating or tissue phosphate content rapidly increases calcitriol production, whereas hyperphosphatemia or hypercalcemia suppresses it. PTH powerfully stimulates calcitriol synthesis. Thus, when hypocalcemia causes a rise in PTH concentration, both the PTH-dependent lowering of circulating P_i and a more direct effect of the hormone on the 1α-hydroxylase lead to increased circulating concentrations of calcitriol.

Integrated Regulation of Extracellular Ca²⁺ Concentration by PTH. Even modest reductions of serum Ca²⁺ stimulate PTH secretion. For minute-to-minute regulation of Ca²⁺, adjustments in renal Ca²⁺ handling more than suffice to maintain plasma calcium homeostasis. With more prolonged hypocalcemia, the renal 1α-hydroxylase is induced, enhancing the synthesis and release of calcitriol that directly stimulates intestinal calcium absorption (Figure 44–3). In addition, delivery of calcium from bone into the extracellular fluid is augmented. In the face of prolonged and severe hypocalcemia, new bone remodeling units are activated to restore circulating Ca²⁺ concentrations, albeit at the expense of skeletal integrity.

When plasma Ca²⁺ activity rises, PTH secretion is suppressed, and tubular Ca²⁺ reabsorption decreases. The reduction in circulating PTH promotes renal phosphate conservation, and both the decreased PTH and the increased phosphate depress calcitriol production and thereby decrease intestinal Ca²⁺ absorption. Finally, bone remodeling is suppressed. These integrated physiological events ensure a coherent response to positive or negative excursions of plasma Ca²⁺ concentrations. In humans, the importance of other hormones, such as FGF23 and calcitonin, to this scheme remains unsettled. They may modulate the Ca²⁺–parathyroid–vitamin D axis or, in the case of FGF23, serve as primary regulators (Razzaque, 2009).

History. Vitamin D is the name applied to two related fat-soluble substances, cholecalciferol (vitamin D₃) and ergocalciferol (vitamin D₂) (Figure 44–4), that share the capacity to prevent or cure rickets. Prior to the discovery of vitamin D, a high percentage of urban children living in temperate zones developed rickets. Some researchers believed that the disease was due to lack of fresh air and sunshine; others claimed a dietary factor was responsible. Mellanby and Huldschinsky showed both notions to be correct; addition of cod liver oil to the diet or exposure to sunlight prevented or cured the disease. In 1924, it was found that ultraviolet irradiation of animal rations was as efficacious at curing rickets as was irradiation of the animal itself. These observations led to the elucidation of the structures of chole- and ergocalciferol and eventually to the discovery that these compounds require further processing in the body to become active. The discovery of metabolic activation is attributable primarily to studies conducted in the laboratories of DeLuca and Kodicek (DeLuca, 1988). The biological actions of vitamin D are mediated by the vitamin D receptor (VDR), a nuclear receptor, that was cloned in 1987 (McDonnell et al., 1987).

Chemistry and Occurrence. Ultraviolet irradiation of several animal and plant sterols results in their conversion to compounds possessing vitamin D activity. The principal provitamin found in animal tissues is 7-dehydrocholesterol, which is synthesized in the skin. Exposure of the skin to sunlight converts 7-dehydrocholesterol to cholecalciferol (vitamin D₃) (Figure 44–4). Ergosterol, which is present only in plants and fungi, is the provitamin for vitamin D₂ (ergocalciferol). Ergosterol and vitamin D₂ differ from 7-dehydrocholesterol and vitamin D₃, respectively, by the presence of a double bond between C22 and C23 and a methyl group at C24. Vitamin D₂ is the active constituent of a number of commercial vitamin preparations and is in irradiated bread and irradiated milk. In humans there is no practical difference between the antirachitic potencies of vitamin D₂ and vitamin D₃. Therefore, "vitamin D" is used here as a collective term for vitamins D₂ and D₃.

Human Requirements and Units. Although sunlight provides adequate vitamin D supplies in the equatorial belt, in temperate climates insufficient cutaneous solar radiation especially in winter may necessitate dietary vitamin D supplementation. It was assumed that vitamin D deficiency had been eliminated as a significant medical problem in the U.S. However, upward adjustment of estimates of normal vitamin D levels, improved analytical methodology, and more comprehensive data collection have led to an appreciation of the common finding of low circulating levels of vitamin D and a re-emergence of vitamin D–dependent rickets. Potential factors contributing to the rise of vitamin D deficiency include diminished consumption of vitamin D–fortified foods owing to concerns about fat intake; reduced intake of calcium-rich foods, including milk, in adolescents and young women of reproductive age; increased use of sunscreens and decreased exposure to sunlight to reduce the risk of skin cancer and prevent premature aging from exposure to ultraviolet radiation; and an increased prevalence and duration of exclusive breast-feeding (the combination of human milk, a poor source of vitamin D, and the high prevalence of low circulating vitamin D levels in U.S. women, particularly African-American mothers) (Bodnar et al., 2007).

There is no consensus regarding optimal vitamin D intake, and determination of vitamin D requirements is remarkably unsupported by clinical measurements. The recommended dietary allowance of vitamin D for infants and children is 400 IU, or 10 μg, daily. The basis for this dose was that it approximates that in a teaspoon (5 mL) of cod liver oil, which had long been considered safe and effective in preventing rickets (Vieth, 1999). Because adults require less vitamin D than infants, the adult dose was set arbitrarily at 200 IU.

The Food and Nutrition Board of the Institute of Medicine has developed Dietary Reference Intakes (DRI) for vitamins (Institute of Medicine, 2003). In both premature and normal infants, intake of 200 units per day of vitamin D from any source is considered adequate for optimal growth. Recently, the American Academy of Pediatrics recommended an intake of 400 IU of vitamin D per day from infants through adolescence (Wagner and Greer, 2008). Beyond adolescence, this amount probably may be sufficient, although nutritionists now recommend high daily intake, up to 4000 IU/day (Bischoff-Ferrari et al., 2009). There is some evidence that vitamin D requirements increase during pregnancy and lactation (Hollis and Wagner, 2004), where vitamin D doses of <1000 IU/day may be inadequate to maintain normal circulating 25-hydroxyvitamin D concentrations. Doses of ≤10,000 IU/day of vitamin D for up to 5 months do not elevate circulating 25-hydroxyvitamin D to concentrations >90 ng/mL.

Absorption, Fate, and Excretion. Vitamins D₂ and D₃ are absorbed from the small intestine, although vitamin D₃ may be absorbed more efficiently. Most of the vitamin appears first within chylomicrons in lymph. Bile is essential for adequate absorption of vitamin D; deoxycholic acid is the major constituent of bile in this regard (Chapter 46). The primary route of vitamin D₃ excretion is the bile; with a smaller percentage found in the urine. Patients who have intestinal bypass surgery or otherwise have severe shortening or inflammation of the small intestine may fail to absorb vitamin D sufficiently to maintain normal levels; hepatic or biliary dysfunction also may seriously impair vitamin D absorption.

Absorbed vitamin D circulates in the blood in association with vitamin D–binding protein, a specific α-globulin. The vitamin disappears from plasma with a t_1/2 of 20–30 hours but is stored in fat depots for prolonged periods.

Metabolic Activation. Whether derived from diet or endogenously synthesized, vitamin D requires modification to become biologically active. The primary active metabolite of the vitamin is calcitriol [1α,25-dihydroxyvitamin D, 1,25(OH)₂D], the product of two successive hydroxylations of vitamin D (Figure 44–4).

25-Hydroxylation of Vitamin D. The initial step in vitamin D activation occurs in the liver, where cholecalciferol and ergocalciferol are hydroxylated in the 25-position to generate 25-OH-cholecalciferol (25-OHD, or calcifediol) and 25-OH-ergocalciferol, respectively. 25-OHD is the major circulating form of vitamin D₃; it has a biological t_1/2 of 19 days, and normal steady-state concentrations are 15-50 ng/mL. Reduced extracellular Ca²⁺ levels stimulate 1α-hydroxylation of 25-OHD, increasing the formation of biologically active 1,25(OH)₂D₃. In contrast, when Ca²⁺ concentrations are elevated, 25-OHD is inactivated by 24-hydroxylation. Similar reactions occur with 25-OH-ergocalciferol. Normal steady-state concentrations of 25-OHD in human beings are 15-50 ng/mL, although concentrations <25 ng/mL may be associated with increased circulating PTH and greater bone turnover.

1α-Hydroxylation of 25-OHD. After production in the liver, 25-OHD enters the circulation and is carried by vitamin D–binding globulin. Final activation to calcitriol occurs primarily in the kidney but also takes place in other sites, including keratinocytes and macrophages (Hewison et al., 2004). The enzyme system responsible for 1α-hydroxylation of 25-OHD (CYP1α, 25-hydroxyvitamin D₃-1α-hydroxylase, 1α-hydroxylase) is associated with mitochondria in proximal tubules.

Vitamin D 1α-hydroxylase is subject to tight regulatory controls that result in changes in calcitriol formation appropriate for optimal calcium homeostasis. Dietary deficiency of vitamin D, calcium, or phosphate enhances enzyme activity. 1α-Hydroxylase is potently stimulated by PTH and probably also by prolactin and estrogen. Conversely, high calcium, phosphate, and vitamin D intakes suppress 1α-hydroxylase activity. Regulation (Figure 44–5) is both acute and chronic, the latter owing to changes in protein synthesis. PTH increases calcitriol production rapidly via a cyclic AMP–dependent pathway. Hypocalcemia can activate the hydroxylase directly in addition to affecting it indirectly by eliciting PTH secretion (Bland et al., 1999). Hypophosphatemia greatly increases 1α-hydroxylase activity. Calcitriol controls 1α-hydroxylase activity by a negative-feedback mechanism that involves a direct action on the kidney, as well as inhibition of PTH secretion. The plasma t_1/2 of calcitriol is estimated at 3-5 days in humans.

24-Hydroxylase. Calcitriol and 25-OHD are hydroxylated to 1,24,25(OH)₂D and 24,25(OH)₂D, respectively, by another renal enzyme, 24-hydroxylase, whose expression is induced by calcitriol and suppressed by factors that stimulate the 25-OHD-1α-hydroxylase. Both 24-hydroxylated compounds are less active than calcitriol and presumably represent metabolites destined for excretion.

Intestinal Absorption of Calcium. Calcium is absorbed predominantly in the duodenum, with progressively smaller amounts in the jejunum and ileum. Studies of Ca²⁺ uptake by isolated cells reflect these differences and suggest that elevated amounts of transport are likely due to greater transport by each duodenal cell. The colon also contributes to calcium absorption because ileostomy reduces absorption.

In the absence of calcitriol, calcium absorption is inefficient and proceeds in a thermodynamically passive manner through the lateral intercellular spaces (paracellular pathway). Calcitriol increases the transcellular movement of Ca²⁺ from the mucosal to the serosal surface of the duodenum. Transcellular Ca²⁺ movement involves three processes: Ca²⁺ entry across the mucosal surface, diffusion through the cell, and energy-dependent extrusion across the serosal cell membrane. Calcium is also secreted from serosal to mucosal compartments. Thus, net calcium absorption is the difference between the two oppositely oriented vectorial processes. The complex mechanisms and the proteins mediating calcium absorption are still incompletely understood (Perez et al., 2008). Evidence implicates TRPV6 Ca²⁺ channels in mediating mucosal calcium entry in the intestine (Bianco et al., 2007). In humans, TRPV6 is expressed in the duodenum and proximal jejunum. A calcium-poor diet upregulates intestinal TRPV6 expression in mice (Van Cromphaut et al., 2001). This effect is greatly reduced in VDR knockout mice, suggesting that TRPV6 mediates calcium entry and is vitamin D dependent.

Ca²⁺ absorption is potently augmented by calcitriol. It is likely that calcitriol enhances all three steps involved in intestinal Ca²⁺ absorption: entry across mucosal brush border membranes, diffusion through the enterocytes, and active extrusion across serosal plasma membranes. Calcitriol upregulates the synthesis of calbindin-D_9K and calbindin-D_28K and the serosal plasma membrane Ca–ATPase. Calbindin-D_9K enhances the extrusion of Ca²⁺ by the Ca–ATPase, whereas the precise function of calbindin-D_28K is unsettled.

Mobilization of Bone Mineral. Although vitamin D–deficient animals show obvious deficits in bone mineral, there is little evidence that calcitriol directly promotes mineralization. Thus, even though VDR knockout mice exhibit severely impaired bone formation and mineralization, these deficiencies can be entirely corrected by a high-calcium diet. These results support the view that the primary role of calcitriol is to stimulate intestinal absorption of calcium, which, in turn, indirectly promotes bone mineralization. Indeed, children with rickets caused by mutations of the VDR have been treated successfully with intravenous infusions of Ca²⁺ and phosphate. In contrast, physiological doses of vitamin D promote mobilization of Ca²⁺ from bone, and large doses cause excessive bone turnover. Although calcitriol-induced bone resorption may be reduced in parathyroidectomized animals, the response is restored when hyperphosphatemia is corrected. Thus, PTH and calcitriol act independently to enhance bone resorption.

Calcitriol increases bone turnover by multiple mechanisms (Suda et al., 2003). Mature osteoclasts apparently lack the VDR. Acting by a non-VDR mechanism, calcitriol promotes the recruitment of osteoclast precursor cells to resorption sites, as well as the development of differentiated functions that characterize mature osteoclasts.

Osteoblasts, the cells responsible for bone formation, express the VDR, and calcitriol induces their production of several proteins, including osteocalcin, a vitamin K–dependent protein that contains γ-carboxyglutamic acid residues, and interleukin-1 (IL-1), a lymphokine that promotes bone resorption (Spear et al., 1988). Thus the current view is that calcitriol is a bone-mobilizing hormone but not a bone-forming hormone. Osteoporosis is a disease in which osteoclast responsiveness to calcitriol or other bone-resorbing agents is profoundly impaired, leading to deficient bone resorption.

Renal Retention of Calcium and Phosphate. The effects of calcitriol on the renal handling of Ca²⁺ and phosphate are of uncertain importance. Calcitriol increases retention of Ca²⁺ independently of phosphate. The effect on Ca²⁺ is thought to proceed in distal tubules, whereas enhanced phosphate absorption occurs in proximal tubules.

Other Effects of Calcitriol. It now is evident that the effects of calcitriol extend well beyond calcium homeostasis. Receptors for calcitriol are distributed widely throughout the body (Pike and Shevde, 2005). Calcitriol affects maturation and differentiation of mononuclear cells and influences cytokine production and immune function (Nagpal et al., 2005). One focus of research is the potential use of calcitriol to inhibit proliferation and to induce differentiation of malignant cells (van den Bemd et al., 2000). The possibility of dissociating the hypercalcemic effect of calcitriol from its actions on cell differentiation has encouraged the search for analogs that might be useful in cancer therapy. Calcitriol inhibits epidermal proliferation and promotes epidermal differentiation and therefore is a potential treatment for psoriasis vulgaris (Kragballe and Iversen, 1993) (Chapter 65). Calcitriol also affects the function of skeletal muscle, brain (Carswell, 1997), and blood pressure (Nagpal et al., 2005).

History and Source. Copp observed in 1962 that perfusion of canine parathyroid and thyroid glands with hypercalcemic blood caused a transient hypocalcemia that occurred significantly earlier than that caused by total parathyroidectomy. He concluded that the parathyroid glands secreted a calcium-lowering hormone (calcitonin) in response to hypercalcemia and in this way normalized plasma Ca²⁺ concentrations. The physiological relevance of calcitonin has been challenged vigorously: calcitonin normally circulates at remarkably low levels; surgical removal of the thyroids has no appreciable effect on calcium metabolism; and conditions associated with profound elevations of serum calcitonin concentration are not accompanied by hypocalcemia (Hirsch and Baruch, 2003). The primary interest in calcitonin arises from its pharmacological use in treating Paget's disease and hypercalcemia and in its diagnostic use as a tumor marker for medullary carcinoma of the thyroid.

The thyroid parafollicular C cells are the site of production and secretion of calcitonin. Human C cells, which are derived from neural crest ectoderm, are distributed widely in the thyroid, parathyroid, and thymus. In non-mammalian vertebrates, calcitonin is found in ultimobranchial bodies, which are separate organs from the thyroid gland.

The calcitonin gene is localized on human chromosome 11p and contains six exons (Figure 44–6). The primary transcript is alternatively spliced in a tissue-specific manner. In thyroid C cells, the calcitonin/calcitonin gene-related peptide (CGRP) pre-mRNA is processed primarily with common exons 2 and 3 to include exon 4. This leads to production of the 32-amino-acid peptide calcitonin, along with a flanking 21-amino-acid peptide called katacalcin, whose physiological significance is unknown. In neuronal cells, in contrast, most of the calcitonin/CGRP pre-mRNA, is processed to exclude exon 4, resulting in inclusion of exons 5 and 6 with common exons 2 and 3, which ultimately gives rise to CGRP. This results in the production of the 37-amino-acid CGRP. Calcitonin is the most potent peptide inhibitor of osteoclast-mediated bone resorption and helps to protect the skeleton during periods of "calcium stress," such as growth, pregnancy, and lactation. CGRP and the closely related peptide adrenomedullin are potent endogenous vasodilators.

Chemistry and Immunoreactivity. Calcitonin is a single-chain peptide of 32 amino acids with a disulfide bridge linking the cysteine residues in positions 1 and 7 (Figure 44–7). In all species, 8 of the 32 residues are invariant, including the disulfide bridge and a carboxyl-terminal proline amide; both structural features are essential for biological activity. The residues in the middle portion of the molecule (positions 10–27) are more variable and apparently influence potency and/or duration of action. Calcitonins derived from salmon and eel differ from the human hormone by 13 and 16 amino acid residues, respectively, and are more potent than mammalian calcitonin. Salmon calcitonin is used therapeutically in part because it is cleared more slowly from the circulation.

Regulation of Secretion. The biosynthesis and secretion of calcitonin are regulated by the plasma Ca²⁺ concentration. Calcitonin secretion increases when plasma Ca²⁺ is high and decreases when plasma Ca²⁺ is low. Multiple forms of calcitonin are found in plasma, including high-molecular-weight aggregates or cross-linked products. Assays for the intact monomeric peptide are now available. The circulating concentrations of calcitonin are low, normally <15 and 10 pg/mL for males and females, respectively. The circulating t_1/2 of calcitonin is ~10 minutes. Abnormally elevated levels of calcitonin are characteristic of thyroid C-cell hyperplasia and medullary thyroid carcinoma.

Calcitonin secretion is stimulated by a number of agents, including catecholamines, glucagon, gastrin, and cholecystokinin, but there is little evidence for a physiological role for secretion in response to these stimuli.

Mechanism of Action. Calcitonin actions are mediated by the calcitonin receptor (CTR), which is a member of the PTH/secretin subfamily of GPCRs (Lin et al., 1991). Six human CTR subtypes occur through alternative splicing of coding and noncoding exons (Purdue et al., 2002). These isoforms exhibit distinct ligand-binding specificity and/or signal-transduction pathways and are distributed in a tissue-specific pattern. Of the more abundant isoforms, hCTRI1^-, which lacks a 16-amino acid insert in the first intracellular loop, preferentially couples with the G_s–adenylyl cyclase pathway (Gorn et al., 1995). The hCTRI1⁺ isoform, which includes this casette, does not couple with PLC (Naro et al., 1998) and therefore does not activate PKC or trigger an increase in Ca²⁺. Calcitonin receptors can dimerize with RAMPs (receptor-activity modifying proteins) to create receptors with high affinity for amylin (Hay et al., 2005) (Chapter 43).

The hypocalcemic and hypophosphatemic effects of calcitonin are caused predominantly by direct inhibition of osteoclastic bone resorption. Although calcitonin inhibits the effects of PTH on osteolysis, it inhibits neither PTH activation of bone cell adenylyl cyclase nor PTH-induced uptake of Ca²⁺ into bone. Calcitonin interacts directly with receptors on osteoclasts to produce a rapid and profound decrease in ruffled border surface area, thereby diminishing resorptive activity.

Depressed bone resorption reduces urinary excretion of Ca²⁺, Mg²⁺, and hydroxyproline. Plasma phosphate concentrations are lowered owing also to increased urinary phosphate excretion. Direct renal effects of calcitonin vary with species. Acute administration of pharmacological doses of calcitonin increases urinary calcium excretion, whereas calcitonin inhibits renal calcium excretion at physiological concentrations. In humans, calcitonin increases fractional urinary calcium excretion in a dose-dependent manner in subjects given a modest calcium load (Carney, 1997).

FGF23 was discovered by positional cloning as the gene responsible for autosomal dominant hypophosphatemic rickets (ADHR). It was simultaneously cloned as one factor causing tumor-induced osteomalacia (TIO). FGF23, a protein of 251 amino acids, is produced primarily by several bone cells including osteoblasts, osteocytes, and lining cells.

FGF23 is secreted in response to dietary phosphorus load, and its main function is the promotion of urinary phosphate excretion and the suppression of active vitamin D production by the kidney. Emerging evidence suggests that FGF23 is the principal regulator of proximal renal tubule phosphate reabsorption and of 1,25-dihydroxyvitamin D synthesis. Serum levels of FGF23 rise precipitously in patients with chronic kidney disease and are inversely correlated with the degree of hyperphosphatemia. Elevated FGF23 may contribute to impaired production of 1,25-dihydroxyvitamin D.

Klotho was originally identified as a gene mutated in mice that developed accelerated aging. Further analysis revealed a pattern of altered mineral ion metabolism and diminished bone density similar to that found in FGF23 knockout mice or patients with ADHR. The similar phenotypes of Klotho-deficient and FGF23-null mice suggested that Klotho and FGF23 function in a common signaling pathway. Klotho acts as a cofactor in FGF signaling, facilitates the interactions of FGF23 with its receptor, and determines its tissue specificity (Razzaque, 2009). Exogenous FGF23 administration reduces serum P_i and calcitriol synthesis. Vitamin D analogs are commonly used in the management of chronic kidney disease. Although no clinical agents based on FGF23 have yet been developed, bioactive fragments or FGF23 inhibitors might become useful in counterbalancing the hyperphosphatemic actions of vitamin D therapy.

Skeletal Organization. It is useful to consider the appendicular, or peripheral, skeleton as separate from the axial, or central, skeleton because their turnover rates differ. Appendicular bones make up 80% of bone mass and are composed predominantly of compact cortical bone. Axial bones, such as the spine and pelvis, contain substantial amounts of trabecular bone within a thin cortex. Trabecular bone consists of highly connected bony plates that resemble a honeycomb. The intertrabecular interstices contain bone marrow and fat. Alterations in bone turnover are observed first and foremost in axial bone both because bone surfaces, where bone remodeling occurs, are more densely distributed in trabecular bone and because marrow precursor cells that ultimately participate in bone turnover lie in close proximity to trabecular surfaces.

Bone Mass. Bone mineral density (BMD) and fracture risk in later years reflect the maximal bone mineral content at skeletal maturity (peak bone mass) and the subsequent rate of bone loss. Major increases in bone mass, accounting for ~60% of final adult levels, occur during adolescence, mainly during years of highest growth velocity. Bone acquisition is largely complete by age 17 in girls and by age 20 in boys. Inheritance accounts for much of the variance in bone acquisition; other factors include circulating estrogen and androgens, physical activity, and dietary calcium.

Bone mass peaks during the third decade, remains stable until age 50, and then declines progressively. Similar trajectories occur for men and women of all ethnic groups. The fundamental accuracy of this model has been amply confirmed for cortical bone, although trabecular bone loss at some sites probably begins prior to age 50. In women, loss of estrogen at menopause accelerates the rate of bone loss. Primary regulators of adult bone mass include physical activity, reproductive endocrine status, and calcium intake. Optimal maintenance of BMD requires sufficiency in all three areas, and deficiency of one is not compensated by excessive attention to another.

Bone Remodeling. Growth and development of endochondral bone are driven by a process called modeling. Once new bone is laid down, it is subject to a continuous process of breakdown and renewal called remodeling, by which bone mass is adjusted throughout adult life (Ballock and O'Keefe, 2003). Remodeling is carried out by myriad independent "bone remodeling units" throughout the skeleton (Figure 44–8). Remodeling proceeds on bone surfaces, ~90% of which are normally inactive, covered by a thin layer of lining cells. In response to physical or biochemical signals, recruitment of marrow precursor cells to the bone surface results in their fusion into the characteristic multinucleated osteoclasts that resorb, or excavate, a cavity into the bone.

Osteoclast production is regulated by osteoblast-derived cytokines such as IL-1 and IL-6. Studies have begun to clarify the mechanisms through which osteoclast production is regulated (Suda et al., 1999). The receptor for activating NF-κB (RANK) is an osteoclast protein whose expression is required for osteoclastic bone resorption. Its natural ligand, RANK ligand (RANKL; previously called osteoclast differentiation factor), is a membrane-spanning osteoblast protein. On binding to RANK, RANKL induces osteoclast formation (Khosla, 2001) (Figure 44–9). RANKL initiates the activation of mature osteoclasts, as well as the differentiation of osteoclast precursors. Osteoblasts produce osteoprotegerin (OPG), which acts as a decoy ligand for RANKL. Under conditions favoring increased bone resorption, such as estrogen deprivation, OPG is suppressed, RANKL binds to RANK, and osteoclast production increases. When estrogen sufficiency is reestablished, OPG increases and competes effectively with RANKL for binding to RANK. In certain model systems, OPG is superior to bisphosphonates in suppressing bone resorption and hypercalcemia (Morony et al., 2005).

Completion of the resorption phase is followed by invasion of preosteoblasts derived from marrow stroma into the base of the resorption cavity. These cells develop the characteristic osteoblastic phenotype and begin to replace the resorbed bone by elaborating new bone matrix constituents, such as collagen and osteocalcin. Once the newly formed osteoid reaches a thickness of ~20 μm, mineralization begins. A complete remodeling cycle normally requires ~6 months.

If the replacement of resorbed bone precisely matched the amount that was removed, remodeling would not change net bone mass. However, small bone deficits persist on completion of each cycle, reflecting inefficient remodeling dynamics. Consequently, lifelong accumulation of remodeling deficits underlies the well-documented phenomenon of age-related bone loss, a process that begins shortly after growth stops. Alterations in remodeling activity represent the final pathway through which diverse stimuli, such as dietary sufficiency, exercise, hormones, and drugs, affect bone balance. Changes in the hormonal milieu often lead to an increase in the activation of remodeling units. Examples include hyperthyroidism, hyperparathyroidism, and hypervitaminosis D. Other factors that may impair osteoblast function include high doses of corticosteroids or ethanol. Finally, estrogen deficiency augments osteoclastic resorptive capacity by a proapoptotic action (Manolagas et al., 2002).

At any given time, a transient deficit in bone, the remodeling space, represents sites of bone resorption that have not yet filled in. Stimuli that alter the rate of emergence of new remodeling units either increase or decrease the remodeling space until a new steady state is established at an increased or decreased bone mass.

Symptoms and signs of primary hyperparathyroidism include fatigue, exhaustion, weakness, polydipsia, polyuria, joint pain, bone pain, constipation, depression, anorexia, nausea, heartburn, nephrolithiasis, and hematuria. This condition frequently is accompanied by significant hypophosphatemia owing to the effects of PTH in diminishing renal tubular phosphate reabsorption. Some patients have renal calculi and peptic ulceration, and a few display classical parathyroid skeletal disease. However, most patients show few, if any, symptoms, and those that are present are often vague and nonspecific. Contemporary clinical assays that distinguish between full-length PTH(1–84) and "intact" PTH [PTH(1–84) + PTH(7–84)] obviate many of the difficulties with previous assays and, in conjunction with an elevated serum calcium, possess a diagnostic accuracy of >90% (Jüppner and Potts, 2002).

Familial benign hypercalcemia (or familial hypocalciuric hypercalcemia) is a genetic disorder generally accompanied by extremely low urinary calcium excretion. Familial benign hypercalcemia results from heterozygous mutations in the Ca²⁺-sensing receptor (Pollak et al., 1996). Hypercalcemia usually is mild, and circulating PTH often is normal to slightly elevated. The importance of making this diagnosis lies in the fact that patients mistakenly diagnosed as having primary hyperparathyroidism may undergo surgical exploration without discovery of an adenoma and without therapeutic benefit. Patients do not experience long-term clinical consequences, except for homozygous infants, who may have severe, even lethal, hypercalcemia. Diagnosis is established by demonstrating hypercalcemia in first-degree family members and a decreased fractional excretion of calcium.

Newly diagnosed hypercalcemia in hospitalized patients is caused most often by a systemic malignancy, either with or without bony metastasis. PTH-related protein (PTHrP) is a primitive, highly conserved protein that may be abnormally expressed in malignant tissue, particularly by squamous cell and other epithelial cancers. The substantial sequence homology of the amino-terminal portion of PTHrP with that of native PTH permits it to interact with the PTH-1 receptor in target tissues, thereby causing the hypercalcemia and hypophosphatemia seen in humoral hypercalcemia of malignancy (Grill et al., 1998). Other tumors release cytokines or prostaglandins that stimulate bone resorption. In some patients with lymphomas, hypercalcemia results from overproduction of 1,25-dihydroxyvitamin D by the tumor cells owing to expression of 1α-hydroxylase. A similar mechanism underlies the hypercalcemia that is seen occasionally in sarcoidosis and other granulomatous disorders.

Hypercalcemia associated with malignancy generally is more severe than in primary hyperparathyroidism (frequently with calcium levels that exceed 13 mg/dL) and may be associated with lethargy, weakness, nausea, vomiting, polydipsia, and polyuria. Assays for PTHrP may aid diagnosis.

Vitamin D excess may cause hypercalcemia if sufficient 25-hydroxyvitamin D is present to stimulate intestinal Ca²⁺ hyperabsorption, leading to hypercalcemia and suppressing PTH and 1,25-dihydroxyvitamin D levels. Measurement of 25-hydroxyvitamin D is diagnostic. Occasional patients with hyperthyroidism show mild hypercalcemia, presumably owing to increased bone turnover. Immobilization may lead to hypercalcemia in growing children and young adults but rarely causes hypercalcemia in older individuals unless bone turnover is already increased, as in Paget's disease or hyperthyroidism. Hypercalcemia sometimes is noted in adrenocortical deficiency, as in Addison's disease, or following removal of a hyperfunctional adrenocortical tumor. Hypercalcemia occurs following renal transplantation owing to persistent hyperfunctioning parathyroid tissue that resulted from the previous renal failure.

The differential diagnosis of hypercalcemia may pose difficulties, but advances in serum assays for PTH, PTHrP, and 25-hydroxy- and 1,25-dihydroxyvitamin D permit accurate diagnosis in the great majority of cases.

Combined deprivation of Ca²⁺ and vitamin D, as observed with malabsorption states or dietary deficiency, readily promotes hypocalcemia. When caused by malabsorption, hypocalcemia is accompanied by low concentrations of phosphate, total plasma proteins, and magnesium. Mg²⁺ deficiency may accentuate the hypocalcemia by diminishing the secretion and action of PTH. In infants with malabsorption or inadequate calcium intake, Ca²⁺ concentrations usually are depressed, with attendant hypophosphatemia resulting in rickets.

Hypoparathyroidism is most often a consequence of thyroid or neck surgery but also may be due to genetic or autoimmune disorders. In hypoparathyroidism, hypocalcemia is accompanied by hyperphosphatemia, reflecting decreased PTH action on renal phosphate transport.

Pseudohypoparathyroidism is a diverse family of hypocalcemic and hyperphosphatemic disorders. Pseudohypoparathyroidism results from resistance to PTH rather than PTH deficiency; this resistance is not due to mutations of the PTH receptor but rather to mutations in G_sα (GNAS1), which normally mediates hormone-induced adenylyl cyclase activation (Yu et al., 1999). The variable phenotypes arising from GNAS1 defects apparently are due to differential genomic imprinting of the maternal and paternal alleles (Levine et al., 2003). Multiple hormonal abnormalities have been associated with the GNAS1 mutation, but none is as severe as the deficient response to PTH.

Hypocalcemia is not unusual in the first several days following removal of a parathyroid adenoma. If hyperphosphatemia is also present, the condition is one of functional hypothyroidism owing to temporary failure of the remaining parathyroid glands to compensate for the missing adenomatous tissue. In patients with parathyroid bone disease, postoperative hypocalcemia associated with hypophosphatemia may reflect rapid uptake of calcium into bone, the "hungry bone" syndrome. In this setting, persistent, severe hypocalcemia may require administration of vitamin D and supplemental calcium for several months.

Neonatal tetany resulting from hypocalcemia sometimes occurs in infants of mothers with hyperparathyroidism; indeed, the tetany may call attention to the mother's disorder. This problem disappears when the infant's own parathyroid glands develop sufficiently to respond appropriately.

Hypocalcemia is associated with advanced renal insufficiency accompanied by hyperphosphatemia. Many patients with this condition do not develop tetany unless the accompanying acidosis is treated, which decreases the ionized calcium. High concentrations of phosphate in plasma inhibit the conversion of 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol. Hypocalcemia also can occur following massive transfusions with citrated blood, which chelates calcium.

Hyperphosphatemia is an important component of the bone disease seen in chronic renal failure. In this condition, phosphate retention is primary and reflects the degree of renal compromise. The increased phosphate level reduces the serum Ca²⁺ concentration, which, in turn, activates the parathyroid gland calcium-sensing receptor, stimulates PTH secretion, and exacerbates the hyperphosphatemia. The continuing hyperphosphatemia may be modified by vigorous administration of aluminum hydroxide gel or calcium carbonate supplements. As discussed later, the FDA recently approved therapeutic use of the calcium-sensing receptor agonist cinacalcet to suppress PTH secretion.

The amount of vitamin D necessary to cause hypervitaminosis varies widely. As a rough approximation, continued daily ingestion of ≥50,000 units by a person with normal parathyroid function and sensitivity to vitamin D may result in poisoning. Hypervitaminosis D is particularly dangerous in patients who are receiving digoxin because the toxic effects of the cardiac glycosides are enhanced by hypercalcemia (Chapter 28).

The initial signs and symptoms of vitamin D toxicity are those associated with hypercalcemia. Because hypercalcemia in vitamin D intoxication generally is due to very high circulating levels of 25-OHD, the plasma concentrations of PTH and calcitriol typically (but not uniformly) are suppressed. In children, a single episode of moderately severe hypercalcemia may arrest growth completely for ≥6 months, and the deficit in height may never be fully corrected. Vitamin D toxicity in the fetus is associated with excess maternal vitamin D intake or extreme sensitivity and may result in congenital supravalvular aortic stenosis. In infants, this anomaly frequently is associated with other stigmata of hypercalcemia.

Vitamin D Deficiency. Vitamin D deficiency results in inadequate absorption of Ca²⁺ and phosphate. The consequent decrease of plasma Ca²⁺ concentration stimulates PTH secretion, which acts to restore plasma Ca²⁺ at the expense of bone. Plasma concentrations of phosphate remain subnormal because of the phosphaturic effect of increased circulating PTH. In children, the result is a failure to mineralize newly formed bone and cartilage matrix, causing the defect in growth known as rickets. As a consequence of inadequate calcification, bones of individuals with rickets are soft, and the stress of weight bearing gives rise to bowing of the long bones.

In adults, vitamin D deficiency results in osteomalacia, a disease characterized by generalized accumulation of undermineralized bone matrix. Severe osteomalacia may be associated with extreme bone pain and tenderness. Muscle weakness, particularly of large proximal muscles, is typical and may reflect both hypophosphatemia and inadequate vitamin D action on muscle. Gross deformity of bone occurs only in advanced stages of the disease. Circulating 25-OHD concentrations <8 ng/mL are highly predictive of osteomalacia.

Metabolic Rickets and Osteomalacia. These disorders are characterized by abnormalities in calcitriol synthesis or response.

Hypophosphatemic vitamin D–resistant rickets, in its most common form, is an X-linked disorder (XLH) of calcium and phosphate metabolism. Calcitriol levels are inappropriately normal for the observed degree of hypophosphatemia. Patients experience clinical improvement when treated with large doses of vitamin D, usually in combination with inorganic phosphate. Even with vitamin D treatment, calcitriol concentrations may remain lower than expected. The genetic basis for XLH has been defined. The affected protein, a phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX), is a neutral endoprotease. The substrate for this enzyme likely is involved in renal phosphate transport. Syndromes closely related to XLH, in which phosphate levels are altered without significant net changes in serum concentrations of calcium, PTH, or 1,25(OH)₂D₃, include hereditary hypophosphatemic rickets with hypercalciuria (HHRH) and autosomal dominant hypophosphatemic rickets. The latter disorder maps to chromosome 12p13.3 and is associated with mutations in the gene encoding fibroblast growth factor 23 (White et al., 2001; Bergwitz and Jüppner, 2010).

Vitamin D–dependent rickets (also called vitamin D–dependent rickets type I, VDDR-1 or pseudovitamin D-deficiency rickets (PDDR) is an autosomal recessive disease caused by an inborn error of vitamin D metabolism involving defective conversion of 25-OHD to calcitriol owing to mutations in CYP1α (1α-hydroxylase). The condition responds to physiological doses of calcitriol. An initial dose of 1-3 μg/day is used to heal rickets, after which a maintenance dose of 0.25-1 μg/day can be used. 1αOHD is also effective as slightly higher dosages (2-5 μg/day and 1-2 μg/day, respectively). Other vitamin D analogs upstream of the 1α-hydroxylase have little activity.

Hereditary 1,25-dihydroxyvitamin D resistance (HVDDR, also called vitamin D–dependent rickets type II) is an autosomal recessive disorder that is characterized by hypocalcemia, osteomalacia, rickets, and total alopecia. Multiple heterogenous mutations of the vitamin D receptor cause vitamin D–dependent rickets type II (Malloy et al., 2005). Absolute hormone resistance results from premature stop mutations, missense mutations in the zinc finger DNA-binding domain, mutations of the receptor ligand-binding domain, or mutations that affect heterodimerization of the VDR with the retinoid X receptor (RXR).

Serum abnormalities include low serum concentrations of calcium and phosphate and elevated serum alkaline phosphatase activity. The hypocalcemia leads to secondary hyperparathyroidism with elevated PTH levels and hypophosphatemia. The 25(OH)-vitamin D values are normal, whereas 1,25(OH)₂-vitamin D levels are elevated in type II vitamin D–dependent rickets. This clinical feature distinguishes hereditary vitamin D–dependent rickets type II from CYP1α deficiency (vitamin D–dependent rickets type I), where serum 1,25(OH)2-vitamin D values are depressed. Children affected by vitamin D–dependent rickets type II are refractory even to massive doses of vitamin D and calcitriol, and they may require prolonged treatment with parenteral Ca²⁺. Some remission of symptoms has been observed during adolescence, but the basis of remission is unknown. Notably, missense mutations in the ligand-binding domain have been described that result only in partial hormone resistance. Thus, the use of calcitriol analogs that bind to the VDR at different amino acids may provide therapeutic opportunities not otherwise available. This represents a prime application of pharmacogenetics, where "personalized" treatment based on the specific VDR mutations can be envisioned.

Renal osteodystrophy (renal rickets) refers to the disordered bone morphology that attends chronic kidney disease. It is characterized by abnormalities of bone turnover, mineralization, volume, linear growth, or strength, as well as underlying defects in mineral ion, PTH, or vitamin D metabolism. In the early phase of chronic renal failure, physiological concentrations of 1,25(OH)2D in circulation may become insufficient to regulate parathyroid cell function normally. As a result, the set point of PTH secretion to plasma Ca²⁺ concentration shifts to the right, PTH synthesis becomes enhanced at the transcriptional level, and parathyroid cells begin to proliferate. This resistance of the parathyroid cells to the physiological concentration of 1,25(OH)2D may be partially due to decreased conversion of 25-OHD to calcitriol. Phosphate retention and diminished serum calcium also increase PTH mRNA levels but at the post-transcriptional level. In addition, calcitriol deficiency impairs intestinal Ca²⁺ absorption and mobilization from bone. Hypocalcemia commonly results (although in some patients, prolonged and severe hyperparathyroidism eventually may lead to hypercalcemia). Aluminum deposition in bone, once a cause of renal osteodystrophy, is no longer an issue since aluminum was removed from dialysis solutions.

Osteoporosis can be categorized as primary or secondary. In 1948, Albright and Reifenstein concluded that primary osteoporosis included two separate entities: one related to menopausal estrogen loss and the other to aging. This concept was extended by the recognition that primary osteoporosis represents two fundamentally different conditions: type I osteoporosis, characterized by loss of trabecular bone owing to estrogen lack at menopause, and type II osteoporosis, characterized by loss of cortical and trabecular bone in men and women due to long-term remodeling inefficiency, dietary inadequacy, and activation of the parathyroid axis with age. It is not clear, however, that these two entities are truly distinct. Although many osteoporotic women undoubtedly have experienced excessive menopausal bone loss, it may be more appropriate to consider osteoporosis as the result of multiple physical, hormonal, and nutritional factors acting alone or in concert.

Secondary osteoporosis is due to systemic illness or medications such as glucocorticoids or phenytoin. The most successful approach to secondary osteoporosis is prompt resolution of the underlying cause or drug discontinuation. Whether primary or secondary, osteoporosis is associated with characteristic disordered bone remodeling, so the same therapies can be used.

Paget's Disease. Paget's disease is characterized by single or multiple sites of disordered bone remodeling. The etiology of the disease is uncertain but is thought to be the result of infection with the measles virus of the paramyxovirus family (Roodman and Windle, 2005). It affects up to 2-3% of the population >60 years of age. The primary pathologic abnormality is increased bone resorption followed by exuberant bone formation. However, the newly formed bone is disorganized and of poor quality, resulting in characteristic bowing, stress fractures, and arthritis of joints adjoining the involved bone. Pagetic lesions contain many abnormal multinucleated osteoclasts associated with a disordered mosaic pattern of bone formation. Pagetic bone is thickened and has abnormal microarchitecture. The altered bone structure can produce secondary problems, such as deafness, spinal cord compression, high-output cardiac failure, and pain. Malignant degeneration to osteogenic sarcoma is a rare but lethal complication of Paget's disease.

Renal Osteodystrophy. Bone disease is a frequent consequence of chronic renal failure and dialysis. Pathologically, lesions are typical of hyperparathyroidism (osteitis fibrosa), vitamin D deficiency (osteomalacia), or a mixture of both. The underlying pathophysiology reflects increased serum phosphate and decreased calcium, leading to secondary events that strive to preserve circulating levels of mineral ions at the expense of bone.

Corticosteroids administered at high doses (e.g., 40-80 mg/day of prednisone) may be useful when hypercalcemia results from sarcoidosis, lymphoma, or hypervitaminosis D (Chapter 42). The response to steroid therapy is slow; from 1-2 weeks may be required before plasma Ca²⁺ concentration falls.

Calcitonin (calcimar, miacalcin) may be useful in managing hypercalcemia. Reduction in Ca²⁺ can be rapid, although "escape" from the hormone commonly occurs within several days. The recommended starting dose is 4 units/kg of body weight administered subcutaneously every 12 hours; if there is no response within 1-2 days, the dose may be increased to a maximum of 8 units/kg every 12 hours. If the response after 2 more days still is unsatisfactory, the dose may be increased to a maximum of 8 units/kg every 6 hours. Calcitonin can lower serum calcium by 1-2 mg/dL.

Intravenous bisphosphonates (pamidronate, zoledronate) have proven very effective in the management of hypercalcemia (see further discussion of bisphosphonates later in the chapter). These agents potently inhibit osteoclastic bone resorption. Oral bisphosphonates are less effective for treating hypercalcemia. Therefore, pamidronate (aredia) is given as an intravenous infusion of 60-90 mg over 4-24 hours; the more prolonged infusion (>4 hours) is reserved primarily for patients with renal dysfunction. With pamidronate, resolution of hypercalcemia occurs over several days, and the effect usually persists for several weeks. Zoledronate (zometa) has superseded pamidronate because of its more rapid normalization of serum calcium and longer duration of action.

Plicamycin (mithramycin, mithracin) is a cytotoxic antibiotic that also decreases plasma Ca²⁺ concentrations by inhibiting bone resorption. Reduction in plasma Ca²⁺ concentrations occurs within 24-48 hours when a relatively low dose of this agent is given (15-25 μg/kg of body weight) to minimize the high systemic toxicity of the drug; indeed, its toxicity generally precludes its use.

Oral sodium phosphate lowers plasma Ca²⁺ concentrations and may offer short-term calcemic control of some patients with primary hyperparathyroidism who are awaiting surgery. However, the risk of precipitating calcium phosphate salts in soft tissues throughout the body is of concern. In light of satisfactory responses to other agents, administration of intravenous sodium phosphate is not recommended as a treatment for hypercalcemia.

Once the hypercalcemic crisis has resolved or in patients with milder calcium elevations, therapy turns to more durable resolution of the hypercalcemic state. Parathyroidectomy remains the only definitive treatment for primary hyperparathyroidism. Specific indications for surgery have been proposed (Bilezikian et al., 2002). In the hands of a skilled parathyroid surgeon, resection of a single adenoma (~80% of cases) or of the hyperplastic glands (~15% of cases) cures hyperparathyroidism. Complications include transient postoperative hypocalcemia, which may reflect temporary disruption of blood supply to the remaining parathyroid tissue or skeletal avidity for calcium, and permanent hypoparathyroidism. As described later, a calcium mimetic that stimulates the calcium-sensing receptor is a promising new therapy for hyperparathyroidism that may be used increasingly in the future.

Therapy of hypercalcemia of malignancy ideally is directed at the underlying cancer. When this is not possible, parenteral bisphosphonates often will maintain calcium levels within an acceptable range.

Calcium is used in the treatment of calcium deficiency states and as a dietary supplement. Ca²⁺ salts are specific in the immediate treatment of hypocalcemic tetany regardless of etiology. In severe tetany, symptoms are best brought under control by intravenous medication. Calcium chloride (CaCl₂·2H₂O) contains 27% Ca²⁺; it is valuable in the treatment of hypocalcemic tetany and laryngospasm. The salt is given intravenously and must never be injected into tissues. Injections of calcium chloride are accompanied by peripheral vasodilation and a cutaneous burning sensation. The salt usually is given intravenously in a concentration of 10% (equivalent to 1.36 mEq Ca²⁺/mL). The rate of injection should be slow (not >1 mL/minute) to prevent cardiac arrhythmias from a high concentration of Ca²⁺. The injection may induce a moderate fall in blood pressure owing to vasodilation. Because calcium chloride is an acidifying salt, it is usually undesirable in the treatment of the hypocalcemia caused by renal insufficiency. Calcium gluceptate injection (a 22% solution; 18 mg or 0.9 mEq of Ca²⁺/mL) is administered intravenously at a dose of 5-20 mL for the treatment of severe hypocalcemic tetany; the injection produces a transient tingling sensation when given too rapidly. When the intravenous route is not possible, injections may be given intramuscularly in the gluteal region at a dose of up to 5 mL. A mild local reaction may result. Calcium gluconate injection (a 10% solution; 9.3 mg of Ca²⁺/mL) given intravenously is the treatment of choice for severe hypocalcemic tetany. Patients with moderate-to-severe hypocalcemia may be treated by intravenous infusion of calcium gluconate at a dose of 10-15 mg of Ca²⁺/kg of body weight over 4-6 hours. Because the usual 10-mL vial of a 10% solution contains only 93 mg Ca²⁺, many vials are needed. The intramuscular route should not be employed because abscess formation at the injection site may result.

For control of milder hypocalcemic symptoms, oral medication suffices, frequently in combination with vitamin D or one of its active metabolites. Calcium salts are acidifying, and different forms can be interchanged to avoid gastric irritation. Available Ca²⁺ salts include calcium carbonate, lactate, gluconate, phosphate, citrate, and hydroxyapatite. Calcium carbonate is prescribed most frequently, whereas calcium citrate may be absorbed more efficiently than other salts. However, absorption efficiency for most commonly prescribed calcium products is reasonable, and for many patients, cost and palatability outweigh modest differences in efficacy. Average doses for hypocalcemic patients are calcium gluconate, 15 g/day in divided doses; calcium lactate, 7.7 g plus 8 g lactose with each meal; and calcium carbonate or calcium phosphate, 1-2 g with meals.

Calcium carbonate and calcium acetate are used to restrict phosphate absorption in patients with chronic renal failure and oxalate absorption in patients with inflammatory bowel disease. Acute administration of calcium may be lifesaving in patients with extreme hyperkalemia (serum K⁺>7 mEq/L). Calcium gluconate (10-30 mL of a 10% solution) can reverse some of the cardiotoxic effects of hyperkalemia, providing time while other efforts are taken to lower the plasma K⁺ concentration.

Additional FDA-approved uses of calcium include intravenous treatment for black widow spider envenomation and management of magnesium toxicity. Use of supplemental calcium in the prevention and treatment of osteoporosis is discussed later.

Doxercalciferol (1α-hydroxyvitamin D₂, hectorol) is a prodrug that first must be activated by hepatic 25-hydroxylation to generate the biologically active compound, 1α,25-(OH)₂D₂ (Figure 44–10). The FDA has approved oral and intravenous preparations of 1α-hydroxyvitamin D₂ for use in treating secondary hyperparathyroidism, starting at 10 mg three times per week.

Dihydrotachysterol (dht, roxane) is a reduced form of vitamin D₂. In the liver DHT is converted to its active form, 25-hydroxydihydrotachysterol. DHT is <1% as active as calcitriol in antirachitic assays but is much more effective in mobilizing bone mineral at high doses; it therefore can be used to maintain plasma Ca²⁺ in hypoparathyroidism. DHT is well absorbed from the GI tract and maximally increases serum calcium concentration after 2 weeks of daily administration. The hypercalcemic effects typically persist for 2 weeks but can last for up to 1 month. DHT is available for oral administration in doses ranging from 0.2-1 mg/day (average 0.6 mg/day).

1α-Hydroxycholecalciferol (1-OHD₃, alphacalcidol; one-alpha) was introduced as a substitute for 1,25(OH)₂D₃; alphacalcidol is a synthetic vitamin D₃ derivative that is already hydroxylated in the 1α position and is rapidly hydroxylated by 25-hydroxylase to form 1,25-(OH)₂D₃. It is equal to calcitriol in assays for stimulation of intestinal absorption of Ca²⁺ and bone mineralization and does not require renal activation. It therefore has been used to treat renal osteodystrophy and is available in the U.S. for experimental purposes.

Ergocalciferol (calciferol, drisdol) is pure vitamin D₂. It is available for oral, intramuscular, or intravenous administration. Ergocalciferol is indicated for the prevention of vitamin D deficiency and the treatment of familial hypophosphatemia, hypoparathyroidism, and vitamin D–resistant rickets type II, typically in doses of 50,000-200,000 units/day in conjunction with calcium supplements.

Analogs of Calcitriol. Several vitamin D analogs (Figure 44–10) suppress PTH secretion by the parathyroid glands but have less or negligible hypercalcemic activity. They therefore offer a safer and more effective means of controlling secondary hyperparathyroidism.

Calcipotriol (calcipotriene) is a synthetic derivative of calcitriol with a modified side chain that contains a 22–23 double bond, a 24(S)-hydroxy functional group, and carbons 25-27 incorporated into a cyclopropane ring. Calcipotriol has comparable affinity with calcitriol for the vitamin D receptor, but it is <1% as active as calcitriol in regulating calcium metabolism. This reduced calcemic activity largely reflects the pharmacokinetics of calcipotriol (Kissmeyer and Binderup, 1991). Calcipotriol has been studied extensively as a treatment for psoriasis (Chapter 65), although its mode of action is not known; a topical preparation (dovonex) is available for that purpose. In clinical trials, topical calcipotriol has been found to be slightly more effective than glucocorticoids with a good safety profile.

Paricalcitol (1,25-dihydroxy-19-norvitamin D₂, zemplar) is a synthetic calcitriol derivative that lacks the exocyclic C19 and has a vitamin D₂ rather than vitamin D₃ side chain (Figure 44–10). It reduces serum PTH levels without producing hypercalcemia or altering serum phosphorus (Martin et al., 1998). In an animal model, paricalcitol prevented or reversed PTH-induced high-turnover bone disease (Slatopolsky et al., 2003). Paricalcitol administered intravenously is FDA-approved for treating secondary hyperparathyroidism in patients with chronic renal failure.

22-Oxacalcitriol (1,25-dihydroxy-22-oxavitamin D₃, OCT, maxacalcitol, oxarol) differs from calcitriol only in the substitution of C-22 with an oxygen atom. Oxacalcitriol has a low affinity for vitamin D–binding protein; as a result, more of the drug circulates in the free (unbound) form, allowing it to be metabolized more rapidly than calcitriol with a consequent shorter t_1/2. Oxacalcitriol is a potent suppressor of PTH gene expression and shows very limited activity on intestine and bone. It is a useful compound in patients with overproduction of PTH in chronic renal failure. It should be borne in mind that persuasive evidence supports the use of cinacalcet for treatment of secondary hyperparathyroidism (Drueke and Ritz, 2009). Indeed, emerging findings show the clinical efficacy of cinacalcet for managing primary hyperparathyroidism (Peacock et al., 2009).

Nutritional Rickets. Nutritional rickets results from inadequate exposure to sunlight or deficiency of dietary vitamin D. The condition, once extremely rare in the U.S. and other countries where food fortification with the vitamin is practiced, is now increasing. Infants and children receiving adequate amounts of vitamin D–fortified food do not require additional vitamin D; however, breast-fed infants or those fed unfortified formula should receive 400 units of vitamin D daily as a supplement (Wagner and Greer, 2008). The usual practice is to administer vitamin A in combination with vitamin D. A number of balanced vitamin A and D preparations are available for this purpose. Because the fetus acquires >85% of its calcium stores during the third trimester, premature infants are especially susceptible to rickets and may require supplemental vitamin D.

Treatment of fully developed rickets requires a larger dose of vitamin D than that used prophylactically. One thousand units daily will normalize plasma Ca²⁺ and phosphate concentrations in ~10 days, with radiographic evidence of healing within ~3 weeks. However, a larger dose of 3000–4000 units daily often is prescribed for more rapid healing, particularly when respiration is compromised by severe thoracic rickets.

Vitamin D may be given prophylactically in conditions that impair its absorption (e.g., diarrhea, steatorrhea, and biliary obstruction). Parenteral administration also may be used in such cases.

Treatment of Osteomalacia and Renal Osteodystrophy. Osteomalacia, distinguished by undermineralization of bone matrix, occurs commonly during sustained phosphate depletion. Patients with chronic renal disease are at risk for developing osteomalacia but also may develop a complex bone disease called renal osteodystrophy. In this setting, bone metabolism is stimulated by an increase in PTH and by a delay in bone mineralization that is due to decreased renal synthesis of calcitriol. In renal osteodystrophy, low bone mineral density may be accompanied by high-turnover bone lesions typically seen in patients with uncontrolled hyperparathyroidism or by low bone remodeling activity seen in patients with adynamic bone disease. The therapeutic approach to the patient with renal osteodystrophy depends on its specific type. In high-turnover (hyperparathyroid) or mixed high-turnover disease with deficient mineralization, dietary phosphate restriction, generally in combination with a phosphate binder, is recommended because phosphate restriction is limited by the need to provide adequate protein intake to maintain nitrogen balance. Although highly effective, aluminum is no longer used as a phosphate binder because it promotes adynamic bone disease, anemia, myopathy, and occasionally dementia. Calcium-containing phosphate binders along with calcitriol administration may contribute to oversuppression of PTH secretion and likewise result in adynamic bone disease and an increased incidence of vascular calcification. Highly effective non-calcium-containing phosphate binders have been developed. Sevelamer hydrochloride (renagel), a nonabsorbable phosphate-binding polymer, effectively lowers serum phosphate concentration in hemodialysis patients, with a corresponding reduction in the calcium × phosphate product. Sevelamer hydrochloride consists of cross-linked poly[allylamine hydrochloride] that is resistant to digestive degradation. Its partially protonated amines spaced one carbon from the polymer backbone chelate phosphate ions. Sevelamer is modestly water soluble and only trace amounts are absorbed from the GI tract. However, with continued use, progressive accumulation has been reported in experimental animals. Side effects of sevelamer include vomiting, nausea, diarrhea, and dyspepsia. Sevelamer does not affect the bioavailability of digoxin, warfarin, enalapril, or metoprolol. Lanthanum carbonate (fosrenol) is a poorly permeable trivalent cation that is useful in treating the hyperphosphatemia associated with renal osteodystrophy.

Renal osteodystrophy associated with low bone turnover (adynamic bone disease) is increasingly common and may be due to oversuppression of PTH with aggressive use of either calcitriol or other vitamin D analogs. Although PTH levels generally are low (<100 pg/mL), a high PTH level does not exclude the presence of adynamic bone disease, especially with PTH assays that do not distinguish between biologically active and inactive PTH fragments (Monier-Faugere et al., 2001). Current guidelines suggest that treatment with an active vitamin D preparation is indicated if serum 25-OHD levels are <30 ng/mL and serum calcium is <9.5 mg/dL (2.37 mM). However, if 25-OHD and serum calcium levels are elevated, vitamin D supplementation should be discontinued. If the serum calcium level is <9.5 mg/dL, treatment with a vitamin D analog is warranted irrespective of the 25-OHD level (Eknoyan et al., 2003).

Hypoparathyroidism. Vitamin D and its analogs are a mainstay of the therapy of hypoparathyroidism. Dihydrotachysterol (DHT) has a faster onset, shorter duration of action, and a greater effect on bone mobilization than does vitamin D and traditionally has been a preferred agent. Calcitriol also is effective in the management of hypoparathyroidism and certain forms of pseudohypoparathyroidism in which endogenous levels of calcitriol are abnormally low. However, most hypoparathyroid patients respond to any form of vitamin D. Calcitriol may be preferred for temporary treatment of hypocalcemia while awaiting effects of a slower-acting form of vitamin D.

Miscellaneous Uses of Vitamin D. Vitamin D is used to treat hypophosphatemia associated with Fanconi's syndrome. Large doses of vitamin D (>10,000 units/day) are not useful in patients with osteoporosis and even can be dangerous. However, administration of 400-800 units/day of vitamin D to frail elderly men and women has been shown to suppress bone remodeling, protect bone mass, and reduce fracture incidence (see later section on osteoporosis). Clinical trials suggest that calcitriol may become an important agent for the treatment of psoriasis (Kowalzick, 2001). As such nontraditional uses of vitamin D are discovered, it will become important to develop noncalcemic analogs of calcitriol that achieve effects on cellular differentiation without the risk of hypercalcemia.

Hypervitaminosis D is treated by immediate withdrawal of the vitamin, a low-calcium diet, administration of glucocorticoids, and vigorous fluid support. As noted earlier under hypercalcemia, forced saline diuresis with loop diuretics is also useful. With this regimen, the plasma Ca²⁺ concentration falls to normal, and Ca²⁺ in soft tissue tends to be mobilized. Conspicuous improvement in renal function occurs unless renal damage has been severe.

Calcitonin is effective in disorders of increased skeletal remodeling, such as Paget's disease, and in some patients with osteoporosis. In Paget's disease, chronic use of calcitonin produces long-term reductions of serum alkaline phosphatase activity and symptoms. Development of antibodies to calcitonin occurs with prolonged therapy, but this is not necessarily associated with clinical resistance. Side effects of calcitonin include nausea, hand swelling, urticaria, and, rarely, intestinal cramping. Side effects appear to occur with equal frequency with human and salmon calcitonin. Salmon calcitonin is approved for clinical use. The latter product also is available as a nasal spray, introduced for once-daily treatment of postmenopausal osteoporosis. For Paget's disease, calcitonin generally is administered by subcutaneous injection because intranasal delivery is relatively ineffective owing to limited bioavailability. After initial therapy at 100 units/day, the dose typically is reduced to 50 units three times a week.

The clinical utility of bisphosphonates resides in their direct inhibition of bone resorption. First-generation bisphosphonates contain minimally modified side chains (R₁ and R₂ in Figure 44-11) (medronate, clodronate, and etidronate) or posses a chlorophenol group (tiludronate) (Figure 44-11). They are the least potent and in some instances cause bone demineralization. Second-generation aminobisphosphonates (e.g., alendronate and pamidronate) contain a nitrogen group in the side chain. They are 10-100 times more potent than first-generation compounds. Third-generation bisphosphonates (e.g., risedronate and zoledronate) contain a nitrogen atom within a heterocyclic ring and are up to 10,000 times more potent than first-generation agents.

Bisphosphonates concentrate at sites of active remodeling. Because they are highly negatively charged, bisphosphonates are membrane impermeable but are incorporated into the bone matrix by fluid-phase endocytosis (Stenbeck and Horton, 2000). Bisphosphonates remain in the matrix until the bone is remodeled and then are released in the acid environment of the resorption lacunae beneath the osteoclast as the overlying mineral matrix is dissolved. The importance of this process for the antiresorptive effect of bisphosphonates is evidenced by the fact that calcitonin blocks the antiresorptive action.

Although bisphosphonates prevent hydroxyapatite dissolution, their antiresorptive action is due to direct inhibitory effects on osteoclasts rather than strictly physiochemical effects. The antiresorptive activity apparently involves two primary mechanisms: osteoclast apoptosis and inhibition of components of the cholesterol biosynthetic pathway.

The current model is that apoptosis accounts for the antiresorptive effect of first-generation bisphosphonates, whereas the inhibitory action of aminobisphosphonates proceeds through the latter mechanism. Consistent with this view, the antiresorptive effect of aminobisphosphonates such as alendronate and risedronate, but not of clodronate or etidronate, persists when apoptosis is suppressed (Halasy-Nagy et al., 2001). First-generation bisphosphonates are metabolized into a nonhydrolyzable ATP analog (AppCCl₂p) that accumulates within osteoclasts and induces apoptosis (Rogers, 2003). In contrast, the aminobisphosphonates such as alendronate and ibandronate directly inhibit multiple steps in the pathway from mevalonate to cholesterol and isoprenoid lipids, such as geranylgeranyl diphosphate, that are required for the prenylation of proteins that are important for osteoclast function. The potency of aminobisphosphonates for inhibiting farnesyl synthase correlates directly with their antiresorptive activity (Dunford et al., 2001).

Available Bisphosphonates

Several bisphosphonates are available in the U.S. (Figure 44–11). Etidronate sodium (didronel) is used for treatment of Paget's disease and may be used parenterally to treat hypercalcemia. Because etidronate is the only bisphosphonate that inhibits mineralization, it has been supplanted largely by pamidronate and zoledronate for treating hypercalcemia. Pamidronate (aredia) is approved for management of hypercalcemia and for prevention of bone loss in breast cancer and multiple myeloma, but it also is effective in other skeletal disorders. Pamidronate is available in the U.S. only for parenteral administration. For treatment of hypercalcemia, pamidronate may be given as an intravenous infusion of 60-90 mg over 4-24 hours.

Several newer bisphosphonates have been approved for treatment of Paget's disease. These include tiludronate (skelid), alendronate (fosamax), and risedronate (actonel). Tiludronate is approved for treatment of Paget's disease of bone. Standard dosing is 400 mg/day orally for 3 months. Tiludronate in recommended doses does not interfere with bone mineralization, unlike etidronate. Zoledronate (ZOMETA) is approved for treating Paget's disease and administered as a single 5-mg infusion decreased bone turnover markers for 6 months with no loss of therapeutic effect (Reid et al., 2005). Zometa is widely used for prevention of osteoporosis in prostate and breast cancer patients receiving hormonal therapy. It is also approved for treating hypercalcemia of malignancies and for preventing fractures and skeletal complications in cancer patients with bone metastases. It is widely used to prevent osteoporosis and fractures in breast and prostate cancer patients receiving hormone-antagonist treatment.

The potent bisphosphonate ibandronate is approved for the prevention and treatment of postmenopausal osteoporosis. The recommended oral dose is 2.5 mg daily or 150 mg once monthly, which appears to be as effective as daily treatment and is well tolerated.

Absorption, Fate, and Excretion. All oral bisphosphonates are very poorly absorbed from the intestine and have remarkably limited bioavailability (<1% [alendronate, risedronate] to 6% [etidronate, tiludronate]). Hence these drugs should be administered with a full glass of water following an overnight fast and at least 30 minutes before breakfast. Oral bisphosphonates have not been used widely in children or adolescents because of uncertainty of long-term effects of bisphosphonates on the growing skeleton.

Bisphosphonates are excreted primarily by the kidneys. Adjusted doses for patients with diminished renal function have not been determined; bisphosphonates currently are not recommended for patients with a creatinine clearance of <30 mL/minute.

Adverse Effects. Oral bisphosphonates, including alendronate, ibandronate, and risedronate, can cause heartburn, esophageal irritation, or esophagitis. Other GI side effects include abdominal pain and diarrhea. Symptoms often abate when patients take the medication after an overnight fast, with tap or filtered water (not mineral water), and remain upright. Esophageal complications are infrequent when the drug is taken as described. If symptoms persist despite these precautions, use of a nonsteroidal anti-inflammatory drug or acetaminophen can diminish these symptoms; a proton pump inhibitor at bedtime may be helpful (Chapter 45). Both drugs may be better tolerated on a once-weekly regimen with no reduction of efficacy. Patients with active upper GI disease should not be given oral bisphosphonates.

Serious osteonecrosis of the jaw is associated with use of bisphosphonates (Edwards et al., 2008). In 2008 the FDA issued an alert on the use of bisphosphonates (http://www.fda.gov/Drugs/DrugSafety/PostmarketDrugSafetyInformationforPatientsandProviders/ucm124165.htm) and product labeling was modified to highlight the severe musculoskeletal pain that may occur and its relation to osteonecrosis of the jaw.

Owing to cytokine release, initial parenteral infusion of pamidronate, may cause skin flushing, flu-like symptoms, muscle and joint aches and pains, nausea and vomiting, abdominal discomfort and diarrhea (or constipation) but mainly when given in higher concentrations or at faster rates than those recommended. These symptoms are short lived and generally do not recur with subsequent administration.

Zoledronate has more potent effects on calcium than some other bisphosphonates and is capable of causing severe hypocalcemia. It has been associated with renal toxicity, deterioration of renal function, and potential renal failure. Thus, the infusion should be given over at least 15 minutes, and the dose should be 4 mg. Patients who receive zoledronate should have standard laboratory and clinical parameters of renal function assessed prior to treatment and periodically after treatment to monitor for deterioration in renal function.

Other Therapeutic Uses

Hypercalcemia. The use of pamidronate in the management of malignancy-associated hypercalcemia was described earlier. Zoledronate appears to be more effective than pamidronate and at least as safe, can be infused over 15 minutes rather than 2-4 hours, and is FDA- approved for this indication.

Postmenopausal Osteoporosis. Much interest is focused on the role of bisphosphonates in the treatment of osteoporosis. Clinical trials show that treatment is associated with increased bone mineral density and protection against fracture.

Cancer. Bisphosphonates may also have direct antitumor action by inhibiting oncogene activation and through their anti-angiogenic effects. Randomized clinical trials of bisphosphonates in patients with breast cancer suggest that they delay or prevent development of metastases as a component of endocrine adjuvant therapy (Gnant et al., 2009).

Full-length human recombinant PTH(1-84) is in phase III trials for hypoparathyroidism, for which teriparatide is not approved. Intermittent PTH(1-84) may soon be approved for use in osteoporosis, but its benefits over PTH(1-34) remain to be established.

Absorption, Fate, and Excretion. Pharmacokinetics and systemic actions of teriparatide on mineral metabolism are the same as for PTH. Teriparatide is administered by once-daily subcutaneous injection of 20 μg into the thigh or abdomen. With this regimen, serum PTH concentrations peak at 30 minutes after the injection and decline to undetectable concentrations within 3 hours, whereas the serum calcium concentration peaks at 4-6 hours after administration. Based on aggregate data from different dosing regimens, teriparatide bioavailability averages 95%. Teriparatide clearance averages 62 L/hour in women and 94 L/hour in men, which exceeds normal liver plasma flow, consistent with both hepatic and extrahepatic PTH removal. The serum t_1/2 of teriparatide is ~1 hour when administered subcutaneously versus 5 minutes when administered intravenously. The longer t_1/2 following subcutaneous administration reflects the time required for absorption from the injection site. The elimination of PTH(1-34) and full-length PTH proceeds by nonspecific enzymatic mechanisms in the liver, followed by renal excretion.

Clinical Effects. In postmenopausal women with osteoporosis, teriparatide increases BMD and reduces the risk of vertebral and nonvertebral fractures. Several laboratories have examined the effects of intermittent PTH on BMD in patients with osteoporosis. In these studies, teriparatide increased axial bone mineral, although initial reports of effects on cortical bone were disappointing. Co-administration of hPTH(1-34) with estrogen or synthetic androgen led to impressive gains in vertebral bone mass or trabecular bone. However, in some early studies there was only maintenance or even loss of cortical bone. Vitamin D insufficiency in patients at baseline or pharmacokinetic differences involving bioavailability or circulating half-life may have contributed to observed differences on cortical bone. The most comprehensive studies to date established the value of daily hPTH(1–34) administration on total BMD, with significant elevations of BMD in lumbar spine and femoral neck and with significant reductions of vertebral and nonvertebral fracture risk in osteoporotic women (Neer et al., 2001) and men (Finkelstein et al., 2003).

Candidates for teriparatide treatment include women who have a history of osteoporotic fracture, who have multiple risk factors for fracture, or who failed or are intolerant of previous osteoporosis therapy. Men with primary or hypogonadal osteoporosis are also candidates for treatment with teriparatide. However, the effect on fracture incidence in this group has not been established.

Adverse Effects. In rats, teriparatide increased the incidence of osteosarcoma (Vahle et al., 2004). Because of these findings, a black box warning appears on the FORTEO package insert. The clinical relevance of this finding is unclear, especially because patients with primary hyperparathyroidism have marked elevation of serum PTH without a greater incidence of osteosarcoma. Nonetheless, teriparatide should not be used in patients who are at increased baseline risk for osteosarcoma (including those with Paget's disease of bone, unexplained elevations of alkaline phosphatase, open epiphyses, or prior radiation therapy involving the skeleton). The safety and efficacy of FORTEO have not been evaluated for extended use beyond 2 years of treatment and therapy should be limited to no more than 2 years. A tumor registry-based analysis of the occurrence of osteosarcoma in teriparatide-treated patients is ongoing; cases of osteosarcoma associated with the drug should be reported to the FDA. Increases in bone density acquired during therapy with teriparatide can maintained or extended with subsequent use of bisphosphonates (Black et al., 2005).

Full-length PTH(1-84) has been associated with osteosarcomas in rats (Jolette et al., 2006) although not in humans. The significance and interpretation of the toxicological findings has been questioned (Tashjian and Goltzman, 2008). However, adverse effects include exacerbation of nephrolithiasis and elevation of serum uric acid levels.

In clinical trials, cinacalcet at 20- to 100-mg doses lowered PTH levels in a concentration-dependent manner by 15–50% and the serum calcium × phosphate product by 7% compared with placebo (Franceschini et al., 2003). Cinacalcet also effectively reduced PTH levels in patients with primary hyperparathyroidism and provided sustained normalization of serum calcium without altering bone mineral density (Shoback et al., 2003). Long-term control of PTH levels was achieved during a 3-year study of cinacalcet, suggesting that resistance does not develop. There currently is insufficient information to know whether reducing serum PTH levels with cinacalcet improves outcomes such as risk of cardiovascular events, bone disease, or mortality (Evenepoel, 2008).

Absorption, Fate, and Excretion. Cinacalcet (αR)-(–)α-methyl-N-[3-[3-[trifluoromethylphenyl]propyl]-1-napthalenemethanamine hydro-chloride) exhibits first-order absorption, with maximal serum concentrations achieved 2-6 hours after oral administration. Maximal effects, as defined by the nadir of serum PTH, occur 2–4 hours after administration. After absorption, plasma concentrations of cinacalcet decrease with a t_1/2 of 30-40 hours. Cinacalcet is eliminated primarily by renal excretion, with some 85% recovered in the urine after oral administration; the drug is also metabolized by multiple hepatic cytochromes, including CYPs 3A4, 2D6, and 1A2.

Cinacalcet is available in 30-, 60-, and 90-mg tablets. Optimal doses have not been defined. The recommended starting dose for treatment of secondary hyperparathyroidism in patients with chronic kidney disease on dialysis is 30 mg once daily, with a maximum of 180 mg/day. For treatment of parathyroid carcinoma, a starting dose of 30 mg twice daily is recommended, with a maximum of 90 mg four times daily. The starting dose is titrated upward every 2-4 weeks to maintain the PTH level between 150 and 300 pg/mL (secondary hyperparathyroidism) or to normalize serum calcium (parathyroid carcinoma).

Adverse Reactions. The principal adverse event with cinacalcet is hypocalcemia. Thus, the drug should not be used if the initial serum [Ca²⁺] is <8.4 mg/dL; serum calcium and phosphorus concentrations should be measured within 1 week, and PTH should be measured within 4 weeks after initiating therapy or after changing dosage.

Hypocalcemia can be diminished by initiating therapy with a low dose and gradually titrating it as necessary or adjusting the dose when vitamin D and/or phosphate binders are administered concomitantly. Patients on hemodialysis with low-calcium dialysate need to be monitored closely for hypocalcemia. Seizure threshold is lowered by significant reductions in serum Ca²⁺, so patients with a history of seizure disorders should be monitored especially closely. Finally, adynamic bone disease may develop if the PTH level is <100 pg/mL, and the drug should be discontinued or the dose decreased if the PTH level falls below 150 pg/mL.

Drug Interactions. Potential drug interactions can be anticipated with drugs that interfere with calcium homeostasis or that hinder cinacalcet absorption. Based on these considerations, potentially interfering drugs may include vitamin D analogs, phosphate binders, bisphosphonates, calcitonin, glucocorticoids, gallium, and cisplatin. The other category of drug interactions is for compounds metabolized by CYPs. Caution is recommended when cinacalcet is co-administered with inhibitors of CYP3A4 (e.g., ketoconazole, erythromycin, or itraconazole), CYP2D6 (many β adrenergic receptor blockers, flecainide, vinblastine, and most tricyclic antidepressants), and many other drugs.

Until recently, antiresorptive drugs were the only agents approved in the U.S. for treating osteoporosis. The increase in bone density that they produce is variable and depends on the site and the particular drug; generally, the increase is <10% after 3 years (Figure 44–12). This situation changed in 2002 when the FDA approved the biologically active PTH fragment PTH(1–34) (teriparatide, forteo) for use in treating postmenopausal women with osteoporosis and to increase bone mass in men with primary or hypogonadal osteoporosis. For a review and proposed guidelines for teriparatide/PTH use, see Hodsman et al. (2005). This agent should be used only after consideration of the risks (potential cause of osteosarcoma) and benefits in patients refractory to bisphosphonates or at serious risk of fracture. Therapeutic approaches likely will evolve considerably as newer treatment paradigms are developed and combinations of antiresorptive agents and PTH are introduced, and as the molecular physiology of osteoblast and osteoclast function is elucidated (Grey et al., 2005).

Antiresorptive Agents. Bisphosphonates. Bisphosphonates are the most frequently used drugs for the prevention and treatment of osteoporosis. Second- and third-generation oral bisphosphonates alendronate and risedronate have sufficient potency to suppress bone resorption at doses that do not inhibit mineralization.

Alendronate (fosamax), risedronate (actonel), and ibandronate (boniva) are approved for prevention and treatment of osteoporosis and for the treatment of glucocorticoid-associated osteoporosis. Multiple appropriate formulations are available. Studies of daily treatment with alendronate established its efficacy to increase bone mineral density, decrease bone turnover, and reduce the risk of vertebral fracture among women with osteoporosis (Bone et al., 2004; Liberman et al., 1995; Tonino et al., 2000; Tucci et al., 1996). Similar results were obtained with risedronate (Delmas et al., 2008). The results of 10-year daily treatment with 10 mg of alendronate (with 500 mg of supplemental calcium) reported a 14% increase of lumbar spine BMD, with smaller increments at the trochanter, total hip, and femoral neck (Bone et al., 2004). On reduction and discontinuation of treatment, BMD at the lumbar spine was maintained. Although significant decreases of BMD occurred at the total hip, femoral neck, and forearm, BMD at the lumbar spine, trochanter, total hip, and total body remained significantly above baseline values at year 10.

For patients in whom oral bisphosphonates cause severe esophageal distress despite countermeasures, intravenous zoledronate (reclast) or ibandronate offer skeletal protection without causing adverse GI effects. For treatment of osteoporosis, ibandronate (3 mg) is given intravenously every 3 months; zoledronate is administered as 5 mg once yearly. Zoledronate is the first bisphosphonate to be approved from once-yearly intravenous treatment of osteoporosis. Intravenous ibandronate significantly increased BMD of the lumbar spine. No fracture prevention data for intravenous ibandronate are available. Zoledronate reduces both vertebral and nonvertebral fractures (Black et al., 2007). Optimal duration of treatment and long-term safety remain to be established. A 4-mg formulation (zometa) is available for intravenous treatment of hypercalcemia of malignancy, multiple myeloma, or bone metastasis resulting from solid tumors.

Denosumab. As described earlier, RANKL binds to its cognate receptor RANK on the surface of precursor and mature osteoclasts, and stimulates these cells to mature and resorb bone. OPG, which competes with RANK for binding to RANKL, is the physiological inhibitor of RANKL. Denosumab is an investigational human monoclonal antibody that binds with high affinity to RANKL, mimicking the effect of OPG, and thereby reducing the binding of RANKL to RANK. Denosumab blocks osteoclast formation and activation. It increases BMD and decreases bone turnover markers when given subcutaneously once every 6 months. Denosumab, administered subcutaneously at 60 mg doses every 6 months, resulted in 68%, 41%, and 20% reductions in vertebral, hip and nonvertebral fractures, respectively, in a phase III clinical trial (Cummings et al., 2008). In the 3-year Fracture Reduction Evaluation with Denosumab and Osteoporosis Every Six Months (FREEDOM) Trial, denosumab significantly reduced morphometric vertebral fractures, and decreased hip and nonvertebral fractures and changes in BMD and bone turnover markers (Cummings et al., 2008). This novel therapy may soon gain FDA approval.

Selective Estradiol Receptor Modulators (SERMs). Considerable work has been undertaken to develop estrogenic compounds with tissue-selective activities. One of these, raloxifene (EVISTA), acts as an estrogen agonist on bone and liver, is inactive on the uterus, and acts as an anti-estrogen on the breast (Chapter 40). In postmenopausal women, raloxifene stabilizes and modestly increases BMD and has been shown to reduce the risk of vertebral compression fracture (Ettinger et al., 1999). Raloxifene is approved for both the prevention and treatment of osteoporosis. With the decreased use of estrogen for treating osteoporosis, the SERM raloxifene would seem to be an ideal alternative to HRT because raloxifene reduces the risk of vertebral fractures (albeit without a positive effect on nonvertebral fractures), breast cancer, and coronary events (coronary death, nonfatal myocardial infarction, and hospitalized acute coronary syndromes other than myocardial infarction) (Clemett and Spencer, 2000). The major drawback of raloxifene is that it can worsen vasomotor symptoms.

Estrogen. There is an unambiguous relationship between estrogen deficiency and osteoporosis. Postmenopausal status or estrogen deficiency at any age significantly increases a patient's risk for osteoporosis and fractures. Likewise, overwhelming evidence supports the positive impact of estrogen replacement on the conservation of bone and protection against osteoporotic fracture after menopause (Chapter 40). The outcome of the Women's Health Initiative (WHI) studies, however, strikingly altered the view of the therapeutic use of HRT for long-term prevention or treatment of osteoporosis. Significantly increased risks of heart disease and breast cancer were found (Anderson et al., 2004; Cauley et al., 2003). The consensus among experts now is to reserve HRT only for the short-term relief of vasomotor symptoms associated with menopause. HRT should be limited to osteoporosis prevention in women with significant ongoing vasomotor symptoms who are not at an increased risk for cardiovascular disease. An annual individualized risk-benefit reassessment should be performed on these patients.

Calcium. The physiological roles of Ca²⁺ and its use in the treatment of hypocalcemic disorders were discussed earlier. The rationale for using supplemental calcium to protect bone varies with time of life.

For preteens and adolescents, adequate substrate calcium is required for bone accretion. Controlled trials indicate that supplemental calcium promotes adolescent bone acquisition (Johnston et al., 1992), but its impact on peak bone mass is not known. Higher calcium intake during the third decade of life is positively related to the final phase of bone acquisition (Recker et al., 1992). There is controversy about the role of calcium during the early years after menopause, when the primary basis for bone loss is estrogen withdrawal. Although little effect of calcium on trabecular bone has been reported, reduction in cortical bone loss with calcium supplementation has been observed (Elders et al., 1994), even in populations with high dietary calcium intake. In elderly subjects, supplemental calcium suppresses bone turnover, improves BMD. Although some studies found a concomitant decreases in the incidence of fracture (Dawson-Hughes et al., 1997), three randomized trials found that it did not reduce fracture rate in older women (Jackson et al., 2006).

Patients who are unable or unwilling to increase calcium by dietary means alone may choose from many palatable, low-cost calcium preparations. As noted previously, numerous oral calcium preparations are available. The most frequently prescribed is carbonate, which should be taken with meals to facilitate dissolution and absorption. Traditional dosing of calcium is ~1000 mg/day, nearly the amount present in a quart of milk. Adults >50 years of age need 1200 mg of calcium daily. More may be necessary to overcome endogenous intestinal calcium losses, but daily intakes of ≥2000 mg frequently are reported to be constipating.

Vitamin D and Its Analogs. Modest supplementation with vitamin D (400-800 IU/day) may improve intestinal Ca²⁺ absorption, suppress bone remodeling, and improve BMD in individuals with marginal or deficient vitamin D status. A prospective study found that neither dietary calcium nor vitamin D intake was of major importance for the primary prevention of osteoporotic fractures in women (Michaelsson et al., 2003). However, supplemental vitamin D in combination with calcium reduced fracture incidence in multiple trials (Jackson et al., 2006). Several meta-analyses reported reduction in the rate of fractures when vitamin D and calcium were taken together. Because women commonly consume less than the recommended intake of vitamin D, use of supplemental vitamin D or increased consumption of such vitamin D–rich foods as dark fish may be prudent.

The use of calcitriol to treat osteoporosis is distinct from ensuring vitamin D nutritional adequacy. Here, the rationale is to suppress parathyroid function directly and reduce bone turnover. Calcitriol and the polar vitamin D metabolite 1α-hydroxycholecalciferol are used frequently in Japan and other countries, but experience in the U.S. has been mixed. Higher doses of calcitriol appear to be more likely to improve BMD, but at the risk of hypercalciuria and hypercalcemia; therefore, close scrutiny of patients and dose modification are required. Restriction of dietary calcium may reduce toxicity during calcitriol therapy (Gallagher and Goldgar, 1990). A low incidence of hypercalciuric and hypercalcemic complications of therapy in Japan may reflect relatively poor calcium intakes in that country.

Calcitonin. Calcitonin inhibits osteoclastic bone resorption and modestly increases bone mass in patients with osteoporosis; the largest increases occurred in patients with high intrinsic rates of bone turnover. One study showed that calcitonin nasal spray (200 units/day) reduced the incidence of vertebral compression fractures by ~40% in osteoporotic women (Chesnut et al., 2000).

Thiazide Diuretics. Although not strictly antiresorptive, thiazides reduce urinary Ca²⁺ excretion and constrain bone loss in patients with hypercalciuria. Whether they will prove to be useful in patients who are not hypercalciuric is not clear, but data suggest that they reduce hip fracture risk. No sustained effect is observed (Schoofs et al., 2003). Hydrochlorothiazide, 25 mg once or twice daily, may reduce urinary Ca²⁺ excretion substantially. Effective doses of thiazides for reducing urinary Ca²⁺ excretion generally are lower than those necessary for blood pressure control. For a more detailed discussion of thiazide diuretics, see Chapter 25.

Anabolic Agents. Teriparatide. Teriparatide (forteo) is the only agent currently available that increases new bone formation. It is FDA- approved for treatment of osteoporosis for up to 2 years in both men and postmenopausal women at high risk for fractures. Teriparatide significantly decreased the incidence of vertebral (4-5% vs. 14%) and nonvertebral fractures (3% vs. 6%) compared to placebo in a prospective, randomized control study (FPT) (Neer et al., 2001). Teriparatide increases predominantly trabecular bone at the lumbar spine and femoral neck; it has less significant effects at cortical sites. Although initial studies showed dose-dependent actions, and side effects, because of concern (and black box label warning), teriparatide is approved at the 20 μg dose, administered once daily by subcutaneous injection in the thigh or abdominal wall. The most common adverse effects associated with teriparatide include injection-site pain, nausea, headaches, leg cramps, and dizziness.

Combination Therapies

Osteoporosis. Because teriparatide stimulates bone formation, whereas bisphosphonates reduce bone resorption, it was predicted that therapy combining the two would enhance the effect on BMD more than treatment with either one alone. However, addition of alendronate to PTH treatment provided no additional benefit for BMD and reduced the anabolic effect of PTH in both women and men (Cosman, 2008). Sequential treatment with PTH(1-84) followed by alendronate increased vertebral BMD to a greater degree than alendronate or estrogen alone (Rittmaster et al., 2000). Recent work underscores the beneficial action of alendronate in consolidating the gains in lumbar spine BMD achieved by antecedent teriparatide treatment in men.

The fracture outcome 30 months after cessation of teriparatide therapy was studied in a cohort of patients included in the FPT. Approximately 50% of each group (teriparatide or control) were treated with a bisphosphonate for some period of time during the 30-month period. Patients treated with bisphosphonates after discontinuation of teriparatide showed continued increments in BMD, whereas those who received no treatment had declines (Prince et al., 2005).

Paget's Disease. Although most patients with Paget's disease require no treatment, factors such as severe pain, neural compression, progressive deformity, hypercalcemia, high-output congestive heart failure, and repeated fracture risk are considered indications for treatment. Bisphosphonates and calcitonin decrease the elevated biochemical markers of bone turnover, such as plasma alkaline phosphatase activity and urinary excretion of hydroxyproline. An initial course of bisphosphonate typically is given once daily or once weekly for 6 months. With treatment, most patients experience a decrease in bone pain over several weeks. Such treatment may induce long-lasting remission. If symptoms recur, additional courses of therapy can be effective. When etidronate is given at higher doses (10-20 mg/kg per day) or continuously for >6 months, there is a substantial risk for osteomalacia. At lower doses (5-7.5 mg/kg per day), focal osteomalacia has been observed occasionally. Defective mineralization has not been observed with other bisphosphonates or with calcitonin.

Choice of optimal therapy for Paget's disease varies among patients. Bisphosphonates are the standard therapy. Intravenous pamidronate induces long-term remission following a single infusion. Zoledronate seems to exhibit greater response rates and a longer median duration of complete response (Major et al., 2001). Compared with calcitonin, bisphosphonates have the advantage of oral administration, lower cost, lack of antigenicity, and generally fewer side effects. Resistance to calcitonin develops in most patients. However, calcitonin is highly reliable and may have a distinct skeletal analgesic property. Mithramycin (plicamycin) has been used in difficult cases of Paget's disease that do not respond to bisphosphonates or calcitonin. Therapeutic utility of this agent is limited by a high potential for hemorrhagic and other toxicities, and it is not generally recommended.

Absorption, Distribution, and Excretion. Human beings obtain fluoride predominantly from the ingestion of plants and water, with most absorption taking place in the intestine. The degree of fluoride absorption correlates with its water solubility. Relatively soluble compounds, such as sodium fluoride, are absorbed almost completely, whereas relatively insoluble compounds, such as cryolite (Na₃AlF₆) and the fluoride found in bone meal (fluoroapatite) are absorbed poorly. A second route of absorption is through the lungs, and inhalation of fluoride present in dusts and gases constitutes the major route of industrial exposure.

Fluoride is distributed widely in organs and tissues but is concentrated in bone and teeth, and the skeletal burden is related to intake and age. Bone deposition reflects skeletal turnover; growing bone shows greater deposition than mature bone.

The kidneys are the major site of fluoride excretion. Small amounts of fluoride also appear in sweat, milk, and intestinal secretions; in a very hot environment, sweat can account for nearly 50% of total fluoride excretion.

Pharmacological Actions and Uses. Because it is concentrated in the bone, the radionuclide ¹⁸F has been used in skeletal imaging. Sodium fluoride enhances osteoblast activity and increases bone volume. These effects may be bimodal, with low doses stimulating and higher doses suppressing osteoblasts; if true, this may account for the poorly mineralized and mechanically defective bone seen in some studies. In doses of 30-60 mg/day, fluoride increases trabecular bone mineral density in many, but not all, patients. In one controlled trial, fluoride increased lumbar spine density (cancellous bone) but decreased cortical bone mineral density; these changes were associated with a significant increase in peripheral fractures and stress fractures (Riggs et al., 1990). In one study, sustained-release fluoride, which provided lower blood fluoride levels, increased bone mineral density and decreased fractures (Pak et al., 1994). Intermittent courses of slow-release fluoride also have been evaluated. When the total fluoride dose was kept constant (Balena et al., 1998), there was no difference in outcome between continuous and intermittent fluoride treatment; notably, both regimens increased cancellous and trabecular thickness to the same extent. Unfortunately, increased bone mass is not synonymous with increased bone strength (Riggs et al., 1990). Thus, the apparent effects of fluoride in osteoporosis are slight compared with those achieved with PTH or other agents.

Other pharmacological actions of fluoride can be classified as toxic. Fluoride inhibits several enzyme systems and diminishes tissue respiration and anaerobic glycolysis.

Acute Poisoning. Acute fluoride poisoning usually results from accidental ingestion of fluoride-containing insecticides or rodenticides. Initial symptoms (salivation, nausea, abdominal pain, vomiting, and diarrhea) are secondary to the local action of fluoride on the intestinal mucosa. Systemic symptoms are varied and severe: increased irritability of the central nervous system consistent with the Ca²⁺-binding effect of fluoride and the resulting hypocalcemia; hypotension, presumably owing to central vasomotor depression as well as direct cardiotoxicity; and stimulation and then depression of respiration. Death can result from respiratory paralysis or cardiac failure. The lethal dose of sodium fluoride for humans is ~5 g, although there is considerable variation. Treatment includes the intravenous administration of glucose in saline and gastric lavage with lime water (0.15% calcium hydroxide solution) or other Ca²⁺ salts to precipitate the fluoride. Calcium gluconate is given intravenously for tetany; urine volume is kept high with vigorous fluid resuscitation.

Chronic Poisoning. In humans, the major manifestations of chronic ingestion of excessive fluoride are osteosclerosis and mottled enamel. Osteosclerosis is characterized by increased bone density secondary both to elevated osteoblastic activity and to the replacement of hydroxyapatite by the denser fluoroapatite. The degree of skeletal involvement varies from changes that are barely detectable radiologically to marked cortical thickening of long bones, numerous exostoses scattered throughout the skeleton, and calcification of ligaments, tendons, and muscle attachments. In its severest form, it is a disabling and crippling disease.

Mottled enamel, or dental fluorosis, was first described >60 years ago. In very mild mottling, small, opaque, paper-white areas are scattered irregularly over the tooth surface. In severe cases, discrete or confluent, deep brown- to black-stained pits give the tooth a corroded appearance. Mottled enamel results from a partial failure of the enamel-forming ameloblasts to elaborate and lay down enamel. Because mottled enamel is a developmental injury, fluoride ingestion following the eruption of teeth has no effect. Mottling is one of the first visible signs of excess fluoride intake during childhood. Continuous use of water containing ~1 ppm of fluoride may result in very mild mottling in 10% of children; at 4-6 ppm the incidence approaches 100%, with a marked increase in severity.

Severe dental fluorosis formerly occurred in regions where local water supplies had a very high fluoride content (e.g., Pompeii, Italy, and Pike's Peak, Colorado). Current regulations in the U.S. require lowering the fluoride content of the water supply or providing an alternative source of acceptable drinking water for affected communities. Sustained consumption of water with a fluoride content of 4 mg/L (4 ppm) is associated with deficits in cortical bone mass and increased rates of bone loss over time (Sowers et al., 1991).

Fluoride and Dental Caries. After a new water supply was established, children in Bauxite, Arkansas, had a much higher incidence of caries than those who had been exposed to the former fluoride-containing water. Subsequent studies established definitely that supplementation of water fluoride content to 1.0 ppm is a safe and practical intervention that substantially reduces the incidence of caries in permanent teeth.

There are partial benefits for children who begin drinking fluoridated water at any age; however, optimal benefits are obtained at ages before permanent teeth erupt. Topical application of fluoride solutions by dental personnel appears to be particularly effective on newly erupted teeth and can reduce the incidence of caries by 30-40%. Dietary fluoride supplements should be considered for children <12 years of age whose drinking water contains <0.7 ppm fluoride. Conflicting results have been reported from studies of fluoride-containing toothpastes.

Adequate incorporation of fluoride into teeth hardens the outer layers of enamel and increases resistance to demineralization. Fluoride deposition apparently involves exchange with hydroxyl or citrate anions in the enamel apatite crystal surface. The mechanism by which fluoride prevents caries is not completely understood. There is no convincing evidence that fluoride from any source reduces the development of caries after the permanent teeth are completely formed (usually ~14 years of age).

The fluoride salts usually employed in dentifrices are sodium fluoride and stannous fluoride. Sodium fluoride also is available in a variety of preparations for oral and topical use, including tablets, drops, rinses, and gels.

Since its inception, regulation of the fluoride concentration of community water supplies periodically has encountered vocal opposition, including allegations of putative adverse health consequences of fluoridated water. Careful examination of these issues indicates that cancer and all-cause mortalities do not differ significantly between communities with fluoridated and nonfluoridated water (Richmond, 1985).