Chapter 38: Introduction To Endocrinology: The Hypothalamic-Pituitary Axis

Additional GH species, differing in size or charge, are found in serum, but their physiological significance is unclear. In the circulation, a 55 kDa protein binds approximately 45% of the 22 kDa and 25% of the 20 kDa forms; this binding protein contains the extracellular domain of the GH receptor and apparently arises from proteolytic cleavage. A second protein unrelated to the GH receptor also binds approximately 5-10% of circulating GH with lower affinity. Bound GH is cleared more slowly and has a biological t_1/2 ∼10 times that of unbound GH, suggesting that the bound hormone may provide a GH reservoir that dampens acute fluctuations in GH levels associated with its pulsatile secretion. Alternatively, the binding protein may decrease GH bioactivity by preventing it from binding to its receptor in target tissues. Obesity increases circulating levels of GH binding proteins and thus may affect the clinical response to exogenous GH.

Each of the SST receptor subtypes (abbreviated SSTRs) binds SST with nanomolar affinity; whereas receptor types 1-4 (SSTR1-4) bind the two SSTs with approximately equal affinity, type 5 (SSTR5) has a 10- to 15-fold greater affinity for SST-28. SSTR2 and SSTR5 are the most important for regulation of GH secretion, and recent studies suggest that these two SSTRs form functional heterodimers with distinctive signaling behavior (Grant et al., 2008). SST exerts direct effects on somatotropes in the pituitary and indirect effects mediated via GHRH neurons in the arcuate nucleus. As discussed later, SST analogs play a key role in the pharmacotherapy of syndromes of GH excess and certain cancers.

Ghrelin acts primarily through a GPCR called the GH secretagogue receptor. Although the interaction of ghrelin with this receptor directly stimulates GH release by isolated somatotropes, the major action on GH secretion apparently is through actions on the GHRH neurons in the arcuate nucleus. Apart from its effects on GH secretion, ghrelin also stimulates appetite and increases food intake, apparently by central actions on NPY and agouti-related peptide neurons in the hypothalamus. Thus, ghrelin and its receptor act in a complex manner to integrate the functions of the GI tract, the hypothalamus, and the anterior pituitary (Ghigo et al., 2005). Peptide and nonpeptide agonists (termed GH secretagogues) and antagonists of the GH secretagogue receptor are undergoing evaluation as possible modulators of neuroendocrine function.

A number of putative prolactin-releasing factors have been described, including TRH, VIP, prolactin-releasing peptide, and PACAP, but their physiological roles are unclear. Under certain pathological conditions, such as severe primary hypothyroidism, persistently elevated levels of TRH can induce hyperprolactinemia.

Unlike GH, which plays important roles throughout life in both sexes, prolactin acts predominantly in women, both during pregnancy and in the postpartum period in women who breast-feed. Serum prolactin levels remain low in men but are elevated somewhat in normal cycling females. During pregnancy, the maternal serum prolactin level starts to increase at 8 weeks of gestation, increases to peak levels of 250 ng/mL at term, and declines thereafter to prepregnancy levels unless the mother breast-feeds the infant. Suckling or breast manipulation in nursing mothers transmits signals from the breast to the hypothalamus via the spinal cord and the median forebrain bundle, which, in turn, stimulate circulating prolactin levels. Prolactin levels can rise 10- to 100-fold within 30 minutes of stimulation. This response is distinct from milk letdown, which is mediated by oxytocin release from the posterior pituitary gland. The suckling-induced prolactin secretion involves both decreased secretion of dopamine by tuberoinfundibular neurons and possibly increased release of factors that stimulate prolactin secretion. The suckling response becomes less pronounced after several months of breast-feeding, and prolactin concentrations eventually decline to prepregnancy levels.

Prolactin also is synthesized in lactotropes near the end of the luteal phase of the menstrual cycle and by decidual cells early in pregnancy; the latter source is responsible for the very high levels of prolactin in amniotic fluid during the first trimester of human pregnancy. The function of this prolactin and what regulates its expression are not known.

The mature human GH receptor contains 620 amino acids, approximately 250 of which are extracellular, 24 of which are transmembrane, and ∼350 of which are cytoplasmic. It exists as a preformed homodimer that forms a ternary complex with one molecule of GH. The formation of the GH-GH receptor ternary complex is initiated by high-affinity interaction of GH with one monomer of the GH receptor dimer (mediated by GH site 1), followed by a second, lower affinity interaction of GH with the GH receptor mediated by GH site 2; these interactions induce a conformational change that activates downstream signaling. As discussed later, pegvisomant is a GH analog with amino acid substitutions that disrupt site 2; it binds the receptor and causes its internalization but cannot trigger the conformational change that stimulates downstream events in the signal transduction pathway. Amino acid–substituted versions of human prolactin that act as antagonists at the prolactin receptor are also under development (Goffin et al., 2006).

The ligand-occupied GH receptor dimer lacks inherent tyrosine kinase activity. Rather, it provides docking sites for two molecules of JAK2, a cytoplasmic tyrosine kinase of the Janus kinase family. The juxtaposition of two JAK2 molecules leads to trans-phosphorylation and autoactivation of JAK2, with consequent tyrosine phosphorylation of cytoplasmic proteins that mediate downstream signaling events (Lanning and Carter-Su, 2006). These include STAT proteins (Signal Transducers and Activators of Transcription), Shc (an adapter protein that regulates the Ras/MAPK signaling pathway), and IRS-1 and IRS-2 (insulin-receptor substrate proteins that activate the PI3K pathway). One critical target of STAT5 is the gene encoding IGF-1 (Figure 38–5). The fine control of GH action also involves feedback regulatory events that subsequently turn off the GH signal. As part of its action, GH induces the expression of a family of SOCS (suppressor of cytokine signaling) proteins and a group of protein tyrosine phosphatases that, by different mechanisms, disrupt the communication of the activated GH receptor with JAK2 (Flores-Morales et al., 2006).

Some studies have shown that the GH receptor can translocate to the nucleus and act as a coregulator to activate transcription and cell proliferation (Swanson and Kopchick, 2007). The precise role of this signal transduction pathway in GH physiology and pathophysiology remains to be defined.

The effects of prolactin on target cells also result from interactions with a cytokine receptor that is widely distributed and signals through many of the same pathways as the GH receptor. Alternative splicing of the prolactin receptor gene on chromosome 5 gives rise to multiple forms of the receptor that are identical in the extracellular domain but differ in their cytoplasmic domains. In addition, soluble forms that correspond to the extracellular domain of the receptor are found in circulation. Unlike human GH and placental lactogen, which also bind to the prolactin receptor and thus are lactogenic, prolactin binds specifically to the prolactin receptor and has no somatotropic (GH-like) activity.

After its synthesis and release, IGF-1 interacts with receptors on the cell surface that mediate its biological activities. The type 1 IGF receptor is closely related to the insulin receptor and consists of a heterotetramer with intrinsic tyrosine kinase activity. This receptor is present in essentially all tissues and binds IGF-1 and the related growth factor, IGF-2, with high affinity; insulin also can activate the type 1 IGF receptor but with an affinity approximately two orders of magnitude less than that of the IGFs. The signal transduction pathway for the insulin receptor is described in detail in Chapter 43.

Prolactin receptors are present in many other sites, including the hypothalamus, liver, adrenal, testes, ovaries, prostate, and immune system, suggesting that prolactin may play multiple roles outside of the breast. The physiological effects of prolactin at these sites remain poorly characterized, and specific defects in their function that result from prolactin deficiency have not been defined.

Diagnosis of Somatotropin Hormone Excess. Although acromegaly should be suspected in patients with the appropriate symptoms and signs, diagnostic confirmation requires the demonstration of increased circulating GH or IGF-1. The "gold standard" diagnostic test for acromegaly is the oral glucose tolerance test. Whereas normal subjects suppress their GH level to <1 ng/mL in response to an oral glucose challenge (the absolute value may vary depending on the sensitivity of the assay), patients with acromegaly either fail to suppress or show a paradoxical increase in GH level.

In patients with hyperprolactinemia, the major question is whether conditions other than a prolactin-producing adenoma are responsible for the elevated prolactin level. A number of medications that inhibit DA signaling can cause moderate elevations in prolactin (e.g., antipsychotics, metoclopramide), as can primary hypothyroidism, pituitary mass lesions that interfere with dopamine delivery to the lactotropes, and pregnancy. Thus thyroid function and pregnancy tests are indicated, as is magnetic resonance imaging (MRI) to look for a pituitary adenoma or other defect that might elevate serum prolactin.

Clinical Manifestations of Growth Hormone Deficiency. Clinically, children with GH deficiency present with short stature, delayed bone age, and a low age-adjusted growth velocity. Specific etiologies associated with GH deficiency in children include genetic disorders that affect pituitary development and can cause deficiencies of multiple pituitary hormones, pituitary or hypothalamic tumors, previous CNS irradiation, and infiltrative processes such as histiocytosis. In most patients, however, the deficiency is idiopathic, with normal production of other pituitary hormones and no obvious structural abnormalities.

GH deficiency in adults does not impair linear growth, which ceases with closure of the epiphyses. Rather, GH deficiency in adults is associated with decreased muscle mass and exercise capacity, decreased bone density, impaired psychosocial function, and increased mortality from cardiovascular causes, probably secondary to deleterious changes in fat distribution, increases in circulating lipids, and increased inflammation (Molitch et al., 2006).

Diagnosis of Growth Hormone Deficiency. Because GH secretion is highly pulsatile, random sampling of serum GH is insufficient to diagnose GH deficiency. Whereas tests for substances that provide an estimate of integrated GH levels over time (e.g., IGF-1 and IGFBP-3) are more useful, provocative tests usually are required. After excluding other causes of poor growth, the diagnosis of GH deficiency should be entertained in children with height >2 to 2.5 standard deviations below normal, delayed bone age, a decreased growth velocity, and a predicted adult height substantially below the mean parental height. In this setting, a serum GH level <10 ng/mL following provocative testing (e.g., insulin-induced hypoglycemia, arginine, levodopa, clonidine, or glucagon) indicates GH deficiency; a stimulated value <5 ng/mL reflects severe deficiency.

In adults, overt GH deficiency usually results from pituitary lesions caused by a functioning or nonfunctioning pituitary adenoma, secondary to trauma, or related to surgery or radiotherapy for a pituitary or suprasellar mass (Molitch et al., 2006). Almost all patients with multiple deficits in other pituitary hormones also have deficient GH secretion, and some experts incorporate the number of other pituitary deficiencies into a diagnostic algorithm for diagnosing GH deficiency. Others accept a serum IGF-1 level below the age- and sex-adjusted normal range as indicative of GH deficiency in a patient with known pituitary disease. The converse is not true because an IGF-1 level within the normal range does not exclude adult GH deficiency. Some experts require an inadequate GH response to provocative testing (i.e., a value <5 ng/mL), with either insulin-induced hypoglycemia or a combination of Arg and GHRH as the preferred stimulus for GH secretion. Finally, in patients with known hypothalamic/pituitary disease of recent onset, Arg alone can be used as the stimulus, with the hGH cutoff set at 1.4 ng/mL. The risk of false-positive provocative tests (i.e., a subnormal GH response) is increased in obese subjects.

Chemistry. Structure-function studies of SST and its derivatives established that the amino acid residues in positions 7-10 of the SST-14 peptide (Phe-Trp-Lys-Thr) are the major determinants of biological activity. Residues Trp⁸ and Lys⁹ appear to be essential, whereas conservative substitutions at Phe⁷ and Thr¹⁰ are permissible. Active SST analogs retain this core segment constrained in a cyclic structure, formed either by a disulfide bond (octreotide, lanreotide, vapreotide) or an amide linkage (seglitide, pasireotide) that stabilizes the optimal conformation (Pawlikowski and Melen-Mucha, 2004). The endogenous peptides, SST-14 and SST-28, do not show specificity for SST receptor subtypes except for SST₅, which preferentially binds SST-28. Some SST analogs exhibit greater selectivity. For example, the octapeptides octreotide and lanreotide bind to the SST subtypes with the following order of selectivity: SST₂ > SST₅ > SST₃ ≫ SST₁ and SST₄. The cyclohexapeptide, pasireotide (SOM230), binds with high affinity to all but the SST₄ receptor. Small nonpeptide agonists that exhibit high selectivity for SST receptor subtypes have been isolated from combinatorial chemical libraries; these compounds may lead to a new class of highly selective, orally active SST mimetics (Weckbecker et al., 2003).

A chimeric compound that activates SST receptors and the D₂ receptor (BIM-23A387) is now in clinical trials for therapy of mixed GH- and prolactin-secreting adenomas and for nonfunctioning pituitary adenomas (Pawlikowski and Melen-Mucha, 2004). Based on the apparent formation of heterodimers between the SST₂ and D₂ receptors, such chimeric compounds may have efficacy for certain tumors that do not respond to either classic SST analogs or DA agonists (Florio et al., 2008).

Therapeutic Uses

Currently, the two somatostatin analogs used widely are octreotide and lanreotide, synthetic derivatives that have longer half-lives and bind preferentially to SST₂ and SST₅ receptors. Octreotide (100 μg) administered subcutaneously three times daily is virtually 100% bioactive, peak effects are seen within 30 min, serum t_1/2 is ∼90 min, and duration of action is ∼12 hour. This formulation successfully controls the biochemical parameters of acromegaly in 50-60% of patients.

The need to inject octreotide three times daily poses a significant obstacle to patient compliance. A long-acting, slow-release form (sandostatin-lar depot) in which the active species is incorporated into microspheres of a biodegradable polymer greatly reduces the injection frequency. Administered intramuscularly in a dose of 20 or 30 mg once every 4 week, typically to patients who have tolerated and responded to the shorter-acting formulation, octreotide LAR is at least as effective as the regular formulation (Murray and Melmed, 2008). Like the shorter-acting formulation, the longer-acting formulation of octreotide generally is well tolerated with a similar incidence of side effects (see following discussion). A lower dose of 10 mg per injection should be used in patients requiring hemodialysis or with hepatic cirrhosis.

In addition to its effect on GH secretion, octreotide can decrease tumor size, although tumor growth generally resumes after octreotide treatment is stopped.

Lanreotide is another long-acting octapeptide SST analog that causes prolonged suppression of GH secretion when administered in a 30-mg dose intramuscularly. Although its efficacy appears comparable to that of the long-acting formulation of octreotide, its duration of action is shorter; thus, it is administered at 10- or 14-day intervals.

A supersaturated aqueous formulation of lanreotide, lanreotide autogel (somatuline depot), has recently been approved for use in the U.S. It is supplied in prefilled syringes containing 60, 90, or 120 mg lanreotide and administered by deep subcutaneous injection. Administered once every 4 weeks, lanreotide autogel provides more uniform drug levels than the depot formulation of octreotide. Results of current clinical trials are at least comparable to those with the slow-release octreotide formulation (Murray and Melmed, 2008).

Adverse Effects. GI side effects—including diarrhea, nausea, and abdominal pain—occur in up to 50% of patients receiving octreotide. In most patients, these symptoms diminish over time and do not require cessation of therapy. Approximately 25% of patients receiving octreotide develop gallbladder sludge or even gallstones, presumably due to decreased gallbladder contraction and bile secretion. In the absence of symptoms, gallstones are not a contraindication to continued use of octreotide. Compared with SST, octreotide reduces insulin secretion to a lesser extent and only infrequently affects glycemic control. Inhibitory effects on TSH secretion may lead to hypothyroidism, and thyroid function tests should be evaluated periodically. The incidence and severity of side effects associated with lanreotide are similar to those of octreotide. Perhaps related to its more global inhibition of SST receptors, pasireotide has exhibited a relatively greater impairment in glycemic control than octreotide or lanreotide. The degree to which this may limit its clinical utility is not yet established.

Other Therapeutic Uses. Somatostatin blocks not only GH secretion but also the secretion of other hormones, growth factors, and cytokines. Thus octreotide and the slow-release formulations of SST analogs have been used to treat symptoms associated with metastatic carcinoid tumors (e.g., flushing and diarrhea) and adenomas secreting vasoactive intestinal peptide (e.g., watery diarrhea). Octreotide also is used for treatment of acute variceal bleeding and for perioperative prophylaxis in pancreatic surgery (see Chapter 46 for discussion of the uses of SST analogs in GI disease). Octreotide also has significant inhibitory effects on TSH secretion, and it is the treatment of choice for patients who have thyrotrope adenomas that oversecrete TSH who are not good candidates for surgery. Finally, modified forms of octreotide labeled with indium or technetium have been used for diagnostic imaging of neuroendocrine tumors such as pituitary adenomas and carcinoids (octreoscan); modified forms labeled with β emitters such as ⁹⁰Y have been used in selective destruction of SST₂ receptor-positive tumors.

Novel uses under evaluation include the treatment of eye diseases associated with excessive proliferation and inflammation (e.g., Graves' orbitopathy and diabetic retinopathy), diabetic nephropathy, and various diseases associated with inflammation (e.g., rheumatoid arthritis, inflammatory bowel disease, pulmonary fibrosis, and psoriasis).

Pegvisomant is administered subcutaneously as a 40-mg loading dose under physician supervision, followed by self-administration of 10 mg per day. Based on serum IGF-1 levels, the dose is titrated at 4- to 6-week intervals to a maximum of 40 mg per day. Pegvisomant should not be used in patients with an unexplained elevation of hepatic transaminases, and liver function tests should be monitored in all patients. In addition, lipohypertrophy has occurred at injection sites, sometimes requiring cessation of therapy; this is believed to reflect the inhibition of direct actions of GH on adipocytes. Because of concerns that loss of negative feedback by GH and IGF-1 may increase the growth of GH-secreting adenomas, careful follow-up by pituitary MRI is strongly recommended, although this may change as more data become available (Jimenez et al., 2008). Pegvisomant differs structurally from native GH and induces the formation of specific antibodies in ∼15% of patients despite the covalent coupling of Lys residues to 4-5 molecules of a polyethylene glycol polymer per modified GH molecule. Nevertheless, the development of tachyphylaxis due to these antibodies has not been reported.

In clinical trials, pegvisomant at higher doses decreased serum IGF-1 to normal age- and sex-adjusted levels in >90% of patients and significantly improved clinical parameters such as ring size, soft-tissue swelling, excessive perspiration, and fatigue. Thus pegvisomant provides a highly effective alternative for use in patients who have not responded to SST analogs, either as sole therapy or as a temporizing measure while waiting for radiation therapy to achieve its full effect; it also is receiving increased scrutiny as first-line therapy.

Chemistry. Bromocriptine is a semisynthetic ergot alkaloid (Figure 38–6) that interacts with D₂ receptors to inhibit spontaneous and TRH-induced release of prolactin; to a lesser extent, it also activates D₁ receptors.

Absorption, Distribution, and Elimination. Although a large fraction of the oral dose of bromocriptine is absorbed, only 7% of the dose reaches the systemic circulation because of a high extraction rate and extensive first-pass metabolism in the liver. Bromocriptine has a relatively short elimination t_1/2 (between 2 and 8 hours) and thus is usually administered in divided doses (see Adverse Effects). To avoid the need for frequent dosing, a slow release oral form is available outside of the U.S. Bromocriptine may be administered vaginally (2.5 mg once daily), reportedly with fewer gastrointestinal side effects.

Therapeutic Uses. Bromocriptine normalizes serum prolactin levels in 70-80% of patients with prolactinomas and decreases tumor size in >50% of patients, including those with macroadenomas. Typically, bromocriptine does not cure the underlying adenoma, and hyperprolactinemia and tumor growth recur upon cessation of therapy.

Adverse Effects. Frequent side effects of bromocriptine include nausea and vomiting, headache, and postural hypotension, particularly on initial use. Less frequently, nasal congestion, digital vasospasm, and CNS effects such as psychosis, hallucinations, nightmares, or insomnia are observed. These adverse effects can be diminished by starting at a low dose (1.25 mg) administered at bedtime with a snack. After 1 week, a morning dose of 1.25 mg can be added. If clinical symptoms persist or serum prolactin levels remain elevated, the dose can be increased gradually, every 3-7 days, to 5 mg two or three times a day as tolerated. Patients often develop tolerance to the adverse effects of bromocriptine. Those who do not respond to bromocriptine or who develop intractable side effects often respond better to cabergoline. At higher concentrations, bromocriptine is used in the management of acromegaly, as noted earlier, and at still higher concentrations is used in the management of Parkinson's disease (Chapter 22).

Therapeutic Uses. Cabergoline is FDA-approved for the treatment of hyperprolactinemia and has become the preferred drug in most settings for this disorder. Its greater efficacy in decreasing serum prolactin in patients with hyperprolactinemia may reflect improved adherence to therapy due to decreased side effects. Therapy is initiated at a dose of 0.25 mg twice a week or 0.5 mg once a week. If the serum prolactin remains elevated, the dose can be increased to a maximum of 1.5-2 mg two or three times a week as tolerated; the dose should not be increased more often than once every 4 weeks.

Cabergoline induces remission in a significant number of patients with prolactinomas, and a trial of drug discontinuation is advocated for the subset of patients with normalization of prolactin and disappearance of a detectable pituitary lesion on MRI scanning.

At higher doses, cabergoline is used in some patients with acromegaly and is now under investigation for patients with Cushing's disease due to corticotrope adenomas (Chapter 42).

Adverse Effects. Compared to bromocriptine, cabergoline has a much lower tendency to induce nausea, although it still may cause hypotension and dizziness. Cabergoline has been linked to valvular heart disease, an effect proposed to reflect agonist activity at the serotonin 5-HT_2B receptor. A similar effect has been seen with pergolide (see later). Thus echocardiographic assessment seems appropriate for patients receiving chronic therapy with cabergoline, particularly those on higher doses (Colao et al., 2008b).

Pergolide. Pergolide (permax), an ergot derivative FDA-approved for treatment of Parkinson disease, also was used off label to treat hyperprolactinemia. In part due to concerns of valvular heart disease, pergolide has been withdrawn from the market.

Chemistry. Although the bacterial or yeast systems used to express the recombinant GH subtly affect the structures of these preparations, they all have similar biological actions and potencies (3 IU/mg). An FDA-approved, encapsulated form of somatropin (nutropin depot) injected intramuscularly, either monthly (1.5 mg/kg body weight) or every 2 weeks (0.75 mg/kg body weight), has been discontinued by the manufacturer.

Pharmacokinetics. As a peptide hormone, GH is administered subcutaneously, with a bioavailability of 70%. Although the circulating t_1/2 of GH is only 20 minutes, its biological t_1/2 is considerably longer, and once-daily administration is sufficient. To match the usual pattern of secretion, GH typically is administered at bedtime, although this is not essential.

Indications for Growth Hormone Treatment. GH deficiency in children is a well-accepted cause of short stature, and replacement therapy has been used for half a century to treat children with severe GH deficiency. With the advent of essentially unlimited supplies of recombinant GH, therapy has been extended to children with other conditions associated with short stature despite adequate GH production, including Turner's syndrome, Noonan's syndrome, Prader-Willi syndrome, chronic renal insufficiency, children born small for gestational age, and children with idiopathic short stature (i.e., >2.25 standard deviations below mean height for age and sex but with normal laboratory indices of growth hormone levels).

Attention also has shifted to the proper role of GH therapy in GH-deficient adults. The consensus of experts is that at least the most severely affected GH-deficient adults may benefit from GH replacement therapy (Molitch et al., 2006). The FDA also has approved GH therapy for AIDS-associated wasting and for malabsorption associated with the short bowel syndrome. The latter indication is based on the finding that GH stimulates the adaptation of gastrointestinal epithelial cells.

Contraindications. Based on controlled clinical trials showing increased mortality, GH should not be used in patients with acute critical illness due to complications after open heart or abdominal surgery, multiple accidental trauma, or acute respiratory failure. GH also should not be used in patients who have any evidence of neoplasia, and antitumor therapy should be completed before GH therapy is initiated. Other contraindications include proliferative retinopathy or severe nonproliferative diabetic retinopathy. In Prader-Willi syndrome, sudden death has been observed when GH was given to children who were severely obese or who had severe respiratory impairment.

Therapeutic Uses. In GH-deficient children, somatropin typically is administered in a dose of 25-50 μg/kg per day subcutaneously in the evening; higher daily doses (e.g., 50-67 μg/kg) are employed for patients with Noonan's syndrome or Turner's syndrome, who have partial GH resistance. In children with overt GH deficiency, measurement of serum IGF-1 levels sometimes is used to monitor initial response and compliance; long-term response is monitored by close evaluation of height, sometimes in conjunction with measurements of serum IGF-1 levels (Cohen et al., 2007). Although the most pronounced increase in growth velocity occurs during the first 2 years of therapy, GH is continued until the epiphyses are fused and also may be extended into the transition period from childhood to adulthood.

In view of the effects of GH on bone density and visceral adiposity and the manifestations of GH deficiency in adults, many experts now continue therapy into adulthood for children with GH deficiency (Radovick and DiVall, 2007). However, many patients who were GH deficient in childhood based on provocative testing, especially those with idiopathic, isolated GH deficiency, respond normally to provocative tests as adults. Thus, it is essential to confirm GH deficiency after full growth has been achieved, ideally after discontinuation of GH replacement for at least 1 month, to identify patients who will benefit from continuing GH treatment.

For adults, weight-based dosing largely has been supplanted by initiation with relatively low doses irrespective of weight, followed by dose titration as tolerated to raise IGF-1 levels to the mid-normal range adjusted for age and sex. A typical starting dose is 150-300 μg per day, with higher doses used in younger patients transitioning from pediatric therapy; lower doses are used in older patients (e.g., >60 years of age). Either an elevated serum IGF-1 level or persistent side effects mandates a decrease in dose; conversely, the dose can be increased (typically by 100-200 μg per day) if serum IGF-1 has not reached the normal range after 2 months of GH therapy. Because estrogen inhibits GH action, women taking oral—but not transdermal—estrogen may require larger GH doses to achieve the target IGF-1 level. In the setting of AIDS-related wasting, considerably higher doses (e.g., 100 μg/kg) have been used. Studies are also underway to assess the effect of GH therapy on reducing visceral adiposity and increasing lean body mass in HIV-infected patients with the adipose redistribution syndrome (Grunfeld et al., 2007; Lo et al., 2008).

Based on the known age-related decline in GH levels, the use of GH therapy to ameliorate or even reverse the consequences of aging has been widely promoted. Many of the studies supporting this use were not placebo controlled and involved small numbers of subjects. Moreover, one review concluded that there was no significant improvement in strength or aerobic performance with GH therapy in elderly subjects (Liu et al., 2007). Thus, this remains an area of considerable debate. In violation of regulations and standard medical practice, some athletes also use injected GH preparations as anabolic agents to enhance performance (Gibney et al., 2007). In addition to the parenteral GH preparations, oral preparations containing "stacked" amino acids that reputedly stimulate GH release have been marketed as nutritional supplements. Despite the absence of validation in controlled trials, these formulations are part of multibillion dollar anti-aging and performance-enhancing programs (Olshansky and Perls, 2008).

Adverse Effects of Growth Hormone Therapy. In children, GH therapy is associated with remarkably few side effects. Rarely, generally within the first 8 weeks of therapy, patients develop intracranial hypertension, with papilledema, visual changes, headache, nausea, and/or vomiting. Because of this, funduscopic examination is recommended at the initiation of therapy and at periodic intervals thereafter. Leukemia has been reported in some children receiving GH therapy; a causal relationship has not been established, and conditions associated with GH deficiency (e.g., Down's syndrome, cranial irradiation for CNS tumors) probably explain the apparent increased incidence of leukemia. Despite this, the consensus is that GH should not be administered in the first year after treatment of pediatric tumors, including leukemia, or during the first 2 years after therapy for medulloblastomas or ependymomas. Because an increased incidence of type 2 diabetes mellitus has been reported, fasting glucose levels should be followed periodically during therapy. Finally, too-rapid growth may be associated with slipped epiphyses or scoliosis, although a causal link with GH has not been proven.

Side effects associated with the initiation of GH therapy in adults include peripheral edema, carpal tunnel syndrome, arthralgias, and myalgias, which occur most frequently in patients who are older or obese and generally respond to a decrease in dose. These volume-related adverse effects occur less frequently with standard-dose rather than weight-based regimens. Although there are potential concerns about impaired glucose tolerance secondary to anti-insulin actions of GH, this generally has not been a major problem at the recommended doses. In fact, changes in visceral fat composition associated with GH replacement may improve insulin sensitivity in some patients.

Drug Interactions. The effects of estrogen on GH therapy were noted earlier. This effect is much less marked with transdermal estrogen preparations. Recent studies suggest that GH therapy can increase the metabolic inactivation of glucocorticoids in the liver. Thus, GH may precipitate adrenal insufficiency in patients with occult secondary adrenal insufficiency or in patients receiving replacement doses of glucocorticoids. This has been attributed to the inhibition of the type 1 isozyme of steroid 11β-hydroxysteroid dehydrogenase, which normally converts inactive cortisone into the active 11-hydroxy derivative cortisol (Chapter 42).

Absorption, Distribution, and Elimination. Mecasermin is administered by subcutaneous injection and absorption is virtually complete. As discussed earlier, IGF-1 in circulation is bound by six proteins; a ternary complex that includes IGFBP-3 and the acid labile subunit accounts for >80% of the circulating IGF-1. This protein binding prolongs the t_1/2 of IGF-1 to ∼6 hours. Both the liver and kidney have been shown to metabolize IGF-1.

Therapeutic Uses. Mecasermin is FDA-approved for patients with impaired growth secondary to mutations in the GH receptor or postreceptor signaling pathway, patients with GH deficiency who develop antibodies against GH that interfere with its action, and the very rare patients with IGF-1 gene defects that lead to primary IGF-1 deficiency (Collet-Solberg and Misra, 2008). Typically the starting dose is 40-80 μg/kg per dose twice daily by subcutaneous injection, with a maximum of 120 μg/kg per dose twice daily.

Clinical trials also have examined the efficacy of mecasermin in the much larger cohort of patents with impaired growth secondary to GH deficiency or with idiopathic short stature. In these settings, mecasermin stimulates linear growth but is less effective than conventional therapy using recombinant GH, suggesting direct effects of GH on linear growth independent of IGF-1. Although further study is needed, mecasermin should be reserved for therapy in patients with FDA-approved indications and is unlikely to replace GH as preferred therapy for most patients with GH deficiency or short stature.

Adverse Effects. Side effects of mecasermin include hypoglycemia and lipohypertrophy, both presumably secondary to activation of the insulin receptor. To diminish the frequency of hypoglycemia, mecasermin should be administered shortly before or after a meal or snack. Lymphoid tissue hypertrophy, including enlarged tonsils, also is seen and may require surgical intervention. Other adverse effects are similar to those associated with GH therapy and include intracranial hypertension, slipped epiphyses, and scoliosis.

Contraindications. Mecasermin should not be used for growth promotion in patients with closed epiphyses. It should not be given to patients with active or suspected neoplasia and should be stopped if evidence of neoplasia develops.

LH and FSH were named initially based on their actions on the ovary; appreciation of their roles in male reproductive function came later. LH and FSH are synthesized and secreted by gonadotropes, which make up ∼10% of the hormone-secreting cells in the anterior pituitary. hCG, which is produced only in primates and horses, is synthesized by the syncytiotrophoblast of the placenta. Pituitary gonadotropin production is stimulated by GnRH and further regulated by feedback effects of the gonadal hormones (Figures 38–7 and 40-2). Chapters 40 and 41 provide more detailed descriptions of gonadotropin regulation.

TSH is measured in the diagnosis of thyroid disorders, and recombinant TSH is used in the evaluation and treatment of well-differentiated thyroid cancer, as discussed in Chapter 39.

Other Regulators of Gonadotropin Production. Gonadal steroids regulate gonadotropin production at the level of the pituitary and the hypothalamus, but effects on the hypothalamus predominate. The feedback effects of gonadal steroids are dependent on sex, concentration, and time. In women, low levels of estradiol and progesterone inhibit gonadotropin production, largely through opioid action on the neural pulse generator. Higher and more sustained levels of estradiol have positive feedback effects that ultimately result in the gonadotropin surge that triggers ovulation. In men, testosterone inhibits gonadotropin production, in part through direct actions and in part via its conversion by aromatase to estradiol.

Gonadotropin production also is regulated by the inhibins, which are members of the bone morphogenetic protein family of secreted signaling proteins. Inhibin A and B are made by granulosa cells in the ovary and Sertoli cells in the testis in response to the gonadotropins and local growth factors (Bilezikjian et al., 2006). They act directly in the pituitary to inhibit FSH secretion without affecting that of LH. Inhibin A exhibits variation during the menstrual cycle, suggesting that it acts as a dynamic regulator of FSH secretion.

Chemistry. The structure of gonadorelin (Table 38–3) corresponds to that of native GnRH; it is a decapeptide with blocked amino and carboxyl termini.

Absorption, Distribution, and Elimination. As a peptide, gonadorelin is administered either subcutaneously or intravenously. It is well-absorbed following subcutaneous injection and has a circulating t_1/2 of ∼2-4 minutes. For therapeutic uses, it must be administered in a pulsatile manner to avoid down-regulation of the GnRH receptor.

Diagnostic and Therapeutic Uses. Because of the limited availability of GnRH, many of its diagnostic and therapeutic uses have been discontinued. For diagnostic purposes, gonadorelin was used in the GnRH stimulation test in patients with hypogonadotropic hypogonadism in an effort to differentiate between hypothalamic and pituitary defects. A blood sample was obtained for a baseline LH value, a single 100-μg dose of GnRH was administered subcutaneously or intravenously, and serum LH levels were measured at various times after injection. A normal LH response to >10 mIU/mL indicated the presence of functional pituitary gonadotropes and prior exposure to GnRH. Inasmuch as the long-term absence of GnRH can impair the responsiveness of otherwise normal gonadotropes, the absence of a response did not always indicate intrinsic pituitary disease. Thus some experts had advocated use of multiple doses of GnRH in an effort to restore gonadotrope responsiveness.

GnRH-stimulation testing also was used to determine whether a child with precocious puberty had a GnRH-dependent (central) or GnRH-independent (peripheral) disorder. A GnRH-induced increase in plasma LH to >10 mIU/mL in boys or >7 mIU/mL in girls indicated true precocious puberty rather than a GnRH-independent process. Currently, some experts in the U.S. and elsewhere have used long-lasting GnRH agonists off label as the stimulating agent for diagnostic assessment (see section on Gonadotropin-Releasing Hormone Agonists).

To treat female infertility secondary to impaired GnRH secretion, gonadorelin was administered by infusion in pulses that promoted a physiological cycle. Advantages over gonadotropin therapy included a lower risk of multifetal pregnancies and a decreased need to monitor plasma estradiol levels or follicle size by ovarian ultrasonography. Side effects usually were minimal; the most common was local irritation due to the infusion device. In women, normal cycling levels of ovarian steroids could be achieved, leading to ovulation and menstruation. The manufacturer has discontinued production for this indication, and gonadorelin is no longer generally available in the U.S.

Chemistry. The sequences of the available GnRH agonists are listed in Table 38–3. They contain substitutions of the native sequence at position 6 that protect against proteolysis and substitutions at the carboxyl terminus that improve receptor-binding affinity. Compared to GnRH, these analogs exhibit enhanced potency and prolonged duration of action.

Pharmacokinetics. The GnRH agonists are available in a variety of formulations for diverse applications, including relatively short-term effects (e.g., assisted reproduction technology) and more prolonged action (e.g., depot forms that inhibit gonadotropin secretion in GnRH-dependent precocious puberty). The rates and extents of absorption thus vary considerably. The intranasal formulations are noteworthy because their bioavailability (∼4%) is considerably lower than those of the parenteral formulations.

Clinical Uses. Based on the intermittent supply of gonadorelin, the depot form of the GnRH agonist leuprolide (lupron) has been used diagnostically to differentiate between GnRH-dependent and GnRH-independent precocious puberty (Brito et al., 2004). Leuprolide depot (3.75 mg) is injected subcutaneously, and serum LH is measured 2 hours later. A plasma LH level of >6.6 mIU/mL is diagnostic of GnRH-dependent (central) disease.

Clinically, the various GnRH agonists are used to achieve pharmacological castration in disorders that respond to reduction in gonadal steroids. A clear indication is in children with GnRH-dependent precocious puberty, whose premature sexual maturation can be arrested with minimal side effects by chronic administration of a depot form of a GnRH agonist.

Long-acting GnRH agonists are used for palliative therapy of hormone responsive tumors (e.g., prostate or breast cancer), generally in conjunction with agents that block steroid biosynthesis or action to avoid transient increases in hormone levels (Chapters 40,41,42). The GnRH agonists also are used to suppress steroid-responsive conditions such as endometriosis, uterine fibroids, acute intermittent porphyria, and priapism. They also have been evaluated off label for their potential to preserve follicles in women undergoing therapy with cytotoxic drugs for cancer treatment, although efficacy in this setting has not been established. Depot preparations can be administered subcutaneously or intramuscularly monthly or every 3 months in these settings and may be particularly useful for pharmacological castration in disorders such as paraphilia (an off-label use), where strict patient compliance is problematic.

Finally, the long-lasting GnRH agonists have been used to avoid a premature LH surge, and thus ovulation, in various ovarian stimulation protocols for in vitro fertilization. The specifics of this use are outlined in Chapter 66.

Adverse Effects. The long-acting agonists generally are well tolerated, and side effects are those that would be predicted to occur when gonadal steroidogenesis is inhibited (e.g., hot flashes and decreased bone density in both sexes, vaginal dryness and atrophy in women, and erectile dysfunction in men). Because of these effects, therapy in non-life-threatening diseases such as endometriosis or uterine fibroids generally is limited to 6 months unless add-back therapy with sex steroids is incorporated into the regimen. Postmarketing surveillance noted an apparent increase in the incidence of pituitary apoplexy, a syndrome of headache, neurological manifestations, and impaired pituitary function that usually results from infarction of a pituitary adenoma (Davis et al., 2006); the product labeling for the GnRH agonists has been updated to include this risk. Although definitive evidence of an increased risk of congenital abnormalities is lacking (Elizur and Tulundi, 2008), the GnRH agonists interfered with pregnancy in animal models and thus are considered as contraindicated in pregnant women (FDA Category X). Thus caution should be exercised to avoid the exposure of pregnant women to GnRH agonists.

Specific Drug Formulations and Approved Indications

Leuprolide (lupron, eligard) is formulated in multiple doses for injection: subcutaneous (500 mg/day), subcutaneous depot (7.5 mg/month; 22.5 mg/3 months; 30 mg/4 months; 45 mg/6 months), and intramuscular depot (3.75 mg/month; 11.25 mg/3 months). It is approved for endometriosis, uterine fibroids, advanced prostate cancer, and central precocious puberty.

Goserelin (zoladex) is formulated as a subcutaneous implant (3.6 mg/month; 10.8 mg/12 weeks). It is approved for endometriosis and advanced prostate and breast cancer.

Histrelin (vantas, supprelin la) is formulated as a subcutaneous implant (50 mg/12 months). It is approved for central precocious puberty and advanced prostate cancer.

Nafarelin (synarel) is formulated as a nasal spray (200 μg/ spray). It is approved for endometriosis (400 μg/day) and central precocious puberty (1600 μg/day).

Triptorelin (trelstar depot, trelstar la) is formulated for depot intramuscular injection (3.75 mg/month; 11.25 mg/12 weeks) and approved for advanced prostate cancer.

Buserelin and Deslorelin are not available in the U.S.

Chemistry. The GnRH antagonists are synthetic peptides with modifications of certain residues found in the native GnRH sequence. In addition to a D-amino acid at position 6 and a D-Ala amide at position 10, the antagonists each contain D-chlorophenylalanyl and 3-pyridylalanyl substitutions at positions 2 and 3, respectively, which affect receptor activation and increase inhibitory potency and solubility. The additional substitution at position 8, along with that at position 10, reduces the induction of histamine release and the tendency to trigger immediate hypersensitivity reactions (Reissmann et al., 1995).

Absorption, Distribution, and Elimination. Both available GnRH antagonists are formulated for subcutaneous administration. Bioavailability exceeds 90% within 1-2 hours, and the t_1/2 varies depending on the dose. Once-daily administration suffices for therapeutic effect.

Adverse Effects. In contrast to older antagonists, the FDA-approved drugs did not manifest potent histamine release in preclinical or clinical trials. However, hypersensitivity reactions, including anaphylaxis, have been noted in postmarketing surveillance, some with the initial dose. When used in conjunction with gonadotropin injections for assisted reproduction, the effects of estrogen withdrawal (e.g., hot flashes) are not seen. The product labeling indicates that the GnRH antagonists are contraindicated in pregnant women (FDA Category X), although definitive data linking them to an increased risk of congenital abnormalities is lacking (Elizur and Tulandi, 2008).

The original gonadotropin preparations for clinical therapy were prepared from human urine and included chorionic gonadotropin, obtained from the urine of pregnant women, and menotropins, obtained from the urine of postmenopausal women. Because of their relatively low purity, these gonadotropins were administered intramuscularly to decrease the incidence of hypersensitivity reactions. Subsequently, urine-derived preparations of sufficient purity to be administered subcutaneously were developed. Highly purified preparations of human gonadotropins now are prepared using recombinant DNA technology that exhibit less batch-to-batch variation; this same technology is likely to lead to improved forms of gonadotropins with increased half-lives or higher clinical efficacy. One such "designer" gonadotropin, FSH-CTP, contains the β—subunit of FSH fused to the carboxy-terminal extension of hCG, thereby increasing considerably the t_1/2 of the recombinant protein. In clinical trials, FSH-CTP has been shown to stimulate follicle maturation in vivo when injected weekly (reviewed by Macklin et al., 2006).

Recombinant FSH (rFSH) is prepared by expressing cDNAs encoding the α and β subunits of human FSH in mammalian cell lines, yielding products whose glycosylation pattern mimics that of FSH produced by gonadotropes. The two available rFSH preparations (follitropin α [gonal-f] and follitropin β [follistim, puregon]) differ slightly in their carbohydrate structures; both exhibit less interbatch variability than do preparations purified from urine and can be administered subcutaneously because they are considerably purer. The relative advantages (i.e., efficacy, lower frequency of side effects such as ovarian hyperstimulation) of recombinant FSH versus urine-derived gonadotropins have not been definitively established despite much debate in the published literature (see Chapter 66 for further discussion).

Treatment typically is initiated with hCG (1500-2000 IUs intramuscularly or subcutaneously) three times per week until clinical parameters and the plasma testosterone level indicate full induction of steroidogenesis. Thereafter, the dose of hCG is reduced to 2000 IU twice a week or 1000 IU three times a week, and menotropins (FSH + LH) or recombinant FSH is injected three times a week (typical dose of 150 IU) to fully induce spermatogenesis.

Based on the observation that the normal male infant is exposed to high levels of gonadotropins during the first year of life, it has been proposed that therapy with LH and FSH during infancy may be associated with increased spermatogenesis later in life (Grumbach, 2005), and studies addressing this question are in progress.

The most common side effect of gonadotropin therapy in males is gynecomastia, which occurs in up to a third of patients and presumably reflects increased production of estrogens due to the induction of aromatase. Maturation of the prepubertal testes typically requires treatment for >6 months, and optimal spermatogenesis in some patients may require treatment for up to 2 years. Once spermatogenesis has been initiated, either by this combined therapy in patients with prepubertal disease or in patients who developed hypogonadotropic hypogonadism after sexual maturation, ongoing treatment with hCG alone usually is sufficient to support sperm production.

Therapy usually consists of injections of hCG (3000 IU/m² body surface area) intramuscularly every other day for six doses. If this does not induce testicular descent, orchiopexy should be performed. Some experts prefer surgery as the initial approach based on the association of hCG treatment with germ cell apoptosis in certain experimental settings (Ritzen, 2008).

Estradiol stimulates oxytocin secretion, whereas the ovarian polypeptide relaxin inhibits its release. The inhibitory effect of relaxin appears to be the net result of a direct effect on oxytocin-producing cells and an inhibitory action mediated indirectly by endogenous opiates. Other factors that primarily affect vasopressin secretion also have some impact on oxytocin release; for example, ethanol inhibits release, whereas pain, dehydration, hemorrhage, and hypovolemia stimulate release. Although peripheral actions of oxytocin appear to play no significant role in the response to dehydration, hemorrhage, or hypovolemia, oxytocin may participate in the central regulation of blood pressure.

Pharmacological doses of oxytocin can inhibit free water clearance by the kidney through arginine vasopressin-like activity at vasopressin V₂ receptors, occasionally causing water intoxication if administered with large volumes of hypotonic fluid. Based on the behavior of intravenously administered oxytocin during labor induction, the plasma t_1/2 of oxytocin is approximately 12-15 minutes.

Mechanism of Action. Oxytocin acts via a specific GPCR (OXT) closely related to the V_1a and V₂ vasopressin receptors. In the human myometrium, OXT couples to G_q/G₁₁, activating the PLC_β-IP₃-Ca²⁺ pathway and enhancing activation of voltage-sensitive Ca²⁺ channels. Oxytocin also increases local prostaglandin production, which further stimulates uterine contractions. Figure 66–2 summarizes the interactions of signaling pathways affecting myometrial contractililty.

Because of difficulties associated with the measurement of oxytocin levels and because loss of pituitary oxytocin apparently does not compromise labor and delivery, the physiological role of oxytocin in pregnancy has been highly debated. Exogenous oxytocin can initiate or enhance rhythmic contractions at any time, but a considerably higher dose is required in early pregnancy. An eightfold increase in uterine sensitivity to oxytocin occurs in the last half of pregnancy, mostly in the last 9 weeks, and is accompanied by a 30-fold increase in oxytocin receptor numbers between week 32 and term. The finding that the oxytocin antagonist atosiban is effective in suppressing preterm labor further supports the physiological importance of oxytocin in this setting.

Brain regions proposed to be critical in the response to fearful stimuli, including the amygdala, midbrain, and striatum, showed decreased activation in response to stressful stimuli following oxytocin treatment (Baumgartner et al., 2008; Huber et al., 2005). The role of perturbations of oxytocin signaling in mental conditions such as social phobia and autism and the possible therapeutic benefit of drugs that manipulate CNS oxytocin effects are exciting areas of ongoing investigation.

Clinical Therapeutics of Oxytocin

Deficiencies of oxytocin associated with disorders of the posterior pituitary impair milk letdown after delivery and may be one of the earliest signs of pituitary insufficiency secondary to postpartum hemorrhage (Sheehan's syndrome); oxytocin is not used clinically in this setting. Rather, oxytocin is used therapeutically to induce or augment labor and to treat or prevent postpartum hemorrhage, as described in Chapter 66.