Chapter 56: Chemotherapy of Tuberculosis, Mycobacterium Avium Complex Disease, and Leprosy

History. The first successful drug for treating TB was para-amino salicylic acid (PAS), developed by Lehman in 1943. A more dramatic success came when Waksman and Schatz developed streptomycin. Further efforts led to development of thiacetazone by Domagk in 1946, isoniazid at Squibb, Hoffman La Roche, and Bayer in 1952, pyrazinamide by Kushner and colleagues in 1952, and rifamycins by Sensi and Margalith in 1957. Ethambutol was discovered at Lederle Laboratories in 1961. As might be anticipated, the use all of these drugs presents problems of drug resistance, adverse events, and drug interactions. Therefore, newer classes of agents are being developed. Moxifloxacin, PA-824, and TMC-207 have reached advanced clinical testing.

Chemistry. Rifamycins are macrocyclic antibiotics characterized by a chromophoric naphthohydroquinone group that is spanned by a long aliphatic bridge, with an acetyl group at C25. Rifapentine and rifabutin are derivatives of rifampin, whose structure is:

Antibacterial Activity. Rifampin inhibits the growth of most gram-positive bacteria as well as many gram-negative microorganisms such as Escherichia coli, Pseudomonas, indole-positive and indole-negative Proteus, and Klebsiella. Rifampin is very active against Staphylococcus aureus and coagulase-negative staphylococci. The drug also is highly active against Neisseria meningitidis and Haemophilus influenzae. Rifampin inhibits the growth of Legionella species in cell culture and in animal models.

Rifampin inhibits the growth of many M. tuberculosis clinical isolates in vitro at concentrations of 0.06-0.25 mg/L (Heifets, 1991). Rifampin is also bactericidal against M. leprae. M. kansasii is inhibited by 0.25-1 mg/L. Most strains of Mycobacterium scrofulaceum, Mycobacterium intracellulare, and M. avium are suppressed by concentrations of 4 mg/L. Mycobacterium fortuitum is highly resistant to the drug. Rifapentine minimum inhibitory concentrations (MICs) are similar to those of rifampin. Rifabutin inhibits the growth of most MAC isolates at concentrations ranging from 0.25-1 mg/L. Rifabutin also inhibits the growth of many strains of M. tuberculosis at concentrations of ≤0.125 mg/L and in vitro has better MICs than rifampin.

Mutations in genes involved in DNA repair mechanisms will impair the repair of multiple genes, which may lead to hyper-mutable strains (Chapter 48). M. tuberculosis Beijing genotype clinical isolates have been associated with higher rates of simultaneous rifampin and isoniazid resistance associated with mutations in the repair genes mut and ogt (Nouvel et al., 2006; Rad et al., 2003). Inducible or environment-dependent mutators may be a more common phenomenon than these stable mutator phenotypes (Warner and Mizrahi, 2006). Antibiotics, endogenous oxidative and metabolic stressors lead to DNA damage, which induces dnaE2. The induction is associated with error-prone DNA repair. This leads to higher rates of rifampin resistance (Boshoff et al., 2003).

Rifamycins are metabolized by microsomal B-esterases and cholinesterases that remove the acetyl group at position 25, resulting in 25-O-desacetyl rifamycins. Rifampin is also metabolized by hydrolysis to 3-formyl rifampin, whereas rifapentine is metabolized to 3-formyl rifapentine and 3-formyl-25-O-desacetyl-rifapentine. A major pathway for rifabutin elimination is CYP3A. Due to autoinduction, all three rifamycins reduce their own area under the concentration-time curves (AUC) with repeated administration (Table 56–3). They have good penetration into many tissues, but levels in the CNS reach only ~5% of those in plasma, likely due to the activity of P-glycoprotein. The drugs and metabolites are excreted by bile and eliminated via feces, with urine elimination accounting for only one-third and less of metabolites.

The population pharmacokinetics (PK) of rifampin are best described using a one-compartment model with transit compartment absorption (Wilkins et al., 2008), using the PK parameters in Table 56–2. Single-drug formulations increase mean transit time during absorption by ~100% and systemic clearance (SCL) by 24% in comparison to fixed-dose combinations of rifampin and other anti-TB drugs. Thus the absorption of rifampin will be slower, and the peak concentration (C_Pmax) of rifampin lower, with some formulations compared to others (Wilkins et al., 2008).

Rifapentine pharmacokinetics are likewise best described using a one-compartment open model with first-order absorption and elimination (Langdon et al., 2005). The PK parameters are summarized in Table 56–2. However, for each 1-kg weight increase above 50 kg, SCL increases by 0.05 L/hour and V_d by 0.69 L. Thus, C_Pmax and AUC decrease with increasing patient weight above 50 kg.

Rifabutin pharmacokinetics are best described by a two-compartment open model with first-order absorption and elimination. Rifabutin disposition is biexponential. Rifabutin concentrations are substantially higher in tissue than in plasma due to its lipophilic properties, leading to the very high apparent volumes of distribution (Table 56–2). The consequence is that C_Pmax values for rifabutin are lower than one would predict by comparison with other rifamycins. The volume of peripheral compartment decreases by 27% with concomitant azithromycin administration; tobacco smoking increases the volume by 39%.

These results predict that the t_1/2 of a rifamycin is less of an issue in optimizing therapy, and that if patients could tolerate it, higher doses would lead to higher bactericidal activities while suppressing resistance. In a recent clinical study, TB patients in South Africa were treated with 20 mg/kg/day of rifampin for 5 days, and the rate of sputum bacillary decline compared to that for doses of 3 mg/kg/day, 6 mg/kg/day, and 12 mg/kg/day (Diacon et al., 2007). There was a linear increase in the rate of kill, with a 2-fold increase between the 600 mg dose and the 1200 mg dose. Currently, efficacy of higher doses of rifapentine and rifampin is being examined in clinical studies.

Rifampin is also useful for the prophylaxis of meningococcal disease and H. influenzae meningitis. To prevent meningococcal disease, adults may be treated with 600 mg twice daily for 2 days or 600 mg once daily for 4 days; children >1 month of age should receive 10-15 mg/kg, to a maximum of 600 mg. Combined with a β-lactam antibiotic or vancomycin, rifampin may be useful for therapy in selected cases of staphylococcal endocarditis or osteomyelitis, especially those caused by staphylococci "tolerant" to penicillin. Rifampin may also be indicated for the eradication of the staphylococcal nasal carrier state in patients with chronic furunculosis.

Untoward Effects. Rifampin is generally well tolerated in patients. Usual doses result in <4% of patients with TB developing significant adverse reactions; the most common are rash (0.8%), fever (0.5%), and nausea and vomiting (1.5%). Rarely, hepatitis and deaths due to liver failure have been observed in patients who received other hepatotoxic agents in addition to rifampin or who had preexisting liver disease. Chronic liver disease, alcoholism, and old age appear to increase the incidence of severe hepatic problems. GI disturbances have occasionally required discontinuation of the drug. Various nonspecific symptoms related to the nervous system also have been noted.

Hypersensitivity reactions may be encountered. Hemolysis, hemoglobinuria, hematuria, renal insufficiency, and acute renal failure have been observed rarely; these also are thought to be hypersensitivity reactions. High-dose rifampin should not be administered on a dosing schedule of less than twice weekly because this is associated with a flu-like syndrome of fever, chills, and myalgias in 20% of patients so treated. The syndrome also may include eosinophilia, interstitial nephritis, acute tubular necrosis, thrombocytopenia, hemolytic anemia, and shock. Light chain proteinuria has also been documented with rifampin use. Thrombocytopenia, transient leukopenia, and anemia have occurred during therapy. Because the potential teratogenicity of rifampin is unknown and the drug is known to cross the placenta, it is best to avoid the use of this agent during pregnancy.

Rifabutin is generally well tolerated; primary reasons for discontinuation of therapy include rash (4%), GI intolerance (3%), and neutropenia (2%) (Nightingale et al., 1993). Neutropenia occurred in 25% of patients with severe HIV infection who received rifabutin. Uveitis and arthralgias have occurred in patients receiving rifabutin doses >450 mg daily in combination with clarithromycin or fluconazole. Patients should be cautioned to discontinue the drug if visual symptoms (pain or blurred vision) occur. Rifabutin causes an orange-tan discoloration of skin, urine, feces, saliva, tears, and contact lenses, like rifampin. Rarely, thrombocytopenia, a flu-like syndrome, hemolysis, myositis, chest pain, and hepatitis develop in patients treated with rifabutin. Unique side effects include polymyalgia, pseudojaundice, and anterior uveitis.

Rifamycin Overdose. Rifampin overdose is uncommon and has been poorly studied. Doses of up to 12 g have produced serum rifampin concentrations of 400 mg/L with no change in the serum elimination rate. The most prominent symptoms are the orange discoloration of skin, fluids, and mucosal surfaces, leading to the term red-man syndrome. Overdose can be life-threatening; treatment consists of supportive measures; there is no antidote.

Drug Interactions. Because rifampin potently induces CYPs 1A2, 2C9, 2C19, and 3A4, its administration results in a decreased t_1/2 for a number of compounds, including HIV protease and non-nucleoside reverse transcriptase inhibitors, digitoxin, digoxin, quinidine, disopyramide, mexiletine, tocainide, ketoconazole, propranolol, metoprolol, clofibrate, verapamil, methadone, cyclosporine, corticosteroids, coumarin anticoagulants, theophylline, barbiturates, oral contraceptives, halothane, fluconazole, and the sulfonylureas. It leads to therapeutic failure of these agents, with potentially catastrophic consequences. Prior to putting a patient on rifampin, therefore, all the patient's medications should be examined for potential interactions. Rifabutin is a less potent inducer of CYPs than rifampin, both in terms of potency and number of CYP enzymes involved; however, rifabutin does induce hepatic microsomal enzymes and decreases the t_1/2 of zidovudine, prednisone, digitoxin, quinidine, ketoconazole, propranolol, phenytoin, sulfonylureas, and warfarin. It has less effect than rifampin on serum levels of indinavir and nelfinavir. Compared to rifabutin and rifampin, the CYP-inducing effects of rifapentine are intermediate.

Mechanism of Action. Pyrazinamide is "activated" by acidic conditions. Initially it was assumed that the acidic conditions under which pyrazinamide works were inside macrophage phagosomes. However, pyrazinamide may not be very effective within macrophages; rather, the acidic conditions for activation may be at the edges of necrotic TB cavities where inflammatory cells produce lactic acid (Blumberg et al., 2003).

Mycobacterium tuberculosis nicotinamidase, or pyrazinaminidase deaminates pyrazinamide to pyrazinoic acid (POA^-), which is then transported to the extracellular milieu by an efflux pump (Zhang et al., 1999). In an acidic extracellular milieu, a fraction of POA⁻ is protonated to POAH, a more lipid-soluble form that enters the bacillus. The Henderson-Hasselbalch equilibrium (Chapter 2) progressively favors the formation of POAH and its equilibration across membrances as the pH of the extracellular medium declines toward the pK_a of pyrazinoic acid, 2.9, a condition that also enhances microbial killing (Zhang et al., 2002). Although the actual mechanism of microbial kill is still unclear, three mechanisms have been proposed (Zhang et al., 2003; Zimhony et al., 2000):

• inhibition of fatty acid synthase type I leading to interference with mycolic acid synthesis

• reduction of intracellular pH

• disruption of membrane transport by HPOA

Antibacterial Activity. Pyrazinamide exhibits antimicrobial activity in vitro only at acidic pH. At pH of 5.8-5.95, 80-90% of clinical isolates have an MIC of ≤100 mg/L (Salfinger and Heifets, 1988).

Mechanisms of Resistance. Pyrazinamide-resistant M. tuberculosis have pyrazinamidase with reduced affinity for pyrazinamide. This reduced affinity decreases the conversion of pyrazinamide to POA. Single point mutations in the pncA gene are encountered in up to 70% of resistant clinical isolates. The mechanisms contributing to resistance in 30% of resistant clinical isolates is unclear.

Absorption, Distribution, and Excretion. Pyrazinamide oral bioavailability is >90%. Pharmacokinetics are best described by a one-compartment model. GI absorption segregates patients into two groups: fast absorbers (56%) with an absorption rate constant of 3.56/hour and slow absorbers (44%) with an absorption rate of 1.25/hour (Wilkins et al., 2006). The drug is concentrated 20-fold in lung epithelial lining fluid (Conte et al., 2000). Pyrazinamide is metabolized by microsomal deamidase to POA and subsequently hydroxylated to 5-hydroxy-POA, which is then excreted by the kidneys. CL (clearance) and V_d (volume of distribution) increase with patient mass (0.5 L/hour and 4.3 L for every 10 kg above 50 kg), and V_d is larger in males (by 4.5 L) (see Table 56–2) This has several implications: The t_1/2 of pyrazinamide will vary considerably based on weight and gender, and the AUC_0-24 will decrease with increase in weight for the same dose (same mg drug/kg body weight). Pyrazinamide clearance is reduced in renal failure; therefore, the dosing frequency is reduced to three times a week at low glomerular filtration rates. Hemodialysis removes pyrazinamide; therefore, the drug needs to be re-dosed after each session of hemodialysis (Malone et al., 1999b).

Microbial Pharmacokinetics-Pharmacodynamics. Pyrazinamide's sterilizing effect is closely linked to AUC_0-24/MIC (Gumbo et al., 2008). However, resistance suppression is linked to the fraction of time that C_P persists above MIC (T > MIC). Because patient weight impacts both SCL and volume, both AUC and t_1/2 will be impacted by high weight. Clinical trial simulations that account for patient weight reveal that optimal AUC_0-24/MIC and T > MIC are likely to be achieved only by doses much higher than the currently recommended 15-30 mg/kg/day (Gumbo et al., 2008). The safety of such higher doses in actual patients is unclear.

Therapeutic Uses. The co-administration of pyrazinamide with isoniazid or rifampin has led to a one-third reduction in the duration of anti-TB therapy, and a two-thirds reduction in TB relapse. This led to reduction in length of therapy to 6 months, producing the current "short course" chemotherapy. Pyrazinamide is administered at an oral dose of 15-30 mg/kg/day.

Untoward Effects. Injury to the liver is the most serious side effect of pyrazinamide. When a dose of 40-50 mg/kg is administered orally, signs and symptoms of hepatic disease appear in ~15% of patients, with jaundice in 2-3% and death due to hepatic necrosis in rare instances. However, these rates were determined in an era when pyrazinamide was administered for durations much longer than the current 2 months. Elevations of plasma alanine/aspartate aminotransferases are the earliest abnormalities produced by the drug. Regimens employed currently (15-30 mg/kg/day) are much safer. Prior to pyrazinamide administration, all patients should undergo studies of hepatic function, and these studies should be repeated at frequent intervals during the entire period of treatment. If evidence of significant hepatic damage becomes apparent, therapy must be stopped. Pyrazinamide should not be given to individuals with hepatic dysfunction unless this is absolutely unavoidable.

Pyrazinamide inhibits excretion of urate, resulting in hyperuricemia in nearly all patients, and may cause acute episodes of gout. Other untoward effects observed with pyrazinamide include arthralgias, anorexia, nausea and vomiting, dysuria, malaise, and fever. In the U.S., the use of pyrazinamide is not approved during pregnancy because of inadequate data on teratogenicity.

There are minimal data on pyrazinamide overdose, and no antidote has been studied.

Chemistry. Isoniazid (Isonicotinic acid hydrazide), also called INH, is a small water-soluble molecule (MW = 137) that is structurally related to pyrazinamide (see Figure 56–3).

Other products of KatG activation of INH include superoxide, H₂O₂, alkyl hydroperoxides, and the NO radical, which may also contribute to the mycobactericidal effects of INH (Timmins and Deretic, 2006). M. tuberculosis could be especially sensitive to damage from these radicals because the bacilli have a defect in the central regulator of the oxidative stress response, oxyR. Backup defense against radicals is provided by alkyl hydroperoxide reductase (encoded by ahpC), which detoxifies organic peroxides. Increased expression of ahpC reduces isoniazid effectiveness.

Antibacterial Activity. The isoniazid MICs with clinical M. tuberculosis strains vary from country to country. In the U.S., e.g., the MICs are 0.025-0.05 mg/L (Heifets, 1991). Activity against M. bovis and M. kansasii is moderate. Isoniazid has poor activity against MAC. It has no activity against any other microbial genus.

Mechanisms of Resistance. The prevalence of drug-resistant mutants is ~1 in 10⁶ bacilli. Because TB cavities may contain as many as 10⁷ to 10⁹ microorganisms, preexistent resistance can be expected in pulmonary TB cavities of untreated patients. These spontaneous mutants can be selected by monotherapy; indeed, strains resistant to isoniazid will be selected and amplified by isoniazid monotherapy. Thus two or more agents are usually used. Because the mutations resulting in drug resistance are independent events, the probability of resistance to two antimycobacterial agents is small, ~1 in 10¹² (1 × 10⁶ × 10⁶), a low probability considering the number of bacilli involved.

Resistance to INH is associated with mutation or deletion of katG, overexpression of the genes for inhA (confers low-level resistance to INH and some cross-resistance to ethionamide), and ahpC and mutations in the kasA and katG genes. KatG mutants exhibit a high level of resistance to isoniazid (Zhang and Yew, 2009). The most common mechanism of isoniazid resistance in clinical isolates is due to single point mutations in the heme binding catalytic domain of KatG, especially a serine to asparagine change at position 315. Although isolates with this mutation completely lose the ability to form nicotinoyl-NAD⁺/NADP⁺ adducts, they retain good catalase activity and maintain good biofitness. Compensatory mutations in the ahpC promoter occur and increase survival of katG mutant strains under oxidative stress.

KatG 315 mutants have a high probability of co-occurrence with ethambutol resistance (Hazbón et al., 2006; Parsons et al., 2005). Mutations in katG, ahpC, and inhA have also been associated with rpoB mutations (Hazbón et al., 2006). This suggests that mutations at different loci associated with resistance to different drugs may somehow interact to make multiple drug resistance more likely. In the laboratory, efflux pump induction by isoniazid has been demonstrated, and it also confers resistance to ethambutol (Colangeli et al., 2005). In an in vitro pharmacodynamic model, efflux pump-induced resistance developed within 3 days and was followed by development of katG mutations (Gumbo et al., 2007b).

Absorption, Distribution, and Excretion. The bioavailability of orally administered isoniazid is ~100% for the 300 mg dose. The pharmacokinetics of isoniazid are best described by a one-compartment model, with the pharmacokinetic parameters in Table 56–2 (Kinzig-Schippers et al., 2005). The ratio of isoniazid in the epithelial lining fluid to that in plasma is 1-2 and for CSF is 0.9 (Conte et al., 2002). Approximately 10% of drug is bound to protein. From 75-95% of a dose of isoniazid is excreted in the urine within 24 hours, mostly as acetylisoniazid and isonicotinic acid.

Isoniazid is metabolized by hepatic arylamine N-acetyltransferase type 2 (NAT2), encoded by a variety of NAT2* alleles (Figure 56–3). The drug is N-acetylated to N-acetylisoniazid in a reaction that uses acetyl-coA. Isoniazid clearance in patients has been traditionally classified as one of two phenotypic groups: "slow" and "fast" acetylators, as seen in Figure 56–4. Recently, the phenotypic groups have been expanded to fast, intermediate, and slow acetylators, and population pharmacokinetic parameters of isoniazid have been estimated and related to NAT2 genotype; the number of NAT2*4 alleles account for 88% of the variability of INH clearance (Kinzig-Schippers et al., 2005).

The frequency of each acetylation phenotype depends on race but is not influenced by sex or age. Fast acetylation is found in Inuit and Japanese. Slow acetylation is the predominant phenotype in most Scandinavians, Jews, and North African whites. The incidence of "slow acetylators" among various racial types in the U.S. is ~50%. Because high acetyltransferase activity (fast acetylation) is inherited as an autosomal dominant trait, "fast acetylators" of isoniazid are either heterozygous or homozygous. Although it has been useful to categorize different "racial" groups dominated by one or the other of these phenotypes, the more precise approach will be to determine the NAT2*4 alleles for each patient to guide therapy for that patient in the future.

Microbial Pharmacokinetics-Pharmacodynamics. Isoniazid's microbial kill is best explained by the AUC_0-24-to-MIC ratio (Gumbo et al., 2007c). Resistance emergence is closely related to both AUC/MIC and C_max/MIC (Gumbo et al., 2007c). Because AUC is proportional to dose/CL, this means that efficacy is most dependent on drug dose and CL, and thus on the activity of NAT-2 polymorphic forms. This also suggests that dividing the isoniazid dose into more frequent doses may be detrimental in terms of resistance emergence, and more intermittent dosing would be better (Chapter 48).

Therapeutic Uses. Isoniazid is available as a pill, as an elixir, and for parenteral administration. The commonly used total daily dose of isoniazid is 5 mg/kg, with a maximum of 300 mg; oral and intramuscular doses are identical. Children should receive 10-15 mg/kg/day (300 mg maximum). Dosing information in the treatment of M. tuberculosis and M. kansasii infections is given in section II and VI.

Untoward Effects. After NAT2 converts isoniazid to acetylisoniazid, which is excreted by the kidney; acetylisoniazid can also be converted to acetylhydrazine (Roy et al., 2008), and then to hepatotoxic metabolites by CYP2E1. Alternatively, acetylhydrazine may be further acetylated by NAT-2 to diacetylhydrazine, which is nontoxic. In this scenario, rapid acetylators will rapidly remove acetylhydrazine while slower acetylators or induction of CYP2E1 will lead to more toxic metabolites. Rifampin is a potent inducer of CYP2E1, which is why it potentiates isoniazid hepatotoxicity.

Elevated serum aspartate and alanine transaminases are encountered commonly in patients on isoniazid. However, the enzyme levels often normalize even when isoniazid therapy is continued (Blumberg et al., 2003). Severe hepatic injury occurs in ~0.1% of all patients taking the drug. Hepatic damage is rare in patients <20 years old but the incidence increases with age to 1.2% between 35 and 49 years and to 2.3% over 50 years of age. Overall risk is increased by co-administration with rifampin to ~3%. Fatal hepatitis is even rarer (0.02%). Most cases of hepatitis occur 4-8 weeks after the start of therapy.

If pyridoxine is not given concurrently, peripheral neuritis (most commonly paresthesias of feet and hands) is encountered in ~2% of patients receiving isoniazid 5 mg/kg of the drug daily. Neuropathy is more frequent in "slow" acetylators and in individuals with diabetes mellitus, poor nutrition, or anemia. Other neurological toxicities include convulsions in patients with seizure disorders, optic neuritis and atrophy, muscle twitching, dizziness, ataxia, paresthesias, stupor, and toxic encephalopathy. Mental abnormalities may appear during the use of this drug, including euphoria, transient impairment of memory, separation of ideas and reality, loss of self-control, and florid psychoses. The prophylactic administration of pyridoxine prevents the development not only of peripheral neuritis, as well as most other nervous system disorders in practically all instances, even when therapy lasts as long as 2 years.

Patients may develop hypersensitivity to isoniazid. Hemato-logical reactions also may occur. Vasculitis associated with antinuclear antibodies may appear during treatment but disappears when the drug is stopped. Arthritic symptoms (back pain; bilateral proximal interphalangeal joint involvement; arthralgia of the knees, elbows, and wrists; and the "shoulder-hand" syndrome) have been attributed to this agent.

Miscellaneous reactions associated with isoniazid therapy include dryness of the mouth, epigastric distress, methemoglobinemia, tinnitus, and urinary retention. In persons predisposed to pyridoxine-deficiency anemia, the administration of isoniazid may result in dramatic anemia. Treatment of the anemia with large doses of vitamin B₆ gradually returns the blood count to normal. A drug-induced syndrome resembling systemic lupus erythematosus has also been reported.

Isoniazid Overdose. Intentional isoniazid overdose occurs most often in young women with concomitant psychiatric problems prescribed isoniazid for latent TB (Sullivan et al., 1998). As little as 1.5 g can be toxic. Isoniazid overdose has been associated with the clinical triad of:

• seizures refractory to treatment with phenytoin and barbiturates

• metabolic acidosis with an anion gap that is resistant to treatment with sodium bicarbonate

• coma

The common early symptoms appear within 0.5-3 hours of ingestion and include ataxia, peripheral neuropathy, dizziness, and slurred speech. The most dangerous are grand mal seizures and coma, encountered when patients ingests ≥30 mg/kg of the drug. Mortality in these circumstances is as high as 20%. Intravenous pyridoxine is administered over 5-15 minutes on a gram-to-gram basis with the ingested isoniazid. If the dose of ingested isoniazid is unknown, then a pyridoxine dose of 70 mg/kg should be used. In patients with seizures, benzodiazepines are utilized.

Isoniazid's toxicity may be interpreted in terms of effects on pyridoxine metabolism. Isoniazid binds to pyridoxal 5′-phosphate to form isoniazid-pyridoxal hydrazones, thereby depleting neuronal pyridoxal 5′-phosphate and interfering with pyridoxal phosphate-requiring reactions, including the synthesis of the inhibitory neurotransmitter, GABA. Decreased levels of GABA lead to cerebral overexcitability and lowered seizure threshold. The antidote is replenishment of pyridoxal 5′-phosphate.

Drug Interactions. Isoniazid is a potent inhibitor of CYP2C19, CYP3A, and a weak inhibitor of CYP2D6 (Desta et al., 2001). However, isoniazid induces CYP2E1. Drugs that are metabolized by these enzymes will potentially be affected. Table 56–4, based on work by Desta et al. (2001), is a summary of drugs that interact with isoniazid via these mechanisms.

Mechanism of Action. Ethambutol inhibits arabinosyl transferase III, thereby disrupting the transfer of arabinose into arabinogalactan biosynthesis, which in turn disrupts the assembly of mycobacterial cell wall (Lewis, 1999). The arabinosyl transferases are encoded by embAB genes.

Antibacterial Activity. Ethambutol has activity against a wide range of mycobacteria but has no activity against any other genus. Ethambutol MICs are 0.5-2 mg/L in clinical isolates of M. tuberculosis, ~0.8 mg/L for M. kansasii, and 2-7.5 mg/L for M. avium (Heifets, 1991; Lewis, 1999). The following species are also susceptible: M. gordonae, M. marinum, M. scrofulaceum, M. szulgai. However, the majority of M. xenopi, M. fortuitum, and M. chelonae have been reported as resistant (Lewis, 1999).

Mechanisms of Resistance. In vitro, mycobacterial resistance to the drug develops via mutations in the embB gene. In 30-70% of clinical isolates that are resistant to ethambutol, mutations are encountered at codon 306 of the embB gene. However, mutations in this codon are also encountered in ethambutol-susceptible mycobacteria, as though this mutation is necessary, but not sufficient, to confer ethambutol resistance (Safi et al., 2008). In addition, enhanced efflux pump activity may induce resistance to both isoniazid and ethambutol in the laboratory.

Absorption, Distribution, Metabolism, and Excretion. The oral bioavailability of ethambutol is ~80%. Approximately 10-40% of the drug is bound to plasma protein. Ethambutol drug concentrations have been modeled using a two-compartment open model, with first-order absorption and elimination (Peloquin et al., 1999; Zhu et al., 2004). The decline in ethambutol is biexponential, with a t_1/2 of 3 hours in the first 12 hours, and a t_1/2 of 9 hours between 12 and 24 hours, due to redistribution of drug. Clearance and V_d are greater in children than in adults on a per kilogram basis. Slow and incomplete absorption is common in children, so that good peak concentrations of drug are often not achieved with standard dosing (Zhu et al., 2004). In addition, these C_max values are not very impressive given the typical MIC values for most clinical isolates of mycobacteria. See Table 56–2 for PK data on this drug.

Alcohol dehydrogenase oxidizes ethambutol to an aldehyde, which is then oxidized by aldehyde dehydrogenase to dicarboxylic acid. However, 80% of the drug is not metabolized at all and is renally excreted. Therefore, in renal failure ethambutol should be dosed at 15-25 mg/kg, three times a week instead of daily, even in patients receiving hemodialysis.

Microbial pharmacokinetics-pharmacodynamics. Ethambutol's microbial kill of M. tuberculosis is optimized by AUC/MIC, while that against disseminated MAC is optimized by C_max/MIC (Srivastava et al., 2010; Deshpande et al., 2010). Thus, to optimize microbial kill, high intermittent doses such as 25 mg/kg every other day to 50 mg/kg twice a week may be superior to daily doses of 15 mg/kg.

Therapeutic Uses. Ethambutol is available for oral administration in tablets containing the D-isomer. It is used for the treatment of TB, disseminated MAC, and in M. kansasii infection. Ethambutol is administered at 15-25 mg/kg per day for both adults and children.

Untoward Effects. Ethambutol produces very few serious untoward reactions. Fewer than 2% of patients who receive daily doses of 15 mg/kg of ethambutol have adverse reactions: ~1% experience diminished visual acuity, 0.5% a rash, and 0.3% drug fever. Other side effects that have been observed are pruritus, joint pain, GI upset, abdominal pain, malaise, headache, dizziness, mental confusion, disorientation, and possible hallucinations. Numbness and tingling of the fingers owing to peripheral neuritis are infrequent. Anaphylaxis and leukopenia are rare. Therapy with ethambutol results in an increased concentration of urate in the blood in ~50% of patients, owing to decreased renal excretion of uric acid.

The most important side effect is optic neuritis, resulting in decreased visual acuity and loss of ability to differentiate red from green. The incidence of this reaction is proportional to the dose of ethambutol and is observed in 15% of patients receiving 50 mg/kg/day, in 5% of patients receiving 25 mg/kg/day, and in <1% of patients receiving daily doses of 15 mg/kg. The intensity of the visual difficulty is related to the duration of therapy after the decreased visual acuity first becomes apparent and may be unilateral or bilateral. Tests of visual acuity and red-green discrimination prior to the start of therapy and periodically thereafter are thus recommended. Recovery usually occurs when ethambutol is withdrawn; the time required is a function of the degree of visual impairment.

Cases of ethambutol overdose are rare; drug interactions involving ethambutol are not significant.

Bacterial Resistance. Primary resistance to streptomycin is found in 2-3% of M. tuberculosis clinical isolates. Streptomycin and the two other aminoglycosides inhibit protein synthesis by binding to the 30S ribosomal subunit and causing misreading of the genetic code during translation. The 30s ribosomal unit is made of the 16S mRNA (encoded by rpsL), which binds to the ribosomal protein S12 (encoded by rrs) to optimize tRNA binding and mRNA decoding. Mutations in rpsL and rrs are associated with high-level aminoglycoside resistance in mycobacteria. However, mutations in these genes are only encountered in half of clinical isolates with aminoglycoside resistance. GidB is an rRNA methyltransferase for 16S rRNA, and mutations in gidB gene are associated with low-level streptomycin resistance (Okamoto et al., 2007). The gidB mutations lead to high-level streptomycin resistant mutants at a rate 2000 times that in wild type. Mutations in gidB are encountered in 33% of streptomycin-resistant clinical isolates of M. tuberculosis. Finally, efflux pump–mediated resistance was recently demonstrated in clinical isolates with low-level streptomycin-resistant M. tuberculosis and interacted with chromosomal mutations in gidB (Spies et al., 2008). Thus resistance to aminoglycosides involves several genetic loci, as well as efflux pumps.

Therapeutic Uses. Therapeutic uses of aminoglycosides in treatment of mycobacterial infections are discussed later.

Mechanism of Action. The biochemical basis for the antimicrobial actions of clofazimine remains to be established (Anonymous, 2008a). Possible mechanisms of action include:

• membrane disruption

• inhibition of mycobacterial phospholipase A₂

• inhibition of microbial K⁺ transport

• generation of hydrogen peroxide

• interference with the bacterial electron transport chain

However, it is known that clofazimine has both antibacterial activity as well as anti-inflammatory effects via inhibition of macrophages, T cells, neutrophils, and complement.

Antibacterial Activity. The MICs for M. avium clinical isolates are 1-5 mg/L. The MICs for M. tuberculosis are ~1.0 mg/L. The compound also is useful for treatment of chronic skin ulcers (Buruli ulcer) produced by Mycobacterium ulcerans. It has activity against many gram-positive bacteria with an MIC ≤1.0 mg/L against S. aureus, coagulase-negative Staphylococci, Streptococcus pyogenes, and Listeria monocytogenes. Gram-negative bacteria have MICs >32 mg/L.

Bacterial Resistance. Mechanisms of resistance are unknown.

Absorption, Distribution, and Excretion. Clofazimine's oral bioavailability is highly variable, 45-60%; bioavailability is increased 2-fold by high-fat meals and decreased 30% by antacids (Nix et al., 2004). After a single dose, clofazimine is best modeled using a one-compartment model and has a prolonged absorption phase; after 200 mg of clofazimine, the t_max is 5.3-7.8 hours. After prolonged repeated dosing, the t_1/2 is ~70 days. For PK data, see Table 56–2 and Nix et al. (2004). As a result of the good penetration into many tissues, a reddish black discoloration of skin and body secretions may occur and take a long time to resolve. Crystalline deposits of the drug have been encountered in many tissues at autopsy (Anonymous, 2008a). Clofazimine is metabolized in the liver in four steps: hydrolytic dehalogenation, hydrolytic deamination, glucuronidation, and hydroxylation.

Dosing. Clofazimine is administered orally at doses up to 300 mg a day.

Untoward Effects. GI problems are encountered in 40-50% of patients and include abdominal pain, diarrhea, nausea, and vomiting. In patients who have died following the abdominal pain, crystal deposition in intestinal mucosa, liver, spleen, and abdominal lymph nodes has been demonstrated (Anonymous, 2008a). Body secretion discoloration, eye discoloration, and skin discoloration occur in most patients and can lead to depression in some patients.

Drug interactions. Anti-inflammatory effects may be inhibited by dapsone.

Fluoroquinolones are DNA gyrase inhibitors. Their chemistry, spectrum of activity, pharmacology, and adverse events are discussed in greater detail in Chapter 52. Drugs such as ofloxacin and ciprofloxacin have been second-line anti-TB agents for many years, but they are limited by the rapid development of resistance. Adding C8 halogen and C8 methoxy groups markedly reduces the propensity for drug resistance. Of the C8 methoxy quinolones, moxifloxacin (approved by the Food and Drug Administration for nontubercular infections) is furthest along in clinical testing as an anti-TB agent. Moxifloxacin is being studied to replace either isoniazid or ethambutol.

Microbial Pharmacokinetics-Pharmacodynamics Relevant to TB. Fluoroquinolone microbial kill is best explained by AUC_0-24/MIC ratio. In preclinical models, moxifloxacin AUC_0-24/MIC exposures equivalent to those from the standard 400-mg dose were associated with good microbial kill but amplified the drug-resistant subpopulation, so that resistance emerged in 7-13 days with monotherapy (Gumbo et al., 2004). This time to emergence of resistance harmonizes well with speed of resistance emergence in patients (Ginsburg et al., 2003). Moxifloxacin exposure best associated with minimizing emergence of resistance was an AUC_0-24/MIC of 53. Clinical trial simulations revealed that doses >400 mg a day might better achieve this AUC/MIC, which experiments in mice substantiate (Almeida et al., 2007). Given that rifamycins reduce moxifloxacin AUC, these results point to a potential concern of quinolone resistance. Unfortunately, the safety of moxifloxacin doses >400 mg has not been established.

Therapeutic Uses in Treatment of TB. In TB patients, moxifloxacin (400 mg/day) has bactericidal effects similar to that of standard doses of isoniazid (Johnson et al., 2006). When replacing ethambutol in the standard multi-drug regimen, 400 mg/day of moxifloxacin produces faster sputum conversion at 4 weeks than ethambutol (Burman et al., 2006b). In a comparison study of moxifloxacin, gatifloxacin, ofloxacin, and ethambutol as the fourth drug administered concurrently with isoniazid, rifampin, and pyrazinamide (Rustomjee et al., 2008b), moxifloxacin led to faster rates of bacterial kill during the early phases, gatifloxacin was equivalent by the eighth week, and ofloxacin was no better than ethambutol. Moxifloxacin is currently being studied in a phase 3 trial that may eventually lead to 4-month duration of anti-TB therapy compared to the current 6 months.

Drug Interactions Relevant to TB. In a study of volunteers treated with rifampin, moxifloxacin, or both drugs, rifampin reduced the moxifloxacin AUC_0-24 by 27% via induction of sulfate conjugation (Weiner et al., 2007). In another study, rifapentine reduced moxifloxacin AUC_0-24 by 17% (Dooley et al., 2008). These studies suggest that the most important cause of pharmacokinetic variability for moxifloxacin is concomitantly administered drugs for tuberculosis.

Mechanism of Action. TMC-207 acts by targeting subunit c of the ATP synthase of M. tuberculosis, leading to inhibition of the proton pump activity of the ATP synthase (Andries et al., 2005; Koul et al., 2007). Thus, the compound targets bacillary energy metabolism.

Antibacterial Activity. The TMC-207 MIC for M. tuberculosis is 0.03-0.12 mg/L. It has good activity against MAC, M. leprae, M. bovis, M. marinum, M. kansasii, M. ulcerans, M. fortuitum, M. szulgai, and M. abscessus (Andries et al., 2005; Huitric et al., 2007).

Bacterial Resistance. The proportion of M. tuberculosis mutants resistant to four times the MIC is 5 × 10⁻⁷ to 2 × 10⁻⁸. Resistance is associated with two point mutations: D32V and A63P. This region of the gene encodes the membrane-spanning domain of the ATP synthase c subunit.

Absorption, Distribution, and Excretion. After oral ingestion of 400 mg of TMC-207, the t_max was 4 hours, the C_max was 5.5 mg/L after 400 mg/day, and the AUC_0-24 was 65 mg·h/L. Based on these data, the CL is ~6.2 L/h, although systemic clearance reportedly is "triexponential." Population pharmacokinetic studies have not been published.

Pharmacokinetics, Efficacy, and Therapeutic Use. The anti-TB activity of TMC-207 is correlated with time above MIC. In murine TB, TMC-207 had superior bactericidal activity compared to isoniazid and rifampin, and accelerated sterilization when combined with rifampin, isoniazid, and pyrazinamide (Andries et al., 2005). In patients with drug-susceptible TB, the rate of sputum bacillary decline was similar to rifampin and isoniazid (Rustomjee et al., 2008a). A regimen of TMC207 400 mg daily for 2 weeks followed by 200 mg three times day thereafter was added to a background second-line regimen of either kanamycin or amikacin, ofloxacin with or without ethambutol in patients with TB resistant to both isoniazid and rifampin (MDR-TB), and led to an 8-week sputum conversion of ~50% with TMC207 compared to 9% without (Diacon et al., 2009).

Untoward Effects. The adverse events encountered with TMC207 are mild and include nausea in 26% of patients and diarrhea in 13% of patients, with others such as arthralgia, pain in extremities, and hyperuricemia in a small proportion of patients (Diacon et al., 2009). However, only a limited number of patients have been exposed to this drug, so that the full side-effect profile is unclear.

Mechanism of Action. PA-824 inhibits M. tuberculosis mycolic acid and protein synthesis at the step between hydroxymycolate and ketomycolate (Stover et al., 2000). Similar to the structurally related metronidazole, PA-824 is a pro-drug that requires activation by the bacteria via a nitro-reduction step that requires, among other factors, a specific glucose-6-phosphate dehydrogenase, FGD1, and the reduced deazaflavin co-factor F₄₂₀ (Bashiri et al., 2008). Another mechanism involves generation of reactive nitrogen species such as NO by PA-824's des-nitro metabolite, which then augment the kill of intracellular nonreplicating persistent bacilli by the innate immune system (Singh et al., 2008).

Antibacterial Activity. In vitro, the drug kills both nonreplicating M. tuberculosis that are under anaerobic conditions as well as replicating bacteria in ambient air. The MICs of PA-824 against M. tuberculosis range from 0.015–0.25 mg/L, but the drug lacks activity against other mycobacteria.

Bacterial Resistance. The proportion of mutants resistant to 5 mg/L of PA-824 is 10^-6. Resistance arises due to changes in structure of FGD, which is due to a variety of point mutations in fgd gene. However, resistant isolates have also been identified that lack fgd mutations, so that resistance may also be due to other mechanisms (Stover et al., 2000).

Pharmacokinetics and Efficacy. In murine and guinea pig TB, PA-824 was equivalent to standard doses of isoniazid (Stover et al., 2000). In a recent murine TB study, 100 mg/kg/day of PA-824 with pyrazinamide and rifampin showed total sterilization at 2 months and no relapse, versus 15% relapse with the standard isoniazid, rifampin, and pyrazinamide regimen (Tasneen et al., 2008). Phase 1 studies have been performed, but the pharmacokinetic data have not been published. Phase 2 studies are in progress.

Mechanism of Action. Mycobacterial EthaA, an NADPH-specific, FAD-containing monooxygenase, converts ethionamide to a sulfoxide, and then to 2-ethyl-4-aminopyridine (Vannelli et al., 2002). Although these products are not toxic to mycobacteria, it is believed that a closely related and transient intermediate is the active antibiotic. Ethionamide inhibits mycobacterial growth by inhibiting the activity of the inhA gene product, the enoyl-ACP reductase of fatty acid synthase II (Larsen et al., 2002). This is the same enzyme that activated isoniazid inhibits. Although the exact mechanisms of inhibition may differ, the results are the same: inhibition of mycolic acid biosynthesis and consequent impairment of cell-wall synthesis.

Antibacterial Activity. The multiplication of M. tuberculosis is suppressed by concentrations of ethionamide ranging from 0.6-2.5 mg/L. A concentration of ≤10 mg/L will inhibit ~75% of photochromogenic mycobacteria; the scotochromogens are more resistant.

Bacterial Resistance. Resistance occurs mainly via changes in the enzyme that activates ethionamide, and mutations are encountered in a transcriptional repressor gene that controls its expression, etaR. Mutations in inhA gene lead to resistance to both ethionamide and isoniazid.

Absorption, Distribution, and Excretion. The oral bioavailability of ethionamide approaches 100%. The pharmacokinetics are adequately explained by a one-compartment model with first-order absorption and elimination (Zhu et al., 2002); see PK values in Table 56–2. After oral administration of 500 mg of ethionamide, a C_max of 1.4 mg/L is achieved in 2 hours. The t_1/2 is ~2 hours. The concentrations in the blood and various organs are approximately equal. Ethionamide is cleared by hepatic metabolism; six metabolites have been identified. Metabolites are eliminated in the urine, with <1% of ethionamide excreted in an active form.

Therapeutic Uses. Ethionamide is administered only orally. The initial dosage for adults is 250 mg twice daily; it is increased by 125 mg/day every 5 days until a dose of 15-20 mg/kg/day is achieved. The maximal dose is 1 g daily. The drug is best taken with meals in divided doses to minimize gastric irritation. Children should receive 10-20 mg/kg/day in two divided doses, not to exceed 1 g/day.

Untoward effects. Approximately 50% of patients are unable to tolerate a single dose larger than 500 mg because of GI upset. The most common reactions are anorexia, nausea and vomiting, gastric irritation, and a variety of neurologic symptoms. Severe postural hypotension, mental depression, drowsiness, and asthenia are common. Convulsions and peripheral neuropathy are rare. Other reactions referable to the nervous system include olfactory disturbances, blurred vision, diplopia, dizziness, paresthesias, headache, restlessness, and tremors. Pyridoxine (vitamin B₆) relieves the neurologic symptoms, and its concomitant administration is recommended. Severe allergic skin rashes, purpura, stomatitis, gynecomastia, impotence, menorrhagia, acne, and alopecia have also been observed. A metallic taste also may be noted. Hepatitis has been associated with the use of the ethionamide in ~5% of cases. Hepatic function should be assessed at regular intervals in patients receiving the drug.

Mechanism of Action. PAS is a structural analog of para-aminobenzoic acid, the substrate of dihydropteroate synthase (folP1/P2). As a result, PAS is thought to be a competitive inhibitor folP1. However, in vitro the inhibitory activity against folP1 is very poor. However, mutation of the thymidylate synthase gene (thyA) results in resistance to PAS, but only 37% of the PAS-resistant clinical isolates or spontaneous mutants encode a mutation in thyA gene, or in any genes encoding enzymes in the folate pathway or biosynthesis of thymine nucleotides (Mathys et al., 2009). Unidentified actions of PAS likely play more important roles in its anti-TB effects.

Antibacterial Activity. PAS is bacteriostatic. In vitro, most strains of M. tuberculosis are sensitive to a concentration of 1 mg/L. It has no activity against other bacteria.

Bacterial Resistance. Mutations in thyA gene lead to drug resistance in a minority of drug-resistant isolates.

Absorption, Distribution, and Excretion. PAS oral bioavailability is >90%. PAS pharmacokinetics are described by a one-compartment model (Peloquin et al., 2001); see the PK values in Table 56–2. The C_max increases 1.5-fold and AUC 1.7-fold with food compared to fasting (Peloquin et al., 2001). These results mean that PAS should be administered with food, which also greatly reduces gastric irritation. Protein binding is 50-60%. PAS is N-acetylated in the liver to N-acetyl PAS, a potential hepatotoxin. Over 80% of the drug is excreted in the urine; >50% is in the form of the acetylated compound. Excretion of PAS acid is reduced by renal dysfunction; thus the dose must be reduced in renal dysfunction.

Therapeutic Uses. PAS is administered orally in a daily dose of 12 g. The drug is best administered after meals, with the daily dose divided into three equal portions. Children should receive 150-300 mg/kg/day in 3-4 divided doses.

Untoward Effects. The incidence of untoward effects associated with the use of PAS is ~10-30%. GI problems predominate and often limit patient adherence. Hypersensitivity reactions to PAS are seen in 5-10% of patients and manifest as skin eruptions, fever, eosinophilia, and other hematological abnormalities.

Mechanism of Action. Cycloserine and d-alanine are structural analogs; thus cycloserine inhibits alanine racemase which converts L-alanine to d-alanine and d-alanine: d-alanine ligase, stopping reactions in which d-alanine is incorporated into bacterial cell-wall synthesis (Anonymous, 2008b).

Antibacterial Activity. Cycloserine is a broad-spectrum antibiotic. It inhibits M. tuberculosis at concentrations of 5-20 mg/L. It has good activity against MAC, enterococci, E. coli, S. aureus, Nocardia species, and Chlamydia.

Mechanisms of Resistance. Mutations involved in cycloserine resistance to pathogenic Mycobacteria are currently unknown. However, resistance in clinical isolates of M. tuberculosis has been detected in 10-82% of isolates (Anonymous, 2008b).

Absorption, Distribution, and Excretion. Oral cycloserine is almost completely absorbed. The population pharmacokinetics are best described using a one-compartment model with first-order absorption and elimination (Zhu et al., 2001). The drug's t_1/2 is 9 hours. C_max in plasma is reached in 45 minutes in fasting subjects, but is delayed for up to 3.5 hours with a high-fat meal. See Table 56–2 for PK values. Cycloserine is well distributed throughout body. There is no appreciable barrier to CNS entry for cycloserine, and cerebrospinal fluid (CSF) concentrations are approximately the same as those in plasma. About 50% of cycloserine is excreted unchanged in the urine in the first 12 hours; a total of 70% is recoverable in the active form over a period of 24 hours. The drug may accumulate to toxic concentrations in patients with renal failure. About 60% of it is removed by hemodialysis, and the drug must be re-dosed after each hemodialysis session (Malone et al., 1999a).

Therapeutic Uses. Cycloserine is available for oral administration. The usual dose for adults is 250-500 mg twice daily.

Untoward Effects. Neuropsychiatric symptoms are common and occur in 50% of patients on 1 g/day, so much so that the drug has earned the nickname "psych-serine." Symptoms range from headache and somnolence to severe psychosis, seizures, and suicidal ideas. Large doses of cycloserine or the concomitant ingestion of alcohol increases the risk of seizures. Cycloserine is contraindicated in individuals with a history of epilepsy and should be used with caution in individuals with a history of depression.

Capreomycin (capastat) is an antimycobacterial cyclic peptide. It consists of four active components: capreomycins IA, IB, IIA, and IIB. The agent used clinically contains primarily IA and IB. Antimycobacterial activity is similar to that of aminoglycosides as are adverse effects, and capreomycin should not be administered with other drugs that damage cranial nerve VIII.

Bacterial resistance to capreomycin develops when it is given alone; such microorganisms show cross-resistance with kanamycin and neomycin. The adverse reactions associated with the use of capreomycin are hearing loss, tinnitus, transient proteinuria, cylindruria, and nitrogen retention. Severe renal failure is rare. Eosinophilia is common. Leukocytosis, leukopenia, rashes, and fever have also been observed. Injections of the drug may be painful. Capreomycin is a second-line antituberculosis agent. The recommended daily dose is 1g (no more than 20 mg/kg) per day for 60-120 days, followed by 1 g two to three times a week.

Mechanism of Action. Dapsone is a structural analog of para-aminobenzoic acid (PABA) and a competitive inhibitor of dihydropteroate synthase (folP1/P2) in the folate pathway, shown in Figure 56–5. The effect on this evolutionarily conserved pathway also explains why dapsone is a broad-spectrum agent with antibacterial, anti-protozoal, and antifungal effects.

The anti-inflammatory effects of dapsone occur via inhibition of tissue damage by neutrophils (summarized by Wolf et al., 2002). First, dapsone inhibits neutrophil myeloperoxidase activity and respiratory burst. Second, it inhibits activity of neutrophil lysosomal enzymes. Third, it may also act as a free radical scavenger, counteracting the effect of free radicals generated by neutrophils. Fourth, dapsone may also inhibit migration of neutrophils to inflammatory lesions (Wolf et al., 2002). Dapsone is extensively used for acne, but this therapy is not recommended.

Antimicrobial Effects. Antibacterial. Dapsone is bacteriostatic against M. leprae at concentrations of 1-10 mg/L. More than 90% of clinical isolates of MAC and M. kansasii have an MIC of ≤8 mg/L, but the MICs for M. tuberculosis isolates are high. It has little activity against other bacteria.

Anti-parasitic. Dapsone is also highly effective against Plasmodium falciparum with IC₅₀ of 0.006-0.013 mg/mL (0.6-1.3 mg/L) even in sulfadoxine-pyrimethamine–resistant strains. Dapsone has an IC₅₀ of 0.55 mg/L against Toxoplasma gondii tachyzoites.

Antifungal. Dapsone is effective at concentrations of 0.1/mg/L against the fungus Pneumocystic jiroveci.

Drug Resistance. Resistance to dapsone in P. falciparum, P. jiroveci, and M. leprae results primarily from mutations in genes encoding dihydropteroate synthase (Figure 56–5). In P. falciparum mutations occur at several positions such as 436, 437, 540, 58, and 613. In P. jiroveci isolates, mutations are often amino acid substitutions at positions 55 and 57. In M. leprae, mutations are encountered at codons 53 and 55.

Absorption, Distribution, and Excretion. After oral administration, absorption is complete; the elimination t_1/2 is 20-30 hours. The population pharmacokinetics of dapsone are shown in Table 56–2 (Simpson et al., 2006). CL increases 0.03 L/hour and V_d 0.7 L increases for each 1-kg increase in body weight above 62.3 kg. Dapsone undergoes N-acetylation by NAT2. N-oxidation to dapsone hydroxylamine is via CYP2E1, and to a lesser extent by CYP2C. Dapsone hydroxylamine enters red blood cells, leading to methemoglobin formation. Sulfones tend to be retained for up to 3 weeks in skin and muscle and especially in liver and kidney. Intestinal reabsorption of sulfones excreted in the bile contributes to long-term retention in the bloodstream; periodic interruption of treatment is advisable for this reason. Epithelial lining fluid to plasma ratio is between 0.76 and 2.91; CSF-to-plasma ratio is 0.21-2.01 (Gatti et al., 1997). Approximately 70-80% of a dose of dapsone is excreted in the urine as an acid-labile mono-N-glucuronide and mono-N-sulfamate.

Therapeutic Uses. Dapsone is administered as an oral agent. Therapeutic uses of dapsone in the treatment of leprosy are described later. Dapsone is combined with chlorproguanil for the treatment of malaria. Dapsone is also used for P. jiroveci infection and prophylaxis, and for the prophylaxis for T. gondii, as discussed in chapters devoted to these infections. The anti-inflammatory effects are the basis for therapy for pemphigoid, dermatitis herpetiformis, linear IgA bullous disease, relapsing chondritis, and ulcers caused by the brown recluse spider (Wolf et al., 2002).

Dapsone and Glucose-6-Phosphate Dehydrogenase Deficiency. Glucose-6-phosphate dehydrogenase (G6PD) protects red cells against oxidative damage. However, G6PD deficiency is encountered in nearly half a billion people worldwide, the most common of 100 variants being G6PD-A^-. Dapsone, an oxidant, causes severe hemolysis in patients with G6PD deficiency. Thus, G6PD deficiency testing should be performed prior to use of dapsone wherever possible.

Other Untoward Effects. Hemolysis develops in almost every individual treated with 200-300 mg of dapsone per day. Doses of ≤100 mg in healthy persons and ≤50 mg in healthy individuals with a G6PD deficiency do not cause hemolysis. Methemoglobinemia also is common. A genetic deficiency in the NADH-dependent methemoglobin reductase can result in severe methemoglobinemia after administration of dapsone. Isolated instances of headache, nervousness, insomnia, blurred vision, paresthesias, reversible peripheral neuropathy (thought to be due to axonal degeneration), drug fever, hematuria, pruritus, psychosis, and a variety of skin rashes have been reported. An infectious mononucleosis-like syndrome, which may be fatal, occurs occasionally.

Prophylaxis. After infection with M. tuberculosis, ~10% of people will develop active disease over a lifetime. The highest risk of re-activation TB is in patients with Mantoux tuberculin skin test reaction ≥5 mm who also fall into one of the following categories: recently exposed to TB, have HIV co-infection, have fibrotic changes on chest radiograms, or are immunosuppressed due to HIV infection, post-transplantation, or are taking immunosuppressive medications for any reason. If the tuberculin skin test is ≥10 mm, high risk of TB is encountered in recent (≤5 years) immigrants from areas of high TB prevalence, children <4 years of age, children exposed to adults with TB, intravenous drug users, as well as residents and employees of high-risk congregate settings. Any person with a skin test >15 mm is also at high risk of disease. In these patients at high risk of active TB, prophylaxis is recommended to prevent active disease. Prophylaxis consists of oral isoniazid, 300 mg daily or twice weekly, for 6 months in adults. Those who cannot take isoniazid should be given rifampin, 10 mg/kg daily, for 4 months. In children, isoniazid 10-15 mg/kg daily (maximum 300 mg) is administered, or 20-30 mg/kg two times a week directly observed, for 9 months. In children who cannot tolerate isoniazid, rifampin 10-20 mg/kg daily for 6 months is recommended.

The first 2 months of the four-drug regimen is termed the initial phase of therapy, and the last 4 months the continuation phase of therapy. Rifapentine (10 mg/kg once a week) may be substituted for rifampin in the continuation phase in patients with no evidence of HIV infection or cavitary TB. Pyridoxine, vitamin B₆, (10-50 mg/day) should be administered with isoniazid to minimize the risks of neurological toxicity in patients predisposed to neuropathy (e.g., the malnourished, elderly, pregnant women, HIV-infected individuals, diabetic patients, alcoholic patients, and uremic patients). To ensure compliance, therapy is administered as directly observed therapy (DOT). Although DOT is the standard of care, an analysis of a series of 11 randomized clinical trials found no difference in outcome between DOT and self-administered therapy (Volmink and Garner, 2007).

The duration of therapy for drug-susceptible pulmonary TB is 6 months. A 9-month duration should be used for patients with cavitary disease who are still sputum culture positive at 2 months. HIV-infected patients with CD4+ lymphocyte cell counts <100/mm³ are at increased risk of developing rifamycin resistance. Therefore, daily therapy is recommended during the continuation phase. Most cases of extrapulmonary TB are treated for 6 months. TB meningitis is an exception that requires a 9- to 12-month duration. In addition, corticosteroids are recommended for TB pericarditis, and results of a meta-analysis suggest they should also be used in TB meningitis (Prasad and Singh, 2008).

In documented drug resistance, therapy should be based on evidence of susceptibility and should include:

• at least three drugs to which the pathogen is susceptible, with at least one of the injectable anti-TB agents

• in the case of MDR-TB, use of four to six medications for better outcomes

• at least 18 months of therapy (Blumberg et al., 2003; Orenstein et al., 2009)

The addition to the regimen of a fluoroquinolone and surgical resection of the main lesions have been associated with improved outcome (Chan et al., 2004). There are currently no data to support intermittent therapy.

M. intracellulare often infects immunocompetent patients. The first decision after isolating MAC from pulmonary specimens is to determine whether disease is actually present or if the organism is merely part of environmental contamination. Criteria in favor of therapy includes bacteriological evidence, which consists of positive cultures from at least two sputums, or one positive culture from bronchoalveolar lavage or pulmonary biopsy with a positive culture or histopathological features, and clinical evidence of infection, and radiological evidence of infection such as pulmonary cavitation, nodular lesions, and/or bronchiectasis (Griffith et al., 2007).

In newly diagnosed patients with MAC pneumonia, triple drug therapy is recommended. These include a rifamycin, ethambutol, and a macrolide (Griffith et al., 2007; Kasperbauer and Daley, 2008). For the macrolides, either oral clarithromycin or azithromycin may be used. Rifampin is often the rifamycin of choice. Clarithromycin, 1000 mg, or azithromycin, 500 mg, are combined with ethambutol, 25 mg/kg, and rifampin, 600 mg, and administered three times a week for nodular and bronchiectatic disease. Therapy is continued for 12 months after the last negative culture. The same drugs are administered for patients with cavitary disease, but the dosing regimens are azithromycin 250 mg, ethambutol 15 mg/kg, and rifampin 600 mg. Parenteral streptomycin or amikacin at 15 mg/kg are recommended as a fourth drug. The effect of the aminoglycosides on clinical outcomes is unclear. Duration of therapy is as for nodular disease. In advanced pulmonary disease or during re-treatment, rifabutin 300 mg daily may replace rifampin. Because clarithromycin susceptibility correlates with outcome, risk of failure is high when high clarithromycin MICs are documented. Patients at risk for failure also include those with cavitary disease, presumably due to higher bacillary load. Even with these therapies, long-term success is still fairly limited. Only half of patients have successful outcomes as defined by both culture conversion and clinical outcomes.

The symptoms and laboratory findings of disseminated disease are nonspecific and include fever, night sweats, weight loss, elevated serum alkaline phosphates, and anemia at the time of diagnosis. However, when disease occurs in patients already on antiretroviral therapy, it may manifest as a focal disease of the lymph nodes, osteomyelitis, pneumonitis, pericarditis, skin or soft-tissue abscesses, genital ulcers, or CNS infection (DHHS Panel, 2008). In addition to a compatible clinical picture, isolation of MAC from cultures of blood, lymph node, bone marrow, or other normally sterile tissue or body fluids is required for diagnosis.

Prophylactic Therapy. The goals of prophylactic therapy are to prevent the development of disease during the time when a patient's CD4 count is low. Monotherapy with either oral azithromycin 1200 mg once a week or clarithromycin 500 mg twice a day is started when patients present with a CD4 count <50/mm³ (DHHS Panel, 2008). For patients intolerant to macrolides, rifabutin 300 mg a day is administered. Once the CD4 count is >100 per mm³ for ≥3 months, MAC prophylaxis should be discontinued.

Definitive and Suppressive Therapy. In patients with disease due to MAC, the goals of therapy include suppression of symptoms and conversion to negative blood cultures. The infection itself is not completely eradicated until immune reconstitution. Therapy is divided into initial therapy and chronic suppressive therapy. Recommended therapy consists of a combination of clarithromycin 500 mg twice a day with ethambutol 15 mg/kg daily, administered orally (DHHS Panel, 2008). Azithromycin 500-600 mg daily is an acceptable alternative to clarithromycin, especially in those patients in whom clarithromycin would adversely interact with other drugs. The addition of rifabutin 300 mg a day may improve outcomes. Mortality in disseminated MAC is high in patients with either a CD4 cell count <50/mm³ or a MAC burden of >2 log₁₀ CFU/mm³ of blood, or in the absence of effective antiretroviral therapy. In these patients, a fourth drug may be added, based on susceptibility testing. Potential fourth agents include amikacin, 10-15 mg/kg intravenously daily; streptomycin, 1 g intravenously or intramuscularly daily; ciprofloxacin, 500-750 mg orally twice daily; levofloxacin, 500 mg orally daily; or moxifloxacin, 400g orally daily. Patients should be continued on suppressive therapy until all three of the following criteria are met:

• therapy duration of at least 12 months

• CD4 count >100/mm³ for at least 6 months

• asymptomatic for MAC infection

Four major clinical types of leprosy impact therapy. At one end of the spectrum is tuberculoid leprosy, also termed paucibacillary leprosy because the bacterial burden is low and M. leprae is rarely found in smears. On the other end of the spectrum is the lepromatous form of the disease (Levis and Ernst, 2005). This is characterized by a disseminated infection and a high bacillary burden. Two major intermediate forms of the disease are recognized: borderline (dimorphous) tuberculoid disease, which has features of both tuberculoid and lepromatous leprosy, and indeterminate disease, which has early hypopigmented lesions without features of the lepromatous and tuberculoid leprosy.

Mycobacterium leprae was discovered by Armauer Hansen in 1873. The M. leprae genome has undergone reductive evolution and has radically downsized its genome (Cole et al., 2001). As a result, M. leprae cannot produce ATP from NADH or utilize acetate or galactose as carbon sources; moreover, it has lost the anaerobic electron transfer system and cannot survive under hypoxic conditions. It has a long doubling time (14 days) and is an obligate intracellular pathogen. As a result, M. leprae is difficult to culture on synthetic media, an impediment to basic research on the disease.

Pauci-Bacillary Leprosy. The WHO regimen consists of a single dose of oral rifampin, 600 mg, combined with dapsone, 100 mg, administered under direct supervision once every month for 6 months, and dapsone, 100 mg a day, in between for 6 months. In the U.S., the regimen consists of dapsone, 100 mg, and rifampin, 600 mg, daily for 6 months, followed by dapsone monotherapy for 3-5 years.

Multibacillary Therapy. The WHO recommends the same regimen as for paucibacillary leprosy, with two major changes. First, clofazimine, 300 mg a day, is added for the entirety of therapy. Second, the regimen lasts 1 year instead of 6 months. In the U.S., the regimen is also the same as for paucibacillary, but dual therapy continues for 3 years, followed by dapsone monotherapy for 10 years. Clofazimine (an orphan drug) is added when there is dapsone resistance or chronically reactional patients.

The duration of therapy for multi-bacillary leprosy is a drawback. Studies in murine leprosy, and in patients, have demonstrated that viable bacilli are killed within 3 months of therapy (Ji et al., 1996) suggesting that the length of current therapy for multibacillary leprosy may be unnecessarily long. Recently, the WHO proposed that all forms of leprosy be treated with the same dose as for paucibacillary leprosy; a clinical trial was promising (Kroger et al., 2008). This new shorter regimen promises to reduce duration of therapy radically.

Treatment of Reactions in Leprosy. Patients with tuberculoid leprosy may develop "reversal reactions," manifestations of delayed hypersensitivity to antigens of M. leprae. Cutaneous ulcerations and deficits of peripheral nerve function may occur. Early therapy with corticosteroids or clofazimine is effective. Reactions in the lepromatous form of the disease (erythema nodosum leprosum) are characterized by the appearance of raised, tender, intracutaneous nodules, severe constitutional symptoms, and high fever. This reaction may be triggered by several conditions but is often associated with therapy. It is thought to be an Arthus-type reaction related to release of microbial antigens in patients harboring large numbers of bacilli. Treatment with clofazimine or thalidomide is effective.

Mycobacteria other than those already discussed can be recovered from a variety of lesions in humans. Because they frequently are resistant to many of the commonly used agents, they must be examined for sensitivity in vitro and drug therapy selected on this basis. Therapy for infections from these organisms is summarized in Table 56–5. In some instances, surgical removal of the infected tissue followed by long-term treatment with effective agents is necessary. M. kansasii causes disease similar to that caused by M. tuberculosis, but it may be milder. The microorganisms may be resistant to isoniazid. Therapy with isoniazid, rifampin, and ethambutol has been successful.