Understanding Mycobacterial Infections and Tuberculosis Treatment

1. Oula Mohammed M.B.Ch.B/ MSc. Clinical pharmacology

2. Mycobacteria are slender, rod-shaped ‘acid-fast bacilli’ that cause a variety of diseases, including TB, leprosy, and localized or disseminated M. avium- intracellulare infections. Mycobacterial infections classically result in the formation of slow-growing, granulomatous lesions that cause tissue destruction anywhere in the body.

3. They are resistant to most antibiotics because of the following: 1. They grow slowly; antibiotics that are most active against growing bacteria are relatively ineffective. 2. Mycobacterial cells can be dormant and thus completely resistant to many drugs or killed only very slowly. 3. They are intracellular pathogens, and organisms residing within macrophages are inaccessible to drugs that penetrate these cells poorly. 4. The lipid-rich mycobacterial cell wall, which contains mycolic acids, which are long-chain, β-hydroxylated fatty acids. This cell wall is impermeable to many agents.

4. Mycobacterium tuberculosis, one of a number of mycobacteria, can lead to serious infections of the lungs, genitourinary tract, skeleton, and meninges.

5. Mycobacterium tuberculosis can cause latent tuberculosis infection (LTBI) and the disease known as tuberculosis (TB). [Note: In LTBI, the patient is infected with M. tuberculosis without signs or symptoms of active TB disease.] TB is the leading infectious cause of death worldwide, and a quarter of the world’s population is infected with TB. Increasing in frequency are diseases caused by nontuberculosis mycobacteria (NTM). These species intracellulare, M. chelonae, kansasii, and M. fortuitum. Finally, M. leprae causes leprosy. include M. M. avium- abscessus, M.

6. The goals of TB therapy are to kill tubercle bacilli rapidly, to eliminate persistent bacilli and prevent relapse, and to prevent transmission. The organism grows slowly; thus, the disease must be treated with multidrug therapy for at least6 months to eradicate the pathogen. • First-line agents are Isoniazid (known as INH), rifampicin (and other rifamycins), pyrazinamide and, ethambutol. • Second-line agents include streptomycin or amikacin ,paraaminosalicylic acid, ethionamide,bedaquiline capreomycin, cycloserine, flouroquiolones as ciprofloxacin & macrolides.

9. Second-line medications are either less effective, more toxic, or have not been studied extensively. They are useful in patients who cannot tolerate the first-line drugs or who are infected with myobacteria that are resistant to the first- line agents. Isoniazid and rifampicin are the two most active drugs. An isoniazid-rifampicin combination administered for 9 months will cure 95–98% of cases of tuberculosis caused by susceptible strains. The addition of pyrazinamide to an isoniazid-rifampin combination for the first 2 months allows the total duration of therapy to be reduced to 6 months without loss of efficacy. First 2 months duration of therapy to 6 months 9 months until susceptibility Treatment of multidrug-resistant TB (MDR-TB) typically lasts for about 2 years. In practice, therapy is initiated with a four-drug regimen of isoniazid, rifampicin, pyrazinamide, and ethambutol until susceptibility of the clinical isolate has been determined.

11. Strategies for addressing drug resistance Populations of M. tuberculosis contain small numbers of organisms that are naturally resistant to a particular drug. Under selective pressure from inadequate treatment, especially from monotherapy, these resistant organisms can emerge as the dominant population. Multidrug therapy is employed to suppress these resistant organisms. The first-line drugs isoniazid, rifampin, ethambutol, and pyrazinamide are preferred because of their high efficacy and acceptable incidence of toxicity. Rifabutin or rifapentine may replace rifampin under certain circumstances. Active disease always requires treatment with multidrug regimens, and preferably three or more drugs with proven in vitro activity against the isolate. Improvement can occur in the first several weeks of treatment, therapy is continued much longer to eradicate persistent organisms and to prevent relapse. Patient adherence can be low when multidrug regimens last for 6 months or longer. One successful strategy for achieving better treatment completion rates is directly observed therapy, (DOT). Patients take the medications under observation of a member of the health- care team.

12. Isoniazid(INH) Isoniazid is the most active drug for the treatment of tuberculosis caused by susceptible strains. It is the hydrazide of isonicotinic acid & a synthetic analog of pyridoxine (Vit.B6), It is small (MW 137) & freely soluble in water. Isoniazid penetrates into macrophages and is active against both extracellular and intracellular organisms. Isoniazid inhibits synthesis of mycolic acids, which are essential components of mycobacterial cell walls. Isoniazid is a prodrug that is activated by the mycobacterial catalase-peroxidase (katG). The activated form of isoniazid forms a covalent complex with two enzymes: an enoyl acyl carrier protein reductase (InhA) and a beta-ketoacyl carrier protein synthetase (KasA), this complex blocks mycolic acid synthesis and kills the cell.

14. Isoniazid is specific for treatment of M. tuberculosis. It is bactericidal against sensitive strains of M. tuberculosis and some strains of M. kansasii. It has little activity against M. avium-intracellulare and is not active against M. leprae or most other bacteria. Resistance is associated with several different chromosomal mutations, each of which results in one of following: 1. Mutation or deletion of catalase-peroxidase (producing mutants incapable of prodrug activation) 2. Varying mutations of the acyl carrier proteins 3. Overexpression of the target enzyme InhA.

15. • Orally administered isoniazid is readily absorbed. Absorption • is impaired if isoniazid is taken with food. • The drug diffuses into all body fluids, cells, and caseous material (necrotic tissue resembling cheese that is produced in tubercles). Drug levels in the cerebrospinal fluid (CSF) are about the same as those in the serum. • Metabolism of isoniazid, especially acetylation by liver N-acetyltransferase, is genetically determined. • Patients may be rapid or slow acetylators: The average plasma concentration of isoniazid in rapid acetylators is about one third to one-half of that in slow acetylators, and average half-lives are 90 minutes and 3 to 4 hours, respectively. • More rapid clearance of isoniazid by rapid acetylators is usually of no therapeutic consequence when appropriate doses are administered daily. • Excretion is through glomerular filtration, predominantly as metabolites. Severely depressed renal function results in accumulation of the drug, primarily in slow acetylators.

16. The incidence of adverse effects is low. Except for hypersensitivity, adverse effects are related to the dosage and duration of administration. Peripheral neuritis (manifesting as paresthesias of the hands and feet), which is the most common adverse effect, appears to be due to a relative pyridoxine (vitamin B6) deficiency as isoniazid inhibits conversion of pyridoxine to its active form and promotes its excretion. Most of the toxic reactions are corrected by supplementation of pyridoxine. Central nervous system toxicity, which is less common, includes memory loss, psychosis, and seizures. These may also respond to pyridoxine. Hepatitis and idiosyncratic hepatotoxicity, severe side effect associated with isoniazid. It has been suggested that this is caused by a toxic metabolite of monoacetylhydrazine, formed during the metabolism of isoniazid. Its incidence increases among patients with increasing age, among patients who also take rifampicin, or among those who drink alcohol daily. There is histologic evidence of hepatocellular damage and necrosis. contraindicates further use of the drug. Immunologic reactions , fever and skin rashes are occasionally seen. Drug-induced systemic lupus erythematosus has been reported. Potentially fatal hepatitis is the most Development of isoniazid hepatitis

17. Because isoniazid inhibits metabolism of phenytoin and carbamazepine, isoniazid can potentiate the adverse effects of these drug (for example, nystagmus and ataxia). Slow acetylators are particularly at risk.

19. Rifamycins ( rifampicin, rifabutin and rifapentine) Rifampicin (rifampin in USA), rifabutin, and rifapentine are all considered to be rifamycins, a group of structurally similar macrocyclic antibiotics, which are first-line drugs for tuberculosis. Any of these rifamycins must always be used in conjunction with at least one other antituberculosis drug to which the isolate is susceptible. Rifampicin Rifampicin is derived from the soil mold streptomyces.It has a broader antimicrobial activity than isoniazid and is used in the treatment of a number of different bacterial infections. Because resistant strains rapidly emerge during therapy, it is never given as a single agent in the treatment of active tuberculosis. Rifampicin blocks the process of RNA transcription by interacting with the β-subunit of bacterial (but not human) DNA-dependent RNA polymerase, inhibiting MRNA synthesis by suppressing the initiation step.

20. Rifampicin is bactericidal for both intracellular and extracellular mycobacteria, including M. tuberculosis, and atypical mycobacteria, such as M. kansasii and Mycobacterium avium complex (MAC). It is effective against many gram-positive and gram-negative organisms and is frequently used prophylactically for individuals exposed to meningitis caused by meningococci or Haemophilus influenzae. Rifampicin is the most active antileprosy drug at present, but to delay the emergence of resistant strains, it is usually given in combination with other drugs (dapsone and clofazimine). Resistance to rifampicin can be caused by a mutation in the affinity of the bacterial DNA-dependent RNA polymerase for the drug or by decreased permeability.

21. Absorption is adequate after oral administration. It readily penetrates most tissues and penetrates into phagocytic cells. It can kill organisms that are poorly accessible to many other drugs, such as intracellular organisms and those sequestered in abscesses and lung cavities. The drug is taken up by the liver and undergoes enterohepatic cycling. Rifampin can induce hepatic cytochrome P450 enzymes and transporters, leading to numerous drug interactions. Rifampin itself can induce the hepatic mixed-function oxidases, leading to a shortened half-life over the first 1 to 2 weeks of dosing. Elimination of metabolites and the parent drug is via the bile into the feces or via the urine. [Note: Urine, feces, and other secretions have should be forewarned. Tears may even stain soft contact lenses orange-red.] an orange-red color, so patients

22. Adverse effects Rifampicin is generally well tolerated. The most common adverse reactions include nausea, vomiting, and rash. Rifampicin causes a harmless orange color to urine, sweat, tears, and contact lenses. Hepatitis and death due to liver failure is rare; however, the drug should be used with caution in patients who are alcoholic, elderly, or have chronic liver disease due to the increased incidence of severe hepatic dysfunction when rifampicin is administered alone or concomitantly with isoniazid. A hypersensitivity reaction, manifesting as a flulike illness with chills, fever, fatigue, and headache, develops in as many as 50% of persons taking rifampicin. This reaction is more common in those who take large doses once or twice a week than in those who take smaller doses every day and sometimes extending to acute renal failure, hemolytic anemia, and shock. Rifampin strongly induces most cytochrome P450 isoforms, which increases the metabolism of numerous other drugs including methadone, anticoagulants, cyclosporine, some anticonvulsants, protease inhibitors, some nonnucleoside reverse transcriptase inhibitors, contraceptives, warfarin and many others resulting in significant lowering of serum levels of these drugs.

23. Rifabutin Rifabutin is a derivative of rifampicin. It has significant activity against M tuberculosis, M avium-intracellulare, and M fortuitum. Its activity is similar to that of rifampicin, and cross-resistance with rifampicin is virtually complete. Rifabutin is both substrate and inducer of cytochrome P450 enzymes. Because it is a less potent inducer(approximately 40% less), rifabutin is indicated in place of rifampicin for treatment of tuberculosis in HIV-infected patients who are receiving concurrent antiretroviral therapy with a protease inhibitor or nonnucleoside reverse transcriptase inhibitor (eg, efavirenz)drugs that also are cytochrome P450 substrates. Rifabutin has adverse effects similar to those of rifampicin but can also cause uveitis, skin hyperpigmentation, and neutropenia.

24. Rifapentine Rifapentine has activity comparable to that of rifampicin but has a longer half-life than rifampin and rifabutin, which permits weekly dosing. However, for the intensive phase (initial 2 months) of the short-course therapy for tuberculosis, rifapentine is given twice weekly. In the subsequent phase, rifapentine is dosed once per week for 4 months. Toxicity is similar to that of rifampicin.

25. Pyrazinamide Pyrazinamide is a synthetic, orally effective, bactericidal, antitubercular agent used in combination with isoniazid, rifampin, and ethambutol. Most of the clinical benefit from pyrazinamide occurs early in treatment. Therefore, this drug is usually discontinued after 2 months of a 6-month regimen. The precise mechanism of action is unclear. Pyrazinamide is taken up by macrophages and exerts its activity against mycobacteria residing within the acidic environment of lysosomes. The drug is converted to pyrazinoic acid (the active form of the drug) by mycobacterial pyrazinamidase. Pyrazinoic acid inhibits the growth of M. tuberculosis, possibly by inhibiting fatty acid synthesis. Resistance may be due to impaired uptake of pyrazinamide or mutations in pyrazinamidase that impair conversion of pyrazinamide to its active form. Pyrazinamide is well absorbed from the gastrointestinal tract and widely distributed in body tissues, including inflamed meninges. It undergoes extensive metabolism. Major adverse effects of pyrazinamide include hepatotoxicity (in 1–5% of patients), nausea, vomiting, drug fever, and hyperuricemia. The latter may provoke acute gouty arthritis.

26. Ethambutol Ethambutol is a butanol (butyl alcohol) derivative that is bacteriostatic and specific for most strains of M. tuberculosis and M. kansasii. Ethambutol inhibits arabinosyl transferase, an enzyme that is important for the synthesis of the mycobacterial arabinogalactan cell wall. Resistance is not a serious problem if the drug is employed with other antitubercular agents. Ethambutol can be used in combination with pyrazinamide, isoniazid, and rifampicin to treat tuberculosis. Absorbed on oral administration, ethambutol is well distributed throughout the body. Penetration into the central nervous system (CNS) is therapeutically adequate in tuberculous meningitis. Both parent drug and metabolites are excreted by glomerular filtration and tubular secretion.

27. The most important adverse effect is optic neuritis, which results in diminished visual acuity and loss of ability to discriminate between red and green (red-green color blindness). The risk of optic neuritis increases with higher doses and in patients with renal impairment. Visual acuity should be periodically examined. Discontinuation of the drug results in reversal of the optic symptoms. In addition, urate excretion is decreased by the drug; thus, gout may be exacerbated Ethambutol is relatively contraindicated in children too young to permit assessment of visual acuity and red-green color discrimination.

29. Alternative Second-Line Drugs for Tuberculosis Streptomycin This antibiotic is discussed with the aminoglycosides, which are protein synthesis inhibitors. Most tubercle bacilli are inhibited by streptomycin. Resistance is due to a point mutation in either the gene encoding the S12 ribosomal protein gene or the gene encoding 16S ribosomal rRNA, which alters the ribosomal binding site. Streptomycin penetrates into cells poorly and is active mainly against extracellular tubercle bacilli. Streptomycin crosses the blood-brain barrier and achieves therapeutic concentrations with inflamed meninges. Streptomycin sulfate is used when an injectable drug is needed or desirable, principally in individuals with severe, possibly life-threatening forms of tuberculosis, e.g., meningitis and disseminated disease, and in the treatment of infections resistant to other drugs. Streptomycin is ototoxic and nephrotoxic. Vertigo and hearing loss are the most common adverse effects and may be permanent. Toxicity is dose-related, and the risk is increased in the elderly. As with all aminoglycosides, the dose must be adjusted according to renal function.

30. Ethionamide This is a structural analog of isoniazid with the same mechanism of action. It is effective after oral administration and is widely distributed throughout the body, including the CSF. Metabolism is extensive, and the urine is the main route of excretion. Adverse effects that limit its use include gastric irritation, hepatotoxicity, peripheral neuropathies, and optic neuritis. Supplementation with vitamin B6 (pyridoxine) may lessen the severity Hypothyroidism, gynecomastia, alopecia, impotence, and CNS effects also have been reported. of the neurologic side effects. Capreomycin This peptide inhibits protein synthesis. It is obtained from Streptomyces capreolus. It is administered parenterally. Capreomycin is primarily reserved for the treatment of multidrug-resistant tuberculosis. Careful monitoring of the patient is necessary to prevent its nephrotoxicity and ototoxicity. Para-aminosalicylic acid Para-aminosalicylic acid (PAS) works via folic acid inhibition. While largely replaced by ethambutol for drug susceptible TB, PAS remains an important component of many regimens for MDR-TB.

31. Cycloserine An orally effective, tuberculostatic agent appears to antagonize the steps in bacterial cell wall synthesis involving D-alanine. It distributes well throughout body fluids, including the CSF. Cycloserine is metabolized, and both parent and metabolite are excreted in urine. Accumulation occurs with renal insufficiency. Adverse effects involve CNS disturbances, and epileptic seizure activity may be exacerbated. Peripheral neuropathies are also a problem, but they respond to pyridoxine. Amikacin It is an aminoglycoside antibiotic. Amikacin is indicated for treatment of tuberculosis suspected or known to be caused by streptomycin-resistant or multidrug-resistant strains. Bedaquiline Bedaquiline, a diarylquinoline, is an ATP synthase inhibitor. It is approved for the treatment of MDR-TB. Bedaquiline is administered orally, and it is active against many types of mycobacteria. Bedaquiline has a boxed warning for QT prolongation, and monitoring of the electrocardiogram is recommended. Elevations in liver enzymes have also been reported and liver function should be monitored during therapy. This agent is metabolized via CYP3A4, and administration with strong CYP3A4 inducers (for example, rifampin) should beavoided.

32. Fluoroquinolones As ciprofloxacin are broad-spectrum bactericidal drugs that inhibit bacterial topoisomerases as DNA gyrase which is essential for DNA replication. They are used in the treatment of Mycobacterium avium intracellulare infections and drug- resistant tuberculosis. Macrolides The macrolides, such as azithromycin and clarithromycin, are part of the regimen that includes ethambutol and rifabutin used for the treatment of infections by M. avium-intracellulare complex. Azithromycin is preferred for HIV-infected patients because it is least likely to interfere with the metabolism of antiretroviral drugs.