This article throws light upon the top five anti-tuberculosis agents. The five agents are: 1. Isoniazid (INH) 2. Rifampicin (RIF) 3. Ethambutol (EMB) 4. Pyrazinamide (PZA) 5. Streptomycin (STR).
Contents
Anti-Tuberculosis Agent # 1. Isoniazid (INH):
INH (Fig. 6.1a) is the synthetic hydrazide of isonicotic acid discovered in 1952 and the first- line anti-tuberculosis medication used in the prevention and treatment of TB. INH is the cornerstone of the therapy and should be included in all regimens unless a high degree of INH resistance exists. This drug is never used on its own to treat TB because resistance develops quickly.
INH is highly selective and acts almost exclusively against M. tuberculosis, M. bovis and M. Africanism. This remarkable selectivity in its action is thought to be mediated by the bacterial enzyme catalase peroxidase which catalyses the reaction converting INH to a potent bactericidal derivative.
INH is bactericidal at MIC levels of less than 0.1 µg/ml for 80% of susceptible strains of M. tuberculosis.
Mechanism of Action:
The exact mechanism of the action of INH has not been fully elucidated, but several mechanisms including interference with the metabolism of bacterial proteins, nucleic acids, carbohydrates and lipids have been proposed. INH is a pro-drug and must be activated by bacterial catalase.
It is activated by catalase-peroxidase enzyme katG to form isonicotinic acyl anion or radical. These forms will then react with a NADH radical to form isonicotinic acyl-NADH complex. This complex will then binds tightly to ketoenoylreductase known as InhA and prevent access of the natural enoyl-AcpM substrate.
This mechanism inhibits the synthesis of mycolic acid in the mycobacterial cell wall. INH combines with an enzyme which interferes with the cell metabolism of the bacteria. As a result of the disruption in its metabolism and without a cell wall the bacteria die.
INH reaches therapeutic concentration in serum, cerebrospinal fluid (CSF) and within caseous granulomas and metabolised in the liver via acetylation. INH is bactericidal to rapidly-dividing mycobacteria, but is bacteriostatic if the Mycobacterium is slow-growing. Susceptible bacteria may undergo 1 or 2 divisions before multiplication is arrested.
Resistance to INH:
INH inhibits the biosynthesis of mycolic acids present in the cell wall of M. tuberculosis. This renders the mycobacterial cell wall defective, thereby penetratable to toxic oxygen. KatG is the only enzyme in M. tuberculosis capable of activating INH. Expression of either the KatG or an alkyl hydro peroxidase AhpC is considered sufficient to protect the bacilli against toxic peroxides.
Side Effects and Toxicity:
Adverse reactions include rash, abnormal liver functions, hepatitis, sideroblastic anemia, peripheral neuropathy, mild central nervous system (CNS) effects and drug interactions resulting in increased phenytoin (dilantin) or disulfiran (antabuse) levels.
Peripheral neuropathy and CNS effects are associated with the use of INH and are due to pyridoxine (vitamin B6) depletion. Headache, poor concentration, poor memory and depression have all been associated with INH use.
The frequency of these side effects is not known and the association with INH is not well validated. The presence of these symptoms is not frequently disabling and is not a reason to stop treatment with INH and the patients are strongly encouraged to continue treatment despite these symptoms.
The hepatoxicity associated with INH results from the toxic effect of an intermediate product produced by N-hydroxylation of mono-acetyl-hydrazine, one of the metabolites of INH, by the liver cytochrome P-450 mixed function oxidase system. Hepatoxicity, nausea, vomiting, abdominal pains and appetite loss can be avoided with close clinical monitoring of the patient.
Anti-Tuberculosis Agent # 2. Rifampicin (RIF):
RIF (Fig. 6.1 b) is the second major anti-tuberculosis agent which is used in conjunction with other anti-tuberculosis agents in the treatment of TB. RIF is a semi synthetic derivative of one of a group of structurally similar, complex macro cyclic antibiotics produced by Streptomyces mediterranei.
RIF inhibits the growth of most Gram-positive bacteria as well as many Gram- negative bacteria. RIF inhibits M. tuberculosis at concentrations ranging from 0.005 – 0.2 µg/mL in vitro. RIF is soluble in organic solvents and in water at acidic pH.
Mechanism of Action:
RIF may be bacteriostatic or bactericidal in action, depending on the concentration of the drug attained at the site of infection and the susceptibility of the infecting organism.
RIF inhibits deoxyribonucleic acid (DNA)-dependent ribonucleic acid (RNA)-polymerase of the Mycobacterium by forming a stable drug-enzyme complex, leading to the suppression of the initiation of chain formation in RNA synthesis. More specifically, the (J-subunit of this complex enzyme is the site of the action of the drug, although RIF binds only to the holoenzyme.
Resistance to RIF:
Mutation in RNA polymerase beta subunit gene (rpoB) is the major mechanism of resistance to RIF with high frequencies of 90% or more. Evaluation of the relationship between RIF’s susceptibility and genetic alteration in rpoB gene also showed that 95% of the RIF-resistant M. tuberculosis isolates involved genetic alterations in an 81-base pair core region of rpoB gene.
This region is called the rifampicin resistance-determining region. Moreover; these genetic alterations in the rpoB gene are suspected as being the resistance mechanisms to RIF.
Side Effects and Toxicity:
In humans, acute overdose with RIF, i.e. up to 12 g has not been fatal, however one fatality has been reported following ingestion of a single 60 g dose of the drug.
The lethal dose (LD50) of RIF in mice is 0.885 g/kg. The most important complication of RIF is liver toxicity, which occurs 4 times more frequently in regimens containing both INH and RIF than in those containing INH alone.
Anti-Tuberculosis Agent # 3. Ethambutol (EMB):
EMB (Fig. 6.1c) is a synthetic anti-tuberculosis agent prescribed to treat TB. It is active in vitro and in vivo against M. tuberculosis, M. bovis, M. marinum, M. avium and M. intracellular.
EMB is usually given in combination with other drugs such as INH, RIF and PZA, at a daily dose of 25 mg/kg to humans during the first 2 months of well- supervised therapy and at 15 mg/kg for longer, often less well supervised periods.
Mechanism of Action:
EMB may be bacteriostatic or bactericidal in action, depending on the concentration of the drug attained at the site of infection and the susceptibility of the organism.
Although the exact mechanism has not yet been fully determined, the drug appears to inhibit the synthesis of one or more metabolites in susceptible bacteria resulting in the impairment of cellular metabolism, arrest of multiplication, and cell death. EMB is active against susceptible bacteria only when they are undergoing cell division.
Resistance to EMB:
EMB has been shown to inhibit the incorporation of mycolic acids into the cell wall. It has also been shown to inhibit the transfer of arabinogalactan into the cell wall of mycobacteria.
Only the dextroisomer of EMB is biologically active, an observation consistent with the idea that the drug binds to a specific drug target, which is assumed to be arabinosyl transferase. The EmbB gene, encoding arabinosyl transferase which catalyses cell wall synthesis, is mutated in EMB- resistant strains.
Side Effects and Toxicity:
A single administration of EMB has low toxicity in mice. In humans the adverse effects of EMB include dermatitis, pruritis, headache, dizziness, fever and mental confusion.
Anti-Tuberculosis Agent # 4. Pyrazinamide (PZA):
PZA (Fig. 6.1 d) is a derivative of niacin-amide and is a synthetic anti-tuberculosis drug used to treat TB in patients. Currently, PZA is considered as a first line drug, and only used in combination with other drugs such as INH and RIF in the treatment of M. tuberculosis.
PZA has no other medical uses and is not used to treat other mycobacteria which are resistant to the drug. PZA is used in the first two to four months of treatment to reduce the duration of treatment required.
Mechanism of Action:
PZA may be bacteriostatic or bactericidal in action, depending on the concentration of the drug attained at the site of infection and the susceptibility of the organism.
The drug is active in vitro and in vivo at a slightly acidic pH. PZA stops the growth of M. tuberculosis, which has an enzyme pyrazinamidase that is only active at acidic pH. PZA converts the enzyme to the active form, pyrazinoic acid (POA). POA inhibits the enzyme fatty acid synthetase 1, which is required by the bacterium to synthesise fatty acids.
In addition, POA lowers the pH of the environment below a certain level which is optimal for M. tuberculosis growth. Mutations of the pyrazinamidase gene (pncA), are responsible for PZA resistance in M. tuberculosis. This appears to contribute to the drug’s anti-mycobacterial activity in vitro.
Resistance to PZA:
The mechanism of action and the resistance of M. tuberculosis to PZA have also been partially identified. PZA is crucial for achieving sterilization by killing persisting semi-dormant bacilli in the lungs.
Its activity depends on the presence of a bacterial amidase, which converts PZA to POA, which is the active antibacterial molecule. PZA-resistant bacilli lack this amidase activity. The pncA gene encoding for this has been identified and the mutation to this pncA gene has been associated with resistance to PZA.
Side Effects and Toxicity:
The most frequent adverse effect of PZA is hepatoxicity. Hepatoxicity becomes a problem when PZA is given in large doses and for long periods. Hepatoxicity may appear at any time during therapy. When PZA is used in short-course therapy no increase in the incidence of hepatotoxicity is noted.
The original dose for PZA was 40 – 70 mg/kg and the incidence of drug-induced hepatoxicity has fallen significantly since the recommended dose has been reduced. In the standard four-drug regime (INH, RIF, PZA and EMB), PZA is the most common cause of drug-induced hepatitis. It is not possible to clinically distinguish pyrazinamide-induced hepatitis from hepatitis caused by INH or RIF.
Another common side effect of PZA is joint pains (arthralgia), which can be distressing to patients, but never harmful. Other side effects include nausea and vomiting, anorexia, sideroblastic anemia, skin rash, urticaria, pruritus, hyperuricemia, dysuria, interstitial nephritis, malaise, rarely porphyria and fever. In mice, PZA has a LD50 of 3.4 g/kg when administered orally.
Anti-Tuberculosis Agent # 5. Streptomycin (STR):
STR (Fig. 6.1 e) is an aminoglycoside antibiotic which is particularly active against M. tuberculosis as well as against many Gram-negative bacteria. STR is bactericidal for the tubercle bacillus in vitro. Concentrations as low as 0.4 µg/mL inhibit growth of the tubercle bacillus in vitro. STR is an alternative to EMB in the four-drug protocols for the treatment of TB. STR is easily soluble in water.
Mechanism of Action:
STR, like other aminoglycosides, is actively transported across the bacterial cell membrane by an oxygen-dependent system. Once inside the bacteria, it binds to the polysomes and inhibits the synthesis of proteins.
The drug binds to the 30S subunit of the bacterial ribosome which consists of 21 proteins and a single 16S molecule of RNA. Protein synthesis in the bacteria is blocked by inhibiting the movement of the peptidylTrna associated with translocation and this stimulates tRNA errors.
Resistance to STR:
Most STR resistance strains have a mutation on the rrs and rspL genes encoding a 16S rRNA and a 12S ribosomal subunit protein respectively. In contrast to other bacteria, which have multiple copies of rRNA genes, M. tuberculosis complex members have only one copy. Hence, single nucleotide changes can potentially produce antibiotic resistance.
Side Effects and Toxicity:
The toxic effects of STR are manifested mainly on vestibular rather than auditory function in human beings. An acute toxic effect following intracisternal injection in animals is clonic-convulsion. Other acute toxic ejects following cutaneous or intravenous injections are nausea, vomiting and ataxia.