Chemical formula: C₆H₇N₃O Molecular mass: 137.139 g/mol PubChem compound: 3767
Isoniazid has no significant antibacterial action against any micro-organisms except the mycobacteria; against mycobacterium tuberculosis it is bacteriostatic in extremely low concentrations.
Isoniazid is a highly active tuberculostatic drug, and at high concentrations it is bactericidal to mycobacterium tuberculosis, possibly acting by interference with the synthesis of mycolic acid (a constituent of the bacterial cell wall).
Isoniazid is used mainly in the treatment of pulmonary tuberculosis but it appears to be effective also in the treatment of extrapulmonary lesions, including meningitis and genito-urinary disease.
Readily and completely absorbed after oral administration.
Readily diffuses into all tissues and fluids including the cerebrospinal fluid. Isoniazid is retained in the skin and in infected tissue; it crosses the placenta and is secreted in the milk of lactating mothers.
Isoniazid does not appear to be bound in the blood.
Isoniazid is not appreciably protein-bound and diffuses readily throughout the body. It affects intracellular as well as extracellular bacilli.
Plasma elimination half-life, in rapid acetylators about 1.2 hours and in slow acetylators about 3.5 hours.
The primary metabolic route involves acetylation the rate of which is determined genetically.
Acetylation, hydrolysis and glycine conjugation, hydrazone formation, and n-methylation; acetylation is polymorphic and two groups of acetylators have been identified, rapid and slow acetylators. The rate of hydrolysis is more rapid in the rapid acetylators than in the slow ones. The metabolites formed include acetyl isoniazid, isonicotinic acid, isonicotinuric acid, isonicotinoyl-hydrazones of pyruvic and glutaric acids, and n-methylisoniazid.
Over 90% of a dose is excreted in the urine in 24 hours, most being excreted in the first 12 hours, 4-32% is unchanged, but no more than 10% of a dose is excreted in the faeces.
Not applicable since isoniazid tablets have been used in clinical practice for many years and its effects in man are well known.
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