Within1 to 2 hours after oral administration, isoniazid produces peak blood levels which decline to 50 percent or less within 6 hours. It diffuses readily into all body fluids (cerebrospinal, pleural, and ascitic fluids), tissues, organs, and excreta (saliva, sputum, and feces). The drug also passes through the placental barrier and into milk in concentrations comparable to those in the plasma. From 50 to 70 percent of a dose of isoniazid is excreted in the urine within 24 hours.

Isoniazid is metabolized primarily by acetylation and dehydrazination. The rate of acetylation is genetically determined.

Approximately 50 percent of Blacks and Caucasians are "slow inactivators", and the rest are "rapid inactivators"; the majority of Eskimos and Orientals are "rapid inactivators."

The rate of acetylation does not significantly alter the effectiveness of isoniazid. However, slow acetylation may lead to higher blood levels of the drug and thus, to an increase in toxic reactions.

Pyridoxine (vitamin B6) deficiency is sometimes observed in adults with high doses of isoniazid and is considered probably due to its competition with pyridoxal phosphate for the enzyme apotryptophanase.

Mechanism of Action

Isoniazid inhibits the synthesis of mycoloic acids, an essential component of the bacterial cell wall. At therapeutic levels isoniazid is bacteriocidal against actively growing intracellular and extracellular Mycobacterium tuberculosis organisms lsoniazid resistant Mycobacterium tuberculosis bacilli develop rapidly when lsoniazid monotherapy is administered.

Microbiology

Two standardized in vitro susceptibility methods are available for testing isoniazid against Mycobacterium tuberculosis organisms. The agar proportion method (CDC or NCCLS M24-P) utilizes middlebrook 7H10 medium impregnated with isoniazid at two final concentrations, 0.2 and 0.1 mcg/mL. MIC₉₉ values are calculated by comparing the quantity of organisms growing in the medium containing drug to the control cultures. Mycobacterial growth in the presence of drug ³1% of the control indicates resistance.

The radiometric broth method employs the BACTEC 460 machine to compare the growth index from untreated control cultures to cultures grown in the presence of 0.2 and 1.0 mcg/mL of isoniazid. Strict adherence to the manufacturer's instructions for sample processing and data interpretation is required for this assay.

Mycobacterium tuberculosis isolates with an MIC₉₉ £ 0.2 mcg/mL are considered to be susceptible to isoniazid. Susceptibility test results obtained by the two different methods discussed above cannot be compared unless equivalent drug concentrations are evaluated.

The clinical relevance of in vitro susceptibility for mycobacterium species other than M. tuberculosis using either the BAGTEC or the proportion method has not been determined.

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