Isoniazid Injection

Isoniazid acts against actively growing tubercle bacilli.

Within one to two hours after oral administration, isoniazid produces peak blood levels which decline to 50 percent or less within six hours. It diffuses readily into all body fluids (cerebrospinal, pleural, and ascitic), 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 in 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 acetylators and the rest are rapid acetylators; the majority of Eskimos and Orientals are rapid acetylators.

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

Pyridoxine (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 mycolic acids, an essential component of the bacterial cell wall. At therapeutic levels isoniazid is bacteriocidal against actively growing intracellular and extracellular Mycobacterium tuberculosis organisms.

Isoniazid resistant Mycobacterium tuberculosis bacilli develop rapidly when isoniazid 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 1.0 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 ≥1% 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 manufacturers 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 BACTEC or the proportion method has not been determined.

Consult your pharmacist.

In the US -

Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088 or at www.fda.gov/medwatch.

In Canada - Call your doctor for medical advice about side effects. You may report side effects to Health Canada at 1-866-234-2345.