Isoniazid 300 mg, Pyridoxine Hcl 3 mg Tablets

Pyridoxal phosphate and pyridoxamine phosphate are active vitamin B6 coenzymes involved in amino-acid metabolism. B6 also contributes to neurotransmitter synthesis, glycogen metabolism and hemoglobin formation.

Vitamin B6 is absorbed in the jejunum. Phosphorylated forms are dephosphorylated before free B6 enters by passive diffusion. The NIH review describes similar absorption from foods and supplements; much of a large pharmacological dose is rapidly eliminated in urine. This physiology does not supply a measured half-life for this formulation.

Within 1 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 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 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.

Isoniazid inhibits the synthesis of mycoloic acids, an essential component of the bacterial cell wall. At therapeutic levels isoniazid is bactericidal against actively growing intracellular and extracellular Mycobacterium tuberculosis organisms.

Resistance to isoniazid occurs because of mutations in the katG, inhA, kasA and ahpC genes. Resistance in M. tuberculosis develops rapidly when isoniazid monotherapy is administered.

For specific information regarding susceptibility test criteria and associated test methods and quality control standards recognized by FDA for this drug, please see: www.fda.gov/STIC.