Pyrazinamide is a synthetic analogue of nicotinamide. Its exact mechanism of action is not known, although its target appears to be the mycobacterial fatty acid synthetase involved in mycolic acid biosynthesis. Pyrazinamide requires an acidic environment, such as that found in the phagolysosomes, to express its tuber-culocidal activity. Thus, pyrazinamide is highly effective on intracellular mycobacteria. The mycobacterial enzyme pyrazinamidase converts pyrazinamide to pyrazi-noic acid, the active form of the drug. A mutation in the gene (pncA) that encodes pyrazinamidase is responsible for drug resistance; resistance can be delayed through the use of drug combination therapy.
Pyrazinamide is well absorbed from the GI tract and is widely distributed throughout the body. It penetrates tissues, macrophages, and tuberculous cavities and has excellent activity on the intracellular organisms; its plasma half-life is 9 to 10 hours in patients with normal renal function. The drug and its metabolites are excreted primarily by renal glomerular filtration.
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