Paraaminosalicyclic Acid

Para-aminosalicylic Acid (PAS), like the sulfonamides (see Chapter 44), is a structural analogue of p-aminobenzoic acid (PABA). It is a folate synthesis antagonist that interferes with the incorporation of PABA into folic acid. PAS is bacteriostatic, and in vitro, most strains of M. tuberculosis are sensitive to a concentra tion of 1^g/mL. The antibacterial activity of PAS is highly specific for M. tuberculosis; it is not effective against other mycobacterium species.

PAS is readily absorbed from the GI tract and is widely distributed throughout body fluids except cerebrospinal fluid. It penetrates tissues and reaches high concentrations in the tuberculous cavities and caseous tissue. Peak plasma levels are reached within 1 to 2 hours of drug administration, and the drug has a halflife of about an hour. PAS is primarily metabolized by hepatic acetylation. When combined with isoniazid, PAS can function as an alternative substrate and block hepatic acetylation of isoniazid, thereby increasing free isoniazid levels. Both the acetylated and unaltered drug are rapidly excreted in the urine. The concentration of PAS in urine is high and may result in crystalluria.

Use of PAS has diminished over the years following the introduction of more effective drugs, such as ri-fampin and ethambutol. At present, therapy with PAS is limited to the treatment of MDR tuberculosis. Problems with primary resistance, poor compliance due to GI intolerance, and lupuslike reactions have further discouraged its use.

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