INHIBITION OF PNEUMOCYSTIS-CARINII DIHYDROPTEROATE SYNTHETASE BY PARA-ACETAMIDOBENZOIC ACID - POSSIBLE MECHANISM OF ACTION OF ISOPRINOSINE IN HUMAN-IMMUNODEFICIENCY-VIRUS INFECTION

Isoprinosine has been reported to decrease progression to AIDS, primar ily by preventing Pneumocystis carinii pneumonia (PCP), in human immun odeficiency virus-infected patients, but the mechanism of action is un known. para-Acetamidobenzoic acid (PAcBA), one component of isoprinosi ne, is structurally related to para-aminobenzoic acid (PABA), a precur sor of de novo folate synthesis. This pathway is known to be important for P. carinii because sulfonamides, which are effective anti-P. cari nii agents, inhibit incorporation of PABA into folate precursors by th e enzyme dihydropteroate synthetase (DHPS). Inhibition of P. carinii D HPS by PAcBA was investigated by using two assays. In short-term cultu res of P. carinii from rats, [H-3]PABA incorporation into reduced fola tes was inhibited by both isoprinosine (mean +/- standard error 50% in hibitory concentration [IC50], 20 +/- 8.4 muM) and PAcBA free acid (IC 50, 240 +/- 100 muM); a soluble PAcBA salt was more potent than PAcBA free acid alone (IC50, 29 +/- 48 muM). The activity of PAcBA free acid was confirmed in a cell-free DHPS inhibition assay (IC50, 120 +/- 120 muM). Inosine and dimethylaminopropanol, two other components of isop rinosine, were poor inhibitors of PABA incorporation (IC50, > 1,000 mu M). PAcBA free acid also showed activity in inhibiting the DHPS of Tox oplasma gondii, but was a poor inhibitor of the DHPSs of Escherichia c oli and Saccharomyces cerevisiae. In a rat model of PCP, the PAcBA sal t administered intraperitoneally demonstrated no activity against esta blished PCP either alone or when used in combination with trimethoprim ; the lack of efficacy in this model may be due to the rapid metabolis m of the drug. Prevention of PCP by PaCBA through inhibition of P. car inii DHPS may explain the activity of isoprinosine in decreasing the p rogression to AIDS in human immunodeficiency virus-infected patients.