DAPSONE ACETYLATION BY HUMAN LIVER ARYLAMINE N-ACETYLTRANSFERASES ANDINTERACTION WITH ANTIOPPORTUNISTIC INFECTION DRUGS

Dapsone is used in the treatment of Pneumocystis carinii pneumonia, an opportunistic infection that afflicts acquired immunodeficiency syndr ome (AIDS) patients. Inhibition of N-acetyltransferase (NAT)-dependent acetylation of dapsone could increase peak plasma concentrations of d apsone and shift the biotransformation pathway to the P450-mediated fo rmation of a toxic metabolite of dapsone, the hydroxylamine. Therefore , we have determined using human liver cytosol and bacterially express ed NATs, the NAT isoform responsible for acetylating dapsone and the p otential for antiopportunistic infection drugs to inhibit this metabol ic pathway, Formation of monoacetyldiaminodiphenylsulfone (MADDS) was quantitated by HPLC/UV detection at 270 nm after incubation of dapsone with 100 mu M acetyl coenzyme A regenerating system and human liver c ytosol. The mean +/- SD apparent K-M for the formation of MADDS in thr ee different human livers predicted to be fast acetylators based on ge notyping was 98 +/- 17.6 mu M, and the V-max was 190 +/- 20 pmol/min/m g cytosol protein. Eadie-Hofstee transformation of the substrate veloc ity data was linear, indicating acetylation by a kinetically single en zyme. Sulfamethazine (250 mu M) inhibited dapsone acetylation by 100% and 80%, respectively, at dapsone concentrations of 3 and 100 mu M, in both fast- and slow-acetylating liver cytosol preparations, whereas p ara-aminobenzoic acid (100 mu M) did not inhibit MADDS formation at ei ther of these dapsone concentrations Lineweaver-Burk plots of dapsone acetylation in the presence of 0, 25, and 50 mu M sulfamethazine showe d an increase in the apparent K-M, with increase in sulfamethazine con centration with no change in the V-max, indicating competitive inhibit ion of dapsone acetylation by sulfamethazine. The apparent K-M of daps one acetylation by bacterially expressed NAT1 and NAT2 enzymes was 687 and 136 mu M, respectively. Human liver cytosol preparations, predict ed to be slow acetylators based on genotyping, acetylated dapsone at a significantly lower rate when compared with fast acetylator human liv er cytosols. At clinically relevant concentrations, pyrimethamine, but not other antiopportunistic infection drugs (atovaquone, sulfadiazine , clarithromycin, trimethoprim, ketoconazole, and fluconazole), signif icantly but modestly (23%) inhibited MADDS formation in human liver cy tosols. These data indicate that NAT2 is the predominant liver NAT iso form acetylating dapsone in vivo and that coadministration with antiop portunistic infection drugs should not significantly inhibit this acet ylation pathway.