IN-VITRO FORMATION, DISPOSITION AND TOXICITY OF N-ACETOXY-SULFAMETHOXAZOLE, A POTENTIAL MEDIATOR OF SULFAMETHOXAZOLE TOXICITY

Variation in the formation and disposition of the hydroxylamine of (SM X-HA) is thought to play an important role in the pathogenesis of sulf amethoxazole(SMX)-induced idiosyncratic adverse drug reactions. We hyp othesized that, in analogy to carcinogenic arylamines, SMX-HA might be further converted to an electrophilic N-acetoxy metabolite which coul d play a role in mediating SMX toxicity. Accordingly, we chemically sy nthesized N-acetoxy-SMX, and examined the characteristics of its forma tion, metabolism, cytotoxicity and mutagenicity in human and bacterial test systems. The human arylamine N-acetyltransferases, (NAT)1 and NA T2, were capable of converting SMX-HA to N-acetoxy-SMX. NAT1 and NAT2 possessed similar affinities for SMX-HA (apparent K-m values of 650 an d 520 mu M, respectively), but the apparent maximal velocity of the NA T1-mediated acetylation was higher than that of NAT2. (1332 vs. 37 nmo l/min/U of immunoreactive NAT protein). Human peripheral blood mononuc lear cells 12,000 x g supernatant fractions converted N-acetoxy-SMX ma inly back to SMX-HA, and also to a lesser extent to SMX, at clinically relevant concentrations. Similar pathways were observed in human hepa tic cytosolic fractions. In a cytotoxicity assay, N-acetoxy-SMX was si gnificantly more toxic to human peripheral blood mononuclear cells tha n SMX-HA (16.6 vs. 11.5% dead cells at a concentration of 300 mu M). N -acetoxy-SMX was weakly mutagenic to the Salmonella typhimurium TA100 strain in the Ames test. These data suggest that the N-acetoxy metabol ites of sulfonamides could potentially play a role in mediating sulfon amide idiosyncratic adverse drug reactions.