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 (SMX-HA) is thought to play an important role in the pathogenesis of sulfamethoxazole (SMX)-induced idiosyncratic adverse drug reactions. We hypothesized that, in analogy to carcinogenic arylamines, SMX-HA might be further converted to an electrophilic N-acetoxy metabolite which could play a role in mediating SMX toxicity. Accordingly, we chemically synthesized N-acetoxy-SMX, and examined the characteristics of its formation, metabolism, cytotoxicity and mutagenicity in human and bacterial test systems. The human arylamine N-acetyl-transferases, (NAT)1 and NAT2, were capable of converting SMX-HA to N-acetoxy-SMX. NAT1 and NAT2 possessed similar affinities for SMX-HA (apparent Km values of 650 and 520 microM, respectively), but the apparent maximal velocity of the NAT1-mediated acetylation was higher than that of NAT2. (1332 vs. 37 nmol/min/U of immunoreactive NAT protein). Human peripheral blood mononuclear cells 12,000 x g supernatant fractions converted N-acetoxy-SMX mainly back to SMX-HA, and also to a lesser extent to SMX, at clinically relevant concentrations. Similar pathways were observed in human hepatic cytosolic fractions. In a cytotoxicity assay, N-acetoxy-SMX was significantly more toxic to human peripheral blood mononuclear cells than SMX-HA (16.6 vs. 11.5% dead cells at a concentration of 300 microM). N-acetoxy-SMX was weakly mutagenic to the Salmonella typhimurium TA100 strain in the Ames test. These data suggest that the N-acetoxy metabolites of sulfonamides could potentially play a role in mediating sulfonamide idiosyncratic adverse drug reactions.
Nakamura
H
H
Department of Pediatrics, University of Toronto, Ontario, Canada.
Uetrecht
J
J
Cribb
A E
AE
Miller
M A
MA
Zahid
N
N
Hill
J
J
Josephy
P D
PD
Grant
D M
DM
Spielberg
S P
SP
D000123:Acetyltransferases / Q000378:metabolism; D001711:Biotransformation; D003600:Cytosol / Q000201:enzymology; D006801:Humans; D006877:Hydroxamic Acids / Q000378:metabolism; D066298:In Vitro Techniques; D007963:Leukocytes, Mononuclear / Q000378:metabolism; D008099:Liver / Q000201:enzymology; D009152:Mutagenicity Tests; D011994:Recombinant Proteins / Q000378:metabolism; D012486:Salmonella typhimurium / Q000235:genetics; D013420:Sulfamethoxazole / Q000031:analogs & derivatives* / Q000378:metabolism / Q000493:pharmacokinetics / Q000633:toxicity*
D006877:Hydroxamic Acids; C095797:N-acetoxysulfamethoxazole; D011994:Recombinant Proteins; D000123:Acetyltransferases; D013420:Sulfamethoxazole