IN VITRO HEPATIC OXIDATIVE BIOTRANSFORMATION OF TRIMETHOPRIM

Abstract

Introduction: Trimethoprim-sulfamethoxazole (TMP-SMX) is a commonly used antibiotic often associated with idiosyncratic adverse drug reactions (IADRs). Historically, bioactivation of SMX to a reactive intermediate has been implicated in IADRs. Recently, the bioactivation of TMP has been described as TMP N-acetyl cysteine (NAC) adducts were identified following human liver microsome (HLM) incubations and in the urine of children taking TMP, suggesting cytochrome P450 enzymes (P450s) catalyze TMP bioactivation. In this study, we identified the P450s involved in the formation of six TMP primary metabolites. Methods: A panel of characterized HLMs (n=16), c-DNA expressed P450s, and pooled HLMs in the presence and absence of selective P450 inhibitors were incubated with therapeutic concentrations of TMP. Reactive metabolites were trapped with NAC, and metabolite formation was quantified by UPLC/MS/MS. Correlation coefficients between the rates of metabolite formation and P450 marker reaction rates were determined using least-squares regression analysis and evaluated at α=0.05. Results: 1-NO-TMP, Cα-NAC-TMP and Cα-OH-TMP were formed by CYP3A4 and inhibited by ketoconazole (CYP3A inhibitor). 4'-demethylation was catalyzed by several P450s including CYP3A4, correlated with multiple CYPs, and inhibited primarily with ketoconazole suggesting CYP3A4 contributed to 4'-demethylation. 3-NO-TMP was formed by CYP1A and inhibited by α-naphthoflavone (CYP1A inhibitor). 3'-demethylation was catalyzed by multiple P450s including CYP2C9, correlated with CYP2C9 activity and inhibited by sulphafenazole (CYP2C9 inhibitor). Conclusion: These findings suggest that P450s were responsible for the primary metabolism of TMP with CYPs 2C9 and 3A4 being the most significant contributors to TMP primary metabolism. Factors modulating activity of these enzymes may affect the risk of IADRs.