Effects of CYP2C9 genetic polymorphism and drug-drug interactions on trimethoprim metabolism

The aim of the study was to investigate how CYP2C9 genetic polymorphism and drug-drug interactions with psychiatric drugs affect trimethoprim metabolism. We conducted in vitro enzymatic incubation assays using rat liver microsomes (RLM), human liver microsomes (HLM) and recombinant human CYP2C9 variants, and carried out in vivo pharmacokinetics experiments using Sprague-Dawley rats. The analytes were analyzed using ultra performance liquid chromatography tandem mass spectrometry (UPLC-MS/MS). We determined the kinetic parameters of 12 CYP2C9 variants using trimethoprim as a substrate. The results showed significant decreases in catalytic activity for CYP2C9.3, CYP2C9.16, CYP2C9.42 and CYP2C9.48 compared to CYP2C9.1. Notably, CYP2C9.42 exhibited the lowest catalytic activity, only 28.19 % of CYP2C9.1. Fluoxetine was found to be the most potent among the 17 investigated psychiatric drugs in inhibiting trimethoprim metabolism, with a half-maximal inhibitory concentration (IC 50 ) values of 7.12 μM in RLM and 50.61 μM in HLM, respectively. The combination of trimethoprim and fluoxetine increased the AUC (0-∞) and AUC (0-t) of trimethoprim by 1.97- and 2.23-fold, respectively. Mechanistically, fluoxetine exhibited a competitive inhibition in RLM and a combination of competitive and non-competitive inhibition in HLM on trimethoprim metabolism. Our findings highlighted the significant impacts of CYP2C9 gene polymorphisms and fluoxetine on trimethoprim metabolism, providing insights for the precise administration of trimethoprim.