Abstract

To evaluate the inhibitory effects of trimethoprim and sulfamethoxazole on cytochrome P450 (P450) isoforms, selective marker reactions for CYP1A2, CYP2A6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4 were examined in human liver microsomes and recombinant CYP2C8 and CYP2C9. The in vivo drug interactions of trimethoprim and sulfamethoxazole were predicted in vitro using [I]/([I] +K_i) values. With concentrations ranging from 5 to 100 μM, trimethoprim exhibited a selective inhibitory effect on CYP2C8-mediated paclitaxel 6α-hydroxylation in human liver microsomes and recombinant CYP2C8, with apparent IC₅₀(K_i) values of 54 μM (32 μM) and 75 μM, respectively. With concentrations ranging from 50 to 500 μM, sulfamethoxazole was a selective inhibitor of CYP2C9-mediated tolbutamide hydroxylation in human liver microsomes and recombinant CYP2C9, with apparent IC₅₀ (K_i) values of 544 μM (271 μM) and 456 μM, respectively. With concentrations higher than 100 μM trimethoprim and 500 μM sulfamethoxazole, both drugs lost their selectivity for the P450 isoforms. Based on estimated total hepatic concentrations (or free plasma concentrations) of the drugs and the scaling model, one would expect in vivo in humans 80% (26%) and 13% (24%) inhibition of the metabolic clearance of CYP2C8 and CYP2C9 substrates by trimethoprim and sulfamethoxazole, respectively. In conclusion, trimethoprim and sulfamethoxazole can be used as selective inhibitors of CYP2C8 and CYP2C9 in in vitro studies. In humans, trimethoprim and sulfamethoxazole may inhibit the activities of CYP2C8 and CYP2C9, respectively.