Inhibitory effects of trospium chloride on cytochrome P450 enzymes in human liver microsomes

Trospium chloride, an atropine derivative used for the treatment of urge in continence, was tested for inhibitory effects on human cytochrome P-450 enz ymes. Metabolic activities were determined in liver microsomes from two don ors using the following selective substrates: dextromethorphan (CYP2D6), de nitronifedipine (CYP3A4), caffeine (CYP1A2), chlorzoxazone (CYP2E1), S-(+)- mephenytoin (CYP2C19), S-(-)-warfarin (CYP2C9) and coumarin (CYP2A6). Incub ations with each substrate were carried out without a possible inhibitor an d in the presence of trospium chloride at varying concentrations (37-3000 m u M) at 37 degrees in 0.1 M KH2PO4 buffer containing up to 3% DMSO. Metabol ite concentrations were determined by high-performance liquid chromatograph y (HPLC) in all cases except CYP2A6 where direct fluorescence spectroscopy was used. First, trospium chloride IC50 values were determined for each sub strate at respective K-M concentrations. Trospium chloride did not show rel evant inhibitory effects on the metabolism of most substrates (IC50 values considerably higher than 1 mM). The only clear inhibition was seen for the CYP2D6-dependent high-affinity O-demethylation of dextromethorphan, where I C50 values of 27 mu M and 44 mu M were observed. Therefore, additional dext romethorphan concentrations (0.4-2000 mu M) were tested. Trospium chloride was a competitive inhibitor of the reaction with Ki values of 20 and 51 mu M, respectively. Thus, trospium chloride has negligible inhibitory effects on CYP3A4, CYP1A2, CYP2E1, CYP2C19, CYP2C9 and CYP2A6 activity but is a rea sonably potent inhibitor of CYP2D6 in vitro. Compared to therapeutic trospi um chloride peak plasma concentrations below 50 nM, the 1000-times higher c ompetitive inhibition constant K-i however suggests that inhibition of CYP2 D6 by trospium chloride is without any clinical relevance.