Inhibitory effect of loperamide on cytochrome P450 3A4 in human liver microsomes

Background : Loperamide, a peripherally acting opioid receptor agonist with a methadone-like structure and antidiarrheal action, is mainly metabolized to desmethylloperamide through N-demethylation pathway. It has been reported that CYP2C8 and CYP3A4 might play a crucial role in loperamide Ndemethylation in therapeutic concentrations and loperamide might be an inhibitor of CYP3A4. However, there has been no available data to reveal it. The inhibitory effect of loperamide on CYP3A4 using midazolam 1'-hydroxylation was evaluated in vitro by human liver microsomes. Methods : Various concentrations of loperamide or ketoconazole (a selective and potent inhibitor of CYP3A4) were co-incubated with midazolam in human liver microsomes. CYP3A4-catalyzed midazolam metabolite, 1'-hydroxymidazolam was analyzed by high-performance liquid chromatography (HPLC) to determine the inhibitory potency of loperamide on midazolam 1'-hydroxylation using IC₅₀ (the concentration of inhibitor representing 50% inhibitory potency) and Ki (apparent inhibitory constant) values. Results : Loperamide inhibited concentration-dependently CYP3A4-catalyzed midazolam 1'-hydroxylation with apparent IC₅₀ values of 0.78 μM in human liver microsomes. Graphical analysis showed that loperamide had a potent inhibitory effect on midazolam 1'-hydroxylation with a Ki value of 0.54 μM in a competitive manner. Conclusions : Loperamide had a potent inhibitory effect on CYP3A4-catalyzed midazolam 1'-hydroxylation in human liver microsomes. However the predicted inhibition of loperamide within therapeutic ranges on the metabolism 1'-hydroxylation showed the low possibility of in vivo drug interaction of loperamide with the drug metabolized by CYP3A4.