Impact of loperamide on the pharmacokinetics and tissue disposition of ritonavir-boosted oral docetaxel therapy; a preclinical assessment

An oral docetaxel formulation boosted by the Cytochrome P450 (CYP) 3 A inhibitor ritonavir, ModraDoc006/r, is currently under clinical investigation. Based on clinical data, the incidence of grade 1–2 diarrhea is increased with this oral docetaxel formulation compared to the conventional intravenous administration. Loperamide, a frequently used diarrhea inhibitor, could be added to the regimen as symptomatic treatment. However, loperamide is also a substrate of the CYP3A enzyme, which could result in competition between ritonavir and loperamide for this protein. Therefore, we were interested in the impact of coadministered loperamide on the pharmacokinetics of ritonavir-boosted oral docetaxel. We administered loperamide simultaneously or with an 8-hour delay to humanized CYP3A4 mice (with expression in liver and intestine) receiving oral ritonavir and docetaxel. Concentrations of docetaxel, ritonavir, loperamide and two of its active metabolites were measured. The plasma exposure (AUC and Cmax) of docetaxel was not altered during loperamide treatment, nor were the ritonavir plasma pharmacokinetics. However, the hepatic and intestinal dispositions of ritonavir were somewhat changed in the simultaneous, but not 8-hour loperamide treatment groups, possibly due to loperamide-induced delayed drug absorption. The pharmacokinetics of loperamide itself did not seem to be influenced by ritonavir. These results suggest that delayed loperamide administration can be added to ritonavir-boosted oral docetaxel treatment, without affecting the overall systemic exposure of docetaxel.