Oral co-administration of elacridar and ritonavir enhances plasma levels of oral paclitaxel and docetaxel without affecting relative brain accumulation

Background: The intestinal uptake of the taxanes paclitaxel and docetaxel is seriously hampered by drug efflux through P-glycoprotein (P-gp) and drug metabolism via cytochrome P450 (CYP) 3A. The resulting low oral bioavailability can be boosted by co-administration of P-gp or CYP3A4 inhibitors. Methods: Paclitaxel or docetaxel (10 mg/kg) was administered to CYP3A4-humanised mice after administration of the P-gp inhibitor elacridar (25mgkg(-1)) and the CYP3A inhibitor ritonavir (12.5mgkg(-1)). Plasma and brain concentrations of the taxanes were measured. Results: Oral co-administration of the taxanes with elacridar increased plasma concentrations of paclitaxel (10.7-fold, Po0.001) and docetaxel (four-fold, Po0.001). Co-administration with ritonavir resulted in 2.5-fold (paclitaxel, Po0.001) and 7.3-fold (docetaxel, Po0.001) increases in plasma concentrations. Co-administration with both inhibitors simultaneously resulted in further increased plasma concentrations of paclitaxel (31.9-fold, Po0.001) and docetaxel (37.4-fold, Po0.001). Although boosting of orally applied taxanes with elacridar and ritonavir potentially increases brain accumulation of taxanes, we found that only brain concentrations, but not brain-to-plasma ratios, were increased after co-administration with both inhibitors. Conclusions: The oral availability of taxanes can be enhanced by co-administration with oral elacridar and ritonavir, without increasing the brain penetration of the taxanes.