Enhancement of the Oral Availability of Cabazitaxel Using the Cytochrome P450 3A (CYP3A) Inhibitor Ritonavir in Mice.

There is currently great interest in developing oral taxanes due to their lower costs and greater patient friendliness. We here wanted to test whether oral ritonavir, a cytochrome P450 3A (CYP3A) inhibitor, could boost the pharmacokinetics and tissue distribution of orally administered cabazitaxel (10 mg/kg) in male wild-type, Cyp3a^-/-, and Cyp3aXAV (transgenic overexpression of human CYP3A4 in liver and intestine) mice. Ritonavir was initially administered at a dose of 25 mg/kg, but lower dosages of 10 and 1 mg/kg were also studied to assess the remaining amount of boosting, aiming to minimize possible side effects. Compared to the respective vehicle groups, plasma exposure of cabazitaxel (AUC_0-24h) was enhanced 2.9-, 10.9-, and 13.9-fold in wild-type mice and 1.4-, 10.1-, and 34.3-fold in Cyp3aXAV mice by treatment with 1, 10, and 25 mg/kg ritonavir, respectively. Upon treatment with 1, 10, and 25 mg/kg of ritonavir, the peak plasma concentration (C_max) was increased by 1.4-, 2.3-, and 2.8-fold in wild-type mice, while it increased by 1.7-, 4.2-, and 8.0-fold in Cyp3aXAV mice, respectively. AUC_0-24h and C_max remained unchanged in Cyp3a^-/-. Biotransformation of cabazitaxel to its active metabolites still took place when coadministered with ritonavir, but this process was delayed due to the Cyp3a/CYP3A4 inhibition. These data indicate that CYP3A is the primary limiting factor in the plasma exposure to cabazitaxel and that cabazitaxel oral bioavailability could be dramatically enhanced by coadministration of an effective CYP3A inhibitor such as ritonavir. These findings could be a starting point for the setup of a clinical study, which would be needed to verify the boosting of cabazitaxel by ritonavir in humans.