Ritonavir is the most potent cytochrome P450 (CYP) 3A4 inhibitor in clinical use and is often applied as a booster for drugs with low oral bioavailability due to CYP3A4-mediated biotransformation, as in the treatment of HIV (e.g., lopinavir/ritonavir) and more recently COVID-19 (Paxlovid or nirmatrelvir/ritonavir). Despite its clinical importance, the exact mechanism of ritonavir-mediated CYP3A4 inactivation is still not fully understood. Nonetheless, ritonavir is clearly a potent mechanism-based inactivator, which irreversibly blocks CYP3A4. Here, we discuss four fundamentally different mechanisms proposed for this irreversible inactivation/inhibition, namely the (I) formation of a metabolic-intermediate complex (MIC), tightly coordinating to the heme group; (II) strong ligation of unmodified ritonavir to the heme iron; (III) heme destruction; and (IV) covalent attachment of a reactive ritonavir intermediate to the CYP3A4 apoprotein. Ritonavir further appears to inactivate CYP3A4 and CYP3A5 with similar potency, which is important since ritonavir is applied in patients of all ethnicities. Although it is currently not possible to conclude what the primary mechanism of action in vivo is, it is unlikely that any of the proposed mechanisms are fundamentally wrong. We, therefore, propose that ritonavir markedly inactivates CYP3A through a mixed set of mechanisms. This functional redundancy may well contribute to its overall inhibitory efficacy.

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 (AUC0–24h) 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. AUC0–24h 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.

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Niraparib (Zejula), a selective oral PARP1/2 inhibitor registered for ovarian, fallopian tube, and primary peritoneal cancer treatment, is under investigation for other malignancies, including brain tumors. We explored the impact of the ABCB1 and ABCG2 multidrug efflux transporters, the OATP1A/1B uptake transporters, and the CYP3A drug-metabolizing complex on oral niraparib pharmacokinetics, using wild-type and genetically modified mouse and cell line models. In vitro, human ABCB1 and mouse Abcg2 transported niraparib moderately. Compared to wild-type mice, niraparib brain-to-plasma ratios were 6- to 7-fold increased in Abcb1a/1b –/– and Abcb1a/1b;Abcg2–/– but not in single Abcg2–/– mice, while niraparib plasma exposure at later time points was ∼2-fold increased. Niraparib recovery in the small intestinal content was markedly reduced in the Abcb1a/1b-deficient strains. Pretreatment of wild-type mice with oral elacridar, an ABCB1/ABCG2 inhibitor, increased niraparib brain concentration and reduced small intestinal content recovery to levels observed in Abcb1a/1b;Abcg2–/– mice. Oatp1a/1b deletion did not significantly affect niraparib oral bioavailability or liver distribution but decreased metabolite M1 liver uptake. No significant effects of mouse Cyp3a ablation were observed, but overexpression of transgenic human CYP3A4 unexpectedly increased niraparib plasma exposure. Thus, Abcb1 deficiency markedly increased niraparib brain distribution and reduced its small intestinal content recovery, presumably through reduced biliary excretion and/or decreased direct intestinal excretion. Elacridar pretreatment inhibited both processes completely. Clinically, the negligible role of OATP1 and CYP3A could be advantageous for niraparib, diminishing drug–drug interaction or interindividual variation risks involving these proteins. These findings may support the further clinical development and application of niraparib.