CLINICAL PHARMACOLOGY
Mechanism of Action
Cabazitaxel is a microtubule inhibitor. Cabazitaxel binds to tubulin and promotes its assembly into microtubules while simultaneously inhibiting disassembly. This leads to the stabilization of microtubules, which results in the inhibition of mitotic and interphase cellular functions.
Pharmacodynamics
Cabazitaxel demonstrated antitumor activity against advanced human tumors xenografted in mice. Cabazitaxel is active in docetaxel-sensitive tumors. In addition, cabazitaxel demonstrated activity in tumor models insensitive to chemotherapy including docetaxel.
Pharmacokinetics
A population pharmacokinetic analysis was conducted in 170 patients with solid tumors at doses ranging from 10 to 30 mg/m2 weekly or every three weeks.
Absorption
Based on the population pharmacokinetic analysis, after an intravenous dose of cabazitaxel 25 mg/m2 every three weeks, the mean Cmax in patients with metastatic prostate cancer was 226 ng/mL (CV 107%) and was reached at the end of the one-hour infusion (Tmax). The mean AUC in patients with metastatic prostate cancer was 991 ng∙h/mL (CV 34%).
No major deviation from the dose proportionality was observed from 10 to 30 mg/m2 in patients with advanced solid tumors.
Distribution
The volume of distribution (Vss) was 4,864 L (2,643 L/m2 for a patient with a median BSA of 1.84 m2) at steady state.
In vitro, the binding of cabazitaxel to human serum proteins was 89 to 92% and was not saturable up to 50,000 ng/mL, which covers the maximum concentration observed in clinical trials. Cabazitaxel is mainly bound to human serum albumin (82%) and lipoproteins (88% for HDL, 70% for LDL, and 56% for VLDL). The in vitro blood-to-plasma concentration ratio in human blood ranged from 0.90 to 0.99, indicating that cabazitaxel was equally distributed between blood and plasma.
Metabolism
Cabazitaxel is extensively metabolized in the liver (> 95%), mainly by the CYP3A4/5 isoenzyme (80% to 90%), and to a lesser extent by CYP2C8. Cabazitaxel is the main circulating moiety in human plasma. Seven metabolites were detected in plasma (including the 3 active metabolites issued from O-demethylation), with the main one accounting for 5% of cabazitaxel exposure. Around 20 metabolites of cabazitaxel are excreted into human urine and feces.
Based on in vitro studies, the potential for cabazitaxel to inhibit drugs that are substrates of other CYP isoenzymes (1A2,-2B6,-2C9, -2C8, -2C19, -2E1, -2D6, and 3A4/5) is low. In addition, cabazitaxel did not induce CYP isozymes in vitro.
Elimination
After a one-hour intravenous infusion [14C]-cabazitaxel 25 mg/m2, approximately 80% of the administered dose was eliminated within 2 weeks. Cabazitaxel is mainly excreted in the feces as numerous metabolites (76% of the dose); while renal excretion of cabazitaxel and metabolites account for 3.7% of the dose (2.3% as unchanged drug in urine).
Based on the population pharmacokinetic analysis, cabazitaxel has a plasma clearance of 48.5 L/h (CV 39%; 26.4 L/h/m2 for a patient with a median BSA of 1.84 m2) in patients with metastatic prostate cancer. Following a one-hour intravenous infusion, plasma concentrations of cabazitaxel can be described by a three-compartment pharmacokinetic model with alpha-, beta-, and γ- half-lives of 4 minutes, 2 hours, and 95 hours, respectively.
Renal Impairment
Cabazitaxel is minimally excreted via the kidney. No formal pharmacokinetic trials have been conducted with cabazitaxel in patients with renal impairment. The population pharmacokinetic analysis carried out in 170 patients including 14 patients with moderate renal impairment (30 mL/min ≤ CLcr < 50 mL/min) and 59 patients with mild renal impairment (50 mL/min ≤ CLcr < 80 mL/min) showed that mild to moderate renal impairment did not have meaningful effects on the pharmacokinetics of cabazitaxel. No data are available for patients with severe renal impairment or end-stage renal disease [see Use in Special Populations].
Hepatic Impairment
No formal trials in patients with hepatic impairment have been conducted. As cabazitaxel is extensively metabolized in the liver, hepatic impairment is likely to increase the cabazitaxel concentrations [see Warnings and Precautions, and Use in Special Populations].
Drug interactions
As cabazitaxel is mainly metabolized by CYP3A in vitro, strong CYP3A inducers or inhibitors are expected to affect the pharmacokinetics of cabazitaxel.
Prednisone or prednisolone administered at 10 mg daily did not affect the pharmacokinetics of cabazitaxel.
In vitro, cabazitaxel did not inhibit the multidrug-resistance protein 1 (MRP1) or 2 (MRP2). In vitro cabazitaxel inhibited the transport of P-gp and BRCP, at concentrations at least 38 fold what is observed in clinical settings. Therefore, the in vivo risk of cabazitaxel to inhibit MRPs, P-gp, or BCRP is unlikely at the dose of 25 mg/m2.
In vitro, cabazitaxel is a substrate of P-gp, but not a substrate of MRP1, MRP2, or BCRP.
NONCLINICAL TOXICOLOGY
Carcinogenesis, Mutagenesis, Impairment of Fertility
Long-term animal studies have not been performed to evaluate the carcinogenic potential of cabazitaxel.
Cabazitaxel was positive for clastogenesis in the in vivo micronucleus test, inducing an increase of micronuclei in rats at doses ≥ 0.5 mg/kg. Cabazitaxel increased numerical aberrations with or without metabolic activation in an in vitro test in human lymphocytes though no induction of structural aberrations was observed. Cabazitaxel did not induce mutations in the bacterial reverse mutation (Ames) test. The positive in vivo genotoxicity findings are consistent with the pharmacological activity of the compound (inhibition of tubulin depolymerization).
Cabazitaxel may impair fertility in humans. In a fertility study performed in female rats at cabazitaxel doses of 0.05, 0.1, or 0.2 mg/kg/day there was no effect of administration of the drug on mating behavior or the ability to become pregnant. There was an increase in pre-implantation loss at the 0.2 mg/kg/day dose and an increase in early resorptions at doses ≥ 0.1 mg/kg/day (approximately 0.02—0.06 times the human clinical exposure based on Cmax). In multi-cycle studies following the clinically recommended dosing schedule, atrophy of the uterus was observed at the 5 mg/kg dose level (approximately the AUC in patients with cancer at the recommended human dose) along with necrosis of the corpora lutea at doses ≥ 1 mg/kg (approximately 0.2 times the AUC at the clinically recommended human dose).
Cabazitaxel did not affect mating performances or fertility of treated male rats at doses of 0.05, 0.1, or 0.2 mg/kg/day. In multiple-cycle studies following the clinically recommended dosing schedule, however, degeneration of seminal vesicle and seminiferous tubule atrophy in the testis were observed in rats treated intravenously with cabazitaxel at a dose of 1 mg/kg (approximately 0.2—0.35 times the AUC in patients with cancer at the recommended human dose), and minimal testicular degeneration (minimal epithelial single cell necrosis in epididymis) was observed in dogs treated with a dose of 0.5 mg/kg (approximately one-tenth of the AUC in patients with cancer at the recommended human dose).
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