PURPOSE Paclitaxel is used for the treatment of several solid tumors and displays a high inter-individual variation in exposure and toxicity. Neurotoxicity is one of the most prominent side-effects of paclitaxel. This study explores potential predictive pharmacokinetic and pharmacogenetic determinants for the onset and severity of neurotoxicity. EXPERIMENTAL DESIGN In an exploratory cohort of patients (n=261) treated with paclitaxel, neurotoxicity incidence and severity, pharmacokinetic parameters and pharmacogenetic variants were determined. Paclitaxel plasma concentrations were measured by HPLC or LC-MS/MS, and individual pharmacokinetic parameters were estimated from previously developed population pharmacokinetic models by non-linear mixed effects modeling (NONMEM). Genetic variants of paclitaxel pharmacokinetics tested were CYP3A4*22, CYP2C8*3, CYP2C8*4, and ABCB1 3435 C>T. The association between CYP3A4*22 and neurotoxicity observed in the exploratory cohort was validated in an independent patient cohort (n=239). RESULTS Exposure to paclitaxel (logAUC) was correlated with severity of neurotoxicity (P <0.00001). Female CYP3A4*22 carriers were at increased risk of developing neurotoxicity (P = 0.043) in the exploratory cohort. CYP3A4*22 carrier status itself was not associated with pharmacokinetic parameters (CL, AUC, Cmax, or T>0.05) of paclitaxel in males or females. Other genetic variants displayed no association with neurotoxicity. In the subsequent independent validation cohort, CYP3A4*22 carriers were at risk of developing grade 3 neurotoxicity (odds ratio = 19.1; P = 0.001). CONCLUSIONS Paclitaxel exposure showed a relationship with the severity of paclitaxel-induced neurotoxicity. In this study, female CYP3A4*22 carriers had increased risk of developing severe neurotoxicity during paclitaxel therapy. These observations may guide future individualization of paclitaxel treatment.
Purpose Docetaxel is extensively metabolized by CYP3A4 in the liver, but mechanisms by which the drug is taken up into hepatocytes remain poorly understood. We hypothesized that (i) liver uptake of docetaxel is mediated by the polymorphic solute carriers OATP1B1 and OATP1B3, and (ii) that inherited genetic defects in this process may impair systemic drug elimination. Methods Transport of docetaxel was studied in vitro using various cell lines stably transfected with OATP1B1*1A (wildtype), OATP1B1*5 [c.521T>C (V174A); rs4149056], OATP1B3, or the mouse transporter Oatp1b2. Docetaxel clearance was evaluated in wildtype and Oatp1b2-knockout mice, as well as in 141 white patients with multiple variant transporter genotypes. Results Docetaxel was found to be a substrate for OATP1B1, OATP1B3, and Oatp1b2, but was not transported by OATP1B1*5. Deficiency of Oatp1b2 in mice was associated with an 18-fold decrease in docetaxel clearance (P=0.0099), which was unrelated to changes in intrinsic metabolic capacity in mouse liver microsomes. In patients, however, none of the studied common reduced-function variants in OATP1B1 or OATP1B3 were associated with docetaxel clearance (P>0.05). Conclusions The existence of at least two potentially redundant uptake transporters in the human liver with similar affinity for docetaxel supports the possibility that functional defects in both of these proteins may be required to confer substantially altered disposition phenotypes. In view of the established exposure-toxicity relationships for docetaxel, we suggest that extreme caution is warranted if docetaxel has to be administered together with agents that potently inhibit both OATP1B1 and OATP1B3.
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