AimTaxanes are anti-cancer agents used to treat several types of solid tumours. They are metabolized by cytochrome P450 (CYP) 3A, displaying a large pharmacokinetic (PK) variability. In this study, we evaluated the endogenous CYP3A4 marker 4-hydroxycholesterol (4-OHC) as a potential individual taxane PK predictor. MethodsSerum 4-OHC and cholesterol concentrations were determined in 291 paclitaxel and 151 docetaxel-treated patients, and were subsequently correlated with taxane clearance. ResultsIn the patients treated with paclitaxel, no clinically relevant correlations between the 4-OHC or 4-OHC : cholesterol ratio and paclitaxel clearance were found. In the patients treated with docetaxel, 4-OHC concentration was weakly correlated with docetaxel clearance in males (r = 0.35 P = 0.01, 95% CI 0.08, 0.58). Of the 10% patients with taxane outlier clearance values, 4-OHC did correlate with docetaxel clearance in males (r = 0.76, P = 0.03, 95% CI 0.12, 0.95). ConclusionThere was no clinical correlation between paclitaxel clearance and the CYP3A4 activity markers 4-OHC or the 4-OHC : cholesterol ratio. A weak correlation was observed between 4-OHC and docetaxel clearance, but only in males. This endogenous CYP3A4 marker has limited predictive value for taxane clearance in patients.