Purpose: To evaluate covariate effects on the pharmacokinetics of temozolomide in cancer patients, and to explore the dose-pharmacokinetics-toxicity relationship of temozolomide.
Methods: Non-linear mixed-effects modeling approach was used to analyze the data from 445 patients enrolled in eleven Phase I and Phase II clinical trials. All patients in the phase I trials had advanced cancer. Patients in the phase II trials had anaplastic astrocytoma (AA), glioblastoma multiforme (GBM) or malignant melanoma (MM). A sparse sampling scheme was prospectively developed using Phase I data and was successfully implemented in Phase II trials. Population factors included age, gender, height (HT), weight (WT), body surface area (BSA), serum creatinine (Sr.Cr.), estimated creatinine clearance, serum chemistry data as indices of hepatic function and disease, smoking status, and selected concomitant medications. Descriptive statistics were used to summarize the toxicity and temozolomide dose and exposure relationship.
Results: The pharmacokinetics of temozolomide follows a one-compartment model with first order absorption and elimination. Temozolomide clearance (CL) increased with BSA for both genders. The population mean clearance for GBM or AA patients was 11.2 L/hr for male with BSA equal to 2.0 m2, and 8.8 L/hr for female with BSA equal to 1.7 m2. The mean clearance for MM patients was slightly higher. The inter-subject variability in clearance was 15%, and the residual variability was 26%. Other factors investigated in this analysis had little effect on clearance. The overall incidence of neutropenia and thrombocytopenia were 5-8%. Temozolomide dose and AUC did not predict nadir neutrophil and platelet counts due to large variability in counts.
Conclusions: The current dose regimen is administered according to BSA which is the most important factor influencing temozolomide clearance. No further dose adjustment is required.