Purpose: Paclitaxel (Taxol) is an effective agent against a broad range of human cancers. Studies on the metabolism and disposition of paclitaxel have shown that it is primarily eliminated via hepatic metabolism by P450 enzymes (2C8 and 3A4) to essentially inactive metabolites, and that biliary and gut transport by P-glycoprotein (PGP) as well as urinary elimination of the parent compound play relatively minor roles. Recent studies in vitro have shown that paclitaxel treatment increases the level of CYP2C8 and CYP3A4 in human hepatocytes as well as PGP in colon tumor cells. The data suggest that previous paclitaxel exposure may influence metabolism and elimination of subsequent doses. Further, since weekly paclitaxel dose schedules are becoming more common as opposed to the original every 21-day dosing, the likelihood of enzyme induction from previous doses impacting that from subsequent doses is increased.
Methods: To study the potential for such sequence-dependent alterations in paclitaxel pharmacokinetics, we carried out pharmacokinetic studies in mouse plasma and tissues following day 1 and days 1 and 5 dosing at 20 mg/kg. Paclitaxel concentrations were determined by a sensitive LC/MS/MS assay out to 16 h post-dosing in plasma, liver, kidney, gut and heart. The effect of paclitaxel treatment on hepatic expression of PGP and P450 isoforms (CYP2C and CYP3A) was determined to elucidate the mechanism by which paclitaxel disposition is altered by previous drug exposure.
Results: Pharmacokinetic analysis of the data showed that plasma and tissue AUC values after treatment on day 5 following a dose on day 1 were between 50% and 74% of those determined following a single dose on day 1. The terminal elimination half-life was not different. Activity and protein levels for CYP2C in liver were elevated at 24 and 96 h after paclitaxel dosing. Cremophor EL, a carrier solvent for paclitaxel, also caused elevated CYP2C activity. Neither CYP3A nor PGP levels in liver were altered by paclitaxel or Cremophor EL treatment at the 24-h and 96-h time points. The levels of 6alpha-OH-paclitaxel in feces were increased on day 5 as opposed to day 1 while paclitaxel levels in feces were unchanged.
Conclusions: The results of our studies showed that paclitaxel pharmacokinetics are altered by previous paclitaxel exposure up to 96 h earlier.