Metabolism of docetaxel in mice.

Previous studies have shown by quantification of the parent drug and the known metabolites M-1, M-2, M-3 and M-4 that the mass balance of docetaxel in mice and humans is not complete. We therefore used reversed-phase high-performance liquid chromatography (HPLC) with photodiode array (PDA) detection and tandem mass spectrometry to trace and identify putative metabolites in the feces and bile of mice injected intravenously with docetaxel. HPLC-PDA revealed two metabolic products in the feces and more than ten potential new metabolites in the bile. Mass spectrometry was performed on docetaxel reference compound, on the known metabolites M-1, M-2, M-3 and M-4, and on HPLC eluate fractions containing metabolic products, six fractions originating from the bile and two from the feces. The mass spectra of the most abundant unknown metabolite in the bile and the feces were identical, and indicated that this structure contained a carboxyl moiety at the tert-butyl group. Under the conditions of storage this product degraded to metabolite M-4. All other unknown metabolites found in the bile samples were oxidized products, with the oxidations in both the C-13 side chain and the baccatin structure, the latter being a new finding.

Efficacy of novel P-glycoprotein inhibitors to increase the oral uptake of paclitaxel in mice.

P-glycoprotein inhibitors can increase the oral bioavailability of paclitaxel. We have now explored the mechanisms that determine the efficacy of several novel P-glycoprotein inhibitors to increase the absorption of paclitaxel from the gut lumen of mice in both in vivo and in vitro experiments. The inhibitors studied were cyclosporin A, PSC 833, GF120918, LY335979 and R101933. Mass balance studies showed that GF120918 was the most effective inhibitor, resulting in almost complete uptake of paclitaxel. PSC 833 was slightly less effective, whereas cyclosporin A and LY335979 were moderately effective. R101933 had only marginal effects. These findings were in line with in vitro transport experiments using LLC-mdr1a cells. By studying the intra-intestinal kinetics of the agents we found that cyclosporin A, PSC 833 and GF120918 rapidly passed the stomach and traveled concurrently with paclitaxel through the intestines, whereas LY335979 and R101933 delayed stomach emptying. Moreover, these latter compounds appear to be more readily absorbed when released into the intestines thus reducing local intestinal concentrations. Due to their combined effects on absorption and metabolic elimination of paclitaxel, cyclosporin A and PSC 833 resulted in the highest paclitaxel levels in plasma. In conclusion, our models provide insight into the factors that determine the suitability of P-glycoprotein inhibitors to enable oral paclitaxel therapy and will be useful in selecting candidate inhibitors for clinical testing.

Metabolism of paclitaxel in mice.

Previous mass balance studies in humans and mice have shown that the fecal and urinary recovery of paclitaxel and known metabolites (3' -hydroxypaclitaxel, 6alpha-hydroxypaclitaxel and 3',6alpha-dihydroxypaclitaxel) was not complete. Obviously this discrepancy is caused by the existence of other yet unknown metabolites. Mdr1a/1b(-/-) mice excrete very low quantities of unchanged paclitaxel. We have therefore used these mice receiving i.v. [3H]paclitaxel to further study the metabolic fate of paclitaxel. The major part of the radiolabel, being 70%, was excreted in the feces. A lipophilic sample, containing about 70% of the radioactivity present in the feces sample, was obtained by diethyl ether extraction. The aqueous residue containing about 30% of the radioactivity was further extracted using methanol. The high-performance liquid chromatography (HPLC) chromatograms of the lipophilic and aqueous sample revealed two and five putative new metabolites of paclitaxel, respectively. The HPLC fractions containing substantial amounts of radioactivity were subjected to tandem mass spectrometry. Two novel monohydroxylated paclitaxel structures were identified, which are probably 2m-hydroxypaclitaxel and 19-hydroxypaclitaxel, structures previously identified in rats. Including these metabolites, about 60% of the mass balance of paclitaxel could be quantified.

Low systemic exposure of oral docetaxel in mice resulting from extensive first-pass metabolism is boosted by ritonavir.

P-glycoprotein seems to be the most important factor limiting the oral absorption of paclitaxel. We have now explored the mechanisms responsible for the low oral bioavailability of docetaxel, a structurally related taxane drug. The recovery of 33% of oxidative metabolites and only 39% of unchanged drug in the feces of FVB wild-type mice receiving 10 mg/kg of oral docetaxel indicates that the major part of the oral dose has been absorbed. The feces and bile of mice receiving 10 mg/kg of i.v. docetaxel contained large amounts of metabolites and only minor quantities of unchanged drug, highlighting the importance of metabolism as an elimination route for this drug. In wild-type and P-glycoprotein knockout mice, dose escalation of p.o. administered docetaxel from 10 to 30 mg/kg resulted in a more than proportional increase in plasma levels, which suggested saturation of first-pass metabolism. Moreover, coadministration of 12.5 mg/kg of the HIV protease inhibitor ritonavir, also a strong inhibitor of cytochrome P4503A4 with only minor P-glycoprotein inhibiting properties, increased the plasma levels after oral docetaxel by 50-fold. In vitro transport studies across monolayers of LLC-PK1 cells (parental and transduced with MDR1 or Mdr1a) suggested that docetaxel is a weaker substrate for P-glycoprotein than paclitaxel is. In conclusion, docetaxel is well absorbed from the gut lumen in mice despite the presence of P-glycoprotein in the gut wall. Subsequent first-pass extraction is the most important factor determining its low bioavailability. The inhibition of docetaxel metabolism by ritonavir provides an interesting strategy to improve the systemic exposure of oral docetaxel.

Entrapment by Cremophor EL decreases the absorption of paclitaxel from the gut.

BACKGROUND: Recent studies in mice and patients have shown that the low oral bioavailability of paclitaxel can be increased by coadministration of P-glycoprotein blockers. However, in patients an increase in the oral paclitaxel dose from 60 to 300 mg/m(2) does not result in proportionally higher plasma levels. We hypothesized that the surfactant Cremophor EL, present in the formulation of paclitaxel, may be responsible for this nonlinear absorption by entrapping paclitaxel within the intestinal lumen, probably by inclusion in micelles. METHODS: Paclitaxel was administered to mdr1ab P-glycoprotein knockout mice with either the conventional (controls) or a seven-fold higher amount of Cremophor EL (test group). Plasma, gastrointestinal tissues with their contents and faeces were collected and analysed by high-performance liquid chromatography to determine the levels of paclitaxel and Cremophor EL. The critical micellar concentrations of Cremophor EL in the contents of the small intestine were also established by an in vitro assay. RESULTS: Paclitaxel recoveries in the faeces of the control and test groups were 7.6% and 35.8%, respectively. The peak plasma level and plasma AUC were reduced in the test group by about 75% and 40%, respectively. Only in mice from the test group did substantial quantities of paclitaxel together with Cremophor EL reach the caecum, thus passing through the small intestine. The concentration of Cremophor EL in the distal part of the small intestine and the caecum was 15 times higher in the test group and well above the critical micellar concentration of Cremophor EL. CONCLUSIONS: These results show that Cremophor EL prevents efficient uptake of paclitaxel from the gut, probably by entrapment within micelles. Other formulations should be developed for oral therapy with paclitaxel.