THE DISPOSITION AND METABOLISM OF [C-14] PIRITREXIM IN DOGS AFTER INTRAVENOUS AND ORAL-ADMINISTRATION

The disposition of [C-14]piritrexim ([C-14]PTX) in male dogs after iv and po doses of 1.8 mg/kg was examined. After either route of administration, greater than 90% of the dose was recovered in the excreta within 72 hr, approximately 20% was recovered in urine and 70% in feces. [C-14]PTX was extensively metabolized by dogs; unchanged drug accounted for less than 15% of the dose in the excreta. The O-demethylated metabolites, 2'- and 5'-demethyl PTX, the glucuronide conjugate of 2'-demethyl PTX, and the sulfate conjugate of 5'-demethyl PTX were the major metabolites. Unchanged drug accounted for a large proportion of the drug-related radiocarbon in plasma. The average plasma half-life of PTX after iv administration was 2.6 +/- 0.3 hr, and the average total body clearance was 0.33 +/-0.13 liter/hr/kg. After po administration, peak plasma concentrations of 0.9 +/- 0.3-mu-g/ml occurred about 1.1 hr after the dose; the absolute oral bioavailability of PTX was 0.63 +/- 0.14. Because the O-demethyl metabolites were active dihydrofolate reductase inhibitors, 2'- and 5'-demethyl PTX were synthesized, and the pharmacokinetics and bioavailability of these compounds in dogs after iv and po administration (5 mg/kg) were examined. The plasma concentration-time data for both compounds after iv doses were described by a two-compartment model, with t1/2-beta = 1.3 and 0.8 hr for the 2'- and 5'-demethyl compounds, respectively. Neither compound showed significant advantages over PTX in terms of pharmacokinetics or bioavailability.