Pharmacology and Toxicity of a Potent “Nonclassical” 2,4-Diamino Quinazoline Folate Antagonist, Trimetrexate, in Normal Dogs

The pharmacology of trimetrexate (JB-11, NSC 249008, 2,4-diamino-5-methyl-6-[(3,4,5-trimethoxyanilino)methyl]quinazoline), an antitumor agent effective against several mouse tumors, was studied in normal dogs. A high-performance liquid chromatographic technique with electrochemical detection, dihydrofolate reductase inhibition assay, and 14C-labeled drug were used to measure plasma disappearance, tissue distribution, excretion, and metabolism of the drug at doses from 0.5 to 6 mg/kg. Doses of 2 mg/kg were well tolerated without toxicity. Higher doses (3 to 6 mg/kg) produced mainly intestinal toxicity without significant hematological or liver abnormalities. The 6-mg/kg dose caused severe bloody diarrhea. After administration of 3 mg/kg, plasma concentrations of trimetrexate were 1 µm and were equal to or greater than 0.1 µm at 1 and 24 hr, respectively. The predominant pharmaco-kinetics of trimetrexate plasma disappearance was an elimination phase with a t ½ of 3.5 hr. Concentrations in the cere-brospinal fluid were 2 to 5% of that in plasma and were maximum within 1 to 2 hr after i.v. administration. Highest tissue concentrations of drug were measured in liver and kidney; lowest were found in brain and lung. A dose equivalent to 3 mg/kg in humans (on a sq m basis) should produce adequate plasma concentrations (greater than 0.1 µm) for therapeutic effects.