Azahomoaminopterin, a New Type of Folic Acid Antimetabolite

Several new folic acid and aminopterin analogues were synthesized, in which the aliphatic chain binding the pteridine and aromatic nucleus was extended by 1 nitrogen atom. Condensation of 2-amino-4-hydroxypteridine-6-aldehyde with p -hydrazinobenzoylglutamate gave a hydrazone whose carbon-nitrogen double bond was reduced by sodium borohydride to form 11-azahomofolic acid. 11-Azahomopteroic acid was prepared by the condensation of 2-amino-4-hydroxypteridine-6-aldehyde with p -hydrazinobenzoic acid and subsequent reduction of the hydrazone with sodium borohydride. 11-Azahomoaminopterin and 4-aminoazahomopteroic acid were prepared in the same manner, using 2,4-diaminopteridine-6-aldehyde as the starting compound. The 5,6,7,8-tetrahydro derivatives of azahomofolic acid and azahomoaminopterin were prepared by catalytic hydrogenation of the parent compounds in glacial acetic acid. Azahomopteroic acid and related compounds showed no significant inhibitory effect on HeLa cells grown in tissue culture, or any toxicity for mice in doses up to 100 mg/kg. Azahomoaminopterin, its parent hydrazone, and analogous pteroic acid derivatives exhibited a marked cytostatic effect on HeLa cells and were toxic for mice in doses exceeding 100 mg/kg, whereas 4-aminoazahomopteroic acid and its parent hydrazone showed no toxicity when administered in the same doses. Among the enzymes interconverting folic acid or involving tetrahydrofolate as a coenzyme, thymidylate synthetase was inhibited slightly by azahomoaminopterin and more strongly by its tetrahydro derivative, the inhibition being noncompetitive versus tetrahydrofolate. Folate reductase from mouse liver was inhibited by all the analogues containing an NH 2 group in position 4 (azahomoaminopterin and related substances). Azahomofolic acid and its derivatives were ineffective. Other tetrahydrofolate-interconverting enzyme systems and purine biosynthesis were not inhibited by any of the compounds studied.