Pneumocystis carinii and Toxoplasma gondii dihydrofolate reductase inhibitors and antitumor agents: Synthesis and biological activities of 2,4-diamino-5-methyl-6-[(monosubstituted anilino)methyl]pyrido[2,3-d]pyrimidines

Thirteen 2,4-diamino-5-methyl-6-[(monosubstituted anilino)methyl]pyrido[2,3 -d]pyrimidines 5-17 were synthesized as potential Pneumocystis carinii (pc) and Toxoplasma gondii (tg) dihydrofolate reductase (DHFR) inhibitors and a s antitumor agents. Compounds 5-17 were designed to investigate the structu re-activity relationship of monomethoxy and monohalide substitution in the phenyl ring and N10-methylation of the C9-N10 bridge. The synthetic route t o compounds 5-12 involved the reductive amination of a common intermediate, 2,4-diamino-5-methylpyrido[2,3-d]pyrimidine-6-carbonitrile (18), with the appropriate anilines. N10-Methylation was achieved by reductive methylation . In contrast to previous reports of trimethoprim, the removal of methoxy a nd chloro groups from the phenyl ring in the 2,4-diamino-5-methyl-6-[(subst ituted anilino)methyl]pyrido[2,3-d]pyrimidine series generally did not decr ease DHFR inhibitory activity. The monosubstituted phenyl analogues 5-12 we re as potent against pcDHFR and tgDHFR as the previously reported disubstit uted phenyl analogues. N10-Methylation generally resulted in a marginal inc rease in potency against both pcDHFR and tgDHFR. Compounds 5, 7, and 9 were evaluated and shown to inhibit the growth of T. gondii cells in culture at nanomolar concentrations. Compounds 6-8, 9, 11, and 16 were selected by th e National Cancer Institute for evaluation in an in vitro preclinical antit umor screening program. All six compounds showed GI(50) values in the 10(-7 )-10(-9) M range in more than 20 cell lines.