Structure-Based Design and Synthesis of Lipophilic 2,4-Diamino-6-Substituted Quinazolines and Their Evaluation as Inhibitors of Dihydrofolate Reductases and Potential Antitumor Agents

The synthesis and biological activities of 14 6-substituted 2,4-diaminoquinazolines are reported. These compounds were designed to improve the cell penetration of a previously reported series of 2,4-diamino-6-substituted-pyrido[2,3-<i>d</i>]pyrimidines which had shown significant potency and remarkable selectivity for <i>Toxoplasma gondii</i> dihydrofolate reductase (DHFR), but had much lower inhibitory effects on the growth of <i>T</i>. <i>gondii</i> cells in culture. The target N9−H analogues were obtained via regiospecific reductive amination of the appropriate benzaldehydes with 2,4,6-triaminoquinazoline, which, in turn, was synthesized from 2,4-diamino-6-nitroquinazoline. The N9−CH<sub>3</sub> analogues were synthesized via a regiospecific reductive methylation of the corresponding N9−H precursors. The compounds were evaluated as inhibitors of DHFR from human, <i>Pneumocystis carinii</i>, <i>T</i>. <i>gondii</i>, rat liver, <i>Lactobacillus</i> <i>casei</i>, and <i>Escherichia</i> <i>coli</i>, and selected analogues were evaluated as inhibitors of the growth of tumor cells in culture. These analogues displayed potent <i>T</i>. <i>gondii</i> DHFR inhibition as well as inhibition of the growth of <i>T</i>. <i>gondii</i> cells in culture. Further, selected analogues were potent inhibitors of the growth of tumor cells in culture in the in vitro screening program of the National Cancer Institute with GI<sub>50</sub>s in the nanomolar and subnanomolar range. Crystallographic data for the ternary complex of hDHFR−NADPH and 2,4-diamino-6-[<i>N</i>-(2‘,5‘-dimethoxybenzyl)-<i>N</i>-methylamino]pyrido[2,3-<i>d</i>]pyrimidine, <b>1c</b>, reveal the first structural details for a reversed N9−C10 folate bridge geometry as well as the first conformational details of a hybrid piritrexim−trimetrexate analogue.