Structural studies on bioactive compounds. 34. Design, synthesis, and biological evaluation of triazenyl-substituted pyrimethamine inhibitors of Pneumocystis carinii dihydrofolate reductase

The triazenyl-pyrimethamine derivative 3a (TAB), a potent and selective inh ibitor of Pneumocystis carinii DHFR, was selected as the starting point for a lead optimization study. Molecular modeling studies, corroborated by a r ecent crystal structure determination of the ternary complex of P. carinii DHFR-NADPH bound to TAB, predicted that modifications to the acetoxy residu e of the lead inhibitor could exploit binding opportunities in the vicinity of an active site pocket bounded by residues Ile33, Lys37, and Leu72. Subs titutions in the benzyl moiety with electron-donating and electron-withdraw ing groups were predicted to probe face-edge interactions with amino acid P he69 unique to the P. carinii enzyme. New triazenes 10a-v and 12a-f were pr epared by coupling the diazonium tetrafluoroborate salt 6b of amino-pyrimet hamine with substituted benzylamines or phenethylamines. The most potent of the new inhibitors against P. carinii DHFR was the naphthylmethyl-substitu ted triazene 10t (IC50: 0.053 muM), but a more substantial increase in pote ncy against the rat liver DHFR led to a reduction in selectivity (ratio rat liver DHFR IC50/P. carinii DHFR IC50: 5.36) compared to the original lead structure 3a (ratio rat liver DHFR IC50/P. carinii DHFR IC50: 114).