STRUCTURE-ACTIVITY AND STRUCTURE-SELECTIVITY STUDIES ON DIAMINOQUINAZOLINES AND OTHER INHIBITORS OF PNEUMOCYSTIS-CARINII AND TOXOPLASMA-GONDII DIHYDROFOLATE-REDUCTASE

Twenty-eight 2,4-diaminoquinazolines with alkyl, halogen, or alkoxy gr oups at the 5-, 6-, and/or 7-position, eight 2,4 diaminopteridines wit h alkyl and aralkyl groups at the 6- and 7-positions, five no-7,8,9,10 -tetrahydropyrimido[4,5-c]isoquinolines with an alkyl, alkylthio, or a ryl group at the 6-position, and nine 4,6-diamino-1,2-dihydro-s-triazi nes with one or two alkyl groups at the 2-position and a substituted p henyl or naphthyl group at the 1-position were evaluated as inhibitors of dihydrofolate reductase enzymes from Pneumocystis carinii, Toxopla sma gondii, and rat liver. Halogen substitution at the 5- or 6-positio n of 2,4-diaminoquinazoline favored selective binding to the P. carini i enzyme but not the T. gondii enzyme. For example, the 50% inhibitory concentrations of 2,4-diamino-6-chloroquinazoline as an inhibitor of P. carinii, T. gondii, and rat liver dihydrofolate reductase were 3.6, 14, and 29 mu M, respectively, corresponding to 12-fold selectivity f or the P. carinii enzyme but only marginal selectivity for the T. gond ii enzyme. Greater than fivefold selectivity for P. carinii but not T. gondii dihydrofolate reductase was also observed for the 2,4-diaminoq uinazolines with 5-methyl, 5-fluoro, 5- and 6-bromo, 6-chloro, and 5- chloro 6-bromo substitution. In contrast, alkyl and aralkyl substituti on at the 6- and 7-positions of 2,4-diaminopteridines was found to be a favorable feature for selective inhibition of the T. gondii enzyme a nd, in two cases, for both enzymes. Nine of the fifty-one compounds te sted against P. carinii dihydrofolate reductase and four of the thirty compounds tested against T. gondii dihydrofolate reductase displayed fivefold or greater selectivity for the microbial enzyme versus the ra t liver enzyme. The most selective against both enzymes was 2,4-diamin o-6,7-bis(cyclohexylmethyl)pteridine, with a selectivity ratio 2 order s of magnitude greater than the value reported for trimetrexate and pi ritrexim. Since substitution at the 7-position is generally considered to be detrimental to the binding of 2,4-diaminopteridines and related compounds to mammalian dihydrofolate reductase, the selectivity obser ved in this study with the 6,7-bis(cyclohexylmethyl) analog mag repres ent a useful approach to enhancing selective inhibition of the enzyme from nonmammalian species.