Structure-activity and structure-selectivity studies on diaminoquinazolines and other inhibitors of Pneumocystis carinii and Toxoplasma gondii dihydrofolate reductase

Abstract

Twenty-eight 2,4-diaminoquinazolines with alkyl, halogen, or alkoxy groups at the 5-, 6-, and/or 7-position, eight 2,4-diaminopteridines with alkyl and aralkyl groups at the 6-and 7-positions, five 1,3-diamino-7,8,9,10-tetrahydropyrimido[4,5-c]isoquinolines with an alkyl, alkylthio, or aryl group at the 6-position, and nine 4,6-diamino-1,2-dihydro-s-triazines with one or two alkyl groups at the 2-position and a substituted phenyl or naphthyl group at the 1-position were evaluated as inhibitors of dihydrofolate reductase enzymes from Pneumocystis carinii, Toxoplasma gondii, and rat liver. Halogen substitution at the 5-or 6-position of 2,4-diaminoquinazoline favored selective binding to the P. carinii 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 M, respectively, corresponding to 12-fold selectivity for the P. carinii enzyme but only marginal selectivity for the T. gondii enzyme. Greater than fivefold selectivity for P. carinii but not T. gondii dihydrofolate reductase was also observed for the 2,4-diaminoquinazolines with 5-methyl, 5-fluoro, 5-and 6-bromo, 6-chloro, and 5-chloro-6-bromo substitution. In contrast, alkyl and aralkyl substitution 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 and, in two cases, for both enzymes. Nine of the fifty-one compounds tested 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 rat liver enzyme. The most selective against both enzymes was 2,4-diamino-6,7-bis(cyclohexylmethyl)pteridine, with a selectivity ratio 2 orders of magnitude greater than the value reported for trimetrexate and piritrexim. 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 observed in this study with the 6,7-bis(cyclohexylmethyl) analog may represent a useful approach to enhancing selective inhibition of the enzyme from nonmammalian species.

Identification of Cryptosporidium parvum Dihydrofolate Reductase Inhibitors by Complementation in Saccharomyces cerevisiae

“…These lipophilic DHFR inhibitors are assumed to cross the plasma membrane by passive or facilitated diffusion and, therefore, to obviate drug susceptibility and resistance problems associated with carrier-or receptor-mediated folate transport mechanisms (7). Many of these compounds have previously been assayed against P. carinii DHFR and T. gondii DHFR-TS in vitro (48,49,(51)(52)(53)(54)(55)(56). For example, the IC 50 of PY875 against the P. carinii DHFR enzyme in vitro was ϳ7 M (51), suggesting that PY875 might also inhibit the growth of Pc-yeast; Table 6 shows this to indeed be the case.…”

“…The activities of many of these drugs against these parasites or their purified DHFR enzymes was known already. For example, members of the PY series had been tested in vitro to measure inhibition of the P. carinii or T. gondii enzyme (48,49,(52)(53)(54)56). We knew at the outset which compounds were effective in vitro against the DHFRs of other pathogens, which allowed us to quickly identify drugs that did not effectively penetrate the yeast host.…”

Classification of drug molecules considering their IC50 values using mixed-integer linear programming based hyper-boxes method

“…Representative compounds from each data set are shown in Figure 2 . The experimental IC 50 values for the dihydrofolate reductase (DHFR) inhibitor set were calculated and reported [ 16 , 19 , 22 , 26 ] for the DHFR enzyme from three different species: P. carinii (PC), T. gondii (TG) and rat liver (RL), where the activity of the DHFR inhibitors to the enzymes from different species differ. Therefore, activities of the inhibitors towards the enzymes from these three species for DHFR inhibitors are studied separately in our study.…”

6,7-disubstituted 2,4-diaminopteridines: novel inhibitors of Pneumocystis carinii and Toxoplasma gondii dihydrofolate reductase

“…Studies published to date, which have evaluated a total of around 200 entities against P. carinii DHFR and T. gondii DHFR, suggest that while the relationship between structure and activity against the two enzymes is distinct, there is a possibility that the identification of entities with potent activity against both enzymes is achievable (7,14,15). A series of 15 6-substituted 2,4-diaminopyrimidines, which are closely related to the structures described in this study, have been shown to contain nanomolar inhibitors of both P. carinii DHFR and T. gondii DHFR (7).…”