Modified 2,4-diaminopyrimidine-based dihydrofolate reductase inhibitors as potential drug scaffolds against Bacillus anthracis

Abstract

The current Letter describes the synthesis and biological evaluation of dihydrophthalazine-appended 2,4-diaminopyrimidine (DAP) inhibitors (1) oxidized at the methylene bridge linking the DAP ring to the central aromatic ring and (2) modified at the central ring ether groups. Structures 4a–b incorporating an oxidized methylene bridge showed a decrease in activity, while slightly larger alkyl groups (CH2CH3 vs CH3) on the central ring oxygen atoms (R2 and R3) had a minimal impact on the inhibition. Comparison of the potency data for previously reported RAB1 and BN-53 with the most potent of the new derivatives (19b and 20a–b) showed similar values for inhibition of cellular growth and direct enzymatic inhibition (MICs 0.5–2μg/mL). Compounds 29–34 with larger ester and ether groups containing substituted aromatic rings at R3 exhibited slightly reduced activity (MICs 2–16μg/mL). One explanation for this attenuated activity could be encroachment of the extended R3 into the neighboring NADPH co-factor. These results indicate that modest additions to the central ring oxygen atoms are well tolerated, while larger modifications have the potential to act as dual-site inhibitors of dihydrofolate reductase (DHFR).