Key interactions of pyrimethamine derivatives specific to wild-type and mutant P. falciparum dihydrofolate reductase based on 3D-QSAR, MD simulations and quantum chemical calculations.

Plasmodium falciparum dihydrofolate reductase-thymidylate synthase ( Pf DHFR-TS) is an important target enzyme in malarial chemotherapy. An understanding of how novel inhibitors interact with wild-type (wt Pf DHFR), quadruple-mutant (qm Pf DHFR), and human ( h DHFR) enzymes is required for the development of these compounds as antimalarials. This study is focused on a series of des -Cl and m -Cl phenyl analogs of pyrimethamine with various flexible 6-substituents. The interactions of these compounds with DHFR enzymes were investigated by 3 D-QSAR, MD simulations, MM-PBSA, and DFT calculations. CoMFA and CoMSIA models were developed with good predictive abilities for wt Pf DHFR and qm Pf DHFR. For h DHFR, CoMSIA models combined with clogP descriptor were successfully derived. Binding free energy using MM-PBSA and comparison of per residue decomposition energy analyses with the DFT method at M06-2X/6-31G ++(d,p) level of theory indicated that Asp54 and Phe58 play important roles in the binding of the most potent compound in the series (compound 27) with both wt Pf DHFR and qm Pf DHFR, whereas Arg59 and Arg122 were additionally found to interact with this inhibitor in qm Pf DHFR. For h DHFR, the residues Glu30 and Phe34 but not Arg70, equivalent to Asp54, Phe58, and Arg122 in Pf DHFR, also play role in compound 27 binding through strong hydrophobic interactions (Phe34) and hydrogen bond network with Glu30, Ile7, and Val115. From the key interactions identified in the DHFR-inhibitor complexes, a general scheme is proposed for designing new inhibitors selective for Pf DHFR that is important for the development of novel antifolate antimalarials.