Antimalarial Inhibitors Targeting Serine Hydroxymethyltransferase (SHMT) with in Vivo Efficacy and Analysis of their Binding Mode Based on X‑ray Cocrystal Structures

Abstract

Target-based approaches toward new antimalarial treatments are highly valuable to prevent resistance development. We report several series of pyrazolopyran-based inhibitors targeting the enzyme serine hydroxymethyltransferase (SHMT), designed to improve microsomal metabolic stability and to identify suitable candidates for in vivo efficacy evaluation. The best ligands inhibited Plasmodium falciparum (<i>Pf</i>) and Arabidopsis thaliana (<i>At</i>) SHMT in target assays and <i>Pf</i>NF54 strains in cell-based assays with values in the low nanomolar range (3.2–55 nM). A set of carboxylate derivatives demonstrated markedly improved in vitro metabolic stability (<i>t</i>_1/2 > 2 h). A selected ligand showed significant in vivo efficacy with 73% of parasitemia reduction in a mouse model. Five new cocrystal structures with <i>Pv</i>SHMT were solved at 2.3–2.6 Å resolution, revealing a unique water-mediated interaction with Tyr63 at the end of the <i>para</i>-aminobenzoate channel. They also displayed the high degree of conformational flexibility of the Cys364–loop lining this channel