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

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 (Pf) and Arabidopsis thaliana (At) SHMT in target assays and PfNF54 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 (t1/2 > 2 h). A selected ligand showed significant in
vivo efficacy with 73% of parasitemia reduction in a mouse model.
Five new cocrystal structures with PvSHMT were solved
at 2.3–2.6 Å resolution, revealing a unique water-mediated
interaction with Tyr63 at the end of the para-aminobenzoate
channel. They also displayed the high degree of conformational flexibility
of the Cys364–loop lining this channel.