As an organism that undergoes rapid growth and cell division, the malaria parasite is very reliant on two major cellular homeostasis pathways: protein translation and proteasomal degradation.
The Plasmodium falciparum proteasome is a potential antimalarial drug target for compounds with activity against multiple life cycle stages. Starting from inhibitors developed to treat cancer, new derivatives were designed and synthesized with the aim of increasing potency against the Plasmodium proteasome and decreasing activity against the human enzyme. Biochemical and cellular assays identified compounds that exhibit selectivity and potency at different stages of the parasite’s lifecycle. We demonstrated curative oral efficacy in the P. falciparum SCID mouse model. Cryo-electron microscopy revealed the new inhibitors bind in a hydrophobic pocket that is structurally different in the human proteasome – underpinning their selectivity.
We have also identified a series of nucleoside sulfamates from phenotypic screening that exhibit high potency against P. falciparum blood stage cultures, high selectivity against mammalian cell lines and long half-lives in blood. An exemplar compound demonstrates multi-stage activity and oral efficacy in the P. falciparum SCID mouse model. Using in vitro evolution of resistance, we have identified the target as P. falciparum tyrosyl-tRNA synthetase (PfTyr-RS). PfTyr-RS catalyses the formation of a highly stable inhibitory sulfamate conjugate, via a mechanism we call reaction-hijacking. Enzyme kinetics and X-ray crystallographic studies of plasmodium and human Tyr-RS reveal that differential flexibility of a loop over the catalytic site determines differential susceptibility to inhibition by nucleoside sulfamates. The work points to the potential for the design of bespoke nucleoside sulfamates, with tuneable specificity for applications in a broad range of infectious diseases.