Pharmacological validation of N-myristoyltransferase as a drug target in Leishmania donovani

Visceral leishmaniasis (VL), caused by the protozoan parasites Leishmania donovani and L. infantum, is responsible for ~30,000 deaths annually. Available treatments are inadequate and there is a pressing need for new therapeutics. N-Myristoyltransferase (NMT) remains one of the few genetically validated drug targets in these parasites. Here, we sought to pharmacologically validate this enzyme in Leishmania. A focused set of 1,600 pyrazolyl sulfonamide compounds was screened against L. major NMT in a robust high-throughput biochemical assay. Several potent inhibitors were identified with marginal selectivity over the human enzyme. There was little correlation between the enzyme potency of these inhibitors and their cellular activity against L. donovani axenic amastigotes and this discrepancy could be due to poor cellular uptake due to the basicity of these compounds. Thus, a series of analogues were synthesised with less basic centres. Although most of these compounds continued to suffer from relatively poor anti-leishmanial activity, our most potent inhibitor of LmNMT (DDD100097, Ki 0.34 nM), showed modest activity against L. donovani intracellular amastigotes (EC50 2.4 µM) and maintained a modest therapeutic window over the human enzyme. Two un-biased approaches, namely screening against our cosmid-based overexpression library and thermal proteome profiling (TPP), confirm that DDD100097 (compound 2) acts on-target within parasites. Oral dosing with compound 2 resulted in a 52% reduction in parasite burden in our mouse model of VL. Thus, NMT is now a pharmacologically validated target in Leishmania. The challenge in finding drug candidates remains to identify alternative strategies to address the drop-off in activity between enzyme inhibition and in vitro activity while maintaining sufficient selectivity over the human enzyme, both issues that continue to plague studies in this area.