Pharmacological validation of N-myristoyltransferase as a drug target in <i>Leishmania donovani</i>

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

Discovery of a benzothiophene-flavonol halting miltefosine and antimonial drug resistance in Leishmania parasites through the application of medicinal chemistry, screening and genomics

Leishmaniasis, a major health problem worldwide, has a limited arsenal of drugs for its control. The appearance of resistance to first- and second-line anti-leishmanial drugs confirms the need to develop new and less toxic drugs that overcome spontaneous resistance. In the present study, we report the design and synthesis of a novel library of 38 flavonol-like compounds and their evaluation in a panel of assays encompassing parasite killing, pharmacokinetics, genomics and ADME-Toxicity resulting in the progression of a compound in the drug discovery value chain. Compound 19, 2-(benzo[b]thiophen-3-yl)- 3-hydroxy-6-methoxy-4H-chromen-4-one, exhibited a broad-spectrum activity against Leishmania spp. (EC50 1.9 mM for Leishmania infantum, 3.4 mM for L. donovani, 6.7 mM for L. major), Trypanosoma cruzi (EC50 7.5 mM) and T. brucei (EC50 0.8 mM). Focusing on anti-Leishmania activity, compound 19 challenge in vitro did not select for resistance markers in L. donovani, while a Cos-Seq screening for dominant resistance genes identified a gene locus on chromosome 36 that became ineffective at concentrations beyond EC50. Thus, compound 19 is a promising scaffold to tackle drug resistance in Leishmania infection. In vivo pharmacokinetic studies indicated that compound 19 has a long half-life (intravenous (IV): 63.2 h; per os (PO): 46.9 h) with an acceptable ADME-Toxicity profile. When tested in Leishmania infected hamsters, no toxicity and limited efficacy were observed. Low solubility and degradation were investigated spectroscopically as possible causes for the sub-optimal pharmacokinetic properties. Compound 19 resulted a specific compound based on the screening against a protein set, following the intrinsic fluorescence changes