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Leishmaniases are a collection of neglected tropical diseases caused by kinetoplastid parasites in the genus Leishmania . Current chemotherapies are severely limited, and the need for new antileishmanials is of pressing international importance. Bromodomains are epigenetic reader domains that have shown promising therapeutic potential for cancer therapy and may also present an attractive target to treat parasitic diseases. Here, we investigate Leishmania donovani bromodomain factor 5 ( Ld BDF5) as a target for antileishmanial drug discovery. Ld BDF5 contains a pair of bromodomains (BD5.1 and BD5.2) in an N-terminal tandem repeat. We purified recombinant bromodomains of L. donovani BDF5 and determined the structure of BD5.2 by X-ray crystallography. Using a histone peptide microarray and fluorescence polarization assay, we identified binding interactions of Ld BDF5 bromodomains with acetylated peptides derived from histones H2B and H4. In orthogonal biophysical assays including thermal shift assays, fluorescence polarization, and NMR, we showed that BDF5 bromodomains bind to human bromodomain inhibitors SGC-CBP30, bromosporine, and I-BRD9; moreover, SGC-CBP30 exhibited activity against Leishmania promastigotes in cell viability assays. These findings exemplify the potential BDF5 holds as a possible drug target in Leishmania and provide a foundation for the future development of optimized antileishmanial compounds targeting this epigenetic reader protein.

There has been a very limited number of high-throughput screening campaigns carried out with Leishmania drug targets. In part, this is due to the small number of suitable target genes that have been shown by genetic or chemical methods to be essential for the parasite. In this perspective, we discuss the state of genetic target validation in the field of Leishmania research and review the 200 Leishmania genes and 36 Trypanosoma cruzi genes for which gene deletion attempts have been made since the first published case in 1990. We define a quality score for the different genetic deletion techniques that can be used to identify potential drug targets. We also discuss how the advances in genome-scale gene disruption techniques have been used to assist target-based and phenotypic-based drug development in other parasitic protozoa and why Leishmania has lacked a similar approach so far. The prospects for this scale of work are considered in the context of the application of CRISPR/Cas9 gene editing as a useful tool in Leishmania.

The protozoan parasite Toxoplasma gondii secretes a family of serine-threonine protein kinases into its host cell in order to disrupt signaling and alter immune responses. One prominent secretory effector is the rhoptry protein 18 (ROP18), a serine-threonine kinase that phosphorylates immunity related GTPases (IRGs) and hence blocks interferon gamma-mediated responses in rodent cells. Previous genetic studies show that ROP18 is a major virulence component of T. gondii strains from North and South America. Here, we implemented a high throughput screen to identify small molecule inhibitors of ROP18 in vitro and subsequently validated their specificity within infected cells. Although ROP18 was not susceptible to many kinase-directed inhibitors that affect mammalian kinases, the screen identified several sub micromolar inhibitors that belong to three chemical scaffolds: oxindoles, 6-azaquinazolines, and pyrazolopyridines. Treatment of interferon gamma-activated cells with one of these inhibitors enhanced immunity related GTPase recruitment to wild type parasites, recapitulating the defect of Δ rop18 mutant parasites, consistent with targeting ROP18 within infected cells. These compounds provide useful starting points for chemical biology experiments or as leads for therapeutic interventions designed to reduce parasite virulence.