Discovery of Potent Antimalarial Type II Kinase Inhibitors with Selectivity over Human Kinases.

While progress has been made in the effort to eradicate malaria, the disease remains a significant threat to global health. Acquired resistance to frontline treatments is emerging in Africa, urging a need for the development of novel antimalarial agents. Repurposing human kinase inhibitors provides a potential expedited route given the availability of a diverse array of kinase-targeting drugs that are approved or in clinical trials. Phenotypic screening of a library of type II human kinase inhibitors identified compound 1 as a lead antimalarial, which was initially developed to target human ephrin type A receptor 2 (EphA2). Here, we report a structure-activity relationship study and lead optimization of compound 1, which led to compound 33, with improved antimalarial activity and selectivity.

Human Polo-like Kinase Inhibitors as Antiplasmodials.

Protein kinases have proven to be a very productive class of therapeutic targets, and over 90 inhibitors are currently in clinical use primarily for the treatment of cancer. Repurposing these inhibitors as antimalarials could provide an accelerated path to drug development. In this study, we identified BI-2536, a known potent human polo-like kinase 1 inhibitor, with low nanomolar antiplasmodial activity. Screening of additional PLK1 inhibitors revealed further antiplasmodial candidates despite the lack of an obvious orthologue of PLKs in Plasmodium. A subset of these inhibitors was profiled for their in vitro killing profile, and commonalities between the killing rate and inhibition of nuclear replication were noted. A kinase panel screen identified PfNEK3 as a shared target of these PLK1 inhibitors; however, phosphoproteome analysis confirmed distinct signaling pathways were disrupted by two structurally distinct inhibitors, suggesting PfNEK3 may not be the sole target. Genomic analysis of BI-2536-resistant parasites revealed mutations in genes associated with the starvation-induced stress response, suggesting BI-2536 may also inhibit an aminoacyl-tRNA synthetase.

Cyclic Tetrapeptide HDAC Inhibitors with Improved Plasmodium falciparum Selectivity and Killing Profile.

Cyclic tetrapeptide histone deacetylase inhibitors represent a promising class of antiplasmodial agents that epigenetically disrupt a wide range of cellular processes in Plasmodium falciparum. Unfortunately, certain limitations, including reversible killing effects and host cell toxicity, prevented these inhibitors from further development and clinical use as antimalarials. In this study, we present a series of cyclic tetrapeptide analogues derived primarily from the fungus Wardomyces dimerus that inhibit P. falciparum with low nanomolar potency and high selectivity. This cyclic tetrapeptide scaffold was diversified further via semisynthesis, leading to the identification of several key structural changes that positively impacted the selectivity, potency, and in vitro killing profiles of these compounds. We confirmed their effectiveness as HDAC inhibitors through the inhibition of PfHDAC1 catalytic activity, in silico modeling, and the hyperacetylation of histone H4. Additional analysis revealed the in vitro inhibition of the most active epoxide-containing analogue was plasmodistatic, exhibiting reversible inhibitory effects upon compound withdrawal after 24 or 48 h. In contrast, one of the new diacetyloxy semisynthetic analogues, CTP-NPDG 19, displayed a rapid and irreversible action against the parasite following compound exposure for 24 h.

A CCaMK-CYCLOPS-DELLA Complex Activates Transcription of RAM1 to Regulate Arbuscule Branching.

Intracellular arbuscular mycorrhiza symbiosis between plants and glomeromycotan fungi leads to formation of highly branched fungal arbuscules that release mineral nutrients to the plant host. Their development is regulated in plants by a mechanistically unresolved interplay between symbiosis, nutrient, and hormone (gibberellin) signaling. Using a positional cloning strategy and a retrotransposon insertion line, we identify two novel alleles of Lotus japonicus REDUCED ARBUSCULAR MYCORRHIZA1 (RAM1) encoding a GRAS protein. We confirm that RAM1 is a central regulator of arbuscule development: arbuscule branching is arrested in L. japonicus ram1 mutants, and ectopic expression of RAM1 activates genes critical for arbuscule development in the absence of fungal symbionts. Epistasis analysis places RAM1 downstream of CCaMK, CYCLOPS, and DELLA because ectopic expression of RAM1 restores arbuscule formation in cyclops mutants and in the presence of suppressive gibberellin. The corresponding proteins form a complex that activates RAM1 expression via binding of CYCLOPS to a cis element in the RAM1 promoter. We thus reveal a transcriptional cascade in arbuscule development that employs the promoter of RAM1 as integrator of symbiotic (transmitted via CCaMK and CYCLOPS) and hormonal (gibberellin) signals.