Antimalarial drug resistance compels the quest for new compounds that target alternative pathways to current drugs. The Plasmodium cyclic GMP-dependent protein kinase (PKG) has essential functions in all of the major life cycle developmental stages. An imidazopyridine PKG inhibitor scaffold was previously shown to clear P. falciparum infection in a rodent model in vivo and blocked transmission to mosquitoes providing proof of concept for this target. To find new classes of PKG inhibitors to serve as alternative chemical starting points, we performed a high-throughput screen of the GSK Full Diversity Collection using recombinant P. falciparum PKG. We developed a robust enzymatic assay in a 1536-well plate format. Promising compounds were then tested for activity against P. falciparum asexual blood stage growth, selectivity and cytotoxicity. By using a scoring system we selected the 66 most promising PKG inhibitors (comprising nine clusters and seven singletons). Among these, thiazoles were the most potent scaffold with mid-nanomolar activity on P. falciparum blood stage and gamete development. Using Kinobeads profiling we identified additional P. falciparum protein kinases targeted by the thiazoles that mediate a faster speed of the kill than PKG-selective compounds. This scaffold represents a promising starting point to develop a new antimalarial.

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High-throughput screening of the Plasmodium falciparum cGMP-dependent protein kinase identified a thiazole scaffold which kills erythrocytic and sexual stage parasites.

Protein and phosphoinositide kinases have been successfully exploited as drug targets in various disease areas, principally in oncology. In malaria, several protein kinases are under investigation as potential drug targets, and an inhibitor of Plasmodium phosphatidylinositol 4-kinase type III beta (PI4KIIIβ) is currently in phase 2 clinical studies. In this Perspective, we review the potential of kinases as drug targets for the treatment of malaria. Kinases are known to be readily druggable, and many are essential for parasite survival. A key challenge in the design of Plasmodium kinase inhibitors is obtaining selectivity over the corresponding human orthologue(s) and other human kinases due to the highly conserved nature of the shared ATP binding site. Notwithstanding this, there are some notable differences between the Plasmodium and human kinome that may be exploitable. There is also the potential for designed polypharmacology, where several Plasmodium kinases are inhibited by the same drug. Prior to starting the drug discovery process, it is important to carefully assess potential kinase targets to ensure that the inhibition of the desired kinase will kill the parasites in the required life-cycle stages with a sufficiently fast rate of kill. Here, we highlight key target attributes and experimental approaches to consider and summarize the progress that has been made targeting Plasmodium PI4KIIIβ, cGMP-dependent protein kinase, and cyclin-dependent-like kinase 3.

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Plasmodium Kinases as Potential Drug Targets for Malaria: Challenges and Opportunities.

Design, synthesis ADMET and molecular docking of new imidazo[4,5-b]pyridine-5-thione derivatives as potential tyrosyl-tRNA synthetase inhibitors

One of the attractive properties of artemisinins is their extremely fast-killing capability, quickly relieving malaria symptoms. Nevertheless, the unique benefits of these medicines are now compromised by the prolonged parasite clearance times and the increasing frequency of treatment failures, attributed to the increased tolerance of Plasmodium falciparum to artemisinin. This emerging artemisinin resistance threatens to undermine the effectiveness of antimalarial combination therapies. Herein, we describe the medicinal chemistry efforts focused on a cGMP-dependent protein kinase (PKG) inhibitor scaffold, leading to the identification of novel chemical entities with very potent, similar to artemisinins, fast-killing potency against asexual blood stages that cause disease, and activity against gametocyte activation that is required for transmission. Furthermore, we confirm that selective PKG inhibitors have a slow speed of kill, while chemoproteomic analysis suggests for the first time serine/arginine protein kinase 2 (SRPK2) targeting as a novel strategy for developing antimalarial compounds with extremely fast-killing properties.

https://pubs.acs.org/doi/pdf/10.1021/acs.jmedchem.9b01099

Development of Chemical Entities Endowed with Potent Fast-Killing Properties against Plasmodium falciparum Malaria Parasites.

The single-celled apicomplexan parasite Plasmodium falciparum is responsible for the majority of deaths due to malaria each year. The selection of drug resistance has been a recurring theme over the decades with each new drug that is developed. It is therefore crucial that future generations of drugs are explored to tackle this major public health problem. Cyclic GMP (cGMP) signaling is one of the biochemical pathways that is being explored as a potential target for new antimalarial drugs. It has been shown that this pathway is essential for all of the key developmental stages of the complex malaria parasite life cycle. This gives hope that targeting cGMP signaling might give rise to drugs that treat disease, block its transmission and even prevent the establishment of infection. Here we review previous work that has been carried out to develop and optimize inhibitors of the cGMP-dependent protein kinase (PKG) which is a critical regulator of the malaria parasite life cycle.

/pdf/targeting-the-malaria-parasite-cgmp-dependent-protein-kinase-4keg6prmto.pdf

Targeting the Malaria Parasite cGMP-Dependent Protein Kinase to Develop New Drugs.

Trisubstituted thiazoles as potent and selective inhibitors of Plasmodium falciparum protein kinase G (PfPKG).

Potent bicyclic inhibitors of malarial cGMP-dependent protein kinase: approaches to combining improvements in cell potency, selectivity and structural novelty

Denise J. Tsagris

Papers

Trisubstituted thiazoles as potent and selective inhibitors of Plasmodium falciparum protein kinase G (PfPKG).

Potent bicyclic inhibitors of malarial cGMP-dependent protein kinase: approaches to combining improvements in cell potency, selectivity and structural novelty

Potent inhibitors of malarial P. Falciparum protein kinase G: Improving the cell activity of a series of imidazopyridines.

Discovery of potent and selective 5-azaindazole inhibitors of leucine-rich repeat kinase 2 (LRRK2) - Part 1