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

Abstract

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.

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

“…One drawback of specific PKG inhibitors is that they are characterized by a moderate/slow killing rate in parasite reduction ratio (PRR) assays similar to that of pyrimethamine (Baker et al, 2017b;Matralis et al, 2019;Penzo et al, 2019). This is thought to be due to the fact that PKG is active within a very narrow temporal window of blood stage development (perhaps less than three hours) just prior to egress, which is thought to persist until completion of invasion (Taylor et al, 2010;Koussis et al, 2020).…”

“…This kinase and others identified in both chemoproteomic studies are amongst the relatively small number of Plasmodium kinases that have small gatekeeper residues which explains why they can bind to PKG inhibitors that exploit the gatekeeper pocket. These results raise the possibility of optimizing one of the thiazole leads 8-10 which combines the desirable properties of a PKG inhibitor with the fast killing properties conferred by inhibition of an additional target, likely CLK2 (Matralis et al, 2019).…”

A multipass membrane protein interacts with the cGMP-dependent protein kinase to regulate critical calcium signals in malaria parasites

“…Kinobeads were prepared as described (Bantscheff et al, 2011;Bergamini et al, 2012). The chemoproteomic affinity capturing experiments were performed as previously described (Bantscheff et al, 2011;Matralis et al, 2019). Briefly, Kinobeads were incubated with P. falciparum extract (Paquet et al, 2017), which was pre-incubated with compound or DMSO (vehicle control).…”

An Update on Development of Small-Molecule Plasmodial Kinase Inhibitors

“…A variety of scaffolds have been explored for Pf PKG inhibitor development, with the imidazopyridines [71,75,76] and thiazoles [73,77,78] Subsequent studies focussed on improving the ADME properties of this chemical class while retaining potent inhibitory activity. By means of structure-activity relationship and modelling studies, Large and co-workers [75,76] systematically varied the substituents of compound 20 and explored other bicyclic cores.…”

“…Besides blood-stage activity, compound 27 also had potent activity against male and female gametes. Matralis and co-workers [77] specifically focussed on developing a series of thiazole derivatives with a fast-killing profile similar to that of artemisinins. Compound 28 (Figure 11) was the most potent in this series, with in vitro nanomolar activity against Pf PKG, P. falciparum blood-stage parasites and gametocytes.…”