Macrocyclic Peptidomimetic Plasmepsin X Inhibitors with Potent In Vitro and In Vivo Antimalarial Activity

The malaria parasite egress protease SUB1 is activated through precise, plasmepsin X-mediated cleavage of the SUB1 prodomain

BACKGROUND

The malaria parasite Plasmodium falciparum replicates within red blood cells, then ruptures the cell in a process called egress in order to continue its life cycle. Egress is regulated by a proteolytic cascade involving an essential parasite subtilisin-like serine protease called SUB1. Maturation of SUB1 initiates in the parasite endoplasmic reticulum with autocatalytic cleavage of an N-terminal prodomain (p31), which initially remains non-covalently bound to the catalytic domain, p54. Further trafficking of the p31-p54 complex results in formation of a terminal p47 form of the SUB1 catalytic domain. Recent work has implicated a parasite aspartic protease, plasmepsin X (PMX), in maturation of the SUB1 p31-p54 complex through controlled cleavage of the prodomain p31.

METHODS

Here we use biochemical and enzymatic analysis to examine the activation of SUB1 by PMX.

RESULTS

We show that both p31 and p31-p54 are largely dimeric under the relatively acidic conditions to which they are likely exposed to PMX in the parasite. We confirm the sites within p31 that are cleaved by PMX and determine the order of cleavage. We find that cleavage by PMX results in rapid loss of the capacity of p31 to act as an inhibitor of SUB1 catalytic activity and we directly demonstrate that exposure to PMX of recombinant p31-p54 complex activates SUB1 activity.

CONCLUSIONS

Our results confirm that precise, PMX-mediated cleavage of the SUB1 prodomain activates SUB1 enzyme activity.

GENERAL SIGNIFICANCE

Our findings elucidate the role of PMX in activation of SUB1, a key effector of malaria parasite egress.

Peptidic Boronic Acid Plasmodium falciparum SUB1 Inhibitors with Improved Selectivity over Human Proteasome

Plasmodium falciparum subtilisin-like serine protease 1 (PfSUB1) is essential for egress of invasive merozoite forms of the parasite, rendering PfSUB1 an attractive antimalarial target. Here, we report studies aimed to improve drug-like properties of peptidic boronic acid PfSUB1 inhibitors including increased lipophilicity and selectivity over human proteasome (H20S). Structure-activity relationship investigations revealed that lipophilic P3 amino acid side chains as well as N-capping groups were well tolerated in retaining PfSUB1 inhibitory potency. At the P1 position, replacing the methyl group with a carboxyethyl substituent led to boralactone PfSUB1 inhibitors with remarkably improved selectivity over H20S. Combining lipophilic end-capping groups with the boralactone reduced the selectivity over H20S. However, compound 4c still showed >60-fold selectivity versus H20S and low nanomolar PfSUB1 inhibitory potency. Importantly, this compound inhibited the growth of a genetically modified P. falciparum line expressing reduced levels of PfSUB1 13-fold more efficiently compared to a wild-type parasite line.

Emerging Drug Targets for Antimalarial Drug Discovery: Validation and Insights into Molecular Mechanisms of Function

Approximately 619,000 malaria deaths were reported in 2021, and resistance to recommended drugs, including artemisinin-combination therapies (ACTs), threatens malaria control. Treatment failure with ACTs has been found to be as high as 93% in northeastern Thailand, and parasite mutations responsible for artemisinin resistance have already been reported in some African countries. Therefore, there is an urgent need to identify alternative treatments with novel targets. In this Perspective, we discuss some promising antimalarial drug targets, including enzymes involved in proteolysis, DNA and RNA metabolism, protein synthesis, and isoprenoid metabolism. Other targets discussed are transporters, Plasmodium falciparum acetyl-coenzyme A synthetase, N-myristoyltransferase, and the cyclic guanosine monophosphate-dependent protein kinase G. We have outlined mechanistic details, where these are understood, underpinning the biological roles and hence druggability of such targets. We believe that having a clear understanding of the underlying chemical interactions is valuable to medicinal chemists in their quest to design appropriate inhibitors.

Design of Novel Series of Antimalarial PMX Inhibitors with Increased Half-Life via Molecular Property Optimization

Plasmepsin X (PMX) has been identified as a multistage antimalarial target. PMX is a malarial aspartyl protease essential for merozoite egress from infected red blood cells and invasion of the host erythrocytes. Previously, we reported the identification of PMX inhibitors by structure-based optimization of a cyclic guanidine core. Preclinical assessment of UCB7362, which displayed both in vitro and in vivo antimalarial activity, revealed a suboptimal dose paradigm (once daily dosing of 50 mg for 7 days for treatment of uncomplicated malaria) relative to current standard of care (three-dose regime). We report here the efforts toward extending the half-life (t1/2) by reducing metabolic clearance and increasing volume of distribution (Vss). Our efforts culminated in the identification of a biaryl series, with an expected longer t1/2 in human than UCB7362 while maintaining a similar in vitro off-target hit rate.