With emerging resistance to frontline treatments, it is vital that new drugs are identified to target Plasmodium falciparum. One of the most critical processes during the parasites' asexual lifecycle is the invasion and subsequent egress of red blood cells (RBCs). Many unique parasite ligands, receptors and enzymes are employed during egress and invasion that are essential for parasite proliferation and survival, therefore making these processes druggable targets. To identify potential inhibitors of egress and invasion, we screened the Medicines for Malaria Venture (MMV) Pathogen Box, a 400 compound library against neglected tropical diseases, including 125 with antimalarial activity. For this screen, we utilised transgenic parasites expressing a bioluminescent reporter, nanoluciferase (Nluc), to measure inhibition of parasite egress and invasion in the presence of the Pathogen Box compounds. At a concentration of 2 microM, we found 15 compounds that inhibited parasite egress by >40% and 24 invasion-specific compounds that inhibited invasion by >90%. We further characterised 11 of these inhibitors through cell-based assays and live cell microscopy, and found two compounds that inhibited merozoite maturation in schizonts, one compound that inhibited merozoite egress, one compound that directly inhibited parasite invasion and one compound that slowed down invasion and arrested ring formation. The remaining compounds were general growth inhibitors that acted during the egress and invasion phase of the cell cycle. We found the sulfonylpiperazine, MMV020291, to be the most invasion-specific inhibitor, blocking successful merozoite internalisation within human RBCs and having no substantial effect on other stages of the cell cycle. This has significant implications for the possible development of an invasion-specific inhibitor as an antimalarial in a combination based therapy, in addition to being a useful tool for studying the biology of the invading parasite.
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This work was supported by the Victorian Operational Infrastructure Support Program, Australia received by the Burnet Institute, Australia. We acknowledge Medicines for Malaria Venture (MMV) for providing access to the MMV Pathogen Box and the Australian Red Cross Blood Bank for the provision of human blood. M.G.D is a recipient of an Australian Government Research Training Program Scholarship, G.E.W a Peter Doherty - Australian Biomedical Fellowship, B.E.S a Development Grant 1113712, (National Health and Medical Research Council (NHMRC)), Australia ,D.W.W and B.E.S (NHMRC), Australia, Project Grant APP1143974, and T.F.dK-W a NHMRC Senior Research Fellowship. B.E.S. is a Corin Centenary Fellow (Walter and Eliza Hall Institute (WEHI)), Autralia, D.W.W is a University of Adelaide, Australia, Beacon Fellow. We thank Alan Cowman, WEHI, Australia, for providing the R1 peptide and Monash Micro Imaging, Australia for assistance with microscopy.