Effect of antiretroviral protease inhibitors on Plasmodium falciparum erythrocyte egress and invasion

Abstract Background Anti-retroviral protease inhibitors directly inhibit the growth of asexual blood stage malaria parasites; however, this activity is not fully understood. While mode of action hypotheses have included parasite aspartic protease (plasmepsin) inhibition, current data suggest that digestive vacuole plasmepsins I-IV are not essential for asexual parasite survival, that plasmepsins VI-VIII are not expressed in these parasites and that antiretroviral protease inhibitors are poor inhibitors of plasmepsin V. The remaining plasmepsins, IX and X, have recently been shown to be essential for merozoite egress and invasion, playing important roles in the processing of key proteins including the rhoptry bulb protein PfRAP1, and subtilisin-like serine protease PfSUB1, respectively. To further understand the antiplasmodial activity of antiretroviral protease inhibitors, here we investigated the impact of tipranavir, lopinavir, ritonavir and saquinavir on the processing of PfRAP1, the PfSUB1-processed PfMSP1, and the egress and invasion of P. falciparum parasites from human erythrocytes.Methods The effect of tipranavir, lopinavir, ritonavir and saquinavir on P. falciparum parasite egress and invasion was assessed using synchronized asexual blood stage P. falciparum parasites. Schizont rupture and purified merozoite invasion were performed with and without drug and quantified by flow cytometry analysis. The impact of selected antitretroviral protease inhibitors on PfRAP1 and PfMSP1 processing was assessed by Western blot.Results The effect of tipranavir, lopinavir, ritonavir and saquinavir on the egress and invasion of P. falciparum parasites from human erythrocytes varied considerably but was low at concentrations shown to inhibit P. falciparum asexual parasite growth in vitro and negligible at clinically relevant concentrations. While the treatment of parasites with the antiretroviral protease inhibitors appeared to reduce the overall expression of PfRAP1 and PfMSP1, the processing of these proteins was not inhibited by concentrations known to inhibit parasite growth in vitro.Conclusions The limited activity of tipranavir, lopinavir, ritonavir and saquinavir on the egress and invasion of P. falciparum parasites from human erythrocytes and the processing of PfRAP1 and PfMSP1 suggests that processing events mediated by plasmepsin IX and X that are required for merozoite invasion are unlikely to be a primary mode of action for these drugs in these parasites.