Effect of antiretroviral protease inhibitors onPlasmodium falciparumerythrocyte egress and invasion

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 Pf RAP1, and subtilisin-like serine protease Pf SUB1, 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 Pf RAP1, the Pf SUB1-processed Pf MSP1, 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 Pf RAP1 and Pf MSP1 processing was assessed by Wesstern 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 antiretrovial protease inhibitors appeared to reduce the overall expression of Pf RAP1 and Pf MSP1, 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 Pf RAP1 and Pf MSP1 suggests that plasmepsin IX and X are unlikely to be the primary targets of these drugs in these parasites.