The activity of protease inhibitors against Giardia duodenalis and metronidazole-resistant Trichomonas vaginalis

Antiretroviral protease inhibitors were assessed in vitro for their activity against Giardia duodenalis and Trichomonas vaginalis. Kaletra® (a co-formulation of ritonavir and lopinavir) was the most effective overall, with 50% effective drug concentrations (EC50) of 1.1–2.7 μM (ritonavir concentration) against G. duodenalis and 6.8–8 μM against metronidazole-sensitive and clinically metronidazole-resistant T. vaginalis. Minimal inhibitory concentrations were 2–2.5 μM and 10–50 μM for G. duodenalis and T. vaginalis, respectively. Within the range of human plasma concentrations for ritonavir, only G. duodenalis was inhibited. Lopinavir alone was less inhibitory than ritonavir but was associated with a blockage in cytokinesis of G. duodenalis trophozoites. Saquinavir was not effective. These findings are significant considering the association between human immunodeficiency virus and T. vaginalis, and between G. duodenalis and homosexual behaviour.