Several parasites responsible for mammalian diseases are dependent on cysteine protease for various life-cycle functions. Inhibition or decreasing function of these proteases can be useful in the treatment and/or prevention of these parasitic diseases including； toxoplasmosis, malaria, African trypanosomiasis, Chagas disease, leishmaniasis, schistosomiasis, amebiasis, giardiasis, clonorchiasis, opisthorchiasis, paragonimiasis, fasciolopsiasis, lymphatic filariasis, onchocerciasis, dracunculiasis, ascariasis, trichuriasis, strongyloidiasis, trichostrongyliasis, trichomoniasis or cestodiasis. Compounds of formula I of the invention are capable of treating and/or preventing the above-identified diseases:
