SID 26681509 quarterhydrate is a potent, reversible, competitive, and selective inhibitor of human cathepsin L with an IC 50 of 56 nM. SID 26681509 quarterhydrate inhibits in vitro propagation of malaria parasite Plasmodium falciparum and inhibits Leishmania major with IC 50 s of 15.4 μM and 12.5 μM, respectively. SID 26681509 quarterhydrate shows no inhibitory activity against cathepsin G.