SID 26681509 is a highly potent, reversible, competitive, and specific inhibitor of human cathepsin L with an IC50 of 56 nM. SID 26681509 inhibits in vitro propagation of malaria parasite Plasmodium falciparum and inhibits Leishmania major with IC50s of 15.4 μM and 12.5 μM, respectively. SID 26681509 shows no inhibitory activity against cathepsin G.