DPPI 1c hydrochloride is a dicyanopyrrolidine compound that acts as a slow-binding and active site-targeting inhibitor of CD26 (DPP 4), with an IC50 = 106 nM and little activity against DPP II, DPP III, DPP VIII, DPP IX, FAP, or APP even at concentrations as high as 30 ??M.