Description_x000D_ Biochem/physiol Actions_x000D_ Bepristat 1a is a selective reversible inhibitor of protein disulfide isomerase (PDI), an enzyme in the endoplasmic reticulum that catalyzes disulfide bond breakage and reformation to catalyze protein folding. Unlike most PDI inhibitors, Bepristat 1a binds at the substrate-binding site, rather than the catalytic site. It blocked PDI activity with an IC50 value of 700 nM. PDI is up-regulated in several cancers, has been implicated in neurodegenerative processes, and plays an important role in thrombus formation. Bepristat 1a potently inhibited platelet aggregation and thrombus formation in vivo.