PF-06840003 (EOS200271) is a highly selective orally bioavailable IDO-1 inhibitor. Although it has moderate hIDO1 enzyme inhibition (IC50 0.41 μM), it is a highly efficient compound (LE 0.53, LipE 5.1), driven by its tight packing within the enzyme, as well as the high density of hydrogen bonds it forms with hIDO-1 despite its small size.