MK-4101 is a Smoothened (SMO) antagonist (IC50 of 1.1 μM for 293 cells) and also a potent inhibitor of the hedgehog pathway (IC50 of 1.5 μM for mouse cells; IC50 of 1 μM for KYSE180 oesophageal cancer cells) . MK-4101 has robust antitumor activity that inhibits tumor cell proliferation and induces extensive apoptosis[1].