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].