MRT-83 is a potent antagonist of Smo , with an IC 50 in the nanomolar range. MRT-83 also blocks Hedgehog (Hh) signaling. In Vitro MRT-83 displays full antagonist properties with an IC 50 (~3 nM) for inhibiting ShhN (3 nM)-induced proliferation of rat GCPs. MRT-83 also blocks SAG (0.01 μM)-induced proliferation of GCPs (IC 50 ~6 nM). MRT-83 blocks BC binding to HEK-hSmo cells in a dose-dependent manner with an IC 50 of 4.6 nM. MRT-83 abrogates BC binding to cells expressing mouse Smo with an IC 50 of 14 nM, which is in good correlation with its IC 50 in the Shh-light2 and alkaline phosphatase assays. MCE has not independently confirmed the accuracy of these methods. They are for reference only. In Vivo Animals treated with ShhN in the presence of MRT-83 are as healthy as those of the other groups but up-regulation of Ptc transcription in the SVZ of these animals is no longer observed in agreement with a complete inhibition of ShhN-mediated effects (8.7±2.4 Ptc + cells/section, n=9) and is not different from vehicle-mediated effects. MRT-83 but not MRT-36 antagonizes the up-regulation of Ptc transcription induced by ShhN in vivo in the SVZ of the LV . MCE has not independently confirmed the accuracy of these methods. They are for reference only. Form:Solid IC50& Target:Smo