SB 505124 is a selective inhibitor of TGFβR for ALK4, ALK5 with IC50 of 129 nM and 47 nM, respectively, also inhibits ALK7, but does not inhibit ALK1, 2, 3, or 6. 

SB 505124 inhibits the TGF-β1 receptor serine/threonine kinase ALK5 with an IC50 value of 47 nM.Though it is a less potent antagonist of ALK4 (IC50 = 129 nM) and ALK7, SB 505124 selectively and concentration-dependently inhibits ALK4-, ALK5-, and ALK7-dependent activation of downstream SMAD2 and SMAD3 and TGF-β–induced MAP kinase pathway components without altering ALK1-3 or ALK6-induced SMAD signaling.In an assay determining in vitro phosphorylation of SMAD3, SB 505124 is more potent than SB 431452  with IC50 values of 47 versus 94 nM, respectively.