SB 431542 is a potent and selective inhibitor of ALK5 with IC50 of 94 nM, 100-fold more selective for ALK5 than p38 MAPK and other kinases.

SB 431542 is a potent and selective inhibitor of the TGF-β1 receptor ALK5 (IC50 = 94 nM). It is a less potent antagonist of ALK4 (IC50 = 140 nM) and ALK7.It does not affect the BMP receptors ALK2, ALK3, ALK6, or a panel of other kinases tested.SB 431542 specifically blocks Smad signaling, reducing gene expression relevant to fibrosis and cancer.Through its effects on ALK/Smad signaling, SB 431542 suppresses renewal in embryonic and induced pluripotent stem cells and promotes their differentiation.