Structural basis for specificity of TGFß family receptor small molecule inhibitors.

ABSTRACT

Transforming growth factor-ß (TGFß) receptor kinase inhibitors have a great therapeutic potential. SB431542 is one of the mainly used kinase inhibitors of the TGFß/Activin pathway receptors, but needs improvement of its EC(50) (EC(50)=1 µM) to be translated to clinical use. A key feature of SB431542 is that it specifically targets receptors from the TGFß/Activin pathway but not the closely related receptors from the bone morphogenic proteins (BMP) pathway. To understand the mechanisms of this selectivity, we solved the crystal structure of the TGFß type I receptor (TßRI) kinase domain in complex with SB431542. We mutated TßRI residues coordinating SB431542 to their counterparts in activin-receptor like kinase 2 (ALK2), a BMP receptor kinase, and tested the kinase activity of mutated TßRI. We discovered that a Ser280Thr mutation yielded a TßRI variant that was resistant to SB431542 inhibition. Furthermore, the corresponding Thr283Ser mutation in ALK2 yielded a BMP receptor sensitive to SB431542. This demonstrated that Ser280 is the key determinant of selectivity for SB431542. This work provides a framework for optimising the SB431542 scaffold to more potent and selective inhibitors of the TGFß/Activin pathway.