Tethering Small Molecules to a Phage Display Library:  Discovery of a Selective Bivalent Inhibitor of Protein Kinase A

We report a noncovalent tethering methodology for the fragment-based selection of bivalent ligands targeting protein kinases. In this approach, a small-molecule warhead, staurosporine, directs a phage display cyclic peptide library to the active site of cAMP-dependent protein kinase (PKA), allowing for targeted library enrichment. A cyclic peptide discovered through this selection, when covalently attached to a staurosporine derivative, displayed a 90-fold increase in affinity for PKA. Moreover, the bivalent inhibitor was shown to be significantly more selective than the starting warhead when tested against a small panel of kinases. Thus our general methodology allows for covalent linkage of known small-molecule ligands to biological libraries for discovering potent bivalent inhibitors of biological targets.