Selection of cyclic-peptide inhibitors targeting Aurora kinase A: problems and solutions.

The critical role of Aurora kinase in cell cycle progression and its deregulation in cancer has garnered significant interest. As such, numerous Aurora targeted inhibitors have been developed to date, almost all of which target the ATP cleft at the active site. These current inhibitors display polypharmacology; that is, they target multiple kinases, and some are being actively pursued as therapeutics. Currently, there are no general approaches for targeting Aurora at sites remote from the active site, which in the long term may provide new insights regarding the inhibition of Aurora as well as other protein kinases, and provide pharmacological tools for dissecting Aurora kinase biology. Toward this long term goal, we have recently developed a bivalent selection strategy that allows for the identification of cyclic peptides that target the surface of PKA, while the active site is blocked by an ATP-competitive compound. Herein, we extend this approach to Aurora kinase (Aurora A), which required significant optimization of selection conditions to eliminate background peptides that target the streptavidin matrix upon which the kinases are immobilized. Using our optimized selection conditions, we have successfully selected several cyclic peptide ligands against Aurora A. Two of these inhibitors demonstrated IC(50) values of 10 μM and were further interrogated. The CTRPWWLC peptide was shown to display a noncompetitive mode of inhibition suggesting that alternate sites on Aurora beyond the ATP and peptide substrate binding site may be potentially targeted.