The Abl protein-tyrosine kinase regulates intracellular signaling pathways controlling diverse cellular processes and forms oncogenic fusion proteins that drive several cancer types. The kinase activity of Abl is repressed by intramolecular interactions involving its regulatory SH3 and SH2 domains. The SH3 domain interacts with a polyproline type II helix formed by the SH2-kinase linker, while the SH2 domain interacts with the back of the C-lobe of the kinase domain. In addition, the myristoylated N-terminal cap region (Ncap) binds a hydrophobic pocket in the C-lobe of the kinase domain, clamping the SH3 and SH2 domains against the back of the kinase domain. Small molecules that allosterically regulate Abl kinase activity through its non-catalytic domains may represent selective probes of Abl function. In this study, we developed a high-throughput screening (HTS) assay to identify chemical modulators of Abl kinase activity that target the regulatory SH3:linker interaction. The fluorescence polarization (FP) assay employs a purified recombinant Abl protein comprised of the regulatory N-cap, SH3 and SH2 domains plus the SH2-kinase linker (Abl N32L protein) and a short fluorescein-labeled peptide probe that selectively binds to the Abl SH3 domain. During assay development, we found that the peptide probe binds to the recombinant Abl N32L protein in vitro, producing a robust FP signal that can be competitively displaced by excess unlabeled peptide. The FP signal is not observed in control N32L proteins bearing either an inactivating mutation in the SH3 domain or enhanced SH3:linker interaction. A pilot screen of 1200 FDA-approved compounds in the FP assay identified four compounds that specifically reduced the FP signal by at least three standard deviations from the untreated controls. Secondary assays demonstrated that one of the hits, the antithrombotic drug dipyridamole, enhances Abl kinase activity in vitro to a greater extent than the previously described Abl agonist, DPH. Docking studies predicted that this compound binds to a pocket formed at the interface of the SH3 domain and the linker, suggesting that it activates Abl by disrupting this regulatory interaction. Interestingly, this compound does not significantly affect the kinase activity of Hck, a Src family member that is regulated by similar SH3:linker interactions. Treatment with dipyrimadole in combination with DPH led to strong enhancement of Abl kinase activity in transfected cells. The Abl N32L FP assay has been implemented in a fully automated format and used to screen diversity libraries totaling 60,000 compounds. The Z-factor coefficients from this HTS campaign indicated that the FP assay performed well and several additional scaffolds were identified for future development. These results demonstrate that screening assays that incorporate the non-catalytic domains of Abl can identify allosteric small molecule regulators of kinase function, providing a new approach to selective drug discovery for this important kinase system.
Citation Format: Prerna Grover, Haibin Shi, Matthew Baumgartner, David Close, Paul A. Johnston, Carlos J. Camacho, Thomas E. Smithgall. Discovery of small molecule allosteric modulators of Abl kinase activity and function. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr B132.