Abstract 2304: Comprehensive characterization of CDK inhibitors using a complete panel of all 20 human cyclin-dependent kinases
Authors: Müller Daniel , Totzke Frank , Weber Thomas , Gericke Andreas , Krämer Diane , Heidemann-Dinger Carolin , Ketterer Constance , Kubbutat Michael H. 
Published: June 2022

Abstract
The family of human Cyclin dependent kinases (CDKs) comprises 20 different CDKs that play critical roles in the regulation of cell cycle progression, gene transcription and neuronal function. Deregulation of different CDKs is frequently observed in human cancer. Enzymatic kinase activity of CDKs is dependent on the binding of a member of the Cyclin protein family. So far more than 15 Cyclins have been described most of them can bind and activate different CDKs. Current data suggest the physiological relevance of at least 50 different CDK/Cyclin complexes. Since the 20 CDKs share significant structural homology and regulate different function in cell growth and development, selectivity of compounds within the CDK family is of critical importance.
Approval of a first CDK inhibitor (Palbociclib) targeting CDK4/6 for the treatment of ER+/HER+- breast cancer served as a clinical proof that targeting specific members of the CDK protein kinase family is a versatile approach to treat cancer. The approval of the first CDK inhibitor sparked the research and development of other inhibitors targeting different members of the CDK-family. Currently, four additional CDK4/6 inhibitors have been approved and more than 15 CDK inhibitors with limited selectivity are in different preclinical or clinical development phases. However, the critical importance of selectivity within the CDK family is underlined by the fact that the clinical development of four CDK9 inhibitors has been stopped due to the lack of selectivity and high toxicity. Different approaches (including inhibition by covalent binding) have resulted in more selective inhibitors, especially against CDKs like CDK7, CDK9 and CDK12. Although several biochemical activity assays for different CDKs have been set up and used for selectivity testing, so far no panel covering all 20 human CDKs using one assay technology has been described.
We report here the setup of a biochemical in-vitro activity assay panel of 32 CDK/Cyclins complexes generated recombinantly in insect cells covering the complete set of all 20 human CDKs. For all 32 complexes a radiometric biochemical activity assay has successfully been established allowing to characterize inhibitors with respect to their biochemical selectivity applying the same assay technology. Using the comprehensive CDK panel we determined the IC50 values of more than 15 CDK inhibitors that have been either approved or are in different preclinical and clinical development phases. Results will be presented showing the selectivity of these inhibitors not only for all 20 CDKs but also for specific CDKs forming active complexes with two or more different cyclins. This CDK screening panel allows the generation of comparative data on compound selectivity early in development, thereby helping to reduce the risk of designing compounds with suboptimal target selectivity.
Citation Format: Daniel Müller, Frank Totzke, Thomas Weber, Andreas Gericke, Diane Krämer, Carolin Heidemann-Dinger, Constance Ketterer, Michael H. Kubbutat. Comprehensive characterization of CDK inhibitors using a complete panel of all 20 human cyclin-dependent kinases [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2304.