The activity of p90 ribosomal S6 kinase 2 (RSK2) has emerged as an attractive target for cancer therapy due to its role in the regulation of diverse cellular processes, such as cell transformation and proliferation and the maintenance of cancer stem cells (CSCs). Several pan-RSK inhibitors have been identified with BI-D1870 and the simple LJH685 and LJI308 analogs being the most selective, potent, and frequently used small molecules to identify the physiological substrates and functional roles for RSK in cells. However, these RSK inhibitors have poor PK profiles, display high clearance and a short plasma half-life and this has prevented their transition to clinical evaluation. Therefore, we designed and synthesized a series of pteridinones and pyrimidines to evaluate the structural features of BI-D1870 that are required for RSK2 inhibition, with the overall aim of developing a RSK inhibitor with improved drug-like properties. We have identified inhibitors of recombinant RSK2 activity, evaluated their target engagement in cells, and measured their effect on cell viability and cytotoxicity in the molm-13 acute myeloid leukemia (AML) cell line. The results of our initial structure-activity studies indicate that halogen substitutions on the 4-hydroxyanilino ring can greatly influence RSK inhibitory potency, elimination of C7 chirality does not appear to effect RSK inhibitory potency, removal of the N5-methyl maintains RSK inhibitory activity, and the N7-alkyl substitution is required for potent RSK inhibition. We have identified compounds that potently inhibit RSK2 activity in cells using the nanoBRET assay, and coupled with the results from our cellular viability and toxicity studies our data supports that RSK2 inhibition is independent from acute cytotoxicity in molm-13 cells. This finding was surprising as the RSKs phosphorylate a number of substrates involved in many diverse cellular processes critical for cell survival. Our studies have provided important structural information for the further development of RSK inhibitors and potential insights into structural modifications that may limit metabolism and incorporate isoform selectivity. Further studies are required to determine the impact of RSK inhibition in cancer cells as synergistic combination therapies may need to be identified. Alternatively, RSK inhibitors may be more therapeutically effective against specific cancer cell populations, such as in the elimination of CSCs.

Citation Format: Kimberly Casalvieri, Christopher Matheson, Philip Reigan, Donald Backos. Substituted pteridinones as p90 ribosomal S6 protein kinase 2 (RSK2) inhibitors and their evaluation in acute myeloid leukemia [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr C093. doi:10.1158/1535-7163.TARG-19-C093