Abstract LB195: AKT degradation selectively inhibits the growth of PI3K/PTEN pathway mutant cancers with wild type KRAS and BRAF by destabilizing Aurora kinase B

Abstract PI3K/AKT/mTOR signaling pathway is one of the most frequently dysregulated pathway in cancer development and the serine/threonine kinase AKT functions as the key node in this pathway to regulate multiple cellular and physiological processes. AKT inhibitors that competitively bind the ATP pocket have been evaluated in clinical trials and have a tolerable toxicity profile with greater efficacy for tumors with pathway mutations; however, many PI3K/AKT pathway mutant tumors remain resistant. Proteolysis targeting chimeras (PROTACs) are a powerful targeted protein degradation technology that hijacks the cellular ubiquitin-proteasome system to induce selective polyubiquitination and degradation of the target proteins. Using this technology, we designed and developed a library of novel small-molecules putative degraders to degrade AKT by recruiting either the cereblon (CRBN) or von Hippel-Lindau (VHL) E3 ligase, that have the ability to degrade AKT in cells and lower downstream signaling to various extents. Though extensive structure-activity relationship studies on various linkers, E3 ligase ligands, and AKT binding moieties, we identified a novel VHL-recruiting AKT degrader, MS21, and characterized it using a panel of 38 cancer cell lines with diverse genotypes and tissue origins. Our results suggest that efficient pharmacologic degradation of AKT phosphorylated on threonine 308 and serine 473 leads to selective inhibition of the growth of tumor cells with alterations of HER2, PIK3CA, PTEN, or AKT1. In these PI3K/PTEN pathway mutant lines, AKT degradation by MS21 was superior to AKT kinase inhibition for reducing cell growth and sustaining lower signaling over many days, and inhibited tumor cell growths through lowering the level of Aurora Kinase B, which we found to be an AKT substrate protein. AKT degradation but not kinase inhibition profoundly lowered Aurora kinase B (AURKB) protein, which is known to be essential for cell division, and induced G2/M arrest and hyperploidy. PI3K activated AKT phosphorylation of AURKB on threonine 73, which protected it from proteasome degradation. A mutant of AURKB (T73E) that mimics phosphorylation and blocks its degradation rescued cells from growth inhibition by MS21. In addition, resistance to MS21 was found to be associated with low levels of baseline AKT phosphorylation in cells as well as mutation of either KRAS or BRAF, and resistance to MS21 could be overcome by the combination treatment of a MEK inhibitor trametinib with MS21, which not only inhibited MEK but also increased AKT phosphorylation and enhanced AKT degradation. Pan-cancer analysis identified that 19% of cases have PI3K/PTEN pathway mutation without RAS pathway mutation, suggesting that these cancer patients could benefit from AKT degrader therapy that leads to loss of AURKB. Citation Format: Jia Xu, Xufen Yu, Tiphaine C. Martin, Ankita Bansal, Kakit Cheung, Abigail Lubin, Elias Stratikopoulos, Kaitlyn M. Cahuzac, Li Wang, Ling Xie, Royce Zhou, Yudao Shen, Xuewei Wu, Shen Yao, Ruifang Qiao, Poulikos I. Poulikakos, Xian Chen, Jing Liu, Jian Jin, Ramon Parsons. AKT degradation selectively inhibits the growth of PI3K/PTEN pathway mutant cancers with wild type KRAS and BRAF by destabilizing Aurora kinase B [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 LB195.