Abstract 2048: Discovery of induction and release of IL-1b are unique and on-target effects of GSPT1 degradation that provide potential mitigation strategies to hypotension in the CC-90009-AML-001 phase 1 trial

Abstract CC-90009 is a novel cereblon (CRBN) E3 ligase modulator (CELMoD) that has demonstrated antileukemic activity and is under investigation in the CC-90009-AML-001 phase I trial (NCT02848001) in patients with relapsed or refractory acute myeloid leukemia (AML). In preclinical studies, CC-90009 drives the binding of translation termination factor G1 to S phase transition I (GSPT1) to CRBN. This results in ubiquitination and proteasome-dependent degradation of GSPT1, leading to activation of the integrated stress response, inhibition of nonsense-mediated decay, and induction of apoptosis. Hypotension, contemporaneous with blast and white blood cell (WBC) decreases, has emerged as a dose-limiting toxicity (DLT) in this ongoing phase I trial. Based on clinical observations, CC-90009-induced cytokine release during leukemic cell death was investigated as a potential cause of hypotension. Using electrochemiluminescence, flow cytometry and western blots, CC-90009-induced IL-1b release was observed in AML cell lines and primary AML bone marrow mononuclear cells. Induction of IL-1b was typically observed after caspase 3 and 8 activation, suggesting a cell-death-related mechanism consistent with clinical findings of hypotension reported after rapid WBC loss. In vitro models showed compounds leading to GSPT1 degradation were more potent IL-1b inducers compared to compounds with other mechanisms, including standard-of-care agents for AML, other cytotoxic agents, protein translation inhibitors, and unfolded protein response inducers. IL-1b induction as a downstream result of GSPT1 degradation was further confirmed by cells expressing non-degradable GSPT1 mutant protein where compound treatment did not induce IL-1b. CC-90009 activates the GCN2 pathway, resulting in caspase-3 and -8-mediated apoptosis. Caspase 8 is known to directly process pro-IL-1b into mature IL-1b. Using an isogenic MV4-11 Cas9 cell pair with wild-type vs null GCN2 status, GCN2 activation downstream of GSPT1 degradation was shown to be required for pro-IL-1b upregulation and caspase 8 activation. Furthermore, genetic and pharmacologic suppression of caspase 8 revealed that its activity was necessary, but not sufficient, for IL-1b release by CC-90009. Of note, dexamethasone (DEX) dampened CC-90009-mediated IL-1b induction without altering the rate or depth of AML cell killing, providing a mechanism-based strategy to manage CC-90009-induced hypotension. The ongoing trial was amended to allow the use of prophylactic DEX; dose escalation is still ongoing, including dose levels higher than the non-tolerated dose defined prior to mandating the use of prophylactic DEX. The insights derived from this study have facilitated GSPT1 targeting in AML therapy by enabling the testing of higher drug exposures. Citation Format: Tsun-Wen Sheena Yao, Yumin Dai, Carla Guarinos, Manuel Sanchez, Alicia Benitez, Hongbin Wang, Soraya Carrancio, Tonia Buchholz, Gang Lu, Michael Pourdehnad, Daniel Pierce, Jinhong Fan. Discovery of induction and release of IL-1b are unique and on-target effects of GSPT1 degradation that provide potential mitigation strategies to hypotension in the CC-90009-AML-001 phase 1 trial [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2048.