Abstract 1745: Combining HRI/PERK/GCN2 inhibitor CRD-799 with proteasome inhibitors provides a novel approach to overcoming resistance in multiple myeloma treatment

Abstract The introduction of novel treatments for multiple myeloma (MM), particularly proteasome inhibitors (PIs) such as bortezomib (BTZ), has significantly improved patient outcomes. However, many patients eventually relapse, underscoring the urgent need for new therapeutic strategies to overcome PI resistance. The integrated stress response (ISR) is a conserved intracellular signaling network activated by various intrinsic and extrinsic stressors. This response is mediated by four specialized kinases, HRI, PERK, PKR, and GCN2, which converge on the phosphorylation of eIF2α. Phosphorylated eIF2α promotes the translation of ATF4, a critical regulator of cellular survival and homeostasis. Our prior studies demonstrated that PI treatment induces mitochondrial stress, endoplasmic reticulum stress, and amino acid starvation, triggering the activation of HRI, PERK, and GCN2, respectively. Targeting this triple kinase axis has shown potential in overcoming PI resistance. Building on these findings, we developed CRD-799, an orally bioavailable small-molecule inhibitor designed to target HRI, PERK, and GCN2. CRD-799 effectively inhibited ISR kinase activity, leading to complete suppression of eIF2α phosphorylation and ATF4 activation under PI treatment conditions. When combined with BTZ, CRD-799 exhibited potent anti-proliferative effects both in vitro and in vivo (AACR 2024 #1233, Inoue et al.). Further investigation revealed a novel mechanism of cell death induced by the combination of CRD-799 and PIs. Apoptosis inducers, including NOXA and CHOP, were significantly upregulated during combination treatment, yet this process appeared independent of the canonical ATF4-mediated transcriptional pathway. Comprehensive RNA-seq analysis revealed suppression of the canonical ATF4-driven stress response, while ATF3 induction and NOXA-dependent apoptosis were preserved. Notably, the combination treatment activated the IRE1-TRAF2-JNK1/2-c-Jun/ATF3 signaling axis, suggesting its role in driving NOXA-mediated apoptosis independently of the classical ISR-ATF4 pathway. This mechanism may underlie the variability in treatment sensitivity observed across MM models. In summary, the combination of the HRI/PERK/GCN2 inhibitor CRD-799 with PIs offers a promising strategy to overcome PI resistance in MM. These findings provide critical mechanistic insights into the heterogeneity of treatment responses, potentially guiding the personalized application of this combination therapy in clinical settings. Citation Format: Yasumichi Inoue, Koki Hikami, Shunsuke Ebara, Midori Sugiyama, Robert Oda, Akio Mizutani, Misaki Yoshida, Hiroko Yamakawa, Shigehiro Yagishita, Akinobu Hamada, Hidetoshi Hayashi, Takaomi Sanda, Masaki Ri, Shinsuke Iida, Daisuke Morishita. Combining HRI/PERK/GCN2 inhibitor CRD-799 with proteasome inhibitors provides a novel approach to overcoming resistance in multiple myeloma treatment [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 1745.