Abstract 524: Using acquired resistance to explore the mechanism of action of the integrated stress response/GCN2 activator NXP800 - A new developmental agent for platinum-resistant ARID1A mutant ovarian cancer

Abstract Background: NXP800 is a potent, oral activator of the Integrated Stress response (ISR), inhibitor of heat shock factor 1 (HSF1) activation and tumor cell proliferation, which is in early clinical studies in ARID1A-mutated, platinum resistant, clear cell ovarian cancer (NCT05226507). We discovered NXP800 using multiparameter medicinal chemistry optimization of a hit identified from a cell-based phenotypic screen. Owing to the unbiased nature of phenotypic screening, target identification is crucial to understand the biological and therapeutic activity of hit compounds. Methods and Results: : By RNAseq profiling human cancer cells treated with NXP800 we identified changes in expression of genes regulated by HSF1 or ATF4 - effects accompanied by eIF2alpha (eIF2a) phosphorylation and resulting activation of the ISR. To further understand this response, we explored whether NXP800 resistance models could inform on its mechanism of action (MoA). We used ARID1A mutant SK-OV-3 human ovarian carcinoma cells that are 1) highly sensitive to NXP800, 2) model the target patient population and 3) MSI-high so likely to have an elevated mutation rate contributing to acquisition of resistance. We generated two independent NXP800-resistant SK-OV-3 cell lines in which NXP800-mediated ATF4 induction and concomitant inhibition of global translation were abolished. By whole exome sequencing, we identified a heterozygous L99P mutation in the alpha subunit of eIF2B (eIF2Ba), the nucleotide exchange factor for eIF2a. Expression of eIF2BaL99P, but not wild-type, in parental SK-OV-3 cells reduced sensitivity to NXP800 to the same level as cells with NXP800-induced resistance. Using fluorescence recovery after photobleaching to monitor the dynamic association of the eIF2B: eIF2a complex, we elucidated that the eIF2BaL99P mutation reduces NXP800-mediated inhibition of eIF2a-GFP recycling through eIF2B bodies. Phosphorylation of eIF2a is regulated by four stress-controlled kinases GCN2, HRI, PKR and PERK. Using systematic siRNA knockdown or small-molecule inhibitors, we showed that GCN2 alone is required for ISR activation by NXP800 and that ISR induction inhibited HSF1 activation. Furthermore, inactivation of GCN2 reduced the antiproliferative activity of NXP800 to the same extent observed in NXP800-resistant SK-OV-3 cells. In contrast, exposure of resistant or parental SK-OV-3 cells expressing eIF2BaL99P to GCN2 inhibitors did not cause further reduction in NXP800 sensitivity - confirming the importance of the eIF2BaL99P mutation in the resistance mechanism. Conclusions: We have used acquired resistance to understand the MoA of NXP800 as a potent activator of GCN2 and the ISR pathway. Further studies are underway to determine the exact proximal molecular target of NXP800 and the mechanism of GCN2/ISR activation. Citation Format: Marissa V. Powers, Rachel Hodgson, Swee Y. Sharp, Toby Roe, K. Elizabeth Allen, Susan Campbell, Robert te Poele, Matthew Cheeseman, Keith Jones, Paul A. Clarke, Paul Workman. Using acquired resistance to explore the mechanism of action of the integrated stress response/GCN2 activator NXP800 - A new developmental agent for platinum-resistant ARID1A mutant ovarian cancer abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 524.