Abstract 3009: STX-478, a <i>PIK3CA</i> mutant-selective inhibitor, alone and in combination with romidepsin for <i>PIK3CA</i> mutant CRC

Abstract Background: Hyperactivation of PI3K has been linked to enhanced tumor cell growth and survival, highlighting its importance as a therapeutic target in colorectal cancer (CRC). In this study, we aim to explore the effects of STX-478, a mutant-selective, allosteric PIK3CA inhibitor, to assess its impact on PIK3CA mutant CRC. Additionally, this study aims to investigate a potential therapy in combination with STX-478 that may amplify its efficacy. A histone deacetylase inhibitor, romidepsin, was chosen as a therapy of interest due to previous work that has shown enhanced activity in CRC lines in combination with PI3K inhibitors. Methods: CRC cell lines SW48 and SW48PK (Horizon Discovery, SW48 with a PIK3CAH1047R mutation) were utilized in a WST assay to determine the impact of STX-478 treatment. Immunoblotting was performed on SW48 and SW48PK with STX-478 single agent or in combination with romidepsin. ImageJ was used to quantify these blots to evaluate PI3K protein expression. Patient derived cancer organoids (PDCOs) were utilized to determine how growth was affected by STX-478 treatment. PDCO lines were derived from biopsies and surgical procedures, after approved Institutional Review Board protocols and with patient consent. Their response was determined by measuring the longest diameter of any given PDCO at Day 0 versus Day 2. Glass’s Delta was determined to quantify effect size and assess any significant growth reduction. Romidepsin was also tested as a secondary agent in both western blots and diameter analysis. Results: SW48 did not have a significant change in cell viability at high concentrations of STX-478 compared to controls. In contrast, SW48PK showed a significant response to STX-478 treatment at even 500 nM (p&amp;lt;0.001). Immunoblotting showed little difference in phosphorylation of AKT in both SW48 and SW48PK, however, in SW48PK, at low concentrations of STX-478, a decrease of phosphorylated ribosomal S6 was seen. In a PIK3CAH1047R mutant PDCO line, a significant response was seen at 750 nM, (Glass’s delta = 1.16) 1,000 nM (1.29), 1500 nM (1.51), and 2000 nM (1.62). When treated with the combination of STX-478 and romidepsin, PDCOs had a significant response compared to control and single agents (median percent relative change 12%; control: 39.1% p&amp;lt;0.001, STX-478: 27.8%, p&amp;lt;0.001, Romidepsin: 19.4%, p = 0.013). Conclusion: This study demonstrates that STX-478, a mutant-selective, allosteric PIK3CA inhibitor, exhibits varying effects on CRC cell lines in both 2D and 3D culture models. The combination of STX-478 and romidepsin is promising and deserves further validation in vivo. Citation Format: Addison T. Zick, Alexa E. Schmitz, Alec B. Cornelio, Cheri A. Pasch, Dustin A. Deming. STX-478, a PIK3CA mutant-selective inhibitor, alone and in combination with romidepsin for PIK3CA mutant CRC [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 3009.