Abstract A050: Effectiveness of PROTAC BET degraders ARV-825 and ARV-771 in combating chemoresistance in HNSCC

Abstract Introduction: Head and neck cancer is the 6th most prevalent cancer worldwide, with 90% of cases classified as head and neck squamous cell carcinoma (HNSCC). Approximately 50% of HNSCC patients experience tumor relapse despite initially responding to chemoradiation. This may be due to acquired resistance to traditional chemotherapy agents, such as cisplatin. Understanding the mechanisms of cisplatin-resistance and identifying alternative therapeutics are vital to improve HNSCC patient outcomes. Previous studies in our lab displayed that cisplatin-induced senescent HN30 HNSCC cells are sensitized to cell death via ABT-263 (Navitoclax), a senolytic BCL-2/BCL-XL dual inhibitor. However, significant toxicities in ABT-263 limit its clinical utility, thus requiring alternative approaches. We aimed to investigate the effectiveness of ABT-263 as well as the PROTAC BET degraders ARV-825 (BRD4-specific) and ARV-771 (pan-BET) in combating HNSCC cisplatin-resistance. Methods: We established a cisplatin-resistant cell line, HN30R, to examine the efficacy of ABT-263, ARV-825, and ARV-771. DDAOG b-galactosidase assay and Annexin-V/PI FACS were used to observe senescence and apoptosis, respectively. Trypan blue exclusion assay was utilized to observe cell viability while western blotting was implemented to detect BCL-2 family proteins, downstream targets of BRD4, and DNA damage markers. Results: ABT-263 was ineffective in sensitizing HN30R to cisplatin due to increased expression of anti-apoptotic BCL-2 and BCL-XL and lack of senescence induction with cisplatin treatment, thus rendering the senolytic ineffective. ARV-825 and ARV-771 both induced senescence in HN30 and HN30R cells. Treatment with either ARV-825 or ARV-771 resulted in reduced cell viability in both HN30 and HN30R cells, though both recovered proliferation after 6-9 days. Cisplatin + ARV-825 or ARV-771 resulted in decreased cell viability over time and promoted a significant induction in apoptosis compared to either treatment alone. Western blotting revealed that BRD4 is significantly downregulated with both ARV-825 and ARV-771 treatment. HN30 and HN30R cells treated with cisplatin + ARV-825 displayed decreased BRD4 expression and downregulation of its downstream targets c-Myc and Survivin. HN30R cells treated with cisplatin + ARV-825 reflected decreased expression of the DNA repair marker RAD51 and increased gH2AX expression. ARV-825 did not depend on HN30 or HN30R being senescent and thus did not function as a conventional senolytic. Conclusion: The BET degraders ARV-825 and ARV-771 successfully sensitized chemoresistant head and neck cancer cells to cisplatin treatment. This may be due to the downregulation of the proliferation and survival markers c-Myc and Survivin following BRD4 degradation. Increased gH2AX expression and downregulated RAD51 with ARV-825 treatment suggest that accumulated DNA damage may contribute to senescence and apoptosis induction due to impaired BRD4 function. Thus, ARV-825 and ARV-771 may be effective alternative therapeutics to combat chemoresistance in HNSCC. Citation Format: Natalie Luffman, Fereshteh Ahmadinejad, Ryan M. Finnegan, Marissa Raymond, David A. Gewirtz, Hisashi Harada. Effectiveness of PROTAC BET degraders ARV-825 and ARV-771 in combating chemoresistance in HNSCC [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2025 Oct 22-26; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2025;24(10 Suppl):Abstract nr A050.