Abstract 3904: SPOP drives AR-V7-mediated ARPI resistance in prostate cancer.

Abstract Approximately 288,000 men are diagnosed with prostate cancer (PCa) in the United States each year, with cases expected to double by 2040. While metastatic PCa can be treated with androgen deprivation therapy (ADT) and AR pathway inhibitors (ARPI), it inevitably develops into treatment resistant, lethal disease. The most common ARPI resistance mechanism in PCa is the expression of AR splice variant AR-V7 that lacks the ligand binding domain and is constitutively active. Patients with PCa expressing AR-V7 experience inferior overall survival. Speckle-type POZ protein (SPOP) is an adaptor protein of the Cullin3-based E3 ubiquitin ligase that facilitates both degradative and non-degradative polyubiquitination of targets such as AR. Loss-of-function SPOP mutations occur in approximately 10-15% of prostate cancers, and SPOP-mutant PCa is a unique subtype that has been shown to be exquisitely sensitive to ADT and ARPI treatment. This sensitivity is mediated in part by increased androgen availability via upregulation of hydroxysteroid 17-beta dehydrogenase 4 (HSD17B4) and related steroid synthesis pathways, reduced AR degradation, and enhanced AR cofactor activity. The remarkable efficacy of ARPI in ADT-resistant SPOP-mutant prostate cancer is not well understood. We hypothesized a novel mechanism by which wild-type SPOP promotes AR-V7-mediated resistance to ARPI in PCa. This mechanism may explain the exceptional ARPI sensitivity of SPOP-mutant PCa, distinct from its susceptibility to ADT. We further proposed that clinical SPOP inhibition may broaden this enhanced ARPI susceptibility to the 85-90% of PCa expressing wild-type SPOP. Our results show that SPOP drives ARPI resistance in prostate cancer that can be reversed by SPOP inhibitor. Bulk RNA sequencing showed that SPOP inhibitor targets spliceosome machinery and more importantly, abrogated AR-V7 expression both at transcriptional and protein level. Further investigation revealed that wild-type SPOP ubiquitinates spliceosome regulator SFPQ, promoting AR-V7 alternative splicing and ARPI resistance. While SPOP loss is pathogenic in PCa, leveraging the weakness conferred by SPOP mutation in the 85% of patients with SPOP-wild type provides an innovative strategy to sensitize ARPI resistant disease to standard of care treatment and save lives. Citation Format: Joanina K. Gicobi, Susanna M. Gregory, Nicole A. Becker, Elizabeth A. Bering, Jacob J. Orme.. SPOP drives AR-V7-mediated ARPI resistance in prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 3904.