Bering, Elizabeth A.

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.

Abstract Description Background Castration-resistant prostate cancers (CRPC) do not respond to immunotherapy.1,2 Prostate cancer-derived immune factors (IF) like sPD-L1 drive immunosuppression and resistance.3 Circulating tumor-reactive T cells (TTR; CD8+CD11ahigh) and other immune subsets predict progression-free survival in oligometastatic CRPC after radiotherapy.4 A better understanding of the levels and changes in IF in CRPC—a tumor immune “circulome”—may lead to better immunotherapeutic approaches. Methods 79 men with oligometastatic castration-resistant prostate cancer were treated with radiation and blood was collected before and after treatment (NCT02816983). Peripheral blood mononuclear cells (PBMC) were subjected to immunophenotyping, and plasma was subjected to the Olink Explore panel assay measuring levels of 5,416 IFs using a proximity extension assay technology. Results High baseline soluble levels of PD-L1, PD-L2, TIM-3, and other selected immunotherapy-related IFs predicted inferior outcomes, including overall survival. Conversely, high soluble CTLA-4 levels strongly predicted superior outcomes (p < 0.0001). These results additionally correlated with changes in circulating tumor-reactive (TTR) T cells in these patients. Discussion IF influence systemic immunity in CRPC. These results suggest a multifactorial mechanism by which prostate cancers drive systemic immunosuppression and resistance to immunotherapy. Ongoing efforts to address IFs may help overcome CRPC immunotherapy resistance.5 Funding Sources Department of Defense HT9425-24-1-0622, An Atlas of the Tumor Immune Circulome (PI: Orme) Prostate Cancer Foundation Young Investigator Award (PI: Orme) National Cancer Institute K12 CA90628-23 (PI: Orme) Topic Categories Tumor Immunology: Checkpoints, Prevention, and Treatment (TIPT)

Abstract Introduction: Immune checkpoint inhibitors (ICI) are an essential systemic therapy for advanced melanoma. However, most melanomas develop resistance to ICI. Tumor-derived soluble PD-L1 (sPD-L1) and other soluble immunosuppressive factors drive checkpoint inhibitor resistance and correlate with inferior survival. We previously showed that therapeutic plasma exchange (TPE) removes sPD-L1 from circulation. Thus, we hypothesized that TPE-mediated removal of sPD-L1 and other immunosuppressive factors could overcome immunotherapy resistance in refractory melanoma. Methods: In this clinical trial (NCT04581382), we prospectively enrolled eighteen (18) patients with widely metastatic melanoma with progression despite anti-PD-1 ICI and elevated sPD-L1 by ELISA (≥1.7ng/mL). Each patient received radiotherapy to between one and three metastatic lesions (at least two unirradiated lesions) followed by three TPE sessions on consecutive days and re-challenge with checkpoint inhibitor. The primary safety and efficacy endpoints of the study were adverse events (AEs) and sPD-L1 reduction by TPE, respectively. Secondary endpoints included RECIST-based response in unirradiated lesions and overall survival. Correlative studies included kinetics of sPD-L1 and soluble immonsuppressive factors and dynamics of peripheral immune cell phenotypes. Results: Mean age was 62 (SD 13) and seven of eighteen (39%) were female. Mean baseline sPD-L1 was 26.31 ng/mL (40.01). The treatment was well-tolerated with one (6%) patient with grade 3 and 4 adverse events from a central line infection after TPE. Levels of sPD-L1 were significantly reduced by TPE (mean 80.2% reduction, p<0.0001). Two patients (11.1%) experienced complete response (CR), one (5.6%) partial response, three (16.7%) stable disease, and 12 (66.7%) progressive disease. In one case, immunotherapy was discontinued after two years due to no detectable lesions. Changes in tumor-reactive (TTR) GZMB+/CX3CR1+/CD11ahigh and other peripheral immune cell populations predicted overall survival in this cohort. In addition to sPD-L1, other soluble mediators of ICI resistance were also reduced by TPE and predicted overall survival in this cohort. Summary/Conclusion: sPD-L1 and other soluble immunoregulatory signaling molecules are important mediators of ICI resistance. SBRT and TPE can resensitize ICI-refractory melanoma by removing these factors. Patients with persistently elevated or rapid rebound of sPD-L1 following TPE experienced poor response and overall survival. Serial monitoring of sPD-L1 may predict response to ICI and multiple courses of TPE may be necessary. Our findings may apply in other ICI-resistant cancers with elevated sPD-L1. ClinicalTrials.gov registration: NCT04581382, ReCIPE-M1 (Rescuing Cancer Immunotherapy with Plasma Exchange in Melanoma 1).