SPEN inactivation drives resistance to androgen receptor pathway inhibitors in metastatic prostate cancer

Abstract Purpose: Treatment intensification with androgen receptor pathway inhibitors (ARPIs) has become the standard of care for patients with metastatic prostate cancer. However, there remains an unmet need to identify biomarkers for treatment resistance. Here, we identify SPEN inactivation as a driver of ARPI resistance. Experimental Design: Pre-clinical studies were performed in LNCaP and VCaP cell lines. Data from a nationwide prostate cancer clinico-genomic database were extracted. Log-rank test and Cox proportional hazards models were used to compare time to next treatment (TTNT) on ARPI with/without SPEN mutations. SPEN immunohistochemistry was performed on a rapid autopsy metastatic tissue microarray. Results: SPEN was identified as a top enzalutamide resistance hit in an unbiased genome-wide loss-of-function screen. SPEN inactivation results in upregulation of cell cycle proliferation and basal/stem cell activity as well as increased translation of pro-oncogenic genes. In a large patient cohort (N=6828), SPEN mutations are enriched following treatment with ARPIs (2.1% to 3.6%, p=0.001) and correlate with shorter TTNT on ARPI in patients with metastatic hormone-sensitive prostate cancer (6.4 vs 29.7 months, HR 2.67, p=0.02). In a metastatic rapid autopsy cohort (N=181), low SPEN H-score is associated with shorter time on abiraterone (5.0 vs 7.9 months, p=0.023) in metastatic castration-resistant prostate cancer. Conclusions: In real-world cohorts, loss of SPEN function across genomic, transcriptomic, and protein levels is associated with reduced benefit from ARPI therapy in metastatic prostate cancer. These findings identify SPEN inactivation as a clinically relevant biomarker of ARPI resistance that warrants prospective evaluation to guide treatment selection.