SUMMARY Prostate cancer is the second leading cause of cancer death in U.S. men. The frontline treatment for advanced prostate cancer is androgen deprivation therapy (ADT), which has been the standard of care for ~80 years. Recent practice-changing data have demonstrated a survival benefit when ADT is intensified, but the treatment choice for intensification is unclear. Regardless, metastatic disease almost always progresses as castration-resistant prostate cancer (CRPC). A major limitation of new CRPC treatments is that patients are selected based on prior therapies instead of their genetics and disease-driving molecular processes. Essential findings from our previous project period include clear and reproducible clinical evidence that an “adrenal-permissive” HSD3B1(1245C) germline variant encodes for a gain-of-function missense in 3β-hydroxysteroid dehydrogenase-1 (3βHSD1), the rate-limiting steroidogenic enzyme for synthesis of 5α-dihydrotestosterone (DHT) from extragonadal precursor steroids. This adrenal-permissive HSD3B1 is a genetic and metabolic driver of hormone therapy resistance. In this renewal application, we propose to determine the mechanisms and clinical significance of inheritance of the HSD3B1(1245C) variant and move toward its use as a biomarker to tailor castration- resistant and castration-sensitive prostate cancer treatment to the individual patient's genetics and tumor biology. We hypothesize that the germline HSD3B1(1245C) variant is a predictive biomarker that will allow us to identify patients who will benefit from upfront adrenal androgen ablation. Our preliminary data indicate that BMX inhibition of 3βHSD1 phosphorylation will be therapeutic – direct therapeutic inhibition of 3βHSD1 for men with the adrenal-permissive HSD3B1 should reverse the adverse biology and clinical outcomes conferred by extragonadal androgen biosynthesis in CRPC. Our data also indicate that after intensified hormonal therapy, HSD3B1 inheritance is a determinant of systemic metabolic effects, including adverse effects seen in some men, but not others. The potential clinical consequences of the concepts we propose are profound because they couple a germline disease-driving metabolic driver to therapeutic mechanisms for reversal of poor outcomes. Aim 1: Determine whether inheritance of the adrenal-permissive HSD3B1(1245C) allele is a predictive biomarker of improved benefit from adding abiraterone to ADT for non-metastatic castration-sensitive prostate cancer. We will analyze germline DNA available from 959 men enrolled in the STAMPEDE trial. Aim 2: Define the role of phosphorylation in 3βHSD1 activation and determine the role of BMX in the conversion of DHEA to downstream potent androgens. We will determine if the BMX kinase is essential for 3βHSD1 phosphorylation and cellular activation and test its efficacy in a proof-of-concept clinical trial. Aim 3: Determine the mechanistic link between HSD3B1 inheritance and circulating metabolites in men un...