Project Summary Prostate cancer depends on androgens and the androgen receptor (AR) for growth and progression. Metastatic tumors are usually initially treated with androgen deprivation therapy (ADT) by way of medical or surgical castration; however, tumors eventually recur as castration-resistant prostate cancer (CRPC), which progresses due to the intratumoral generation of testosterone and/or dihydrotestosterone (DHT) and AR stimulation. The intratumoral synthesis of potent androgens requires the activity of steroidogenic enzymes. In the prior project period, we identified the first example of a gain-of-function missense in a steroidogenic enzyme that increases what is otherwise the rate-limiting step of DHT synthesis from extragonadal precursor steroids and spurs the development of CRPC. This missense in 3?-hydroxysteroid dehydrogenase-1 (3?HSD1) is encoded by HSD3B1(1245C), a common germline variant. In the prior project period, we also discovered that patients with advanced prostate cancer who inherit the HSD3B1(1245C) genetic variant and receive ADT progress to CRPC more rapidly than patients who inherit the wild-type HSD3B1 enzyme, which has lower activity. Although clinical data across 4 patient cohorts now show that the HSD3B1(1245C) genetic variant is a predictive biomarker of resistance to ADT, the precise clinical utility of this biomarker remains uncertain. The overarching goal of this proposal is to determine how HSD3B1(1245C) should be utilized as a biomarker to identify patients who require more intensive upfront treatment and who are otherwise likely to progress more rapidly to lethal disease. As the HSD3B1(1245C) variant encodes for an enzyme that enables more rapid conversion of extragonadal precursors to DHT, we hypothesize that HSD3B1(1245C) is a predictive biomarker of sensitivity to drugs that block the synthesis or effects extragonadal androgens. We hypothesize that metabolism by 3?HSD1 is a class effect of steroidal CYP17A1 inhibitors, thus making non-steroidal CYP17A1 inhibitors or potent AR antagonists more suitable for treating patients who harbor the HSD3B1(1245C) variant. Finally, we hypothesize that intensification of treatment at the time of ADT in phase III clinical trials improves outcomes for patients with metastatic prostate cancer who inherit the HSD3B1(1245C) variant. In Aim 1, we will determine if inheritance of the HSD3B1(1245C) variant is a predictive biomarker of response to non-steroidal CYP17A1 inhibitors or potent AR antagonists. In Aim 2, we will identify whether susceptibility to 3?HSD metabolism is a class effect of steroidal CYP17A1 inhibitors and how this may determine response for patients who inherit the HSD3B1 variant. In Aim 3, we will determine if patients with prostate cancer who inherit the HSD3B1(1245C) variant specifically benefit from more intensive upfront treatment at the time of ADT. If we are correct, the work in this proposal will change standard clinical practice for the treatment of prostate cancer.