Background: Activating AR mutations ensure a continued AR activation by non-androgen steroid ligands, e.g. progesterone and glucocorticoids. CYP11A1 is the only enzyme that catalyzes the conversion of cholesterol to pregnenolone, from which all steroid hormones (glucocorticoids, mineralocorticoids, and sex steroids) are subsequently derived. ODM-208, an oral, selective inhibitor of CYP11A1, is being evaluated for safety and efficacy as a treatment of mCRPC in the ongoing CYPIDES phase I/II trial in men previously treated with both novel hormonal therapies and taxanes (ClinicalTrials.gov identifier: NCT03436485). Preliminary phase 1 results were previously reported (Fizazi K et al., ASCO GU 2022). Here we confirm that the in-vitro sensitivity of common AR mutations to ODM-208 treatment is mirrored in patient response in CYPIDES phase 1. Methods: ODM-208 was administered at daily doses between 6-150 mg (phase 2 dose: 10 mg) with dexamethasone and fludrocortisone, resulting in maximal suppression in all measured steroids at all doses. AR ligand-binding domain (LBD) mutations (L702H, V716M, W742L, W742C, H875Y, F877L, T878A, T878S, M896T, M896V) were assessed using a BEAMing assay (Sysmex Inostics) from plasma circulating cell-free DNA (cfDNA) collected before the first dose of ODM-208. The activation of wild-type (wt) and LBD mutated AR with various ligands was also studied in vitro using a luciferase reporter assay in AR negative PC3 cells. Results: 17 of 44 patients had at least one AR activating mutation, the most frequent being L702H (n = 11), T878A (n = 10), and H875Y (n = 6). Eleven out of the 16 evaluable patients (68%) with an AR LBD mutation achieved ≥50% reduction in serum PSA compared with 2 out of the 24 evaluable patients (8%) without an AR LBD mutation (P <.0001). The in vitro sensitivity of each of these common AR mutations to a variety of non-androgenic steroids will be presented, along with detailed PSA responses, duration of responses, and safety in mCRPC patients bearing the same mutations. Conclusions: AR activating mutations may permit continued hormone dependence in mCRPC, related to actions of non-androgenic steroid hormones. Treatment with ODM-208 blocked all steroid hormone production and resulted in frequent ≥50% PSA reductions in this group of heavily pretreated mCRPC pts with various AR LBD mutations, some being long-lasting. cfDNA AR mutations are a promising predictive biomarker for ODM-208 efficacy. Dexamethasone did not activate mutated ARs, supporting its selection for glucocorticoid replacement therapy in combination with ODM-208. Clinical trial information: NCT03436485.