Background: AR ligand binding domain (LBD) activating somatic point mutation is a known mechanism of resistance to androgen-receptor signaling inhibitor (ARSI) treatment in mCRPC, leading to a persistent addiction to steroid hormones. Two early phase trials investigating CYP11A1 inhibitors - CYPIDES (ODM208, NCT03436485) and STESIDES (ODM209, NCT03878823) - recruited mCRPC patients with and without pre-specified AR LBD mutation. In CYPIDES Part 1, AR LBD mutations (ARm) strongly predicted for PSA decline with ODM-208 (Fizazi, ASCO GU 2021). Here, we evaluated clinical and genomic data associated with ARm in patients with mCRPC. Methods: We reviewed clinical and molecular data from patients with mCRPC who progressed after ≥1 ARSI and ≥1 line of taxane-based chemotherapy or were ineligible to chemotherapy, pre-screened in CYPIDES and STESIDES trials at our center. Circulating tumor DNA (ctDNA) was tested for AR LBD mutation (Guardant360 CDx and Sysmex OncoBEAM) and for genomic analysis (FoundationOne Liquid CDx, STING trial, NCT04932525) to screen for actionable targets. Results: From March 2020 to January 2022, 272 men were screened. Median age was 62y. Overall, 212 (78%), 202 (74%), and 240 (88%) patients had received previous abiraterone, enzalutamide, and docetaxel respectively. Median PSA at screening was 59 ng/mL (IQR 11; 246). ARm was found in 69 patients (25%). The most frequent mutations were L702H (15%), T878A (12%), H875Y (7%), V716M (2%), F877L (2%), W742C (1%), globally consistent with previous reports. Associations of ARm with relevant baseline characteristics of patients are summarized. Duration of enzalutamide exposure was correlated with the number of AR mutations per patient (p = 0.006) whereas no correlation was found for abiraterone exposure duration (p=0.71). In 173 (64%) patients with available genomic testing, no alterations were found to be mutually exclusive with ARm, including in DNA repair alteration. Conclusions: AR LBD mutations were detected in ctDNA in 25% of men with advanced mCRPC treated with at least one ARSI. Their incidence was associated with a longer duration of treatment with enzalutamide and a longer time from CRPC and prostate cancer diagnosis.