Background: Androgen deprivation therapy (ADT), CYP17 inhibitors (CYP17Is, e.g., abiraterone) and 2^nd generation androgen receptor (AR) antagonists (2GARAs: enzalutamide, apalutamide, darolutamide) are important treatments for advanced prostate cancer (PC). Darolutamide is FDA-approved with ADT in non-metastatic castration-resistant PC (M0-CRPC) and with ADT+docetaxel in metastatic castration-sensitive PC (M1-CSPC). However, Phase III trials evaluating darolutamide excluded patients (pts) previously treated with other 2GARAs or CYP17Is, thus clinical cross-resistance remains to be evaluated. While reported that darolutamide may be active in vitro against AR mutants resistant to enzalutamide, this also has not been examined clinically. We assessed the real-world effectiveness of darolutamide in a racially/ethnically diverse population of M1-CRPC pts with prior exposure to CYP17Is and/or other 2GARAs. Methods: With IRB approval, we abstracted clinical data from advanced PC pts receiving care at Ben Taub Hospital, a safety-net public hospital serving a racially/ethnically diverse patient population. We identified 33 M1-CRPC pts (17 non-Hispanic African American, 14 Hispanic white, 1 non-Hispanic white, 1 Asian) and 4 M0-CRPC pts (2 non-Hispanic African American, 2 Hispanic white) treated with darolutamide. Tissue and longitudinal circulating tumor DNA (ctDNA) next-generation sequencing data (Tempus Labs, Chicago, IL), including of the AR gene, were also included. Progression-free survival (PFS) was estimated with the Kaplan-Meier method. Results: Amongst the 33 M1-CRPC pts, before receiving darolutamide, 21 had progressed on CYP17Is but had not received 2GARAs; 11 had progressed on both classes of agents; and 1 had progressed on 2GARA but had not received CYP17Is. Darolutamide suppressed PSA by at least 50% in 4/33 pts (12%, all previously 2GARA-naïve). PFS at 3, 6 and 12 months was 36%, 10%, and 5%, respectively, for the entire M1-CRPC cohort; 42%, 17%, and 8% for pts with prior resistance to CYP17Is but not to 2GARA; 25%, 0% and 0% for pts with prior resistance to 2GARA; and 31%, 0% and 0% for pts with AR mutations detected before darolutamide treatment, respectively. AR mutations that emerged or increased in ctDNA allele fraction while on darolutamide included L702H, H875Y, H100Q, D891H, T878A, L329W and AR copy number gain, suggesting a possible role in darolutamide resistance. All four M0-CRPC pts (no prior CYP17I or 2GARA exposure) had PSA declines >50% and remain progression-free at 12 months of treatment. Conclusions: Sequential treatment with darolutamide after progression on a CYP17I may benefit some M1-CRPC pts. However, prior resistance to enzalutamide/apalutamide and/or associated AR mutations can significantly decrease the likelihood of benefit from darolutamide.