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Background: Oral androgen biosynthesis inhibitor, abiraterone acetate plus prednisone (AA+P), has shown to improve survival and patient-related outcomes (PROs) in clinical trials. The COSMiC study (Canadian Observational Study in Metastatic Cancer of the Prostate; ClinicalTrials.gov: NCT02364531) set out to prospectively amass real-world data on mCRPC patients (pts) managed with AA+P in communities within Canada. Here, we report the interim analysis of their PROs. Methods: At planned data cutoff in Sept 2017 after a median follow-up of 33.8 months, 264 pts were enrolled in 39 sites across Canada. Their FACT-P (Functional Assessment of Cancer Therapy – Prostate) and MoCA (Montreal Cognitive Assessment) were evaluated at baseline as well as at weeks 12, 24, 48 and 72 after AA+P initiation. A 10-point decrease denotes clinically significant degradation in FACT-P and a total MoCA score of > = 26 is considered normal. Descriptive analysis was utilized with continuous variables. Changes from baseline were summarized using mean (SD). Results: At a median age of 77 among 264 pts, 230, 185, 110 and 63 pts were available for analysis at their week 12, 24, 48, and 72 assessments respectively. The mean baseline FACT-P total score was 111.2 (19.44) with a < 3-point absolute change from baseline at subsequent assessments, denoting no clinically significant change in functional status over time. The mean baseline MoCA score was 25.2 (4.50), yet all subsequent assessments scored above 26 and a mean absolute change from baseline of < 1, showing an absence of cognitive decline over time. PSA value was available for 221 pts, 64.3% (142/221) and 34.4% (76/221) achieved a PSA decline of > 50% and 90% respectively. All-grade treatment-related adverse events were reported in 63 pts, with 11% who have had AA+P discontinuation or interruption. Conclusions: COSMiC represents the largest Canadian mCRPC cohort treated with AA+P with real world prospective evaluation of PROs. This data demonstrated the maintenance in quality of life and cognitive status over the course of the study, and underscores the importance of PRO utilization in this complex patient population. Clinical trial information: NCT02364531.