Background: Real-world progression-free survival (rwPFS) and time to next line of therapy (TTNT) are two endpoints of clinical interest in patients with metastatic breast cancer (MBC). Their validation as intermediate endpoints for overall survival (OS), in a real-world setting, remains not fully established. Methods: We conducted a retrospective cohort study using the nationwide US Electronic Health Record-derived de-identified Flatiron Health database. The study population included pts diagnosed with MBC from Jan 1, 2011 to Feb 30, 2021. rwPFS was defined as the time from start of first-line systemic therapy for MBC to disease progression or death. TTNT was defined as the time from start of first-line systemic therapy until start of next line of therapy. The nonparametric Kendall tau correlation between each surrogate endpoint (rwPFS and TTNT) and OS was evaluated using the Hougaard copula model in the Weibull margin distribution. Kendall’s tau (τ) with its 95%CI was calculated across the entire dataset, and within each disease subgroup defined by receptor (ER and/or PR status defined as hormone receptor [HR], and HER2) status. This work was conducted on behalf of the imCORE network and the Dana-Farber Cancer Institute. Results: Overall, 9,770 patients with MBC were included. Median age was 63 years (IQR: 54-72 years). HR+/HER2- disease represented the most frequent MBC subtype (n=6,287; 64.4%), followed by HER2+ (n=2,096; 21.5%) and triple negative (n=1,387; 14.2%) disease. Median f/u was 41.5 months (95%CI: 40.4 to 42.8). Median OS in the overall population was 32.4 months (95%CI: 31.2 to 33.3). Median rwPFS was 11.5 months (95%CI: 11.1 to 11.9), and median TTNT was 11.1 months (95%CI: 10.7 to 11.5). Across the entire population, the correlation of rwPFS with OS was 0.54 (95%CI: 0.53-0.56), while the correlation of TTNT with OS was 0.47 (95%CI: 0.46-0.48) (Table). Conclusions: rwPFS and TTNT may represent meaningful intermediate endpoints for OS in patients with MBC overall, and within the different disease subgroups.