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Background: The management of HER2+ BC has changed dramatically with the introduction and widespread use of HER2-targeted therapies, especially in the adjuvant setting. It is well-known that de novo metastatic HER2+ breast cancer patients have better outcomes that women with metastatic relapse. This could be related both to a lead time bias, as well as to an ATRESS (adjuvant therapy-related shortening of survival) phenomenon [1]. Indeed, cancer treatments induce increased both clonal selection, as well as globally tumor resistance and agressivity [2]. However, there is relatively limited real-world information on the impact of adjuvant and neoadjuvant anti-HER2 targeted treatments on patterns of recurrence and outcomes of HER2+ MBC. Thus, the purpose of this study was to determine how and how long anti-HER2 targeted treatment in early setting impact outcomes of patients with HER2+ recurring BC. Methods: Since 2014, the 18 French Cancer Centers launched the Epidemiological Strategy and Medical Economics (ESME) program to provide real-world data on MBC patients (pts). All pts who started a 1st-line treatment for MBC between 01-Jan-2008 and 31-Dec-2017 were included. We examined clinical characteristics and outcomes (overall survival [OS] and time to 1st metastasis [TTM]) in patients with HER2+ MBC pretreated with trastuzumab in the (neo)adjuvant setting (aT) compared with trastuzumab-naïve patients and patients with de novo HER2+ MBC. Multivariate analyses adjusted for baseline demographic, prognostic factors, adjuvant treatment received and time to MBC. Results: Among the 23698 pts of the ESME database, 4145 were women treated in 1st line of a HER2+ MBC: 1716 pts (41%) had de novo and 2429 pts (59%) recurrent and 65% had Hormone Receptor (HR) + MBC; 53%, 26% and 21% had respectively 1, 2, or > 2 metastatic sites (64% visceral and 11% brain). With a median follow-up of 60.7 m, median OS is 42.4 m (95% CI: 40.1-45.0) for patients who relapsed. OS is significantly longer in de novo MBC: 64.7 m (95% CI: 60.2-73) (HRadjusted=0.68, 95% CI: 0.62-0.76, p < 0.0001). Among pts with recurrent MBC, 56% (1343) had received adjuvant trastuzumab (aT). As 1st-line therapy for MBC, 86 % of pts received HER2-targeted agents (74% T-based, 24% T-pertuzumab-based). Median TTM was 46.9 months (m): 57.6 m in HR+ and 30.2 m in HR-. Among pts with HR- diseases, 38% relapsed during the first 2 years of follow-up and 30% after 4 years (14.8%% and 56.9% in HR+ respectively). Among pts with recurrent MBC, crude median OS is inferior in pts who received aT, as compared to those who did not: 37.2 m (95% CI: 34.4-40.3) vs. 53.5 m (95% CI: 47.6-60.1), but this difference does not persist after adjustment for age, performans status, disease-free interval and number and type of metastatic sites in the multivariate model (HR=1.05, 95% CI: 0.91-1.22). A short disease-free interval (6-24 months compared to >48) remains, however, a strong adverse prognostic factor (HR=2.1, 95% CI: 1.82-2.50). Conclusions: The receipt of adjuvant trastuzumab does not predict for worse outcomes when adjusted to the other prognostic factors, among patients who relapsed during the 2008-2017 period. An ATRESS phenomenon is not suggested, although we cannot rule it out for those who relapsed within the initial 2 years. The much better prognosis of de novo MBC may be largely linked to lead time biases. Citation Format: Fanny Le Du, Matthieu Carton, Mahasti Saghatchian, David Perol, Barbara Pistilli, Etienne Brain, Delphine Loirat, Laurence Vanlemmens, Thomas Vermeulin, Christelle Levy, Anthony Goncalves, Mony Ung, Marie Robert, Anne Jaffre, Mathieu Robain, Suzette Delaloge, Véronique Dieras. Impact of prior adjuvant trastuzumab (aT) on clinical characteristics, patterns of recurrence and outcomes in 4145 patients with Her2 positive (HER2+) metastatic breast cancer (MBC)- Results from the French ESME UNICANCER program [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P1-18-09.