Abstract GS4-09: Pregnancy Outcome and Safety of Interrupting Therapy for women with endocrine responsIVE breast cancer: Primary Results from the POSITIVE Trial (IBCSG 48-14/BIG 8-13)

Background: Pregnancy after breast cancer (BC) is of substantial importance for many young women at diagnosis and during follow-up. BC treatment including standard endocrine therapy (ET) (5-10 years) may reduce ovarian reserve and the chances of subsequent successful pregnancy, given conception is contraindicated during ET. A temporary interruption of ET to attempt and carry a pregnancy in this population has never been prospectively studied. Methods: POSITIVE is a single-arm, prospective, investigator-initiated, international trial evaluating the safety and pregnancy outcomes of interrupting ET for young women with early-stage hormone-receptor-positive (HR+) BC who desire pregnancy. The primary objective is to assess the risk of BC relapse associated with ET interruption for ~2 years to achieve pregnancy. Women ≤42 years with stage I-III HR+ BC who received adjuvant ET (SERM alone, GnRH analogue plus SERM or AI) for 18 to 30 months and wished to interrupt ET to attempt pregnancy were eligible. The primary endpoint is breast cancer free interval (BCFI) defined as the time from enrollment to the first BC event (local, regional, distant recurrence or a new invasive contralateral BC). Planned sample size was 500 patients. Three interim analyses of BCFI were reviewed by the Data Safety Monitoring Committee (DSMC) to assure a 95% chance of stopping the trial early if the annual BCFI event rate exceeded 4%; with primary analysis triggered after 1600 patient years of follow-up (pyfu) and no more than 46 BCFI events defined as the safety threshold. The DSMC recommended continuing the study following each interim analysis. We now report the primary results. Results: From 12/2014 to 12/2019, 518 women were enrolled. At enrollment, the median age of participants was 37 years (27-43 years); 75.0% were nulliparous, 93.4% had stage I/II disease, 66.3% node-negative. Median time from BC diagnosis to enrollment was 29 months (IQR: 25-32). Tamoxifen alone was the most prescribed ET (41.7%), followed by tamoxifen+ovarian function suppression (35.7%). 62.0% of participants had received neo/adjuvant chemotherapy. At a median follow-up of 41 months (1638 pyfu), 44 participants had experienced a BCFI event, not exceeding the pre-specified safety threshold of 46 events. The 3-year BCFI failure percent was 8.9% (95% CI: 6.3 to 11.6%), similar to the 9.2% (95% CI: 7.6 to 10.8%) calculated in the comparative external control cohort from the SOFT/TEXT trials (Sun et al, Breast 2020). Of 497 women followed for pregnancy status, 368 (74.0%) had at least one pregnancy, 317 (63.8%) had at least one live birth, with a total of 365 babies born. Based on competing risk analysis, 76.3% of patients resumed ET (half within 26 months), 8.3% had BCFI event/death before ET resumption, and 15.4% had not resumed ET yet. Conclusions: The POSITIVE trial demonstrates that for young women with early HR+ BC desiring pregnancy, temporary interruption of ET to attempt pregnancy does not confer a greater short-term risk of recurrence than that observed in a modern historical control group that did not interrupt ET. Most participants have had a live birth. Further follow-up is planned to confirm long-term safety. These results should be considered in counselling BC patients desiring future pregnancy. Citation Format: Ann Partridge, Olivia Pagani, Samuel M. Niman, Monica Ruggeri, Fedro Alessandro A. Peccatori, Hatem A. Azim, Marco Colleoni, Cristina Saura, Chikako Shimizu, Anna Saetersdal, Judith Kroep, Audrey Mailliez, Ellen Warner, Virginia F. Borges, Frédéric Amant, Andrea Gombos, Akemi Kataoka, Christine Rousset-Jablonski, Simona Borstnar, Junko Takei, Jeong Eon Lee, Janice Walshe, Manuel Ruiz Borrego, Halle Moore, Christobel Saunders, Vesna Bjelic-Radisic, Snezana Susnjar, Fatima Cardoso, Karen L. Smith, Teresa Ferreiro Vilarino, Karin Ribi, Kathryn Ruddy, Sarra El-Abed, Martine Piccart, Larissa A. Korde, Aron Goldhirsch, Richard D. Gelber. Pregnancy Outcome and Safety of Interrupting Therapy for women with endocrine responsIVE breast cancer: Primary Results from the POSITIVE Trial (IBCSG 48-14/BIG 8-13) [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr GS4-09.