Abstract PO2-19-02: NRG-BR009: Phase III Trial Evaluating Addition of Adjuvant Chemotherapy to Ovarian Function Suppression + Endocrine Therapy in Premenopausal Women with pN0-1, HR+/HER2- Breast Cancer and Oncotype Recurrence Score ≤25 (OFSET)

Abstract Background: Identifying pts with HR-positive/HER2-negative breast cancer (BC) who benefit from adjuvant chemotherapy (ACT) has been a major research focus over the past 20 years. Development and clinical application of genomic classifiers such as the 21-gene recurrence score (RS) has contributed to the biologic understanding of BC and has refined pt selection for ACT. The TAILORx and RxPONDER clinical trials demonstrated that RS identifies many postmenopausal pts with node-negative and node-positive BC and RS 25, who do not benefit from the addition of ACT to endocrine therapy (ET). However, both trials also showed that certain subsets of premenopausal pts (node-negative/high clinical risk/RS 16-20, node-negative/RS 21-25, and node-positive/RS 25) benefited from the addition of ACT to ET. Most premenopausal pts in these two trials did not receive ovarian function suppression (OFS) as part of their ET regimen. Given the observed benefit from OFS in high-risk premenopausal pts with HR-positive/HER2-negative BC in the SOFT and TEXT trials, many questioned whether all or part of the observed ACT benefit in the TAILORx/RxPONDER trials may have been the result of chemotherapy-induced OFS. To address this question, OFSET is a Phase III, multicenter clinical trial evaluating the addition of ACT to OFS+ET vs. OFS+ET alone. Methods: We hypothesize that the addition of ACT to OFS+ET is superior to OFS+ET in improving invasive breast cancer-free survival (IBCFS) among premenopausal, early-stage BC pts with HR-positive/HER2-negative tumors, and a 21-gene RS between 16-25 (for pN0 pts) and 0-25 (for pN1 pts). Secondary objectives include invasive disease-free survival (IDFS), overall survival (OS), distant recurrence-free interval (DRFI), breast cancer-free interval (BCFI), and health-related quality of life (HRQOL). Eligible pts must be node-negative with RS 16-20 (plus high clinical risk), or RS 21-25, or have 1-3 positive nodes with RS 25. Stratification is by nodal status/RS status (pN0 RS 16-25 vs pN1 RS 0-15 and pN1 RS 16-25), intent to receive CDK4/6 inhibitor (yes; no), and age (18-39 vs ≥40). Pts are randomized after surgery to either OFS+ET or ACT followed by OFS+ET. ET is an aromatase inhibitor per investigator discretion, but tamoxifen is allowed if pts do not tolerate the AI or if OFS is incomplete. Radiation therapy will be administered per investigator discretion according to protocol guidelines. The HRQOL sub-study will assess differences in severe menopausal symptoms, measured by the FACT ESS-19 score, between the arms, as well as increased pain severity (PROMIS) during ACT+OFS+ET compared to OFS+ET. Blood and tumor specimens will be collected for future research. The accrual of 3,960 pts is anticipated to be completed in seven years and seven mos. Based on data from NSABP B-28 and RxPONDER, the 5-year IBCFS of pN1 pts on the ACT+OFS+ET arm is estimated at 92.3%. Based on data from TAILORx, the 5-year IBCFS of pN0 pts on the ACT arm is ~95%. Assuming 56% of the pts to be pN0 and 44% pN1, a 0.5% annual loss-to-follow-up rate, the definitive analyses to detect a hazard ratio: 0.75 with ACT+OFS+ET vs. OFS+ET, with one-sided alpha of 0.025 and 80% power will require 380 IBCFS events, which are expected to occur ~11 years after study initiation. The OFSET study is expected to be activated in the third quarter of 2023. Current accrual (06-19-2023) is 0/3,960 Support: U10CA180868, U10CA180822, UG1CA189867, U24CA196067. NCT05879926. Citation Format: Eleftherios Mamounas, Gong Tang, Shannon Puhalla, Sandra Swain, Patricia Ganz, N. Lynn Henry, Reena Cecchini, Sonya Reid, Priya Rastogi, Charles Geyer, Julia White, Amy Clark, Tufia Haddad, Gregory Vidal, Norman Wolmark. NRG-BR009: Phase III Trial Evaluating Addition of Adjuvant Chemotherapy to Ovarian Function Suppression + Endocrine Therapy in Premenopausal Women with pN0-1, HR+/HER2- Breast Cancer and Oncotype Recurrence Score ≤25 (OFSET) abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO2-19-02.