Abstract GS1-09: Addition of S-1 to endocrine therapy in the post-operative adjuvant treatment of hormone receptor-positive and human epidermal growth factor receptor 2-negative primary breast cancer: A multicenter, open-label, phase 3 randomized trial (POTENT trial)

BACKGROUND: Although long-term prognostic outcomes of primary breast cancer (PBC) patients have been improved remarkably in recent years, the disease recurrence remains a serious problem. We have previously investigated a role for oral fluoropyrimidines in postoperative adjuvant treatments.In this study, we aimed to verify the usefulness of S-1 in combination with adjuvant endocrine therapy for PBC patients having luminal disease. PATIENTS AND METHODS: This open-label, randomized, phase 3 trial was carried out in 139 centers in Japan. StageI-IIIPBC patients, who had hormone receptor (HR)-positive and human epidermal growth factor receptor 2 (HER2)-negativestatus and intermediate or higher risk of recurrence were randomly assigned (1:1) to receive standard endocrine therapy alone (control arm) or endocrine therapy plus S-1 (S-1 arm). Recurrence risk assessment was performed using anatomical stage, pathological findings such as histologic grade, and centrally confirmed proliferative marker status. S-1 was administered postoperatively in combination with standard endocrine therapy. For patients who underwent multi-drug postoperative adjuvant or preoperative neoadjuvant chemotherapy, S-1 was administered following the multi-drug chemotherapy. Cases having no residual cancer in the breast and axillary node after the preoperative chemotherapy were excluded from this study. The S-1 dosage was chosen among 80 mg/day, 100 mg/day, and 120 mg/day according to the body surface area of each patient, and S-1 was administered for one year with a 2 weeks on/1 week off administration schedule. The primary endpoint was invasive disease-free survival (iDFS), defined as time from randomization to invasive disease recurrence, occurrence of second invasive cancer event, or death, and was analyzed on an intent-to-treat basis. Secondary endpoints included DFS, distant DFS, overall survival, and safety profile. RESULTS: From Feb 2012 to Feb 2016, 1959 patients were enrolled and 1932 patients were included in the full analysis set (control arm, 973; S-1 arm, 959). The results of the prespecified interim analysis met the primary end point, and this trial was terminated early. Median follow-up was 51.4 months. S-1 significantly reduced invasive events; 153 iDFS events were reported in the control arm and 99 iDFS events were reported in the S-1 arm [hazard ratio, 0.63 (95%CI, 0.49-0.81); p-value, 0.0003]. The 5-year iDFS estimate was 81.5% in the control arm and 86.9% in the S-1 arm. Distant recurrence as the first disease event was observed in 6.8% of patients in the S-1 arm and in 9.5% of those in the control arm. The safety data in patients treated with S-1 was consistent with the known profile of S-1. The S-1 treatment was well tolerated and manageable. CONCLUSIONS: It was concluded that the postoperative adjuvant use of an oralfluoropyrimidine S-1 significantly reduced iDFS events and improved 5-year iDFS estimate in PBC patients having HR-positive and HER2-negative disease, in the combination with standard endocrine therapy, with a feasible safety profile. Funding: This study was funded by the Comprehensive Support Project (CSP) of the Public Health Research Foundation. The research fund was provided to CSP by Taiho Pharmaceutical Co., Ltd. This trial was conducted as a study of ‘Advanced Medical Care,’ the Ministry of Health, Labour and Welfare, Japan. JRCT ID: jRCTs051180057, UMIN000003969 Citation Format: Masakazu Toi, Shigeru Imoto, Takanori Ishida, Yoshinori Ito, Hiroji Iwata, Norikazu Masuda, Hirofumi Mukai, Shigehira Saji, Akira Shimizu, Takafumi Ikeda, Hironori Haga, Toshiaki Saeki, Kenjiro Aogi, Tomoharu Sugie, Takayuki Ueno, Takayuki Kinoshita, Yuichiro Kai, Masahiro Kitada, Yasuyuki Sato, Kenjiro Jimbo, Nobuaki Sato, Hiroshi Ishiguro, Masahiro Takada, Yasuo Ohashi, Shinji Ohno. Addition of S-1 to endocrine therapy in the post-operative adjuvant treatment of hormone receptor-positive and human epidermal growth factor receptor 2-negative primary breast cancer: A multicenter, open-label, phase 3 randomized trial (POTENT trial) [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr GS1-09.