Abstract P1-01-05: Adjuvant S-1 plus endocrine therapy for estrogen receptor-positive, HER2-negative, primary breast cancer: updated overall survival analysis from the POTENT trial
Authors: Takada Masahiro , Saji Shigehira , Ueno Takayuki , Masuda Norikazu , Ishiguro Hiroshi , Ishida Takanori , Saeki Toshiaki , Imoto Shigeru , Ohno Shinji , Iwata Hiroji , Sugie Tomoharu , Aogi Kenjiro , Mukai Hirofumi , Takayama Shin , Sato Nobuaki , Kai Yuichiro , Kitada Masahiro , Nakamura Rikiya , Matsuyama Yutaka , Toi Masakazu 
Published: March 2023

Abstract
Background: The Phase III POTENT trial demonstrated an improvement in invasive disease-free survival (IDFS) by the addition of S-1, an oral fluoropyrimidine, to adjuvant endocrine therapy in patients with ER-positive/HER2-negative early breast cancer. Because the trial was terminated at the interim analysis as the primary endpoint was met, the result of the overall survival (OS) remains immature. Methods: This multicenter observational study aimed to investigate the survival outcomes of patients who participated in the POTENT trial, in which patients with stage I to IIIB ER-positive, HER2-negative breast cancer without protocol-defined low-risk features received adjuvant endocrine therapy alone or with S-1 for 1 year. Of the full analysis set (FAS) of the POTENT trial (N=1930), patients who withdrew the consent or whose institutions terminated the contract were excluded from this study. The primary endpoint was OS. Secondary endpoints were IDFS and distant recurrence-free survival (DRFS). Results: A total of 337 patients (17%) in the POTENT study were excluded from this analysis (eight patients withdrew consent and 329 patients for institutions whose contract had been terminated). A total of 1593 patients were included in this study (803 in the endocrine therapy alone group and 790 in the endocrine therapy plus S-1 group). The median follow-up was 77.5 months (IQR: 68.8–86.0). The median duration of endocrine therapy was 71 and 69 months in the endocrine therapy alone and endocrine therapy plus S-1 groups, respectively. The patient characteristics were well balanced between the treatment groups, except for the number of lymph nodes involved. The endocrine therapy alone group included more patients with four or more positive nodes than the endocrine therapy plus S-1 group (12% vs. 9%, P=0.01). 58 (7%) patients in the endocrine therapy alone group and 51 (6%) in the endocrine therapy plus S-1 group died (HR 0.89, 95%CI: 0.61–1.30, P=0.54). The 5-year overall survival estimate was 94.7% (95%CI: 92.9–96.1%) in the endocrine therapy alone group and 95.6% (95%CI: 93.8–96.8%) in the endocrine therapy plus S-1 group. IDFS events were observed in 166 patients (21%) in the endocrine therapy alone group and in 135 patients (17%) in the endocrine therapy plus S-1 group (HR 0.80, 95%CI: 0.64–1.01). DRFS events occurred in 123 patients (15%) in the endocrine therapy alone group and in 91 patients (12%) in the endocrine therapy plus S-1 group (HR 0.74, 95%CI: 0.56–0.97). Conclusions: In this observational study, data from 337 patients (17%) were missing from the FAS of the POTENT trial. Both the endocrine therapy alone group and endocrine therapy plus S-1 groups showed favorable OS, and OS was similar between the treatment groups. The benefit of IDFS and DRFS by the addition of S-1 to endocrine therapy were maintained.
Citation Format: Masahiro Takada, Shigehira Saji, Takayuki Ueno, Norikazu Masuda, Hiroshi Ishiguro, Takanori Ishida, Toshiaki Saeki, Shigeru Imoto, Shinji Ohno, Hiroji Iwata, Tomoharu Sugie, Kenjiro Aogi, Hirofumi Mukai, Shin Takayama, Nobuaki Sato, Yuichiro Kai, Masahiro Kitada, Rikiya Nakamura, Yutaka Matsuyama, Masakazu Toi. Adjuvant S-1 plus endocrine therapy for estrogen receptor-positive, HER2-negative, primary breast cancer: updated overall survival analysis from the POTENT trial [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 P1-01-05.