Less Necessity of Adjuvant S-1 Treatment in Non-monarchE-eligible Patients

Background: In monarchE and Postoperative Therapy with Endocrine and TS-1 (POTENT) trials, abemaciclib and S-1 have, respectively, shown to be effective as adjuvant therapies for luminal breast cancer (BC), although whether patients who meet the criteria are at high risk of recurrence compared to non-eligible patients is still unknown. Here, we investigated recurrence risk according to the criteria of each trial in Japanese patients.

Methods: We reviewed the records of 992 patients who received surgery at Chiba University Hospital for stage I-III BC from January 2017 to May 2022 and selected 553 analytic cohort patients and retrospectively analyzed the relapse-free survival of the patients as the primary endpoint. High-recurrence risk was defined according to monarchE trial and POTENT trial.

Results: The 5-year RFS for monarchE cohort 1 and cohort 2 eligible patients were 77.78% and 89.33%, respectively, which were significantly lower than monarchE non-eligible patients (98.31%; p < 0.0001). However, the 5-year RFS rate for POTENT eligible patients (90.51%) was lower than for POTENT non-eligible patients (98.75%; p = 0.0001); excluding those who met the monarchE criteria, the prognosis of POTENT eligible patients had no significant differences from the prognosis of patients with POTENT non-eligible BC (p = 0.3100).

Conclusion: MonarchE criteria accurately identify patients who are prone to relapse. Moreover, although POTENT criteria also suggested a reasonable capacity for recurrence prediction, there was no significant difference in recurrence between POTENT non-eligible patients and the patients who were POTENT but not monarchE eligible. This might offer justification for reconsidering the use of S-1 in monarchE non-eligible patients.