Optimal patient population for adjuvant S-1 therapy in hormone receptor-positive, HER2-negative early breast cancer.

Background: Patients with hormone receptor (HR)-positive and human epidermal growth factor receptor 2 (HER2)-negative breast cancer remain at risk of recurrence even beyond 5 years post-surgery. The monarchE and the POTENT trials have demonstrated the efficacy of adding abemaciclib and S-1, respectively, to adjuvant endocrine therapy for patients with intermediate to high-risk luminal breast cancer. Given the broad eligibility criteria of the POTENT trial, this study aimed to determine the optimal selection criteria for adjuvant S-1 therapy in HR-positive/HER2-negative early breast cancer.

Methods: We analyzed the prognosis of POTENT-eligible patients with HR-positive/HER2-negative early breast cancer who underwent surgery at our institute from 1981 to 2023. Kaplan-Meier curves were established, and differences were assessed using the log-rank test.

Results: The analysis included 2099 patients. Among lymph node-negative patients eligible for the POTENT trial, those with grade 2 tumors < 3 cm demonstrated significantly better disease-free survival (DFS) than those with tumors ≥ 3 cm, while patients with grade 3 tumors < 2 cm demonstrated significantly improved DFS compared with those with tumors ≥ 2 cm. Lymph node-negative patients with "grade 2 & ≥ 2 cm, < 3 cm" and "grade 3 & < 2 cm" disease had significantly better DFS compared with the "remaining POTENT eligible" patients (p = 0.009).

Conclusions: The results of this study showed that the prognosis for lymph node-negative early breast cancer patients classified as "grade 2 & ≥ 2 cm, < 3 cm" and "grade 3 & < 2 cm" was favorable. The benefit of adding S-1 to endocrine therapy to these groups may be marginal.