Impact of relative dose intensity on bone marrow suppression induced by S-1: retrospective observational study.

Background: S-1 (a combination of tegafur, gimeracil, and oteracil) is used to treat various cancers. Bone marrow suppression is a dose-limiting toxicity of S-1. The relationship between relative dose intensity (RDI) and bone marrow suppression has not been investigated. Hence, we aimed to elucidate the threshold for RDI to identify bone marrow suppression induced by S-1.

Methods: In this retrospective cohort study, patients who initiated S-1 treatment at Tokyo Women's Medical University, Medical Center East between June 2015 and June 2017 were included. Bone marrow suppression induced by S-1 was assessed using Common Terminology Criteria for Adverse Events version 4.0. The relationships between grade 3 or higher bone marrow suppression induced by S-1 and RDIs (i.e., 70, 75, and 80%) were investigated using the multivariate Cox proportional hazard model.

Results: We identified 143 patients in this study. The median RDI was 78.8%. Bone marrow suppression induced by S-1 developed in 19 (13.3%) patients. The multivariate Cox proportional hazard model revealed that grade ≥ 2 lymphocytopenia was associated with bone marrow suppression induced by S-1 regardless of the threshold for RDI. In addition, RDI > 75% [hazard ratio (HR) = 1.71, p  < 0.05] and RDI > 80% (HR = 1.65, p  < 0.05) were associated with bone marrow suppression induced by S-1.

Conclusions: Reduced dose of S-1 still has the risk of developing bone marrow suppression. Clinicians should assess RDI to identify high risk patients with bone marrow suppression induced by S-1.