Effect of Liver Dysfunction on S-1 Therapy Induced Adverse Effects: A Retrospective Cohort Study

Background/Aim: Renal dysfunction necessitates S-1 dose reduction. However, decreased dihydropyrimidine dehydrogenase (DPD) activity may lead to adverse events due to 5-FU. The guidelines provided by pharmaceutical companies state that total bilirubin (T-Bil) should be ≤upper limit of normal (ULN)×1.5 as a reference value for safely taking S-1. Nevertheless, the relationship between the degree of liver dysfunction and S-1 dose reduction has not been clearly established. Patients and Methods: This study focused on patients who received S-1 monotherapy for various types of cancer. The primary outcome was defined as the variation between blood sampling results on the test day and the subsequent test. The variation data were categorized based on the difference in T-Bil: Low T-Bil group (≤2.25) and High T-Bil group (>2.25). Results: The number of patients that underwent S-1 monotherapy was 883 and the running number was 7,511; Low T-Bil group included 7,245 and High T-Bil group included 266. Examination of the effect of the T-Bil Group on clinical outcomes revealed a correlation with red blood cell (RBC) count, platelet (PLT) count, and T-Bil level. When the impact of the interaction between the T-Bil Group and any of the clinical outcomes, such as the RBC count, PLT count, and T-Bil level, was determined, each outcome showed a significant decrease in the High T-Bil group compared with the Low T-Bil group. Conclusion: S-1 administration in patients with liver dysfunction accompanied by elevated T-Bil levels may cause thrombocytopenia.