Factors and timing of decreased adherence to S-1 in postoperative docetaxel + S-1 therapy for gastric cancer.

403 Background: The combination of docetaxel + S-1 (DS) therapy in postoperative adjuvant treatment for gastric cancer has proven to be effective and has become a standard therapy. Clarification of factors involved in and the timing of S-1 non-adherence is important to minimize non-adherence. Methods: Among 92 patients who started DS treatment as postoperative adjuvant chemotherapy for gastric cancer between November 1, 2015 and April 30, 2021 at the Cancer Institute Hospital of the Japanese Foundation for Cancer Research, 90 were eligible for this study. The S-1 adherence rate per cycle was defined as the number of times a patient took S-1 divided by 28. In this study, adherence to S-1 was assessed through pill counts and by asking patients about reasons for non-adherence in interviews at a pharmaceutical outpatient clinic. Elicited reasons for non-adherence were categorized and analyzed. Results: Percentage of remaining S-1 was 3.9% in the first cycle, 7.4% in the second cycle, and 2.0% in the seventh cycle. Median relative dose intensity of S-1 was 80.9%. As reasons for non-adherence, patients most commonly cited nausea/vomiting (18.9%), missed dose (17.6%), and diarrhea (9.9%). From the first to the seventh course, the highest percentage of remaining S-1 was in the second course. Conclusions: Nausea/vomiting, missed dose, and diarrhea were identified as the most common factors for decreased adherence to S-1 in postoperative DS therapy for gastric cancer. Between the first and seventh courses, S-1 adherence decreased the most during the second course, when docetaxel was started. Using this information, we intend to develop methods to improve S-1 adherence in the future.