Continuous pharmacist intervention and maintenance of dose intensity in postoperative adjuvant S-1 chemotherapy for gastric cancer: a multicenter retrospective study

Introduction Maintaining dose intensity is essential for the efficacy of adjuvant S-1 chemotherapy in gastric cancer. Although pharmacist interventions can reduce toxicity and improve adherence, multicenter evidence regarding their impact on dose maintenance is limited. Aim This study aimed to evaluate the impact of continuous pharmacist intervention on treatment completion and relative performance (RP) value in patients receiving adjuvant S-1 chemotherapy for gastric cancer. Methods This multicenter retrospective study analysed 225 patients across 12 institutions, categorised into continuous intervention (n=97) and non-continuous intervention (n=128) groups. Continuous intervention was defined as systematic monitoring and management for at least 50% of the treatment duration. Inverse probability of treatment weighting adjusted for baseline covariates. The primary and secondary endpoints were the eight-cycle completion rate and the RP value, respectively. Results Eight-cycle completion rate did not differ significantly between the continuous and non-continuous intervention groups (53.0% vs. 55.2%, p = 0.775), nor did the frequency of treatment modification events, including discontinuation or dose reduction. However, the RP value was significantly higher in the continuous intervention group than in the non-continuous intervention group (73.7% vs. 64.9%, p = 0.038), a finding robustly supported by sensitivity analysis between these two groups (75.1% vs. 61.1%, p = 0.0008). The continuous group maintained a high intervention rate (89.1%) throughout the final cycle, providing significantly more supportive care interventions than the non-continuous intervention group (53.2% vs. 4.9%, p < 0.001). Conclusion Although continuous pharmacist intervention did not improve the binary completion rate, it was essential for maintaining RP values above the clinically essential 70% threshold through proactive toxicity management and dynamic dose optimization. These findings suggest that systematic pharmaceutical care contributes significantly to ensuring the quality and intensity of adjuvant chemotherapy.