Usefulness of an S-1 dosage formula: an exploratory analysis of randomized clinical trial (JCOG1001)

The blood concentration of S-1 and adverse events are affected by renal function. Herein, an S-1 dosage formula was developed based on renal function, indicating the dose for a target blood concentration. This study aimed to explore the usefulness of the formula in adjuvant chemotherapy for gastric cancer.

In this ad hoc analysis of the JCOG1001 trial, which evaluated the role of bursectomy for resectable gastric cancer, the recommended dose of S-1 was calculated using the following formula: 1447.8 × (14.5 + 0.301 × CLcr + 8.23 × SEX [male = 1, female = 0]) × body surface area (BSA) (mg/day). Patients were divided into three groups by comparing the initial S-1 dose determined using BSA with the dose recommended by the formula: underdose (UD), equal dose (ED), and overdose (OD).

Among 686 eligible patients, 58, 304, and 324 patients were classified into the UD, ED, and OD groups. The patients' characteristics in the UD/ED/OD groups were median age (53.5/64.0/67.5 years), male sex (98.3%/75.3%/58.0%), and median BMI (24.8/22.8/22.3), respectively. The planned 1-year adjuvant S-1 therapy was completed in 74.1%/73.7%/68.5%, dose reduction was required in 8.6%/21.1%/30.6%, and treatment schedule was altered in 8.6%/17.1/19.8% in the UD/ED/OD groups, resulting in the 5-year overall survival rates of 77.3%/74.3%/77.0%, respectively. The incidences of grade > 3 anemia, thrombocytopenia, diarrhea, stomatitis, and anorexia were significantly higher in the OD group than in the ED and UD groups.

Dose optimization using an S-1 dosage formula can potentially reduce grade ≥ 3 adverse events for overdosed patients.