Prospective evaluation and refinement of an S-1 dosage formula based on renal function for clinical application

In patients with impaired renal function, S-1-related toxicities increase due to higher exposure of 5-fluorouracil (5-FU). Our previous pharmacokinetic study in 16 cancer patients with various renal functions developed an S-1 dosage formula based on individual creatinine clearance (CLcr) and body surface area (BSA). To evaluate and refine the formula, this prospective study was conducted. Thirty-three patients with various renal functions received S-1 for 4 weeks at doses determined by the nomogram derived from the previously developed formula. A series of blood samples were collected after the first dose to calculate the area under the concentration-time curve (AUC) of 5-FU. Thirty patients with BSA of 1.14-1.84 m² and CLcr of 23.8-96.4 mL/min were assessable for pharmacokinetics. The observed daily AUC ranged from 712.6 to 2868.7 ng·h/mL, and 18 patients achieved the target AUC (1447.8 ± 545.4 ng·h/mL). Three patients experienced S-1-related grade 3 adverse events during the first course. In the population pharmacokinetic analysis from the combined data of 46 patients in this study and the previous study, sex was identified as a statistically significant covariate for 5-FU clearance. Hence, the refined formula includes sex as an additional factor: Recommended daily dose = target AUC × (14.5 + 8.23 × SEX [0 for female and 1 for male] + 0.301 × CLcr) × BSA. Revised nomograms for recommended daily doses derived from the refined formula can be used in clinical practice to achieve the target AUC ensuring efficacy and safety of S-1.