What is the dosing schedule, body‑surface‑area‑based dosage, and monitoring recommendations for the oral S‑1 (tegafur/gimeracil/oteracil) chemotherapy regimen?

S-1 (Tegafur/Gimeracil/Oteracil) Chemotherapy Regimen

S-1 should be dosed at 80-120 mg/day orally based on body surface area (BSA), administered for 28 consecutive days followed by a 14-day rest period, repeated every 6 weeks for 1 year in the adjuvant gastric cancer setting. 1, 2

Body Surface Area-Based Dosing

The standard dosing of S-1 is determined by BSA as follows:

• BSA <1.25 m²: 80 mg/day 1, 2
• BSA 1.25-1.5 m²: 100 mg/day 1, 2
• BSA >1.5 m²: 120 mg/day 1, 2

The total daily dose should be divided into two administrations (morning and evening) taken orally. 1

Treatment Schedule

Standard Regimen (Adjuvant Gastric Cancer)

• Administration: Days 1-28 of each cycle 1, 2
• Rest period: Days 29-42 (14 days) 1, 2
• Cycle length: 6 weeks (42 days) 1, 2
• Total duration: 1 year (8 cycles) 1, 2
• Initiation timing: Within 6 weeks after curative gastrectomy with D2 lymphadenectomy 2

Alternative Schedules

Alternate-day dosing has shown improved compliance with fewer adverse effects: 80-120 mg/day (based on BSA) administered on alternate days for 15 months, which demonstrated a 91.8% completion rate versus 72.2% with standard daily dosing. 3

Combination Regimens

S-1 plus cisplatin for advanced/metastatic gastric cancer:

• S-1: 80-120 mg/day (BSA-adjusted) for 28 days, followed by 14-day rest 4
• Cisplatin: 10 mg/m² twice weekly during S-1 administration 4
• This low-dose cisplatin regimen achieved a 47.1% response rate with median survival of 11.0 months 4

S-1 plus nab-paclitaxel for biliary tract carcinoma:

• S-1: 80-120 mg/day on days 1-14 5
• Nab-paclitaxel: 125 mg/m² on days 1 and 8 5
• 21-day cycle 5

Dose Modifications

Renal function is critical for S-1 dosing. Creatinine clearance <66 mL/min is a significant risk factor for treatment discontinuation and requires dose reduction. 6 Initial overdosing based on inadequate renal function assessment is associated with higher discontinuation rates. 6

Common toxicities requiring dose adjustment:

• Grade 3/4 anorexia (6.0%) 2
• Grade 3/4 nausea (3.7%) 2
• Grade 3/4 diarrhea (3.1%) 2
• Neutropenia and stomatitis in combination regimens 5

Dose and schedule modifications should be made at the clinician's discretion based on toxicity, with particular attention to early management of nausea, which is a significant predictor of discontinuation. 6

Monitoring Recommendations

Pre-treatment Assessment

• Renal function: Measure creatinine clearance to avoid initial overdose 6
• Body surface area: Calculate accurately for proper dose selection 1, 2
• Performance status: Ensure adequate PS for treatment tolerance 1
• Hepatitis B screening: Required before initiation 1

During Treatment

• Clinical symptoms and laboratory monitoring: Assess before each cycle 1
• Imaging studies: CT, endoscopy, or contrast radiography to evaluate response 1
• Toxicity assessment: Monitor for gastrointestinal, hematologic, and dermatologic adverse events 2, 5
• Early nausea management: Proactive treatment of nausea is essential to prevent discontinuation 6

Response Evaluation

Response should be evaluated using Japanese Classification of Gastric Carcinoma criteria or RECIST criteria, with continuation decisions based on both efficacy and toxicity profile. 1

Special Populations

Stage II/III gastric cancer after D2 dissection: S-1 monotherapy for 1 year is the established standard, with 3-year overall survival of 80.1% versus 70.1% for surgery alone (HR 0.68, P=0.003). 2

European patients: May require lower starting doses (35 mg/m² twice daily) compared to Asian patients due to higher toxicity rates, though this achieves similar response rates of 26-32%. 7

Patients with stage I disease: Have higher discontinuation rates and may benefit from closer monitoring or alternative schedules. 6