What is the recommended intravenous fosaprepitant dosing schedule for chemotherapy antiemesis, including timing relative to chemotherapy, subsequent oral dosing, dexamethasone dose adjustment, and pediatric weight‑based dosing?

Intravenous Fosaprepitant Dosing for Chemotherapy Antiemesis

For adults receiving highly or moderately emetogenic chemotherapy, administer fosaprepitant 150 mg IV as a single dose on Day 1 over 20-30 minutes, completing the infusion approximately 30 minutes before chemotherapy, with no subsequent oral aprepitant needed. 1

Adult Dosing Regimens

Single-Dose Fosaprepitant Regimen (Preferred for Simplicity)

Day 1:

• Fosaprepitant 150 mg IV over 20-30 minutes, 30 minutes before chemotherapy 2, 1
• Dexamethasone 12 mg PO or IV (reduced from standard 20 mg due to CYP3A4 interaction) 2, 3
• 5-HT3 antagonist (palonosetron 0.25 mg IV preferred) 2, 3

Days 2-4 (for highly emetogenic chemotherapy):

• Dexamethasone 8 mg PO once daily on Day 2 2
• Dexamethasone 8 mg PO twice daily on Days 3-4 2

This single 150 mg dose was proven noninferior to the standard 3-day oral aprepitant regimen in a large randomized trial 2, 4.

Alternative: Lower-Dose Fosaprepitant with Oral Aprepitant Follow-up

Day 1:

• Fosaprepitant 115 mg IV over 20-30 minutes, 30 minutes before chemotherapy 5, 2
• Dexamethasone 12 mg PO or IV 5
• 5-HT3 antagonist 5

Days 2-3:

• Aprepitant 80 mg PO daily 2, 5
• Dexamethasone 8 mg PO daily 2

Critical Dexamethasone Dose Adjustments

The dexamethasone dose must be reduced by 50% when co-administered with fosaprepitant or aprepitant due to CYP3A4 inhibition. 2, 3, 1 This is a common pitfall—failure to reduce dexamethasone can lead to excessive corticosteroid exposure.

• With fosaprepitant 150 mg: Use dexamethasone 12 mg on Day 1 (instead of 20 mg), then 8 mg on Day 2, then 8 mg twice daily on Days 3-4 2
• With fosaprepitant 115 mg + oral aprepitant: Use dexamethasone 12 mg on Day 1, then 8 mg daily on Days 2-4 2, 3

The higher dexamethasone dosing on Days 3-4 (8 mg twice daily) with the 150 mg fosaprepitant regimen compensates for the waning NK1 antagonist effect while maintaining adequate delayed-phase protection 2.

Pediatric Weight-Based Dosing (≥6 months, ≥6 kg)

Single-Dose Regimen (for single-day chemotherapy)

Administer via central venous catheter:

• Age 12-17 years: Fosaprepitant 150 mg IV over 30 minutes 1
• Age 2 to <12 years: Fosaprepitant 4 mg/kg IV (max 150 mg) over 60 minutes 1, 6
• Age 6 months to <2 years: Fosaprepitant 5 mg/kg IV (max 150 mg) over 60 minutes 1

Dexamethasone: Administer 50% of the recommended corticosteroid dose on Days 1-2 due to drug interaction 1

Three-Day Regimen (for multi-day chemotherapy)

Day 1:

• Age 12-17 years: Fosaprepitant 115 mg IV over 30 minutes 1
• Age 6 months to <12 years: Fosaprepitant 3 mg/kg IV (max 115 mg) over 60 minutes 1

Days 2-3:

• Age 12-17 years: Aprepitant 80 mg PO daily 1
• Age 6 months to <12 years: Aprepitant 2 mg/kg PO (max 80 mg) daily (oral suspension) 1

Dexamethasone: Administer 50% of recommended dose on Days 1-4 1

Key Clinical Considerations

Timing and Administration

• Complete the fosaprepitant infusion approximately 30 minutes before chemotherapy initiation to ensure adequate NK1 receptor blockade 1
• The 150 mg dose provides coverage throughout the entire 0-120 hour risk period without need for additional NK1 antagonist doses 2, 4

Infusion Site Precautions

Avoid infusion into small peripheral veins—fosaprepitant carries risk of infusion site reactions including thrombophlebitis, necrosis, and vasculitis, particularly in patients receiving vesicant chemotherapy 1. Use central access when available in pediatric patients 1.

Drug Interactions

Fosaprepitant/aprepitant is a moderate CYP3A4 inhibitor and inducer, affecting multiple medications 2:

• Warfarin: Monitor INR closely for 2 weeks, particularly Days 7-10 post-fosaprepitant 1
• Hormonal contraceptives: Efficacy reduced during and for 28 days after fosaprepitant; use backup contraception 1
• Contraindicated with: Pimozide, terfenadine, astemizole, cisapride (risk of serious cardiac events) 2, 1

Comparative Efficacy

The single 150 mg fosaprepitant dose demonstrated noninferiority to 3-day oral aprepitant in a large phase III trial (N=2,247), with complete response rates of approximately 89% (acute phase) and 75% (delayed phase) for cisplatin-based chemotherapy 4. Pediatric studies show fosaprepitant significantly reduces vomiting compared to 5-HT3 antagonists alone, with complete response rates of 75-95% in the acute phase 6, 7, 8.