Number of Enhertu Vials Per Infusion Session
The number of vials required per Enhertu infusion session depends on the patient's weight and the prescribed dose, with most patients requiring multiple vials since each vial contains only 100 mg and the standard dose is 5.4 mg/kg every 3 weeks.
Dosing and Vial Calculation
Standard dosing for Enhertu is 5.4 mg/kg administered intravenously every 21 days (3 weeks) 1. Each single-dose vial contains 100 mg of fam-trastuzumab deruxtecan-nxki 1.
Calculation Algorithm:
- Determine patient weight in kilograms
- Calculate total dose needed: Weight (kg) × 5.4 mg/kg
- Calculate number of vials: Total dose (mg) ÷ 100 mg per vial, rounded up to nearest whole number
Practical Examples:
- 60 kg patient: 60 × 5.4 = 324 mg → 4 vials needed
- 70 kg patient: 70 × 5.4 = 378 mg → 4 vials needed
- 80 kg patient: 80 × 5.4 = 432 mg → 5 vials needed
- 84 kg patient (average used in cost calculations): 84 × 5.4 = 453.6 mg → 5 vials needed 2
- 90 kg patient: 90 × 5.4 = 486 mg → 5 vials needed
- 100 kg patient: 100 × 5.4 = 540 mg → 6 vials needed
Reconstitution Details
Each 100 mg vial is reconstituted with 5 mL of Sterile Water for Injection to achieve a final concentration of 20 mg/mL 1. More than one vial is typically needed for a full dose 1.
Important Preparation Considerations
- Reconstituted vials are stable for up to 24 hours refrigerated (2-8°C) per FDA labeling 1
- Recent stability data suggests reconstituted T-DXd maintains physical, chemical, and affinity stability for up to 4 weeks when stored at 2-8°C 3, though this extended stability is not yet reflected in official labeling
- Any unused portion after 24 hours should be discarded per current FDA guidelines 1
- The maximum time from reconstitution through end of administration should not exceed 24 hours 1
Cost Implications
Given that most patients require 4-6 vials per infusion and each vial costs approximately $2,460 (based on Medicare reimbursement of $24.60/mg × 100 mg), drug cost alone ranges from $9,840 to $14,760 per cycle, with the average cost around $11,158.56 for an 84 kg patient 2. This does not include administration costs or facility charges.
The single-dose vial design with mandatory 24-hour discard creates significant waste, as partial vials cannot be saved between patients under current labeling 1, 3.