Medication Safety
Institute for Safe Medication Practice, Horsham, PA
Voxzogo (vosoritide) is a C-type natriuretic peptide indicated to increase linear growth in pediatric patients with achondroplasia with open epiphyzes. A prescriber ordered and a pharmacist dispensed the wrong kit, confusing doses and vial sizes.
The product is packaged in kits that include 10 single-dose vials of medication containing either 0.4 mg, 0.56 mg, or 1.2 mg of lyophilized vosoritide powder and 10 prefilled sterile water for injection diluent syringes containing either 0.5 mL, 0.7 mL, or 0.6 mL, respectively. For example, the Voxzogo 0.4 mg kit contains 10 of the 0.4 mg vials of drug and 0.5 mL prefilled diluent syringes.
After reconstitution, the drug concentrations are 0.4 mg/0.5 mL for the 0.4 mg vial, 0.56 mg/0.7 mL for the 0.56 mg vials, and 1.2 mg/0.6 mL for the 1.2 mg vial. However, the full vial volume after reconstitution cannot be withdrawn (e.g., the manufacturer says that the nominal deliverable volume from the 0.4 mg vial is only 0.4 mL, not 0.5 mL).
The recommended daily S.C. dose of Voxzogo is based on the patient’s actual body weight. The manufacturer provides a dosing table in the prescribing information to help prescribers and pharmacists select the proper dose, dose volume, and vial size for dispensing.
For example, a patient weighing 22 kg to 32 kg should receive 0.4 mg of vosoritide daily. According to the dosing table, this requires administration of 0.5 mL of reconstituted drug from a 0.56 mg vial. Similarly, a patient weighing 33 kg to 43 kg should receive 0.5 mg of vosoritide daily, which requires the caregiver to withdraw and administer 0.25 mL of reconstituted drug from a 1.2 mg vial (not the 0.56 mg vial).
A pharmacist recently reported that a prescriber ordered and a pharmacist dispensed the wrong kit (i.e., the 0.4 mg kit) for a patient who required a 0.4 mg dose. They became confused about the doses and associated vial sizes. They assumed the 0.4 mg vials would yield a 0.4 mg dose, similar to other injectable products.
However, as mentioned above, a 0.4 mg dose requires use of the 0.56 mg Voxzogo kit; the complete vial contents cannot be withdrawn from the 0.4 mg vial to provide the 0.4 mg dose.
The error was discovered when the caregiver tried to draw up 0.5 mL of reconstituted drug for the 0.4 mg dose but could only withdraw about 0.45 mL into the syringe. (Note: Once they have received training from a health care practitioner, caregivers may prepare and administer doses in a home setting.)
Safe practice recommendations
To prevent errors, there are several options to consider.
- Create order sentences within the EHR to guide prescribers to select the appropriate dose based on the patient’s actual body weight.
- Automatically link the appropriate Voxzogo kit to the corresponding order sentence.
- Pharmacies may consider adding a note to the drug file and to the pharmacy shelf that directs pharmacy staff to double-check that the vial size selected is appropriate for the prescribed dose.
- Alert clinic, medical office, and pharmacy staff to the potential for confusion with the kit sizes and corresponding doses and provide education that the manufacturer-supplied dosing table must be used during prescribing and dispensing. Relay this information to caregivers who will administer the drug at home.
- Pharmacists should teach caregivers using the “teach-back” method, which incorporates a return demonstration by the caregivers to confirm their ability to prepare and administer the correct dose. ■