The FDA issued a safety announcement today warning prescribers, pharmacists and patients that differences in dosing regimens between the two oral formulations of the antifungal Noxafil can lead to dosing errors.
Noxafil (posaconazole, Merck) is approved for the treatment of invasive fungal infections caused by Aspergillus and Candida in patients aged 13 years or older with weakened immune systems, according to the FDA. The drug is available in two oral formulations — delayed-release tablets and an oral suspension — as well as an IV solution. The oral suspension, approved in 2006, is also used to treat Candida infections in the mouth or throat.
Figure 1. The outer carton label for the oral suspension of posaconazole.
Figure 2. The outer carton label for the delayed-release tablet formulation of posaconazole.
Since the approval of the delayed-release tablets in November 2013, there were 11 dosing errors reported to the FDA, including one death. In one case report, a pharmacy replaced delayed-release tablets with the oral suspension without changing the dose and frequency of administration, resulting in an underdose. The patient later died from a stroke related to an invasive Aspergillus infection, the FDA said. In the other case reports, several patients experienced adverse reactions such as nausea and vomiting.
The FDA reported that exact substitution (mg for mg) can result in drug levels that are lower or higher than needed to effectively treat infections due to variations in how the drug is absorbed and managed by the body.
“The delayed-release tablet has a higher bioavailability than the oral suspension,” the FDA said. “As a result, the dose and frequency of administration for [posaconazole] depend on the particular formulation used and the indication for use.”
To prevent future dosing errors, the outer cartons and labels for posaconazole were updated to clarify that the two formulations cannot be directly substituted for each other. Instead, a change in dose is required.
The FDA recommends that prescribers specify the dosage form, strength, and frequency of all posaconazole prescriptions, that pharmacists request prescribers to clarify the dosage form, strength, or frequency when it is not specified, and that patients talk to their health care professional before switching formulations.
Patients receiving posaconazole should seek medical care if they develop severe diarrhea or vomiting, notice swelling in an arm or leg, experience shortness of breath, or notice a change in heart rate or heart rhythm.