Check your answers on page 86
Caspofungin is an echinocandin antifungal which can be used to treat invasive fungal infections caused by organisms such as Aspergillus spp, including cultures that exhibit resistance to triazole antifungals.
Caspofungin is one of a relatively small number of drugs for which dosing is based on the derived value of body surface area rather than directly measured parameters such as weight. Several physiological variables responsible for drug deposition can be normalised for by using body surface area for initial dosing, which can reduce the incidence of toxicity and adverse drug reactions for drugs dependent on them.
The following calculation considers the use of caspofungin to treat invasive aspergillosis in a 15-month-old child weighing 9.98 kg with a length of 77.7 cm. To calculate the body surface area, calculate the product of the weight (in kg) and the height (in cm), divide the result of this by 3600 and then calculate the square root of that value (Du Bois and Du Bois, 1989). Body surface area (m2)=height (cm)×weight (kg)3600
QUESTION 1
What is the body surface area for this child?
Doses of caspofungin usually involve a high first dose, often followed by a lower maintenance dose. For children of our patient's age, dosing from the British National Formulary for Children (BNFC) is as follows: ‘Loading dose 70 mg/m2 once daily (max. per dose 70 mg) for 1 day, then maintenance 50 mg/m2 once daily (max. per dose 70 mg); increased if necessary to 70 mg/m2 once daily (max. per dose 70 mg)’ (BNFCa, 2023).
QUESTION 2
Calculate the loading and maintenance doses for this child.
Reconstitution of a 50 mg vial, according to the directions of the Summary of Product Characteristics, will produce a solution containing 5.2 mg/ml (Flynn Pharma Ltd, 2022).
QUESTION 3
What volume of this solution will need to be administered to provide the loading dose?
QUESTION 4
The current drug tariff price for a 50 mg vial of caspofungin is £238.87. Assuming none of a vial can be used to treat additional patients, what is the difference in cost of wasted drug for each maintenance dose compared to the loading dose?