Objective:
Physicians prescribing Factor VIII for haemophilia A patients are presented with an array of dose and dose frequency in the package insert making it difficult to precisely prescribe. To clarify the treatment outcome across products, a model of the published half-life was used to provide the expected % IU/dl +/- SD at 24, 48, 72, and 96 hours post dose at a normalized dose and at the high end of the recommended dosage (dose and freq.).
Methods:
Eleven rFVIII products recently marketed in the US were assessed using a model of the halflife +/- SD to calculate the expected %IU/dl +/- SD at 24, 48, 72, and 96 hrs post-dose. Terminal half-life data for adults was obtained from each published package insert (PI). The one-stage clotting assay data was used for all except Afstyla© (chromogenic assay only). Variance reported as coefficient of variation or confidence intervals was converted to standard deviation.
The first comparison was made using a standard dose of 50 IU/kg across all products. The second comparison used the maximum recommended dose and frequency for routine prophylaxis defined in the PI. Full data sets and curves of %IU/dl+/-SD under standard doses of 50 IU/kg and under maximum dose and frequency for routine prophylaxis have been generated.
Summary:
Graphing the single dose comparison (50 iu/kg) revealed 3 general clusters and one outlier at 24 hrs post dose. The first cluster had 4 products with a mean range of 21-25 %IU/dl at 24 hrs post dose. The second cluster had 5 products with a range of 30-32 %IU/dl at 24 hrs. The third cluster had 1 product (Eloctate©) with 43% IU/dl at 24 hrs. The outlier (Nuwiq©) showed exceptional variance compared to others and removed from discussion.
Expected %IU/dl was recalculated using the most frequent and highest dose recommended for individual products. Assessing the –SD value: 10 of 10 products achieved > 1% trough at 24 hrs; 8 of 10 at 48 hrs; 2 of 10 at 72 hrs; and 1 of 10 at 96 hrs. Note at 72hrs, only 2 of 10 products achieve minimal trough at –SD level when 4 of 10 claim q3day dosing.
Conclusions:
Initial dosing for routine prophylaxis relies on the mean half-life. The PK parameters are based on only 18 to 30 patients under well-controlled conditions. There is waste on both sides of the PK half-life distribution curve. Would it be reasonable to assure minimal breakthrough bleeds and less over-dosage by initiating treatment with a personal PK profile for each patient in order to identify the correct dose and frequency? This model could be used to find optimal dose and frequency with input from a personalized product half-life.