Clinical Implications of Pharmacokinetic Variables in the Management of Patients with Severe Hemophilia A.

The utility of pharmacokinetic (PK) measurements in clinical hemophilia practice remains to be proven. To address this issue data from 152 patients with severe hemophilia A (FVIII <1%) from the ADVATE rAHF-PFM clinical trials were analyzed. Studies involved infusion of 50 IU/Kg rAHF-PFM and FVIII measurement pre-infusion and at 0.25, 0.5, 1, 3, 6, 9, 24, 28, 32 and 48 hrs post-infusion in 100 patients aged ≥12 yrs, and at timepoints 1, 9, 24 and 48 hrs in 52 children ages 1–6 yrs. Data were analyzed by compartmental and non-compartmental methods. In the patients >12 yrs with more than one PK measurement, there was a low within patient variance in respect to between patient variance for clearance, MRT and AUC. This suggests that these parameters are more reliably measured for individual patients. Association of PK parameters with age and observed weight/ideal weight ratio was assessed using regression analysis. Median elimination t½ in patients was significantly shorter in children ages 1–6 yrs versus patients ≥ 12. Approximately 20% of this overall difference was due to fewer sampling timepoints in children but the remainder may reflect true biological differences; t½ increased by 0.41 h·y⁻¹ (CI, 0.08–0.74) in children between 1–6 yrs and remained stable in older subjects. The relationship between PK parameters and age and ideal weight/weight ratio differed according to age [table - signs represent the direction of the change (increase, +; decrease, −); numbers are the associated P values for regression on age and weight/ideal weight for each parameter]. In children ages 1–6 yrs, both t½ and MRT of rAHF-PFM increased significantly with age. In subjects ≥ 12 years old, however, recovery, AUC/dose, rates of clearance and Vss increased significantly with both age and increasing weight/ideal weight. Differences noted in the PK parameters and age and body habitus in the two groups may reflect physical differences such as proportionately larger liver size and weight-adjusted plasma volume in children versus adults but may also reflect other yet undefined age-related biological differences. Initial analysis of bleed event data showed a different rate of bleeding on certain days of the week in children ages 1–6 but not in older subjects. Whether this is due to dosing practices, difference in PK or other factors is currently being investigated. These results have important implications for clinicians managing patients with hemophilia, and treatment regimens tailored to individual PK parameters are more likely to allow a more cost effective use of FVIII. In particular, dosing schedules based on the ratio of actual to ideal weight rather than weight appears to be appropriate. The implications of these PK findings on bleeding events whilst on prophylactic FVIII replacement therapy will be further analysed.