To cite this article: Collins PW, Blanchette VS, Fischer K, Bjö rkman S, Oh M, Fritsch S, Schroth P, Spotts G, Astermark J, Ewenstein B, on behalf of the rAHF-PFM study group. Break-through bleeding in relation to predicted factor VIII levels in patients receiving prophylactic treatment for severe hemophilia A. J Thromb Haemost 2009; 7: 413-20.Summary. Background: The role of prophylactic factor VIII (FVIII) to decrease hemophilic bleeding and arthropathy is well established. The rationale for this strategy is to convert patients with severe hemophilia A to a moderate clinical phenotype by reducing time spent with a FVIII level <1 IU dL . Results: The data demonstrate that increasing time with a FVIII below 1 IU dL )1 is associated with increased total bleeds and hemarthroses. Lack of adherence to the intended frequency of FVIII infusion was the most important determinant of low FVIII and increased bleeding. In children aged 1-6 years, the rate of bleeding was also influenced by FVIII half-life and clearance. Conclusions: These data have important implications for the management of patients with severe hemophilia.

Comparison of the pharmacokinetics (PK) of a coagulation factor between groups of patients can be biased by differences in study protocols, in particular between blood sampling schedules. This could affect clinical dose tailoring, especially in children. The aim of this study was to describe the relationships of the PK of factor VIII (FVIII) with age and body weight by a population PK model. The potential to reduce blood sampling was also explored. A model was built for FVIII PK from 236 infusions of recombinant FVIII in 152 patients (1-65 years of age) with severe hemophilia A. The PK of FVIII over the entire age range was well described by a 2-compartment model and a previously reported problem, resulting from differences in blood sampling, to compare findings from children and adults was practically abolished. The decline in FVIII clearance and increase in half-life with age could be described as continuous functions. Retrospective reduction of blood sampling from 11 to 5 samples made no important difference to the estimates of PK parameters. The obtained findings can be used as a basis for PKbased dose tailoring of FVIII in clinical practice, in all age groups, with minimal blood sampling. (Blood. 2012;119(2): 612-618)

Summary. The pharmacokinetic (PK) response to factor VIII (FVIII) and factor IX varies between patients and this has important clinical implications for treatment. Although PK is affected by patient characteristics, this relationship is too weak to infer a result for an individual and, if required, PK must be measured. An important determinant of the efficacy of prophylaxis is the length of time an individual spends with a low level of coagulation factor. This time is more dependent on the patientÕs coagulation factor half-life and the frequency of dosing than in vivo recovery and dose infused. Improved understanding of the effect of PK and dose frequency on factor levels in patients on prophylaxis will help tailor regimens to individuals better and allow more cost effective use of coagulation factor concentrates. Calculations suggest that adults need less FVIII per kg body weight than children. The effect of half-life on trough levels questions the logic of Monday, Wednesday, Friday dosing and suggests a role for innovative regimens including low-dose daily treatment which leads to either higher trough levels or decreased FVIII requirement. This may expand access to prophylaxis in healthcare systems with limited resources and potentially improve patient outcomes. The ideal trough level will vary between individuals and at different times of their lives and may be <1 IU dL )1. If PK is to be used in routine clinical practice, a simplified method for its measurement is required and this methodology is becoming available.

To cite this article: Collins PW, Bjö rkman S, Fischer K, Blanchette V, Oh M, Schroth P, Fritsch S, Casey K, Spotts G, Ewenstein BM. Factor VIII requirement to maintain a target plasma level in the prophylactic treatment of severe hemophilia A: influences of variance in pharmacokinetics and treatment regimens. J Thromb Haemost 2010; 8: 269-75.Summary. Background: Prophylactic factor (F)VIII has been shown to reduce bleeds and arthropathy in patients with severe hemophilia A. Objectives: Assuming that the trough FVIII level is an important determinant of the efficacy of prophylaxis, this paper addresses the effect of the inter-patient variability in pharmacokinetics and different dosing regimens on trough levels. Methods: Simulations used FVIII half-lives and in vivo recoveries (IVR), observed during clinical trials with Advate [Antihemophilic Factor (Recombinant), Plasma/Albumin-Free Method], and commonly used prophylactic regimens to calculate their effect on FVIII levels during prophylaxis. Results and conclusions: Half-life and dose frequency had a larger effect on trough FVIII and time per week with FVIII < 1 IU dL )1 than IVR and infused dose per kg. The combined effect of these parameters resulted in substantial inter-patient variability in the amount of FVIII required to sustain a desired trough level. Prophylactic regimens based on Monday, Wednesday, Friday dosing were less cost effective in maintaining a desired trough level throughout the week. Dose escalation on Friday to cover the weekend would require potentially harmful doses of FVIII in many patients, especially in young children where more than 50% would require a Friday dose of over 100 IU kg )1 and some would require more than 400 IU kg )1. Knowledge of individual patientsÕ half-lives and alteration of frequency of infusions may allow the more cost-effective use of FVIII and potentially expand access to prophylaxis to a greater number of patients, especially in regions where healthcare resources are scarce.

Summary. Participants in an international conference on prophylactic therapy for severe haemophilia developed a consensus summary of the findings and conclusions of the conference. In the consensus, participants agreed upon revised definitions for primary and secondary prophylaxis and also made recommendations concerning the need for an international system of pharmacovigilance. Considerations on starting prophylaxis, monitoring outcomes, and individualizing treatment regimens were discussed. Several research questions were identified as needing further investigation, including when to start and when to stop prophylaxis, optimal dosing and dose interval, and methods for assessment of long-term treatment effects. Such studies should include carefully defined cohorts, validated orthopaedic and quality-of-life assessment instruments, and cost-benefit analyses.