Less is known about what contributes to variability in the pharmacokinetic handling of FIX. A recent paper suggests that when FIX is infused, much of it goes into the extravascular tissue [21]. In contrast, the amount of FVIII that goes extravascular is negligible. The need for frequent, inconvenient and painful infusions with currently available factor may lead to avoidance or delay in starting prophylaxis or, if a patient is already on prophylaxis, to missed doses, which immediately puts them at risk of bleeding. Many studies have shown that adherence to prophylaxis is far from ideal [22-24].

All of these issues are worse in find more very young children where peripheral venous access is, in the best of cases, difficult and Linsitinib in the worst, impossible. The need for frequent infusions with currently available concentrates also leads to a substantial need for central venous access devices (CVADs; mainly port-a-caths). One study showed that 82% of children ≤5 years of age with severe haemophilia A on full-dose prophylaxis required a CVAD [25]. CVADs, although tremendously helpful, are associated with a substantial rate of mechanical failure, infections and thrombosis [26]. As such, many clinicians and

investigators have adopted escalating-dose prophylaxis in which young children are commenced on once weekly infusions, escalated to twice weekly infusions and eventually (in the case of severe haemophilia A), to every other day or full-dose prophylaxis. One approach escalates all patients regardless of whether they are bleeding, while an alternative approach tailors prophylaxis to bleeding

and only escalates those patients experiencing unacceptable bleeding [25, 27]. Tailoring prophylaxis is crotamiton predicated on the observation that bleeding frequency varies significantly among patients with severe haemophilia A [28, 29]. Both approaches allow patients and families time to psychologically accept peripheral venipunctures and have been demonstrated to reduce the number of CVADs required. With these approaches, recent experience suggests that about 30% of young children with severe haemophilia A need CVADs (personal communication, H.M. Van den Berg). Due to the high cost of factor concentrates and the fact that until now, prophylaxis had to be administered very frequently, prophylaxis remains very expensive – prohibitively expensive for most of the world. Lower dose/lower frequency prophylaxis regimens have shown substantial decreases in bleeding frequency while using much less factor than in full-dose prophylaxis [30]. The short half-life of currently licensed factor concentrates creates a great need and a great opportunity for biologically engineered longer acting factor concentrates. These products might address some of the main limitations of current concentrates and lead to improved adherence to prophylaxis. Several methodologies are currently being used to extend the half-life of factor.