Prolonging the Half-Life of Hemostatic Factors via the Neonatal Fc Receptor
Authors: Peters Robert 
Published: October 2019

Abstract
Hemophilia A and B are bleeding disorders characterized by a deficiency of functional clotting factor VIII (FVIII) or factor IX (FIX), respectively. Hemophilia is treated with replacement therapy, and regular prophylaxis to maintain factor levels at one percent or less. These treatments are associated with improved clinical outcomes, such as reduced number and severity of bleeding episodes and preservation of joint health. However, due to the relatively short half-lives of the clotting factors, they require frequent intravenous injection to maintain prophylaxis, typically three to four times per week for FVIII and two to three times per week for FIX. In addition, hemophilia patients who develop neutralizing antibodies against FVIII or FIX are often treated with recombinant activated FVII (rFVIIa) bypass therapy. However, the half-life of FVIIa is shorter than that of FVIII or FIX, requiring multiple injections as frequently as every two hours to control a bleed, and consequently rFVIIa is not generally used for prophylaxis. Long lasting hemostatic factors that can maintain prophylaxis with reduced frequency of injections, and potentially reduce the number of infusions needed to control a bleed on demand, would be useful to increase the use of prophylaxis and improve treatment outcomes. A number of technologies have been applied to FVIII, FIX, and FVIIa to prolong their half-lives. This includes “monomeric” Fc fusion technology, in which a single clotting factor is recombinantly linked to a dimeric Fc region, which has demonstrated advantages over the traditional dimeric fusion configuration and has been tested through phase III trials for FVIII (rFVIIIFc) and FIX (rFIXFc). Another fusion technology utilizes albumin, which has been successfully applied to both FIX (rIX-FP) and FVIIa (rFVIIa-FP), which are in phase II/III clinical studies or have completed phase I studies, respectively. Both monomeric Fc fusion and albumin fusion utilize the same naturally occurring pathway in order to improve half-life, through binding to the neonatal Fc receptor (FcRn), which, contrary to what the name suggests, is expressed in most tissues in mammals throughout life. FcRn has been shown to be responsible for the long plasma half-lives of both IgG and albumin; upon binding these proteins within endosomal vesicles, FcRn diverts these fusion proteins away from a lysosomal degradation pathway and returns them to circulation, thus prolonging their half-life. The interaction between FcRn and the Fc region of IgG have been extensively characterized at the molecular level, which has elucidated an elegant mechanism for pH-dependent binding involving several histidine residues that results in binding within the slightly acidified endosome at approximately pH 6.0 and release at physiological pH 7.5. Recent studies have implicated a distinct site on FcRn for albumin binding, but also involving several histidine residues leading to a similar pH-dependence. Based on the Fc:FcRn interaction data, several groups have generated mutants that increase the affinity of Fc variants for FcRn while maintaining pH-dependence, and demonstrated this can further extend the half-life of IgG antibodies. However, these variants do not have this effect in the context of all antibodies, and data will be presented that demonstrate this lack of further half-life prolongation in the context of rFIXFc and rFVIIaFc. In all cases, the fusion of a clotting factor to Fc or albumin prolonged the half-life compared to the native clotting factor, with the magnitude of half-life extension depending on the specific protein, but not approaching that of IgG. Binding and structural studies have indicated that these differences are not due to steric hindrance of the Fc:FcRn interaction in an Fc fusion protein compared to IgG. Interestingly, studies performed in mice generated by bone marrow transplants between wild-type and FcRn knockout mice have revealed different contributions by somatic and hematopoietic cell-derived FcRn towards the half-life prolongation for IgG, rFVIIIFc, and rFIXFc. Biodistribution studies with rFVIIIFc have found similar tissue distribution to rFVIII, indicating that the clotting factor portion predominantly influences the tissue distribution and raising the possibility that FcRn is salvaging the Fc fusion proteins and IgG from different clearance pathways, which could account for the half-life differences between the specific clotting factor fusions and IgG.

Disclosures:
Peters: Biogen Idec: Employment, Equity Ownership.