Life in the shadow of a dominant partner: the FVIII-VWF association and its clinical implications for hemophilia A

A novel role for the macrophage galactose-type lectin receptor in mediating von Willebrand factor clearance

Previous studies have shown that loss of terminal sialic acid causes enhanced von Willebrand factor (VWF) clearance through the Ashwell-Morrell receptor (AMR). In this study, we investigated (1) the specific importance of N- vs O-linked sialic acid in protecting against VWF clearance and (2) whether additional receptors contribute to the reduced half-life of hyposialylated VWF. α2-3-linked sialic acid accounts for <20% of total sialic acid and is predominantly expressed on VWF O-glycans. Nevertheless, specific digestion with α2-3 neuraminidase (α2-3Neu-VWF) was sufficient to cause markedly enhanced VWF clearance. Interestingly, in vivo clearance experiments in dual VWF^-/-/Asgr1^-/- mice demonstrated enhanced clearance of α2-3Neu-VWF even in the absence of the AMR. The macrophage galactose-type lectin (MGL) is a C-type lectin that binds to glycoproteins expressing terminal N-acetylgalactosamine or galactose residues. Importantly, the markedly enhanced clearance of hyposialylated VWF in VWF^-/-/Asgr1^-/- mice was significantly attenuated in the presence of an anti-MGL inhibitory antibody. Furthermore, dose-dependent binding of human VWF to purified recombinant human MGL was confirmed using surface plasmon resonance. Additionally, plasma VWF:Ag levels were significantly elevated in MGL1^-/- mice compared with controls. Collectively, these findings identify MGL as a novel macrophage receptor for VWF that significantly contributes to the clearance of both wild-type and hyposialylated VWF.

Lonoctocog alfa (rVIII-SingleChain) for the treatment of haemophilia A

INTRODUCTION

The administration of factor VIII (FVIII) concentrates on-demand or on long-term prophylaxis is the effective and safe standard of care of patients with hemophilia A (HA). Development of neutralizing antibodies against exogenous FVIII and the short half-life of the current available products remain major challenges. There is currently a great interest towards newer FVIII products with the goal of reducing the inhibitor risk and increasing the half-life. Area covered: In this review, the authors describe the efficacy and safety of rVIII-SingleChain (Lonoctocog alfa), the first and only single chain recombinant FVIII (rFVIII) molecule developed for the prevention and treatment of bleeding episodes in HA patients. The pre-clinical and clinical studies of rVIII-SingleChain as well as the results of the AFFINITY trial program in previously treated patients both adults and pediatric are presented and discussed. Expert opinion: The results from PTP studies document the efficacy and safety profile of the rVIII-SingleChain. However, even if rFVIII-SingleChain presents advantageous pharmacokinetic features compared to conventional rFVIII, it should not be considered as an EHL-FVIII while its immunogenicity is currently being studied in PUPs. The slightly better PK profile of rFVIII-SingleChain could however allow a small number of selected patients to be treated with a less intensive regimen.

Solid Phase Stepwise Synthesis of Polyethylene Glycols

Polyethylene glycol (PEG) and derivatives with eight and twelve ethylene glycol units were synthesized by stepwise addition of tetraethylene glycol monomers on a polystyrene solid support. The monomer contains a tosyl group at one end and a dimethoxytrityl group at the other. The Wang resin, which contains the 4-benzyloxy benzyl alcohol function, was used as the support. The synthetic cycle consists of deprotonation, Williamson ether formation (coupling), and detritylation. Cleavage of PEGs from solid support was achieved with trifluoroacetic acid. The synthesis including monomer synthesis was entirely chromatography-free. PEG products including those with different functionalities at the two termini were obtained in high yields. The products were analyzed with ESI and MALDI-TOF MS and were found close to monodispersity.

Emicizumab, a bispecific antibody recognizing coagulation factors IX and X: how does it actually compare to factor VIII?

During the last decade, the development of improved and novel approaches for the treatment of hemophilia A has expanded tremendously. These approaches include factor VIII (FVIII) with extended half-life (eg, FVIII-Fc and PEGylated FVIII), monoclonal antibodies targeting tissue factor pathway inhibitor, small interfering RNA to reduce antithrombin expression and the bispecific antibody ACE910/emicizumab. Emicizumab is a bispecific antibody recognizing both the enzyme factor IXa and the substrate factor X. By simultaneously binding enzyme and substrate, emicizumab mimics some part of the function exerted by the original cofactor, FVIII, in that it promotes colocalization of the enzyme-substrate complex. However, FVIII and the bispecific antibody are fundamentally different proteins and subject to different modes of regulation. Here, we will provide an overview of the similarities and dissimilarities between FVIII and emicizumab from a biochemical and mechanistical perspective. Such insight might be useful in the clinical decision making for those who apply emicizumab in their practice now or in the future, particularly in view of the thrombotic complications that have been reported when emicizumab is used in combination with FVIII-bypassing agents.

Novel approaches to hemophilia therapy: successes and challenges

New therapies for hemophilia A and hemophilia B will likely continue to change clinical practice. Ranging from extended half-life to nonfactor products and gene therapy, these innovative approaches have the potential to enhance the standard of care by decreasing infusion frequency to increase compliance, promoting prophylaxis, offering alternatives to inhibitor patients, and easing route of administration. Each category has intrinsic challenges that may limit the broader application of these promising therapies. To date, none specifically address the challenge of dispersing treatment to the developing world.

Lonoctocog Alfa: A Review in Haemophilia A

Lonoctocog alfa (rVIII-SingleChain; Afstyla^®) is a novel single-chain recombinant factor VIII (FVIII) molecule, with a truncated B-domain and the heavy and light chains covalently linked to form a stable and homogenous drug that binds with high affinity to von Willebrand factor (VWF). Intravenous lonoctocog alfa is approved for the prophylaxis and treatment of bleeding in patients with haemophilia A in several countries worldwide. In two pivotal, multicentre trials, lonoctocog alfa was effective in the treatment of bleeding episodes and as prophylaxis, including for perioperative management in adults, adolescents and children. In terms of haemostatic efficacy in controlling bleeding episodes, overall treatment and investigator-assessed success rates were high across all age groups, with the majority of these bleeds controlled with a single injection of lonoctocog alfa. Low median spontaneous, overall and traumatic annualized bleeding rates were evident with prophylactic lonoctocog alfa regimens in both trials. Lonoctocog alfa was generally well-tolerated, with very low rates of injection-site reactions. No previously treated patient experienced an anaphylactic reaction or developed an inhibitor. In conclusion, lonoctocog alfa is an effective and generally well-tolerated alternative to conventional FVIII products for the treatment and prophylaxis of bleeding, including in the surgical setting, in adults, adolescents and children with haemophilia A.

The impact of von Willebrand factor on factor VIII memory immune responses

Immune tolerance induction (ITI) with aggressive infusion of factor VIII (FVIII) is the current strategy used to eradicate FVIII inhibitors and restore normal FVIII pharmacokinetics in inhibitor patients. Whether the use of FVIII products containing von Willebrand factor (VWF) will affect the efficacy of ITI is still controversial. In this study, we explored the impact of VWF on FVIII memory immune responses in hemophilia A (HA) mice. A T-cell proliferation assay and cytokine profile analysis were used to study FVIII-primed CD4⁺ T cells. When CD4⁺ T cells from primed FVIII^null mice were restimulated with recombinant human FVIII (rhF8) plus recombinant human VWF (rhVWF) in vitro, the percentages of daughter CD4⁺ T cells were significantly decreased compared with the groups cultured with rhF8 only. Levels of interferon-γ and interleukin 10 were significantly lower in the rhF8 plus rhVWF groups than in the rhF8 groups. When memory B-cell pools from primed FVIII^null mice were cultured with rhF8 with or without rhVWF to induce differentiation of memory B cells into antibody-secreting cells (ASCs), the number of ASCs was significantly lower in the rhF8 plus VWF group than in the rhF8 group. When memory B-cell pools were transferred into NSGF8KO mice followed by rhF8 immunization with or without rhVWF, the titers of anti-F8 inhibitors and total immunoglobulin G were significantly higher in the rhF8 group than in the rhF8 plus rhVWF group, with an average difference of 2.23- and 2.04-fold. Together, our data demonstrate that VWF attenuates FVIII memory immune responses in HA mice.

Many factor VIII products available in the treatment of hemophilia A: an embarrassment of riches?

Hemophilia A (HA) is a common bleeding disorder caused by the deficiency of factor VIII (FVIII) with an incidence of ~1 in 5000 male births. Replacement of FVIII is necessary to prevent and treat bleeding episodes. However, with multiple new drugs in addition to old standards, choosing among the different FVIII treatment options is harder than ever. There are FVIII products that are plasma derived or recombinant, FVIII products designed to extend the half-life of FVIII, and the first single-chain FVIII product, recombinant factor VIII single chain (rFVIII-SC). As development of inhibitors to FVIII continues to be a major problem in the care of HA patients, recent studies showing lower rates of inhibitor development with plasma-derived FVIIII products versus recombinant FVIII products have made choosing among the many options now available even more complex. Although still unproven, extended half-life (EHL) products may provide the hope of decreased immunogenicity but need further testing in previously untreated patients (PUPs). This review highlights some of the differences between FVIII products currently available and hopefully assists the clinician to decide which FVIII product to choose for their patients.