The sweet business of VWF clearance Free

In this issue of Blood, Byrne et al¹ report key evidence for a longer half-life of a recombinant protein drug due to an alteration in its oligosaccharide structures. Switching the end sugar structure from mostly α2-6–linked sialic acids to α2-3–linked ones reduces the interaction of von Willebrand factor (VWF) with clearance receptors and increases its half-life in circulation.

VWF is a multimeric glycoprotein that is essential in primary hemostasis. It is mostly produced from endothelial cells as ultralarge multimers. Upon entering the circulation, it is cleaved by ADAMTS13 into a smaller fragment that does not spontaneously bind to platelets but retains the ability to bind on activation. When circulating VWF encounters an injured vessel site, it binds to the exposed subendothelial collagen and becomes activated under blood flow.² Thereby, it can recruit platelets to the injury site to initiate hemostasis. On the other hand, too much or too active VWF in circulation increases the risk of microvascular thrombosis and potentially other thrombotic complications. Thus, it is important to maintain circulating VWF within an appropriate “just right” range. von Willebrand disease (VWD) is an autosomal disease with a prevalence of around 1% and is characterized by mucocutaneous bleeding but can less commonly lead to musculoskeletal bleeding. VWD is the result of deficient and/or defective VWF. Management of VWD focuses on increasing both VWF and factor VIII (FVIII) with either high-dose desmopressin or replacement therapy with VWF concentrate. The latter can be given in a prophylactic or on-demand manner, depending on whether there is recurrent bleeding or in a perioperative or acute bleeding situation.³ 

A major treatment challenge lies in the large interindividual variability in the pharmacokinetics of the patient’s own VWF and FVIII and the infused variable ratio of VWF and FVIII in the concentrates. Compared with plasma-containing VWF concentrates (pdVWF), rVWF (vonicog alfa) has a longer half-life in clinical trials ranging from 17.2 to 22.6 hours, regardless of the subtype of VWD. The half-lives of commercially available pdVWF concentrates are shorter than rVWF:Humate-P (CSL Behring, Marburg, Germany; 10 to 11 hours in a 2.4:1 ratio of VWF:FVIII), Willate (Octapharma, Paramus, NJ; 15.8 hours in a 1:1 ratio of VWF:FVIII), Willfact (LFB Biopharmaceuticals, Letchworth Garden City, United Kingdom; 12 hours with negligible FVIII), and Voncento (CSL Behring; 11.5 to 13.5 hours in a 2.4:1 ratio of VWF:FVIII). The exact target trough and time above a prespecified VWF activity necessary to mitigate recurrent bleeding is under investigation; The general practice now is 2 or 3 infusions per week of a pdVWF or rVWF, which seems to mitigate bleeding similarly in those with severe VWD.

To understand the molecular basis for such differences in half-lives, Byrne and colleagues first compared the glycan structures in both VWF products and were able to confirm that, unlike pdVWF,^4,5 only 2-3–linked sialic acids are present in rVWF produced from Chinese hamster ovary cells. This is consistent with the fact that Chinese hamster ovary cells lack α2-6 sialyltransferases. Studies from several groups have reported that glycans in the A2 domain and around the A1 domain are important for VWF clearance.^6-8 A number of clearance receptors have also been identified. Thus, after the authors showed that rVWF does exhibit a longer half-life than pdVWF in a mouse model, and that removing the terminal sialic acids eliminated the difference in half-life between the 2 products, they conducted a systematic survey of the interactions of VWF products with major clearance receptors and compared their clearance in wild-type mice and in mice missing specific clearance receptors. They also showed that macrophage galactose lectin on macrophages and asialoglycoprotein receptor on hepatocytes exhibit much lower binding to rVWF than pdVWF, findings congruent with the longer half-life of rVWF.

Byrne et al suggested that the longer half-life of rVWF observed in human clinical trials could be attributed to the alterations in the terminal sugar modifications. This is significant because it clearly points to a new approach to extending the half-life of VWF concentrates. As macrophage galactose lectin and asialoglycoprotein receptor are implicated in the clearance of many glycoproteins in circulation, their findings also suggest that the same approach may be applicable to other therapeutic recombinant proteins.

Extending the half-life of a drug and slowing down its clearance from the site of action, which may lead to improved bleeding control, could take many forms. Extension of the half-life of FVIII and FIX concentrates has been done successfully via chemical modification, which includes pegylation (reducing renal clearance) and creation of fusion proteins such as with albumin or the neonatal Fc receptor.⁹ A bispecific nanobody that binds VWF to albumin thus prolonging the plasma survival of VWF via the albumin-FcRn recycling mechanism is under early investigation.¹⁰ Byrne et al have suggested that the approach of altering the end of the polysaccharide structure of VWF may be a valid and viable option for a large, glycosylated, and circulating recombinant protein. In hemophilia A and B, management has largely shifted away from standard half-life to extended half-life products, ultralong half-life products, and non-factor therapies over the last decade. Improved understanding of the clearance mechanisms of VWF is a good first step toward a similar evolution of “standard half-life” VWF replacement concentrates with extended half-life products.

Conflict-of-interest disclosure: R.F.S. has received investigator-initiated funding from Takeda, Octapharma, and LFB/Hema Biologics and received honoraria from Sanofi/Sobi, Octapharma, HEMAb, Guardian Therapeutics, Vega, Pfizer, Bayer, and LFB/Hema Biologics. R.L. declares no competing financial interests.