IIB von Willebrand's disease: pathogenetic and therapeutic studies

Progress in von Willebrand Disease Treatment: Evolution towards Newer Therapies

von Willebrand disease (VWD) is a very heterogenous disease, resulting in different phenotypes and different degrees of bleeding severity. Established therapies (i.e., desmopressin, antifibrinolytic agents, hormone therapy for heavy menstrual bleeding, and von Willebrand factor [VWF] concentrates) may work in some subtypes, but not in all patients. In recent years, progress has been made in improving the diagnosis of VWD subtypes, allowing for more specific therapy. The impact of VWD on women's daily lives has also come to the fore in recent years, with hormone therapy, tranexamic acid, or recombinant VWF as treatment options. New treatment approaches, including the replacement of lacking factor VIII (FVIII) function, may work in those subgroups affected by severe FVIII deficiency. Reducing the clearance of VWF is an alternative treatment pathway; for example, rondaptivon pegol is a VWFA1 domain-binding aptamer which not only improves plasma VWF/FVIII levels, but also corrects platelet counts in thrombocytopenic type 2B VWD patients. These approaches are currently in clinical development, which will be the focus of this review. In addition, half-life extension methods are also important for the improvement of patients' quality of life. Targeting specific mutations may further lead to personalized treatments in the future. Finally, a few randomized controlled trials, although relatively small, have been published in recent years, aiming to achieve a higher level of evidence in future guidelines.

The VWF binding aptamer rondoraptivon pegol increases platelet counts and VWF/FVIII in type 2B von Willebrand disease

Type 2B von Willebrand disease (VWD) is characterized by an increased binding affinity of von Willebrand factor (VWF) to platelet glycoprotein Ib. This can lead to clearance of high-molecular-weight (HMW) multimers and thrombocytopenia with a resulting moderate-severe bleeding phenotype. Rondoraptivon pegol (BT200) is a pegylated aptamer binding to the A1 domain of VWF with a novel mechanism of action: it enhances VWF/factor VIII (FVIII) levels by decreasing their clearance. To study the potential benefit of rondoraptivon pegol in patients with type 2B VWD, we conducted a prospective phase 2 trial. Patients with type 2B VWD received 3 mg rondoraptivon pegol subcutaneously on study days 1, 4, and 7, followed by 6 to 9 mg every week until day 28. Five patients (male:female ratio = 3:2) were included. Rondoraptivon pegol rapidly tripled platelet counts from a median of 60 to 179 × 10E9/L (P < .001). Circulating VWF antigen increased from a median of 64% to 143%, which doubled FVIII activity levels from 67% to 134%. In all thrombocytopenic patients, plasma levels of VWF:GPIbM normalized, VWF ristocetin cofactor and VWF collagen-binding activity increased, and HMW multimers appeared. These pronounced improvements reversed during washout of the drug, thus demonstrating causality. The A1 domain binding aptamer directly corrects the underlying defect of type 2B VWD, thus providing a novel potential option for prophylaxis and treatment of patients with this VWD type. These data provide the basis for a phase 2b/3 trial in such patients. This trial was registered at www.clinicaltrials.gov as #NCT04677803.

Assessment of platelet numbers and morphology in the peripheral blood smear

Automated systems are currently in widespread use for the assessment of patients' complete blood counts. The evaluation of the peripheral blood smear, however, still constitutes a pivotal tool in the evaluation of patients with hematologic disorders. This article focuses on disorders affecting the number or morphology of platelets as assessed by evaluation of a peripheral blood smear and also outlines some of their important clinical findings.

Management of severe chronic thrombocytopenia in von Willebrand's disease type 2B

Two patients with a long history of unexplained thrombocytopenia, eventually diagnosed with von Willebrand's disease (vWD) type 2B are reported. In one patient with platelet counts of 80 x 10(9)/l 1-desamino-8-D-arginine vasopressin (DDAVP) had a favourable effect during bleeding episodes. The second patient received intermediate purity von Willebrand's factor (vWF)/factor VIII concentrate (Haemate HS), which helped haemostasis during tooth extraction. It increased platelet counts from 15 to 30 x 10(9)/l, whereas platelet transfusions produced no increase, nor prevented severe bleeding during abdominal surgery. Thus the treatment of vWD type 2B might depend on the degree of thrombocytopenia. It is recommended that in patients with mild to moderately decreased platelet counts, DDAVP treatment can be tried, whereas in patients with severely decreased platelet counts intermediate purity vWF/factor VIII concentrate substitution is preferred. In addition, vWD type 2B should be considered in the differential diagnosis of any child with chronic thrombocytopenia as the treatment strategy is different.

1-Desamino-8-arginine-vasopressin corrects the hemostatic defects in type 2B von Willebrand's disease

DDAVP is effective treatment in most types of von Willebrand's disease; however, in type 2B von Willebrand's disease the use of DDAVP has been contraindicated due to DDAVP-induced thrombocytopenia. Several reports have confirmed the thrombocytopenic effects of DDAVP and the presence of circulating platelet aggregates in type 2B von Willebrand's disease. We have infused three type 2B patients with DDAVP. The three patients had different mutations of their vWf. All three patients had a missense mutation which resulted in a single amino acid substitution in the disulfide loop of the A1 domain. Administration of 20 micrograms of DDAVP resulted in significant elevations of factor VIII, vWf antigen, and ristocetin cofactor levels. In contrast to other studies, DDAVP did not induce or enhance thrombocytopenia in these three patients. When blood was obtained by fingerstick and diluted into sodium oxalate (Unopette) or EDTA (Microvette), the platelet counts did not change over 4 hr. In contrast, blood collected directly into evacuated tubes containing sodium citrate, lithium heparin, or EDTA consistently demonstrated varying degrees of thrombocytopenia and platelet clumping. We also observed a shortening of the pre-infusion bleeding time over the 4 hr period. All three patients have been studied twice and each has shown consistent results. DDAVP appears to be a useful form of treatment in type 2B vWd.

Type IIB von Willebrand's disease: gene mutations and clinical presentation in nine families from Denmark, Germany and Sweden

Type IIB of von Willebrand's disease (vWD) is a variant in which the structurally abnormal von Willebrand factor (vWF) shows an increased affinity for the platelet vWF receptor, glycoprotein Ib (GPIb). This may sometimes give rise to platelet aggregation and thrombocytopenia in vivo. In 20 patients from nine unrelated families with type IIB vWD from Denmark, Germany and Sweden we studied the molecular defect by amplification and direct sequencing of parts of exon 28 which encode for the vWF domain that interacts with platelet GPIb. Three different point mutations were identified one of which has not previously been reported. Fifteen patients from five families were heterozygous for the Arg543-->Trp substitution. The mutation had occurred independently in all five families and in two of them represented a de novo mutation. In one of these families the father, though asymptomatic and with normal laboratory test results, carried the mutation in heterozygous form. In three families, four affected members were found to be heterozygous for the Arg543-->Cys substitution. The mutations were of different origin at least in two of the families. The third substitution, Val551-->Leu, which has not previously been described, was found in one patient and was due to a de novo mutation. In most of the patients spontaneous thrombocytopenia had been recorded on at least one occasion. Five of the patients with the Arg543-->Trp substitution and the one with the Val555-->Leu substitution had all had bleeding associated with thrombocytopenia in the neonatal period of early infancy.

Identification of a point mutation in type IIB von Willebrand disease illustrating the regulation of von Willebrand factor affinity for the platelet membrane glycoprotein Ib-IX receptor

von Willebrand factor (vWF) supports platelet adhesion on thrombogenic surfaces by binding to platelet membrane glycoprotein (GP) Ib in the GP Ib-IX receptor complex. This interaction is physiologically regulated so that it does not occur between circulating vWF and platelets but, rather, only at a site of vascular injury. The abnormal vWF found in type IIB von Willebrand disease, however, has a characteristically increased affinity for GP Ib and binds to circulating platelets. We have analyzed the molecular basis of this abnormality by sequence analysis of a type IIB vWF cDNA and have identified a single amino acid change, Trp550 to Cys550, located in the GP Ib-binding domain of the molecule comprising residues 449-728. Bacterial expression of recombinant fragments corresponding to this vWF domain yielded molecules that, whether containing a normal Trp550 or a mutant Cys550 residue, bound directly to GP Ib in the absence of modulators and with similar affinity. In contrast, mammalian cell expression of the same segment of sequence yielded molecules that, when containing the normal Trp550, did not bind to GP Ib directly but, like native vWF, bound in the presence of ristocetin. However, molecules containing the point mutation (Cys550) behaved like type IIB vWF--namely, bound to GP Ib even without ristocetin modulation and, in the presence of ristocetin, had 10-fold higher affinity than molecules with normal sequence. These results identify a region of vWF that, although not thought to be directly involved in binding to GP Ib, may modulate the interaction through conformational changes.

Effect of a factor VIII concentrate on type IIB von Willebrand's disease-associated thrombocytopenia presenting during pregnancy in identical twin mothers

Marked thrombocytopenia developed during pregnancy in both identical twins mothers who had systemic lupus erythematosus (SLE) and also type IIB von Willebrand's disease (vWD). The proband's platelet count decreased in the third trimester of pregnancy. Large-dose gamma-globulin and prednisolone treatments were performed because of the suspicion of immune thrombocytopenic reaction associated with SLE. These treatments were not effective. Her platelet count returned to the normal range immediately after delivery. Postpartum examinations revealed the decreased ristocetin cofactor activity and the deficiency of large von Willebrand factor (vWF) multimers in preserved plasma samples from the third trimester. These abnormal findings improved after delivery. Investigation of family members revealed that the proband had inherited type IIB vWD from her mother. The other twin, who was also under treatment for SLE, became pregnant about 1 year after delivery in the proband and followed almost the same course as that observed in the proband. As bleeding tendency was observed a few days before delivery, a factor VIII concentrate (Haemate P) was administered to compete with her variant vWF. This concentrate could prevent the further decrease in her platelet count, thereby correcting the hemorrhagic tendency. It seems evident that factor VII concentrate would be effective in treating thrombocytopenia associated with type IIB vWD.

Platelet aggregation and pseudothrombocytopenia induced by 1-desamino-8-D-arginine vasopressin (DDAVP) in type IIB von Willebrand's disease patient

Our study shows that the thrombocytopenia described in type IIB von Willebrand's disease (vWd) after 1-desamino-8-D-arginine vasopressin (DDAVP) infusion is, at least partially, a pseudothrombocytopenia. There was a discrepancy in platelet counts in blood anticoagulated with EDTA (less than 10 x 10(3)/microliters) or citrate (55 x 10(3)/microliters) in one patient with type IIB vWd and chronic thrombocytopenia (80 x 10(3)/microliters) after DDAVP infusion. Furthermore, DDAVP induced a normalization of patient's prolonged bleeding time. Spontaneous platelet aggregation (SPA) observed in platelet-rich plasma before DDAVP infusion was inhibited completely by monoclonal antibodies which block binding of fibrinogen, vWf and fibronectin to GPIIb-IIIa. SPA was partially inhibited by a monoclonal antibody which blocks the binding of vWf to GPIb. After DDAVP, in contrast, SPA partially persisted in the presence of anti-GPIIb-IIIa monoclonal antibodies but was completely inhibited by anti-GPIb monoclonal antibody. Therefore GPIb and GPIIb-IIIa complex seem to play a different role in SPA before and after DDAVP infusion into type IIB vWd.