Methods

Study design

The pivotal study (ClinicalTrials.gov, NCT02932618; registered October 13, 2016) was a phase 3, prospective, multicenter, uncontrolled, open-label clinical trial (Supplementary Figure 1). Patients in the on-demand treatment arm received rVWF treatment for nonsurgical BEs for 12 months (up to 18 months for some patients who were waiting to enter the continuation study [NCT03879135]).

Patients were recruited into 3 age cohorts: ≥12 to <18 years, ≥6 to <12 years, and <6 years. Enrollment started in the oldest age group and was opened to patients aged ≥6 to <12 years after the data monitoring committee reviewed the PK/PD results from 6 patients in the first age group. Enrollment of patients aged <6 years was opened after the committee reviewed PK results from 6 patients in the ≥6- to <12-year age group. Enrollment of previously untreated patients could not start until 4 previously treated patients were treated with rVWF. A planned interim analysis was conducted when the on-demand treatment arm was complete and is presented in this study.

At baseline visit, PK/PD of rVWF (VWF ristocetin cofactor [VWF:RCo], VWF antigen [VWF:Ag], VWF collagen-binding capacity [VWF:CB], and FVIII coagulation activity [FVIII:C]) were measured following an infusion of 50 ± 5 IU/kg rVWF. For on-demand treatment of nonsurgical BEs, an initial dose of 40 to 60 IU/kg rVWF (up to 80 IU/kg for major BEs) was infused, along with 30 to 45 IU/kg recombinant FVIII (rFVIII; octocog alfa; ADVATE) for patients without hemostatically effective baseline FVIII levels, at the investigator’s discretion. Subsequent rVWF doses were administered every 8 to 24 hours, without or with additional rFVIII, to maintain VWF:RCo and FVIII levels for as long as deemed necessary by the investigator; rFVIII was only to be administered if plasma FVIII levels fell <30 IU/dL during the treatment period.

The study was conducted in accordance with the Declaration of Helsinki and applicable national and local regulatory requirements. The study protocol, protocol amendments, informed consent form, and any other written information were reviewed and approved/given favorable opinion by the ethics committee and applicable regulatory authorities before implementation.

Patient population

Eligible patients were children and adolescents (aged 0 to <18 years) with severe VWD (VWF:RCo <20 IU/dL) who were unable to tolerate or were inadequately responsive to DDAVP. Patients provided assent (if appropriate), and their legally authorized representative(s) provided informed consent. If applicable, patients agreed to use adequate birth control measures and female patients of childbearing potential were required to have a negative pregnancy test before enrollment. Key exclusion criteria included diagnosis of pseudo-VWD or other hereditary or acquired coagulation disorders; a history or presence of a VWF inhibitor or a FVIII inhibitor; known hypersensitivity to any of the components of the study drug; and a medical history of a thromboembolic event. Patients who were pregnant or lactating at the time of informed consent or assent were also excluded. A full list of inclusion and exclusion criteria is provided in Supplementary Table 1.

Study end points

The primary efficacy end point was the number and percentage of pediatric patients with treatment success for rVWF-treated nonsurgical BEs, defined as a mean efficacy rating score of <2.5 on a 4-point scale (1, excellent; 2, good; 3, moderate; and 4, none) (Supplementary Table 2). Severity of BEs was assessed by the investigators and determined based on clinical expertise and judgment. Secondary efficacy end points included the number of treated nonsurgical BEs with an efficacy rating of excellent or good, and the number of infusions, rVWF units, and rFVIII units (if needed) per BE. Safety was assessed in terms of the incidence and severity of treatment-emergent adverse events (TEAEs), the incidence of thrombotic events and severe hypersensitivity reactions, and immunogenicity, which included the development of neutralizing antibodies (inhibitors) to VWF and FVIII, total binding antibodies to VWF, and binding antibodies to Chinese hamster ovary proteins, murine immunoglobulin (Ig)G, and recombinant furin.

Analyses

Final data are presented for the on-demand treatment arm of the phase 3 study (November 6, 2017, to data cutoff December 1, 2023). A target sample size of ≥24 pediatric patients with severe VWD (8 in each age cohort) was based on the Guideline on the Clinical Investigation of Human Plasma Derived von Willebrand Factor Products [30] and was not based on a power calculation for a significance test.

The full analysis set (FAS) included all patients who were enrolled and met all eligibility criteria, had received any amount of study drug, and had provided ≥1 hemostatic assessment within 24 hours of an rVWF infusion, without or with rFVIII. All primary and secondary efficacy end points were assessed in the FAS. The safety analysis set (SAS) was used for the analyses of all safety end points and included all enrolled patients who received ≥1 dose of rVWF, without or with rFVIII.

PK/PD properties were assessed in the PK analysis set, defined as all enrolled patients who were not actively bleeding at the time of the PK infusion, had no BEs during the PK assessment, and had ≥1 quantifiable postdose PK measurement. Patients had an initial PK assessment at the baseline visit after a washout of ≥168 hours before infusion of 50 ± 5 IU/kg rVWF. PD was assessed by evaluating FVIII levels, as a marker of treatment effect. Sparse PK/PD samples were collected preinfusion and over 96 hours at 3 time points after the PK infusion, with patients in the 3 age cohorts randomized separately into 1 of 3 different sampling sequences to ensure that the samples were equally distributed among patients. Median concentration values at each scheduled time point were calculated. Point estimates by age cohort and overall were presented for area under the plasma concentration/time curve from 0 to 96 hours post infusion (AUC0-96h), maximal plasma concentration (_C_max), and time to maximal plasma concentration (_T_max) PK/PD parameters, with and without preinfusion correction. These parameters were derived using the noncompartmental estimation approach for sparse sampling designs.

Descriptive statistics are presented for demographic and baseline characteristics, and primary, secondary, and exploratory end points. Data are presented as observed, with no imputation for missing data. For the primary efficacy end point, point estimates and corresponding 2-sided exact 95% CIs for the percentage of patients with treatment success were calculated using the Clopper–Pearson method. The analysis for the primary outcome measure did not include inferential testing of statistical hypotheses.

Rollover pediatric on-demand treatment cohort in the continuation study

Patients in the on-demand arm who completed the phase 3 study had the option to enter a continuation study (ClinicalTrials.gov, NCT03879135; registered March 18, 2019) (Supplementary Figure 1) to extend the on-demand treatment with the study drug for up to an additional 3 years. This was a phase 3b, prospective, open-label, uncontrolled, multicenter study evaluating the long-term safety and efficacy of rVWF in pediatric and adult patients with severe VWD. This analysis includes all patients who completed the phase 3 study and chose to rollover for extended on-demand treatment (rollover pediatric on-demand treatment cohort) in the phase 3b study. The screening visit for the continuation study coincided with the completion visit of the phase 3 study upon receipt of informed consent. Patients were followed up for a maximum of 3 years (minimum 1 year) or until rVWF became commercially available, whichever occurred first.

Efficacy end points included hemostatic efficacy rating for the on-demand treatment of BEs and study drug consumption. Safety was assessed by the incidence and severity of TEAEs and the incidences of thrombotic events, severe hypersensitivity reactions, and immunogenicity. Safety data were evaluated in the SAS, defined as all patients who received rVWF. Efficacy analyses were conducted on the FAS, defined as all patients in the SAS who met all eligibility criteria. Data are presented descriptively; no formal statistical tests were carried out for this study.