Thrombin generation and bleeding in haemophilia inhibitor patients during immune tolerance induction

The design of a Bayesian platform trial to prevent and eradicate inhibitors in patients with hemophilia

Among individuals with the rare congenital bleeding disorder hemophilia A, the major challenge is inhibitor formation, which is associated with significant morbidity and cost. Yet, as the optimal approach to prevent and eradicate inhibitors is not known, we are at equipoise. Because classic trial design is not practical in a rare disease setting, we designed 2 48-week randomized trials comparing ELOCTATE and emicizumab to prevent and eradicate inhibitors. To achieve statistical efficiency, we incorporated historic data (Bayesian priors) on inhibitor formation to allow preferential randomization to emicizumab, piecewise exponential survival models to determine mean and 95% confidence interval for inhibitor formation in each arm, and simulations to determine the best model design to optimize power. To achieve administrative efficiency, the trials will be performed with the same sites, staff, visit frequency, blood sampling, laboratories, and laboratory assays, with streamlined enrollment so patients developing inhibitors in the first trial may be enrolled on the second trial. The primary end point is the probability of inhibitor formation or inhibitor eradication, respectively. The design indicates early stopping rules for overwhelming evidence of superiority of the emicizumab arms. Simulations indicate that, with 66 subjects, the Prevention Trial will have 84% power to detect noninferiority of emicizumab to ELOCTATE with a margin of 10% if emicizumab is truly 10% superior to ELOCTATE; with 90 subjects, the Eradication Trial will have 80% power to detect 15% superiority of ELOCTATE immune tolerance induction with vs without emicizumab. Thus, a platform design provides statistical and administrative efficiency to conduct INHIBIT trials.

Pharmacodynamic monitoring of factor VIII replacement therapy in hemophilia A: Combining thrombin and plasmin generation

BACKGROUND

Clinical severity of hemophilia A (HA) varies, possibly due to interplay of many factors in the hemostatic pathway. Pharmacokinetic monitoring of factor VIII (FVIII) replacement therapy in HA patients consists of measuring FVIII activity levels and subsequent dose adjustment. The Nijmegen Hemostasis Assay (NHA) measures thrombin generation (TG) and plasmin generation (PG).

OBJECTIVE

To determine differences in TG and PG between HA patients before and during a pharmacokinetic study and identify best parameters to develop a pharmacodynamic model.

METHODS

Twenty-five HA patients (baseline FVIII < 1-9 IU/dL) underwent a pharmacokinetic study with a single dose of 25-50 IU/kg standard half-life FVIII concentrate. At baseline and after administration of FVIII TG and PG parameters were measured with the NHA.

RESULTS

FVIII activity level increased from median 1.0 IU/dL (interquartile range < 1.0-6.0) to 71 IU/dL (62-82) 15 minutes after administration and decreased to 15 IU/dL (10-26) at 24 hours. TG was enhanced simultaneously, with thrombin peak height (TPH) increasing from 22nM (15-35) to 222nM (159-255), and thrombin potential (TP) from 404nM/min (undetectable-876) to 1834nM/min (1546-2353). Twenty-four hours after infusion, TG parameters remained high (TPH 73nM [58.5-126.3]; TP 1394nM/min [1066-1677]) compared to FVIII activity level. PG showed hyperfibrinolysis in severe HA patients compared to mild patients and controls, which normalized after FVIII supplementation.

CONCLUSION

HA patients showed clear differences in baseline TG and PG despite having comparable FVIII activity levels. These results reveal a discrepancy between FVIII activity level and TG, in which the latter may be a better parameter to monitor individualized treatment in HA patients.

The endothelial specific isoform of type XVIII collagen correlates to annual bleeding rate in haemophilia patients

Introduction

The medical need in the haemophilic (HF) field to reduce bleeding incidents requires measurement of the annual bleeding rate (ABR) in haemophiliacs. Vascular rupture is associated with damage to the vascular endothelium causing exposure of the basement membrane. Endothelial cells and matrix impairment may be associated with joint bleeds and later development of HF arthropathy. Imbalanced extracellular matrix turnover is a central pathological feature in many diseases consequent to epithelial or endothelial cell damage. Type XVIII collagen is an essential basement membrane component, with an endothelial specific isoform.

Aim

To quantify the basement membrane specifically for the endothelial cells, as that may have particular relevance to endothelial cell stability and rupture in haemophiliacs. A newly developed ELISA assay detecting endothelial type XVIII collagen (COL-18N) was used to assess the clinical relevance of endothelial basement membrane turnover in patients diagnosed with HF arthropathy and correlation to ABR.

Methods

We developed an ELISA assay for quantification of COL-18N. Serum from 35 male HF patients was investigated using the COL-18N ELISA.

Results

COL-18N correlated to the ABR of haemophiliacs, r = 0.45, P<0.006.

Conclusion

Vascular rupture and consequent bleeding are associated with joint damage and deterioration of life quality in haemophiliacs. Quantification of ABR is an important part in efficacy assessment of different interventions, and the benchmark of these. Objective biomarkers reflecting endothelial dysfunction, vascular leaks and rupture, like the COL-18N biomarker that associate with ABR, may assist in identifying the most optimal treatment and monitoring of HF patients.