Real-world assay variability between laboratories in monitoring of recombinant factor IX Fc fusion protein activity in plasma samples

Recombinant factor IX Fc for the treatment of hemophilia B

Current hemophilia B treatment guidelines recommend routine prophylaxis with factor IX (FIX) replacement products, tailored to maintain plasma activity at levels that will prevent bleeds. However, plasma FIX activity may not be the primary determinant or best indicator of hemostatic efficacy due to its extravascular distribution. FIX replacement therapy has evolved to include extended half-life (EHL) products that provide effective bleed protection when administered at intervals of 7 days or longer. rFIXFc is a recombinant fusion protein with an extended circulation time. rFIXFc has a biodistribution profile consistent with distribution into extravascular space, where it may support hemostasis at sites of vessel injury independent of circulating plasma activity levels. The safety and efficacy of rFIXFc prophylaxis is well established in adults, adolescents and children including previously untreated patients with hemophilia B, with substantial evidence from clinical trials and real-world clinical practice. This review describes the pharmacokinetic characteristics of rFIXFc, summarizes available safety and efficacy data, and evaluates the use of rFIXFc in special populations. Current hemophilia B treatment challenges, including target FIX plasma levels, perioperative use, and management of patients with comorbidities, are discussed together with the potential role of EHL products in the future treatment landscape of hemophilia B.

Recombinant factor IX Fc for major surgery in hemophilia B: factor IX plasma activity levels and effective hemostasis

Background

Major surgical procedures are associated with significant bleeding risk and infectious complications in patients with hemophilia, which may be minimized by factor replacement. Monitoring perioperative factor levels guides dosing to maintain adequate levels for hemostatic control.

Objectives

We report prospectively collected post hoc surgical data in patients with hemophilia B who underwent major surgery with extended half-life recombinant factor IX Fc fusion protein (rFIXFc) in phase 3 studies (B-LONG/Kids B-LONG and B-YOND).

Methods

Achieved FIX plasma levels were described for those who underwent major surgeries with ≥1 peak and/or predose FIX assessment available on the day of surgery (Day 0 [D0]) from the central laboratory. Dosing, injection frequency, adverse events, and hemostatic responses were assessed. Two representative cases were described further including blood loss, transfusions, and concomitant medication assessment.

Results

Of 35 major surgeries, 17 (N = 16 subjects) with sufficient FIX measurements were included in this analysis; 13 of 17 surgeries were orthopedic. On D0, a median loading (preoperative) dose of 101.1 International Units (IU)/kg/injection achieved a median peak FIX of 103.3 IU/dL. Across postoperative Days 1 to 3, 4 to 6, and 7 to 14, the median predose levels were 75.1 IU/dL with 1 injection/d, 71.6 IU/dL with 0 to 1 injection/d, and 43.2 IU/dL with 0 to 1 injection/d, respectively. Hemostasis was rated excellent (14 of 16) or good (2 of 16) across surgeries. Both case studies (knee arthroscopy and ankle fusion) illustrate measured FIX levels with rFIXFc.

Conclusion

The aggregate analysis and representative cases of major surgeries demonstrate that rFIXFc can achieve FIX levels for effective hemostasis during invasive high-risk procedures.

Managing surgery in hemophilia with recombinant factor VIII Fc and factor IX Fc: Data on safety and effectiveness from phase 3 pivotal studies

Background

Surgical procedures impose hemostatic risk to people with hemophilia, which may be minimized by optimal factor (F) replacement therapy.

Methods

This analysis evaluates the efficacy and safety of extended half-life factor replacement recombinant FVIII and FIX Fc fusion proteins (rFVIIIFc and rFIXFc) during surgery in phase 3 pivotal (A-LONG/Kids A-LONG and B-LONG/Kids B-LONG) and extension (ASPIRE and B-YOND) studies. Dosing regimens were determined by investigators. Injection frequency, dosing, blood loss, transfusions, and hemostatic response were assessed.

Results

Forty-five major (n = 31 subjects) and 90 minor (n = 70 subjects) procedures were performed in hemophilia A; 35 major (n = 22) and 62 minor (n = 37) procedures were performed in hemophilia B. Unilateral knee arthroplasty was the most common major orthopedic procedure (hemophilia A: n = 15/34; hemophilia B: n = 8/24). On the day of surgery, median total dose in adults/adolescents was 81 IU/kg for rFVIIIFc and 144 IU/kg for rFIXFc; most major procedures required ≤2 injections (including loading dose). Through days 1-14, most major procedures had ≤1 injection/day. Hemostasis was rated excellent (rFVIIIFc: n = 39/42; rFIXFc: n = 29/33) or good (n = 3/42; n = 4/33) in evaluable major surgeries, with blood loss comparable with subjects without hemophilia. Most minor procedures in adults/adolescents required one injection on the day of surgery, including median loading dose of 51 IU/kg (rFVIIIFc) and 80 IU/kg (rFIXFc). No major treatment-related safety concerns were identified. No subjects developed inhibitors or serious vascular thromboembolic events.

Conclusions

rFVIIIFc and rFIXFc were efficacious and well tolerated for the management of perioperative hemostasis across a wide spectrum of major and minor surgeries in hemophilia.

An Update on Laboratory Diagnostics in Haemophilia A and B

Haemophilia A (HA) and B (HB) are X-linked hereditary bleeding disorders caused by lack of activity of coagulation factors VIII (FVIII) or IX (FIX), respectively. Besides conventional products, modern replacement therapies include FVIII or FIX concentrates with an extended half-life (EHL-FVIII/FIX). Two main strategies for measuring plasma FVIII or FIX activity are applied: the one-stage clotting assay (OSCA) and the chromogenic substrate assay (CSA), both calibrated against plasma (FVIII/FIX) standards. Due to the structural modifications of EHL-FVIII/FIX, reagent-dependent assay discrepancies have been described when measuring the activity of these molecules. Assay discrepancies have also been observed in FVIII/FIX gene therapy approaches. On the other hand, nonfactor replacement by the bispecific antibody emicizumab, a FVIIIa-mimicking molecule, artificially shortens activated partial thromboplastin time–based clotting times, making standard OSCAs inapplicable for analysis of samples from patients treated with this drug. In this review, we aim to give an overview on both, the currently applied and future therapies in HA and HB with or without inhibitors and corresponding test systems suitable for accompanying diagnostics.

Pharmacodynamics of eftrenonacog-alfa (rFIX-Fc) in severe hemophilia B patients: A real-life study

Eftrenonacog-alfa is a recombinant factor IX-Fc fusion protein increasingly prescribed in hemophilia B patients. We aimed to assess its pharmacodynamics (PD) in real-life setting via FIX activity measurement and thrombin generation assay (TGA). Sixty samples from 15 severe hemophilia B treated patients were collected at different time points. FIX activity was measured using product-specific one-stage clotting assay (reference method) and two chromogenic assays (CSA) (Biophen FIX and Rox FIX). TGA was triggered with 1 pM tissue factor. Five parameters were analyzed: lag time (LT), time to peak (TTP), peak height (PH), endogenous thrombin potential (ETP), and velocity. PD models were built to characterize their relationships with FIX activity, using mixed effects models. Mean trough FIX level was estimated at 4.64 (±1.50) IU/dl with a recovery at 0.78 (±0.16) IU/dl per 1 IU/kg injected dose. FIX activity ranged between 1 and 86 IU/dl with 21.5 IU/dl median value. Biophen FIX and Rox FIX allowed reliable measurements except in samples with FIX <20 IU/dl in which values were underestimated (delta >30%). PD models revealed that velocity was the most sensitive TGA parameter to FIX activity followed by PH, ETP, TTP and finally LT. Following FIX activity peak after eftrenonacog-alfa injection, velocity decreased first, followed by PH then ETP. Both CSA failed to accurately measure FIX in severe hemophilia B patients receiving eftrenonacog-alfa throughout the measuring range. TGA could be an additional valuable tool to evaluate hemostasis balance in treated patients.

Real-world assay variability between laboratories in monitoring of recombinant factor IX Fc fusion protein activity in plasma samples

Introduction

Monitoring of factor IX (FIX) replacement therapy in haemophilia B relies on accurate coagulation assays. However, considerable interlaboratory variability has been reported for one‐stage clotting (OSC) assays. This study aimed to evaluate the real‐world, interlaboratory variability of routine FIX activity assays used in clinical haemostasis laboratories for the measurement of recombinant FIX Fc fusion protein (rFIXFc) activity.

Methods

Human FIX‐depleted plasma was spiked with rFIXFc at 0.80, 0.20 or 0.05 IU/mL based on label potency. Participating laboratories tested samples using their own routine OSC or chromogenic substrate (CS) assay protocols, reagents and FIX plasma standards. Laboratories could perform more than one measurement and method, and were not fully blinded to nominal activity values.

Results

A total of 142 laboratories contributed OSC results from 175 sample kits using 11 different activated partial thromboplastin time (aPTT) reagents. The median recovered FIX activity for the 0.80, 0.20 and 0.05 IU/mL samples was 0.72 IU/mL, 0.21 IU/mL and 0.060 IU/mL, respectively. Across all OSC reagents, interlaboratory variability (% CV) per aPTT reagent ranged from 9.4% to 32.1%, 8.2% to 32.6% and 12.2% to 42.0% at the 0.80, 0.20 and 0.05 IU/mL levels, respectively. CS results showed excellent median recoveries at all nominal levels (87.5% to 115.0%; n = 11) with low interlaboratory variability (CV 3.6% to 15.4%).

Conclusion

This large, real‐world data set indicates that rFIXFc activity in plasma samples can be accurately measured with the majority of routine OSC and CS assay methods. Given the variation in FIX assay procedures between sites, it is important that individual laboratories qualify their in‐house methods for monitoring of rFIXFc activity.