Potency estimates for recombinant factor IX in the one-stage clotting assay are influenced by more than just the choice of activated partial thromboplastin time reagent

Field Study and Correlative Studies of Factor IX Variant FIX-R338L in Participants Treated with Fidanacogene Elaparvovec

BACKGROUND

 Fidanacogene elaparvovec, an adeno-associated virus-based gene therapy vector expressing the high-activity factor IX (FIX) variant FIX-R338L, is in development for hemophilia B. One-stage clotting (OS) assays and chromogenic substrate (CS) assays are commonly used to measure FIX-R338L variant activity. Data from ongoing trials suggest FIX activity varies between different OS and CS assays.

MATERIAL AND METHODS

 To better understand FIX-R338L activity in clinical samples, an international multisite field study was conducted across a central laboratory and 18 local laboratories, using standard protocols, reagents, and instrumentation, with individual participant samples from a phase 1/2a study of fidanacogene elaparvovec.

RESULTS

 Unlike the wild-type FIX control, FIX-R338L activity was higher with the OS silica-based assay versus OS ellagic acid-based and CS assays. Variation in FIX activity was greater at the lowest activity levels. Activated FIX (FIXa) in plasma could result in higher OS assay activity or increased thrombin generation, which could overestimate FIX activity. However, FIXa was not detected in the participant samples, indicating that it was not contributing to the OS assay differences. Since individuals on gene therapy may receive exogenous replacement FIX products, replacement products were spiked into patient plasma samples to target a therapeutic concentration. Exogenous FIX was additive to endogenous FIX-R338L, with no interference from FIX-R338L.

CONCLUSION

 These results demonstrate FIX-R338L activity can be measured with OS and CS assays in clinical laboratories and provide insight into assay variability when measuring FIX with endogenously produced FIX-R338L. The findings may help establish best practices for measuring FIX-R338L activity (Clinicaltrials.gov identifier: NCT02484092).

A Review of Factor VIII and Factor IX Assay Methods for Monitoring Extended Half-Life Products in Hemophilia A and B

Clinical trials have used a variety of coagulation factor assay methods to assess treatment with recombinant Factor VIII (rFVIII) and recombinant Factor IX (rFIX) extended half-life (EHL) products. However, diagnostic laboratories may use different reagent combinations for routine use or for field trials of EHL products. The focus of this review is on the choice of one-stage clotting and chromogenic Factor VIII and Factor IX methods and the influence that assay principle and components may have on results, including the effects of different activated partial thromboplastin time reagents and factor-deficient plasma. Our aim is to tabulate the findings for each method and reagent group to give laboratories practical guidance as to how the reagent combinations used in their local laboratory compare to others, for the various EHLs available.

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.

Factor IX assay discrepancies in the setting of liver gene therapy using a hyperfunctional variant factor IX-Padua

BACKGROUND

Limited information exists regarding the factor IX (FIX) coagulant activity (FIX:C) measured by different assays following FIX-Padua gene therapy.

OBJECTIVE

Assess for the first time FIX:C in five commonly used coagulation assays in plasma samples from hemophilia B subjects receiving FIX-Padua gene transfer.

METHODS

FIX:C was compared between central (n = 1) and local laboratories (n = 5) in the study, and across four commonly used FIX:C one-stage assays and one FIX:C chromogenic assay. For comparison, samples of pooled congenital FIX-deficient plasma spiked with purified recombinant human FIX (rHFIX)-Padua protein or rHFIX (nonacog alfa) to obtain FIX:C concentrations from ~20% to ~40% were tested.

RESULTS

FIX:C results at local laboratories strongly correlated with central laboratory results. However, absolute values at the central laboratory were consistently lower than those at local laboratories. Across five different FIX:C assays, a consistent pattern of FIX:C was observed for subjects receiving fidanacogene elaparvovec-expressed gene transfer. Use of Actin FSL activated partial thromboplastin time (APTT) reagent in the central laboratory resulted in lower FIX:C values compared with other APTT reagents tested. The chromogenic assay determined lower FIX:C than any of the one-stage assays. The rHFIX-Padua protein-spiked samples showed similar results. In contrast, FIX:C results for rHFIX-nonacog alfa measured within 25% of expected for all one-stage assays and below 25% in the chromogenic assay.

CONCLUSIONS

Assay-based differences in FIX:C were observed for fidanacogene elaparvovec transgene product and rHFIX-Padua protein, suggesting the variable FIX:C determined with different assay reagents is inherent to the FIX-Padua protein and is not specific to gene therapy-derived FIX-Padua.

The traceability of commercial plasma calibrators to the plasma International Standards for factor VIII and factor IX

Accurate measurement of coagulation factors is essential, especially for diagnosis of deficiency. Clinical laboratories use commercially available plasma calibrators, which should be traceable to the relevant plasma International Standard (IS). This study assessed the relationship between the plasma IS for factors IX (FIX) and VIII (FVIII) and some commonly used commercial calibrators. Calibrators from seven manufacturers were assayed for FIX and FVIII activity by one‐stage clotting assay (OSCA) using different activated partial thromboplastin time (APTT) reagents and deficient plasmas, or chromogenic assay (CA). Results were calculated relative to the 4th IS Factors II,VII,IX,X, Plasma or the 6th IS Factor VIII/VWF, Plasma. Results for each calibrator were similar across the APTT reagents and deficient plasmas used. All calibrators showed a recovery of 90%‐111% of the manufacturers’ values, except calibrator C, which had recovery of around 85%. CA gave similar results, with good recovery for all but calibrator C. Similar low recoveries for OSCA and CA were found for a different lot of calibrator C and for a different calibrator product from manufacturer C. When all calibrators from manufacturer C were assayed by OSCA using the manufacturer's own deficient plasmas and APTT reagents, the mean recovery was still below 90%. Overall, there was good traceability of the international unit between the IS and commercial calibrator plasmas. Calibrators from one manufacturer consistently yielded lower than expected values for FIX and FVIII. This could lead to an over‐estimation of the coagulation factor content in patient samples and demonstrates the importance of careful choice of calibrator.

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.

Activity measurements of dalcinonacog alfa

INTRODUCTION

Many recombinant and modified FIX products have been, and continue to be, developed with the aim of improving treatment for patients with haemophilia B. One such new product is dalcinonacog alfa, a recombinant FIX with modifications to provide improved features such as subcutaneous administration.

AIM

In view of previously observed assay discrepancies with modified FIX therapeutics, the aim of this study was to assess potential discrepancies in potency measurement of dalcinonacog alfa between and within different assay methods.

METHODS

Potency of dalcinonacog alfa was measured against the 5th International Standard (IS) for FIX Concentrate and the 4th IS for FIX Plasma by One-Stage Clotting Assay, using 9 different APTT reagents and 2 commercially available FIX chromogenic kits. Plasma-derived concentrate and recombinant FIX samples were also included for comparison in every assay.

RESULTS

Substantial discrepancies were observed when assaying dalcinonacog alfa using the one-stage clotting assay against both standards. No statistically valid results were obtained when testing dalcinonacog alfa using either chromogenic kit. Increasing the incubation time with the activation reagent in both chromogenic kits resulted in valid assays and increased the potency to become more in line with potencies by one-stage clotting assays. Increasing the incubation time in the chromogenic kits had no effect on the potencies of the plasma-derived or recombinant samples. However, incubation time influenced in the one-stage clotting assay using Dapttin.

CONCLUSIONS

Within and between assay method discrepancy was found when assaying dalcinonacog alfa. Methods for potency labelling and clinical monitoring should be given careful consideration.