Presentation Details
| Accurate Measurement of Factor VIII Activity and Inhibitors in the Presence of Mim8 William Pickering1, Karin Nana Weldingh2, Mary Robinson1, Caroline Cogswell1, Wan Hui Ong Clausen2, Mirella Ezban2, Vlady Ostrow3. 1Labcorp Colorado Coagulation, Englewood, CO, USA.2Novo Nordisk A/S, Søborg, Denmark.3Novo Nordisk Inc., Plainsboro, NJ, USA |
Abstract
Background: Mim8 (denecimig) is an activated factor IX (FIXa)/factor X (FX) bispecific antibody for the prophylactic treatment of hemophilia A (HA) with or without inhibitors. Measuring factor VIII (FVIII) activity and FVIII inhibitors in patients with HA is clinically important but can be challenging in the presence of FVIII mimetics. FVIII mimetics such as Mim8 interfere with activated partial thromboplastin time-based assays, while chromogenic substrate assays (CSAs) containing human or human/bovine proteins exhibit varied sensitivity to FVIII mimetics. In the presence of bispecific antibodies, FVIII activity can only be assessed using CSAs containing bovine FIXa/FX. Objectives: To investigate Mim8 interference with the measurement of FVIII activity in plasma samples containing standard half-life (SHL) and extended half-life (EHL) FVIII products and on the measurement of FVIII inhibitor levels. Methods: To assess FVIII activity of SHL and EHL products, severe HA plasma was spiked with ADVATE®, Novoeight®, Esperoct®, or ELOCTATE® (5, 10, 15, 20, and 100 IU/dL final concentrations) and Mim8 (0, 3, 6, and 12 µg/mL final concentrations). FVIII activity was measured with the fully bovine Chromogenix Coatest® SP4 FVIII (DiaPharma Group, Inc) and mixed human/bovine CRYOcheck™ (Precision BioLogic, Inc) CSAs. Interference was defined as a ≥1.2-fold increase in FVIII activity relative to the corresponding sample without Mim8. To assess FVIII inhibitor levels, a FVIII inhibitor kit (Precision BioLogic, Inc) and 2 FVIII CSA kits using bovine FIXa/FX reagents (Factor VIII Chromogenic Assay [Siemens Healthineers], Chromogenix Coatest® SP4 FVIII [DiaPharma Group, Inc]) were used to test pooled congenital FVIII-deficient plasma (George King Bio-Medical) spiked with Mim8 (5, 10, 20, and 40 µg/mL) and FVIII inhibitor (Affinity Biologicals; 0.2, 1.0, and 4.8 Bethesda units [BU]). Mim8 interference was defined as a negative result (<0.6 BU) becoming positive (0.2 BU FVIII inhibitor samples) or a result falling outside ±25% of the unspiked sample result (1.0 and 4.8 BU FVIII inhibitor samples). Results: Using the bovine CSA, no notable interference with FVIII activity was observed in samples spiked with SHL or EHL product at any FVIII/Mim8 concentration. Using the bovine/human CSA, a Mim8 dose-dependent increase in interference was observed in samples with FVIII concentrations ≤0.20 IU/mL. With both SHL and EHL products, increased FVIII levels correlated with reduced interference in a Mim8 dose-dependent manner. No Mim8 interference was observed at any FVIII inhibitor level, and inhibitor titers measured in all Mim8 spiked samples fell within ±25% of target. Siemens CSA inhibitor samples at 0.2 BU remained negative (<0.6 BU), with no reported results >0.4 BU. Spiked samples at 1.0 and 4.8 BU recovered within −5.0% to −3.0% and −2.1% to 2.4%, respectively, of the corresponding unspiked samples. Unspiked 1.0 and 4.8 BU samples recovered within 1.0% and 19.6% of target, respectively. Conclusions: FVIII activity of SHL and EHL products was accurately measured in the presence of Mim8 using bovine CSAs. Using FVIII CSAs with bovine reagents, FVIII inhibitor levels up to approximately 5.0 BU were accurately measured in HA plasma in the presence of up to 40 µg/mL Mim8.
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No part of this publication may be reproduced, distributed, or transmitted in any form or by any means, including photocopying, recording, or other electronic or mechanical methods, without the prior written permission of the author.