Abstract Number: PB0679
Meeting: ISTH 2021 Congress
Theme: Hemophilia and Rare Bleeding Disorders » Novel Biotherapeutics in Hemophilia
Background: Mim8, a new bispecific antibody, facilitates activation of factor (F)X by binding FIXa and FX. This FVIII mimetic was designed for the subcutaneous prophylactic treatment of patients with haemophilia A (HA) with or without inhibitors. Chromogenic substrate assays (CSAs) are used to monitor the FVIII activity (FVIII:C) of patients with HA.
Aims: We evaluated the reliability of Mim8 measurements made using plasma-calibrated FVIII CSA and the necessity of product-specific calibrator for accuracy.
Methods: Mim8 was spiked into severe HA plasma at 0–20µg/mL. Mim8 mimetic FVIII activity (FVIII:C) was measured on two days by 5 CSA kits (Table 1) using either a Sysmex CS-5100 or an ACL TOP 700. Calibration curves were constructed using human plasma standards. A modified Hyphen CSA was evaluated on three days using a Mim8 reference product (10.26µg/mL) as calibrator and 1/80 plasma dilution
(Table 2).
Results: Mimetic FVIII:C of 33.2IU/dL was observed at the lowest Mim8 concentration (1µg/mL) and increased concurrently with Mim8 concentration, with the Hyphen CSA calibrated using human plasma. Siemens CSA was insensitive to even 20µg/mL. Some mimetic FVIII:C was observed above 5µg/mL with the Coamatic CSA, whilst some mimetic FVIII:C was measured at 1-2µg/mL Mim8 using Precision Biologic and Rossix CSA (Table 1).
The between-day coefficient of variance of a modified Hyphen CSA calibrated with Mim8 was <3% for each Mim8 concentration. Recovery of 20% or less from the target Mim8 concentration was observed for all concentrations evaluated
(Table 2).
Table 1 | Hyphen FVIII:C (IU/dL) |
Siemens FVIII:C (IU/dL) |
Coamatic® FVIII:C (IU/dL) |
Rossix FVIII:C (IU/dL) |
Precision Biologic FVIII:C (IU/dL) |
Source of protein in kit; analyser; dilution | Human FX Human FIXa; CS5100; 1/40 |
Bovine FX Bovine FIXa; CS5100; 1/30 |
Bovine FX Bovine FIXa; ACL TOP 700; 1/15 |
Bovine FX Human FIXa; ACL TOP 700; 1/60* |
Bovine FX Human FIXa; ACL TOP 700; 1/30* |
Mim8 1µg/mL | 33.2 | <3 | <1 | <1* | 1.7* |
Mim8 2µg/mL | 70.0 | <3 | <1 | 1.7* | 2.5* |
Mim8 5µg/mL | 148.3 | <3 | <1 | 3.4* | 5.0 |
Mim8 8µg/mL | 212.4 | <3 | 1.3 | 7.7* | 7.5 |
Mim8 10µg/mL | 247.0 | <3 | 2.0 | 10.9 | 9.2 |
Mim8 15µg/mL | 332.7 | <3 | 2.5 | 17.2 | 17.7 |
Mim8 20µg/mL | 389.1 | <3 | 5.2 | 24.2 | 19.7 |
Mim8 reference 10.26µg/mL | 242.9 | <3 | 3.3 | 11.3 | 10.4 |
Table 2 | Mean Mim8 concentration (µg/mL) | Standard deviation | Coefficient of variance (%) | Difference from expected target (%) |
Mim8 0µg/mL | <0.6 | 0.01 | 43.3 | not calculated |
Mim8 1µg/mL | 0.8 | 0.02 | 2.5 | -20.0 |
Mim8 2µg/mL | 1.9 | 0.04 | 2.2 | -5.2 |
Mim8 5µg/mL | 5.2 | 0.06 | 1.1 | 2.9 |
Mim8 8µg/mL | 8.7 | 0.12 | 1.4 | 8.9 |
Mim8 10µg/mL | 10.8 | 0.10 | 0.9 | 8.2 |
Mim8 15µg/mL | 16.6 | 0.02 | 0.1 | 10.6 |
Mim8 20µg/mL | 21.1 | 0.17 | 0.8 | 5.7 |
Mim8 reference 10.26µg/mL | 11.2 | 0.10 | 0.8 | 8.7 |
Conclusions: Human IXa/X and hybrid bovine X/human IXa CSAs (Table 1) demonstrated sensitivity to the presence of Mim8. Bovine IXa/X CSA were largely insensitive to Mim8 and can likely be used to accurately measure additional recombinant FVIII therapy or FVIII inhibitors in patients treated with Mim8. Mim8 concentration can be accurately determined in a modified Hyphen CSA with Mim8 calibrator.
To cite this abstract in AMA style:
Bowyer A, Ezban M, Kitchen S. Measuring the Chromogenic FVIII Mimetic Activity of the New Bispecific Antibody, Mim8, in Artificially Spiked Severe Haemophilia A Plasma [abstract]. Res Pract Thromb Haemost. 2021; 5 (Suppl 2). https://abstracts.isth.org/abstract/measuring-the-chromogenic-fviii-mimetic-activity-of-the-new-bispecific-antibody-mim8-in-artificially-spiked-severe-haemophilia-a-plasma/. Accessed March 22, 2024.« Back to ISTH 2021 Congress
ISTH Congress Abstracts - https://abstracts.isth.org/abstract/measuring-the-chromogenic-fviii-mimetic-activity-of-the-new-bispecific-antibody-mim8-in-artificially-spiked-severe-haemophilia-a-plasma/