Abstract Number: PB0680
Meeting: ISTH 2021 Congress
Theme: Hemophilia and Rare Bleeding Disorders » Novel Biotherapeutics in Hemophilia
Background: Mim8 is a new human bispecific antibody designed for the subcutaneous prophylactic treatment of patients with haemophilia A (HA) with or without inhibitors. Mim8 mimics the function of activated factor (F)VIII by binding FIXa and FX, to facilitate activation of FX. Mim8 can influence the results of the one-stage activated partial thromboplastin time (APTT)-based FVIII clotting assays (OSA). Therefore, modification of OSA is needed to assess Mim8 drug concentration levels.
Aims: We investigated APTT and OSA using 7 reagents in severe HA (SHA) plasma spiked with Mim8. OSA were calibrated using both human plasma and product-specific calibrators.
Methods: Mim8 was spiked into SHA plasma at 0–20µg/mL. APTT and human plasma calibrated (1/10 dilution) OSA were evaluated on either a Sysmex CS5100 or ACL TOP 700 analyser. Modified OSA were evaluated on three days with each APTT reagent using a Mim8 reference product (at 10.26µg/mL) as calibrator and 1/80 plasma dilution.
Results: The APTT with all reagents was shortened to within normal limits at 1µg/mL Mim8 (Table 1). Standard mimetic FVIII activity OSA were >250IU/dL at Mim8 concentrations above 1µg/mL (data not shown).
Mim8 reference product calibrated modified OSA demonstrated some reagent variation (Table 2). However, the between-day coefficient of variance was <20% for all reagents and Mim8 concentrations. The recovery of Mim8 compared to expected target concentration was within 22% for all reagents and Mim8 concentrations.
| Table 1 | Actin® APTT (s) | Actin® FS APTT (s) | Actin® FSL APTT (s) | Pathromtin® SL APTT (s) | APTT SP APTT (s) |
SynthASil® APTT (s) | SynthAFax® APTT (s) |
| Reference range (s) | 20.9-30.3 | 19.2-27.9 | 25-31.3 | 26.4-37.5 | 25.4-36.9 | 23.2-33.4 | 22.4-32.9 |
| Mim8 0 µg/mL | 105.6 | 82.8 | 92.2 | 138.4 | 102.2 | 119 | 51.3 |
| Mim8 1 µg/mL | 19.7 | 22.4 | 26 | 32.7 | 25.9 | 28.4 | 16.9 |
| Mim8 2 µg/mL | 16.6 | 19.4 | 23.2 | 27.5 | 22.5 | 24.9 | 14.9 |
| Mim8 5 µg/mL | 13.9 | 16.9 | 20.8 | 23.1 | 19.8 | 21.4 | 13.3 |
| Mim8 8 µg/mL | 12.5 | 15.8 | 19.6 | 22 | 18.2 | 20 | 12.7 |
| Mim8 10 µg/mL | 12.2 | 15.4 | 19.3 | 20.7 | 18 | 19.2 | 12.2 |
| Mim8 15 µg/mL | 11.3 | 14.6 | 18.7 | 19 | 16.6 | 18.3 | 12.1 |
| Mim8 20 µg/mL | 11.1 | 14.2 | 18.3 | 18.5 | 16.6 | 17.8 | 11.9 |
APTT of severe haemophilia A plasma spiked with Mim8.
| Table 2 | Actin® (Mim8 µg/mL) |
Actin® FS (Mim8 µg/mL) |
Actin® FSL (Mim8 µg/mL) |
Pathromtin® SL (Mim8 µg/mL) |
APTT SP (Mim8 µg/mL) |
SynthASil® (Mim8 µg/mL) |
SynthAFax® (Mim8 µg/mL) |
| Mim8 0 µg/mL | <0.6 | <0.6 | <0.6 | <0.6 | <1 | <1 | <1 |
| Mim8 1 µg/mL | 0.97 | 1.11 | 0.90 | 0.93 | 0.98 | 1.22 | 1.08 |
| Mim8 2 µg/mL | 2.00 | 2.19 | 1.97 | 2.20 | 2.16 | 2.24 | 2.28 |
| Mim8 5 µg/mL | 4.80 | 4.97 | 4.70 | 4.87 | 5.14 | 4.97 | 5.37 |
| Mim8 8 µg/mL | 8.10 | 8.16 | 8.23 | 8.03 | 8.56 | 8.09 | 8.47 |
| Mim8 10 µg/mL | 9.93 | 9.95 | 9.87 | 9.80 | 9.89 | 10.00 | 10.12 |
| Mim8 15 µg/mL | 15.77 | 16.24 | 16.10 | 15.73 | 14.93 | 15.47 | 15.38 |
| Mim8 20 µg/mL | 20.73 | 22.38 | 22.97 | 20.73 | 20.25 | 19.61 | 21.38 |
| Mim8 reference product 10.26 µg/mL | 10.40 | 10.55 | 10.23 | 10.40 | 10.07 | 10.23 | 11.14 |
Mim8 concentration in spiked severe haemophilia A plasma measured using modified one-stage assays and product-specific standard
Conclusions: The APTT with all reagents was shortened to within normal limits at the lowest Mim8 concentration studied; this corresponded to falsely elevated FVIII:C measurements using standard OSA. Standard OSA should not be performed in patients treated with Mim8. A modified OSA calibrated using a Mim8 calibrator enabled accurate measurement of Mim8 drug concentration and was reproducible with all APTT reagents studied.
To cite this abstract in AMA style:
Bowyer A, Ezban M, Kitchen S. Measuring the FVIII Mimetic Activity of the New Bispecific Antibody, Mim8, in Severe Haemophilia A Plasma Using APTT and One-stage FVIII Assays [abstract]. Res Pract Thromb Haemost. 2021; 5 (Suppl 2). https://abstracts.isth.org/abstract/measuring-the-fviii-mimetic-activity-of-the-new-bispecific-antibody-mim8-in-severe-haemophilia-a-plasma-using-aptt-and-one-stage-fviii-assays/. Accessed November 29, 2021.« Back to ISTH 2021 Congress
ISTH Congress Abstracts - https://abstracts.isth.org/abstract/measuring-the-fviii-mimetic-activity-of-the-new-bispecific-antibody-mim8-in-severe-haemophilia-a-plasma-using-aptt-and-one-stage-fviii-assays/