Abstract Number: PB0680
Meeting: ISTH 2021 Congress
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
|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|
|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|
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, 2023.
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