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Measurement of activated factor IX in factor IX concentrates: correlation with in vivo thrombogenicity
1Division of Haematology, NIBSC, South Mimms, Potters Bar, Hertfordshire, UK.
Insights
A new assay for activated FIX (FIXa) can predict thrombotic risk in high purity FIX (HP FIX) products. Measuring FIXa levels offers a better prediction of thrombogenicity than current in vitro tests.
Area of Science:
- Hematology
- Biochemistry
- Thrombosis Research
Background:
- Current in vitro tests inadequately assess thrombogenicity of high purity Factor IX (HP FIX) concentrates.
- Prothrombin complex concentrates (PCCs) are used in treating Factor IX deficiencies.
Purpose of the Study:
- Develop a chromogenic assay for activated Factor IX (FIXa).
- Evaluate FIXa assay's ability to predict in vivo thrombogenic potential of HP FIX products.
Main Methods:
- Developed a chromogenic assay for FIXa.
- Utilized a modified Wessler stasis model to assess in vivo thrombogenicity.
- Correlated FIXa levels with thrombus scores in the animal model.
Main Results:
- One of seven HP FIX products showed no detectable FIXa and no in vivo thrombogenicity.
- Six HP FIX products with detectable FIXa (0.15-1.2 U/1000 FIX) exhibited varying degrees of in vivo thrombogenicity.
- A significant positive correlation (r = 0.55, p < 0.02) was found between FIXa levels and in vivo thrombogenicity.
- Neither the Neutralizing Anti-Platelet Thrombin Time (NAPTT) nor the Thrombin and Factor Xa Generation Assay (TFCT) correlated significantly with in vivo results.
Conclusions:
- High purity FIX products may retain a residual thrombotic risk.
- Measuring FIXa content is a more reliable predictor of HP FIX thrombogenicity than current in vitro assays.
Abstract:
Current in vitro tests for thrombogenicity of FIX concentrates used for prothrombin complex concentrates (PCCs), are of little value when applied to high purity FIX (HP FIXs). In the present study, we have developed a chromogenic assay for activated FIX (FIXa) and evaluated its ability to predict in vivo thrombogenic potential of HP FIXs in a modified Wessler stasis model. Among the HP FIXs, only 1 out of 7 products had no detectable FIXa; this product also showed no in vivo thrombogenicity. In the other 6 products, FIXa content ranged from 0.15-1.2 U/1000 in FIX, and all showed some evidence of in vivo thrombogenicity, with mean thrombus scores ranging from 0.25-4. There was a significant positive correlation (r = 0.55, p < 0.02) between FIXa levels and in vivo thrombogenicity of HP FIXs. NAPTT data were not significantly correlated with the in vivo results and the TFCT also showed no direct correlation with the mean thrombus score. These results indicate that HP FIXs may still carry a small residual thrombotic risk and measurement of FIXa content of these products may be a better predictor of thrombogenicity than the current in vitro tests.
