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    Limited information exists regarding the factor IX (FIX) coagulant activity (FIX:C) measured by different assays following FIX-Padua gene therapy. Assess for the first time FIX:C in five commonly used coagulation assays in plasma samples from hemophilia B subjects receiving FIX-Padua gene transfer. FIX:C was compared between central (n = 1) and local laboratories (n = 5) in the study, and across four commonly used FIX:C one-stage assays and one FIX:C chromogenic assay. For comparison, samples of pooled congenital FIX-deficient plasma spiked with purified recombinant human FIX (rHFIX)-Padua protein or rHFIX (nonacog alfa) to obtain FIX:C concentrations from ~20% to ~40% were tested. FIX:C results at local laboratories strongly correlated with central laboratory results. However, absolute values at the central laboratory were consistently lower than those at local laboratories. Across five different FIX:C assays, a consistent pattern of FIX:C was observed for subjects receiving fidanacogene elaparvovec-expressed gene transfer. Use of Actin FSL activated partial thromboplastin time (APTT) reagent in the central laboratory resulted in lower FIX:C values compared with other APTT reagents tested. The chromogenic assay determined lower FIX:C than any of the one-stage assays. The rHFIX-Padua protein-spiked samples showed similar results. In contrast, FIX:C results for rHFIX-nonacog alfa measured within 25% of expected for all one-stage assays and below 25% in the chromogenic assay. Assay-based differences in FIX:C were observed for fidanacogene elaparvovec transgene product and rHFIX-Padua protein, suggesting the variable FIX:C determined with different assay reagents is inherent to the FIX-Padua protein and is not specific to gene therapy-derived FIX-Padua. © 2021 Pfizer Inc. Journal of Thrombosis and Haemostasis published by Wiley Periodicals LLC on behalf of International Society on Thrombosis and Haemostasis.


    Mary M Robinson, Lindsey A George, Marcus E Carr, Benjamin J Samelson-Jones, Valder R Arruda, John E Murphy, Denis Rybin, Jeremy Rupon, Katherine A High, Stefan Tiefenbacher. Factor IX assay discrepancies in the setting of liver gene therapy using a hyperfunctional variant factor IX-Padua. Journal of thrombosis and haemostasis : JTH. 2021 May;19(5):1212-1218

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    PMID: 33636038

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