Abstract Number: PB0655
Meeting: ISTH 2021 Congress
Background: Adeno-associated virus (AAV) gene therapies for haemophilia B are being developed to produce sustained increases in factor IX (FIX) serum levels in treated patients. A hyperfunctional FIX variant (FIX-Padua, FIX-R338L) is being utilized across gene therapy programmes to achieve FIX activity levels within the normal range (50%-150%). Despite the broad usage of FIX-R338L in AAV gene therapies, the mechanisms underlying the FIX-R338L gain of function are not yet fully understood.
Aims: To report the development of computational models for characterizing differences in activity between FIX-R338L and FIX-WT.
Methods: Published data on the kinetic constants and respective thrombin generation of FIX-R338L (Samelson-Jones et al 2019) and FIX-R338A (a related variant; Chang et al 1998) and aPTT data were used in concert with published ordinary differential equation-based models of aPTT and thrombin generation to guide the development of accurate FIX-R338L computational models.
Results: Computational modelling showed that KM and kcat differences between the FIX-R338L and FIX-WT derived intrinsic tenases are sufficient to account for observed functional differences between the two variants. To achieve good correspondence with both aPTT and thrombin generation data, the model suggests that catalytic efficiency (kcat/KM) of FIX-R338L tenase must be ~7-fold greater than that of FIX-WT tenase. The modelling suggests that both a lower KM and a greater kcat are required to capture the FIX-R338L gain of function observed in empirical aPTT and thrombin generation data. Computational model outputs are consistent with our empirical findings in clinically relevant FIX assays showing that factor X, but not factor VIII, impacts FIX-R338L activity estimates.
Conclusions: Results to date suggest that factor X levels affect FIX-R338L activity as measured by clinically relevant FIX assays. Iterative evaluation of KM and kcat for FIX-R338L led to refinement of computational models that will serve as powerful tools for understanding the mechanisms that contribute to FIX-R338L gain of function.
To cite this abstract in AMA style:Orfeo T, Gissel M, Everse SJ, Shehu E, Corbau R, Foley JH. Development of Ordinary Differential Equation-based aPTT and Thrombin Generation Models for Characterizing FIX-Padua Gain of Function [abstract]. Res Pract Thromb Haemost. 2021; 5 (Suppl 2). https://abstracts.isth.org/abstract/development-of-ordinary-differential-equation-based-aptt-and-thrombin-generation-models-for-characterizing-fix-padua-gain-of-function/. Accessed February 28, 2024.
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