Computational modeling of factor Xa inhibition by immobilized tissue factor pathway inhibitor

Coating surfaces of implanted devices with anticoagulants can reduce thrombosis and studies using a recombinant form of endogenous tissue factor pathway inhibitor (rTFPI) are promising. The anticoagulant function of immobilized rTFPI is thought to occur primarily by its inhibition of plasma clotting factor Xa (FXa); however the kinetics of this reaction at a surface are as yet unknown. To better understand the surface inhibition reaction under flow conditions, a theoretical model was developed delineating the roles of mass transport and reaction kinetics for an in vitro parallel plate device used in prior experimental studies [Hall et al., J. Biomech. Eng. 120:484-490, 1998]. As a first approximation, the kinetics of inhibition of FXa by rTFPI reported for static, homogeneous systems was considered. The unsteady convection-diffusion equation was solved for different wall-shear rates and inlet concentrations of FXa using the computational fluid dynamics software CFD-ACE (ESI Software Group). The results show that the heterogeneous inhibition reaction is diffusion controlled prior to saturation of the rTFPI. The experimental results compare favorably with the model at the lower shear rates (100-400 s(-1)). At higher shear rates (>400 s(-1)) the theoretical results follow the same trend as the experimental results but show a greater inhibition of FXa, implying an effect of flow or shear on the inhibition reaction.