A recirculatory model for tissue plasminogen activator with saturable endothelial binding

Tissue plasminogen activator (t-PA) binds to the endothelium in vitro [1]. This binding may be crucial in the prevention of thrombus formation. The aim of the present study was to develop a mathematical model to quantify the binding of t-PA to the endothelium in vivo.

Nine healthy male volunteers received a continuous low dose infusion with recombinant t-PA (3.75 μg min⁻¹) and, as control, an infusion with indocycanine green (ICG; 0.5 mg min⁻¹), both for 40 min. A circulatory model was developed using non-linear mixed effect modelling.

Delays were observed in the ICG concentration profile that were not captured by a one- or two-compartment model. The ICG concentration profile was best described by a three-compartment recirculatory model with a total distribution volume of 3.0 l (s.e. mean 0.3, inter-individual coefficient of variation (CV) 17%) and plasma flow trough the ring of 0.77 l min⁻¹ (s.e. mean 0.20, CV 0%; fixed). t-PA antigen, activity and t-PA/PAI-1 complex profile showed a marked delay in increase at the beginning of the infusion, that was not captured by the circulatory model. Incorporating a reversible and concentration-dependent binding component in the model resulted in an accurate description of the t-PA concentration profile. In the resulting model (Figure 1), binding of t-PA was characterized by an affinity constant of 0.29 ml ng⁻¹ (s.e. mean 0.14, CV 114%) and a concentration of binding sites of 99 ng ml⁻¹ (s.e. mean 12, CV 0%).

Figure 1.

Recirculatory model for t-PA with saturable endothelial binding.

Clearance was estimated at 0.41 l min⁻¹ (s.e. mean 0.02, CV 18%), and endogenous production at 2.06 μg min⁻¹ (s.e. mean 0.07, CV 6%).

The in vivo characteristics of the t-PA binding site (Kd ∼ 51 pm) correspond with a high affinity t-PA endothelial binding site described in vitro (Kd 28 pM). The current model provides a means of quantification of t-PA binding and may provide a tool to study the t-PA binding in vivo. Assessing the capacity of the endothelium to bind t-PA and possibly other endogenous substances may serve as a novel tool to assess endothelial function.