Tissue Concentration of Heparin, Not Administered Dose, Correlates with the Biological Response of Injured Arteries in Vivo

Drug activity is often studied in well controlled and characterized cellular environments in vitro. However, the biology of cells in culture is only a part of the tissue behavior in vivo. Quantitative studies of the dose response to drugs in vivo have been limited by the inability to reliably determine or predict the concentrations achieved in tissues. We developed a method to study the dose response of injured arteries to exogenous heparin in vivo by providing steady and predictable arterial levels of drug. Controlled-release devices were fabricated to direct heparin uniformly and at a steady rate to the adventitial surface of balloon-injured rat carotid arteries. We predicted the distribution of heparin throughout the arterial wall by using computational simulations of intravascular drug binding and transport, and we correlated these concentrations with the biologic response of the tissues. This allowed the estimation of the arterial concentration of heparin required to maximally inhibit intimal hyperplasia after injury in vivo, 0.3 mg/ml. This estimation of the required concentration of drug seen by a specific tissue is independent of the route of administration and holds for all forms of drug release. In this way we may now be able to evaluate the potential of widely disparate forms of drug release and to finally create some rigorous criteria by which to guide the development of particular delivery strategies for local diseases.