Intravascular delivery of particulate systems: Does geometry really matter?

In cancer therapy and imaging, the systemic passive delivery of particulate systems has relied on the enhanced permeability and retention (EPR) effect: sufficiently small particles can cross the endothelial fenestrations and accumulate in the tumor parenchyma. The vast majority of man-made particulates exhibit a spherical shape as a result of surface energy minimization during their synthesis. The advent of phage display libraries, which are revealing the extraordinary molecular diversity of endothelial cells, and the development of processes for fabricating particles with shapes other than spherical are opening the path to new design solutions for systemically administered targeted particulates. In this paper, the role of particle geometry (i.e., size and shape) is discussed at the tissue and cellular scales. Emphasis is placed on how the synergistic effect of particle geometry and molecular targeting can enhance the specificity of delivery. The intravascular delivery process has been broken into three events: margination, firm adhesion and control of internalization. Predictions from mathematical models and observations from in-vitro experiments were used to show the relevance of particle geometry in systemic delivery. Rational design of particulate systems should consider, beside the physico-chemical properties of the surface coatings, geometrical features as size and shape. The integration of mathematical modeling with in-vitro and in-vivo testing provides the tools for establishing a rational design of nanoparticles.