Passive versus active targeting. (Left) In passive targeting, particles tend to passively diffuse through the leaky vasculature of the tumor bed and accumulate primarily through the enhanced permeability effect. (Right) In active targeting, once particles have extravasated in the target tissue, the presence of ligands on the particle surface facilitates their interaction with receptors that are present on tumor or other cells, resulting in enhanced accumulation and preferential cellular uptake through receptor-mediated processes. This approach can be used either for vascular targeting and/or tumor cell targeting purposes. Reproduced with permission from .