Tumor vascular endothelium: Barrier or target in tumor directed drug delivery and immunotherapy

The therapy of solid tumors with conventional chemotherapeutics, drug delivery preparations and immunomodulatory agents directed against the tumor cells is corrupted by a major barrier presented by the tumor vasculature. Permeability of the tumor blood vessels for transport of small molecules and macromolecular drug-carrier conjugates is only sufficient in the blood vessels at the tumor-host interface. Downregulation of the expression of adhesion molecules, required for the facilitation of immune cell recruitment, by the tumor vascular endothelium results in an escape of the tumor from host defence. New therapeutic approaches for the treatment of solid tumors are aimed at the tumor vasculature, either at the endothelial cells themselves or at basement membrane or tumor stroma components. Angiogenesis can be directly blocked with angiogenesis inhibitors, while angiogenesis related factors can serve as tumor vasculature specific epitopes for drug delivery strategies. Some glycoproteins expressed by tumor endothelial cells or present in the basement membrane and tumor stroma are also potential tumor selective targets. Therapeutic modalities that are suitable for site specific delivery are agents that increase tumor accumulation of (targeted) chemo/radiotherapeutics through increasing tumor vascular permeability. The observation that for tumor growth the blood supply is a limiting factor, led to the development of strategies to inhibit angiogenesis or block the tumor blood flow. Manipulation of the expression of endothelial cell adhesion molecules by selectively delivering modulatory agents at or in the tumor vascular endothelial cells may induce (bispecific antibody mediated) host defense activity directed against the tumor cells.