Targeting of tumors on the pathophysiological principles and physicochemical aspects of delivery systems

A solid tumor comprises two major cellular components: the tumor parenchyma and the stroma; the latter incorporating the vasculature and other supporting cells. As the tumor grows, in order to meet the metabolic requirements of an expanding population of tumor cells, the pre-existing blood vessels become subject to intense angiogenic pressure. Several factors produced by tumor cells and infiltrating immune-competent effector cells in the tumor parenchyma are believed to signal the development of new capillaries from the pre-existing vessels by capillary sprouting and/or dysregulated intussusceptive microvascular growth. Further, in many solid tumors, endothelial cells destined to create new vessels are recruited not only from nearby vessels, but also to a significant extent from precursor cells within the bone marrow (so-called endothelial progenitor cells), a process referred to as "vasculogenesis".