The concept of using nanocarriers for the selective delivery of therapeutic and imaging agents in cancer has been reinvigorated by the emergence of exciting new approaches for nanocarrier design and fabrication. In addition to liposomes, which are already widely used for clinical purposes [1,2], other nanocarriersincluding polymeric nanoparticles (NPs), dendrimers, block co-polymer micelles, magnetic NPs and various inorganic NPs -are under intense investigation [3][4][5]. Each of these technologies has its merits and liabilities, but all are affected by similar biological parameters that govern the abilities of nanocarriers to distribute into tissues and be taken up by cells. Thus, the design of nanocarriers for potential therapeutic or imaging applications should be guided by consideration of the biological barriers that stand between the NP at its point of entry into the body and its ultimate destination. The biological barrier concept has a long history in the drug delivery field [6], and has proven quite useful in this context.
Basic Characteristics of the Vasculature and Mononuclear Phagocyte SystemAlthough nanocarriers may be used in a variety of local applications (topical, inhalational, etc.), the primary concern here will be with material administered intravenously for systemic use. Thus, the architecture of the vasculature, the composition of the blood, the body's system for particle clearance (the mononuclear phagocyte system), and the permeability characteristics of the vascular endothelium will all be key factors in determining the biodistribution of nanocarriers. Understanding these factors and avoiding the rapid sequestration of administered nanocarriers at undesirable sites represents the first major barrier to the effective use of NP technology for drug delivery. 263 Nanobiotechnology II. Edited by Chad A. Mirkin and Christof M. Niemeyer