Nanoparticles are rapidly coated with blood components [6, 7, 198, 199, 249, 250] in the same manner as other colloidal carriers such as liposomes, microemulsions, erythro-cyte ghosts, etc. This process is called opsonization. These opsonizing molecules trigger a rapid uptake of the colloidal drug carriers mainly by the sessile but also by the wandering macrophages of the RES . As a consequence, shortly after intravenous injection of such carriers about 60 to 90% of the injected dose is distributed into the liver, 2-20% into the spleen, a varying amount into the lungs, and a smaller amount into the bone marrow [7, 198].
Some nanoparticles have the tendency to accumulate in certain solid tumors [25, 264]. This accumulation is due to the so-called EPR effect  that also can be observed with the other particulate colloidal carriers as well as with soluble macromolecules. The EPR effect can be attributed to two main factors [284-286]: First, the endothelium around the blood vessels in tumors often is very discontinuous and leaky and therefore allows the extravasation of larger partic-ulates. Second, no lymphatic drainage exists in these tumors that normally would remove these particulates. However, in order to take good advantage of this effect, the particles have to stay long enough in the circulation to reach the tumor and must not be removed before by the macrophages. A prolonged circulation can be achieved by pegylation (see Section 4.7) or by coating with surfactants (see Section 6.1.2).
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