Passive targeting is defined as a methodology to increase the target/nontarget ratio of the amounts of delivered drugs primarily by minimizing nonspecific interactions with nontarget organs, tissues, and cells. Nanoparticles are usually taken up by the liver, spleen, and other parts of the reticuloendothelial system (RES) (the macrophage is one of these elements). Depending on surface characteristics, nanopar-ticles can be taken up by the RES system. Particles with more hydrophobic surfaces (such as poly(propylene oxide), poly(methyl methacrylate), etc.) will be preferably taken up by the liver followed by the spleen and lungs (Brigger et al. 2002). To maximize circulation time and targeting ability, the nanoparticle size should be less than 100 nm (in diameter) and the surface should be hydrophilic to avoid clearance by macrophages. This can be achieved by coating nanoparticles with hydrophilic coatings (such as poly(ethylene glycol) (PEG), poloxamines, or poloxamers (Storm et al. 1995)). Particles that can avoid being taken up by the RES are called "stealth particles." The enhanced permeability and retention (EPR) effect due to leaky vascular structures and impaired drainage system of tumors creates opportunities to increase the transport of drugs from blood vessels into tumor tissues to retain drugs there. Drugs can be designed to have sizes larger than the pore size of blood vessels in healthy tissues (from 2 to 6 nm), but smaller than the pore size of blood vessels in tumor tissues (ranging from 100 to 780 nm) (Yuan et al. 1995; Hobbs et al. 1998).
The drugs will then accumulate preferentially at the tumor site and side effects of the drugs on healthy tissue will therefore be reduced. In addition, due to the ineffectiveness of the lymphatic system's operation at tumor sites, the drugs can be retained in the tumor interstitium longer, therefore increasing treatment effectiveness.
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Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...