Active Targeting

Active targeting to the tumor can be achieved by molecular recognition of cancer cells either via ligand-receptor or antibody-antigen interactions. Active targeting may also lead to receptor-mediated cell internalization of drug carrier system. Nanoparticles and other polymer drug-conjugates offer numerous opportunities for targeting tumors through surface modifications which allow specific biochemical interactions with the proteins/receptors expressed on target cells.19,20 For active and passive targeting of drug carrier systems, it is essential to avoid their uptake by the reticuloendothelial system (RES) so that they remain in the blood circulation and extravasate in the tumor vasculature. Particles with more hydrophobic surfaces are preferentially taken up by the liver, followed by the spleen and lungs.21,22 Size of nanoparticles as well as their surface characteristics are the key parameters that can alter the biodistribution of nanoparticles. Particles smaller than 100 nm and coated with hydrophilic polymers such as amphiphilic polymeric compounds which are made of polyethylene oxide such as poloxamers, poloxamines, or polyethylene glycol (PEG) are being investigated to avoid their uptake by the RES. To improve the efficacy of targeting cancer chemotherapeutics to the tumor, a combination of passive and active targeting strategy is being investigated where long-circulating drug carriers are conjugated to tumor cell-specific antibody or peptides.23 In addition to the above approach, direct intratumoral injection of the carrier system is feasible if the tumor is localized and can be accessed for administration of a

carrier system.24

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