Targeted drug delivery to tumor sites is one of the ultimate goals in drug delivery. Recent progress in nanoparticle engineering has certainly improved drug targeting, but the results are not as good as expected. This is largely due to the fact that nanoparticles, regardless of how advanced they are, find the target as a result of blood circulation, like the conventional drug delivery systems do. Currently, the nanoparticle-based drug delivery to the target tumor tissues is based on wrong assumptions that most of the nanoparticles, either PEGylated or not, reach the target by the enhanced permeation and retention (EPR) effect. Studies have shown that so-called targeting moieties, i.e., antibodies or ligands, on the nanoparticle surface do not really improve delivery to target tumors. Targeted drug delivery to tumor sites is associated with highly complex biological, mechanical, chemical and transport phenomena, of which characteristics vary spatiotemporally. Yet, most of the efforts have been focused on design and surface manipulation of the drug carrying nanoparticles with relatively little attention to other aspects. This article examines the current misunderstandings and the main difficulties in targeted drug delivery.
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