Micelles

Micelles are self-assembling nanosized colloidal particles with a hydrophobic core and hydrophilic shell (Fig. 2). Micelles have been successfully used to deliver poorly soluble pharmaceuticals, and they are attractive drug carriers for cancer therapy. Polymeric micelles, micelles formed from amphiphilic block co-polymers, possess high stability both in vitro and in vivo and good biocompatibility. Lipid-core micelles are micelles that are formed by conjugating soluble copolymers with lipids. Of these, polyethylene glycol-phosphatidyl ethanolamine conjugate (PEG-PE) lipid-core micelles are of special interest. These micelles can effectively solubilize a broad variety of poorly soluble drugs (anticancer drugs in particular) and diagnostic agents.

Drug-loaded lipid-core micelles can spontaneously target body areas with compromised vasculature (tumors, infarcts) via EPR effect. Lipid-core mixed micelles containing certain specific components (such as positively charged lipids) are capable of escaping endosomes and delivering incorporated drugs directly into the cell cytoplasm. Various specific targeting ligand molecules (such as antibodies) can be attached to the surface of the lipid-core micelles and bring drug-loaded micelles to and into target cells. Lipid-core micelles carrying various reporter (contrast) groups may become the imaging agents of choice in different imaging modalities (89).

Figure 2 Depiction of polymeric micelles as carriers for drug delivery. Source: Reprinted from Ref. 88 with permission from Elsevier.

Micelles prepared from amphiphilic PEG-PE represent a particulate drug delivery system capable of accumulation in tumors via EPR and can be used as a tumor-specific delivery system for poorly soluble anticancer drugs (90). Micelles were prepared by mixing PEG-PE and d-alpha-tocopheryl polyetheyene glycol 1000 succinate (TPGS), and loaded with the poorly soluble anticancer drug camptothecin (CPT). The cytotoxicity of the CPT-loaded mixed micelles against various cancer cells in vitro was remarkably higher than that of the free drug (91). Paclitaxel-loaded mixed polymeric micelles consisting of PEG-PE, solid triglycerides (ST), and cationic lipofectin lipids (LL) were able to escape from endosomal degradation and deliver drug to the cytoplasm of BT-20 cancer cells, increasing anticancer efficiency (92).

Environmentally sensitive micelles prepared from poly(2-ethyl-2-oxazoline)-b-poly(L-lactide) diblock copolymers (PEOz-PLLA) make a promising carrier to transport anticancer drugs specifically to tumor cells and release the drug molecules inside the cell cytosol for improved chemotherapy (93,94). Polymer micelles can also be prepared with cross-linked ionic cores that display high stability. Block ionomer complexes of poly(ethylene oxide)-b-poly(methacrylic acid) copolymer and divalent metal cations have been utilized as micellar templates for the synthesis of the cross-linked micelles. Such micelles represent hydrophilic nanospheres of core-shell morphology. Cisplatin, a potent chemotherapeutic agent, was incorporated into the ionic core of the micelles with remarkably high efficiency (22% w/w) (95).

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