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11/5/2006 11:22:53 AM
Self-Assembling Nanoparticle Releases Drug Into Cell Cytoplasm

A growing body of research leaves no doubt that nanoparticles can transport drugs into cancer cells, but what is less clear is that the drug payload can exit the tiny endosomes that pull nanoparticles into cells. As a result, there is still some concern that only a small portion of a nanoparticle’s drug payload will actually get into the cell’s cytoplasm and attack its intended intracellular molecular target.

Researchers at the University of Ulsan College of Medicine in Seoul, Korea, have developed a self-assembling nanoparticle that can sense the low pH of endosomes and fall apart. The disintegration of these nanoparticles not only releases their drug payload but enables it to exit the endosomes. Yong Woo Cho, Ph.D., led this project, the results of which are reported in the Journal of Controlled Release.

Chitosan, a polymer of the sugar glucosamine obtained from crab and shrimp shells, serves as the starting material for these self-assembling nanoparticles. The investigators modify the polymer by attaching a chemical derivative of the amino acid histidine to each of the sugar units in the chitosan backbone. At neutral pH, histidine is hydrophobic, or poorly soluble in water. The presence of multiple histidines on the water-soluble, or hydrophilic, chitosan backbone creates a molecule that naturally self-assembles into a structure that surrounds the hydrophobic histidines with a protective shell of hydrophilic chitosan.

When added to cells grown in culture, the nanoparticles fuse with the cell membrane, forming endosomes that pinch off inside the cell. At the low pH found inside an endosome, histidine takes on a positive charge and becomes hydrophilic, too. As a result, the physical forces that held together the self-assembling nanoparticle no longer exist and the nanoparticle falls apart. Any drug molecules entrapped within the nanoparticle would then be released into the endosomes.

By itself, nanoparticle disassembly would not be enough to ensure that entrapped drug molecules would escape from an endosome and enter the cell’s cytoplasm. However, positively charged histidine also destabilized the membrane that forms the endosome, allowing drug to escape into the rest of the cell. Experiments with nanoparticles loaded with a fluorescent dye showed clearly that this process was occurring. Subsequent experiments with nanoparticles containing the drug paclitaxel confirmed these results; the released drug successfully stopped cell growth and reproduction.

This work is detailed in a paper titled, “N-acetyl histidine-conjugated glycol chitosan self-assembled nanoparticles for intracytoplasmic delivery of drugs: endocytosis, exocytosis and drug release.” Investigators from Hanyang University in Seoul, Korea, and Yeungnam University in Gyeongsan, Korea, also participated in this study. An abstract of this paper is available through PubMed.

View abstract.

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