The concept of liposome was first developed by Alec Bangham in 1965. Liposomes are spherical-shaped vesicles that can be created from cholesterol and natural non-toxic phospholipids, and their structure is similar to cell membranes. Liposomes have attracted attention as a carrier system for therapeutically active agents, owing to their unique characteristics, including biocompatibility, self-assembly capabilities, scalability, as well as a broad range of physical and chemical properties. Liposomes have shown great therapeutic potential as carriers for payloads and for delivery to targeted sites, which has led to several liposomal formulations designed for the preclinic and clinical trials, with applications ranging from cancer chemotherapy to gene therapy.

The application of liposomal drug delivery system has already had a major impact on many biomedical areas, especially in the pharmaceutical field. Due to extensive developments in liposome technology, many liposome-based drug formulations are available for human use and a variety of products are undergoing different clinical trials. In general, there are three main types of liposomal delivery systems.

※ Conventional Liposomes
Conventional liposomes, the first generation of liposomes, are made of phospholipids and have a high delivery to monocytic and phagocytic cells. Compared with free drugs, conventional liposomal formulations enhance the delivery of drugs to diseased tissues by modifying the pharmacokinetics and biodistribution, thereby reducing the toxicity of the compound in the body. However, the delivery system tends to be quickly eliminated from the bloodstream, thus limiting its therapeutic efficacy.

※ Sterically-stabilized Liposomes
To improve liposome stability, liposome formulations incorporating polyethylene glycol (PEG) have been introduced. It has a pronounced effect on liposome tissue distribution and can greatly improve the pharmacological efficacy of encapsulated antitumor drugs. Although coating liposomes with PEG prolong the circulation time, there may be an offsetting decrease in the ability to interact with the desired target.

※ Ligand-targeted Liposomes
Now Liposomes can be designed to be highly responsive in vivo, with active targeting, increased stealth, and controlled drug-release properties. Ligand-targeted liposomes (LTLs) have shown considerable promise in the treatment of cancer and are poised for clinical development. For example, in some breast cancer cases, human epidermal growth factor receptor 2 (HER2) is overexpressed at levels up to 1,000-fold higher than normal, and LTLs against HER2 have shown successful preclinical outcomes.

LTLs offer great potential for targeted delivery of drugs to designated cell types or organs in vivo, which selectively express or over-express specific ligands. Many types of ligands are available, such as proteins, peptides, and nucleic acids, which enable multiple directions for cancer cell localization.

In addition, the advent of liposomes marks an unparalleled opportunity to advance the treatment of a variety of diseases, including cancer. Compared with other ordinary cytotoxic drugs, liposome formulations have been reported to increase circulation life time, enhance deposition in the infected tissues, prevent the metabolic degradation of drugs, and alter tissue distribution of drugs. It has been found that liposomes get accumulated in tumors in a higher concentration in comparison to normal cells. Moreover, it has been evidenced that the antineoplastic activity of doxorubicin is increased when given as a liposomal formulation, accounting for its anticancer potential. Recent studites demonstrated that a gene transfer liposomal product named Allovectin-7TM was found to be effective against metastatic melanoma, renal cell and colorectal carcinoma, demonstrating its potential in gene therapy.

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