Optimization of cationic liposomal complexes for in vivo applications is complex, involving many diverse components including nucleic acid purification, plasmid design, delivery vehicle formulation, administration route and schedule, dosing, detection of gene expression, and others. Optimizing all components of the delivery system is pivotal and will allow broad use of liposomal complexes to treat human diseases or disorders. This article will highlight the features of liposomes that contribute to successful delivery, gene expression, and efficacy.

Delivery of nucleic acids using liposomes is promising as a safe and nonimmunogenic approach to gene therapy and would overcome the numerous disadvantages of viral vectors. Furthermore, gene therapeutics composed of artificial reagents can be standardized and regulated as drugs rather than as biologics. Cationic lipids have been used for efficient delivery of nucleic acids to cells in tissue culture for several years.[1] Much effort has also been directed toward developing cationic liposomes for efficient delivery of nucleic acids in animals and in humans. Most frequently, the formulations that are best to use for transfection of a broad range of cell types in culture are not optimal for achieving efficacy in animals or people. Functional properties defined in vitro do not assess the stability of the complexes in plasma, pharmacokinetics, biodistribution, and colloidal properties that are essential for optimal activity in vivo. Nucleic acid-liposome complexes must also traverse tight barriers in vivo and penetrate throughout the target tissue to produce efficacy for the treatment of many diseases. These are not issues for achieving efficient transfection of cells in culture with the exception of polarized tissue culture cells. Therefore, optimized in vivo liposomes may differ from those used for efficient tissue culture transfection.

Different types of nucleic acids of unlimited size can be delivered using liposomes. Recent advances have dramatically improved transfection efficiencies and efficacy of liposomal vectors.[2-6] We demonstrated broad efficacy of a robust liposomal delivery system in small and large animal models for lung,[3] breast,[5] head and neck (Hung and Templeton, unpublished data), and pancreatic cancers,[4] and for hepatitis B and C (Clawson and Templeton, unpublished data). This liposomal delivery system is currently used in a clinical trial to treat non-small-cell lung cancer and will be used in upcoming clinical trials to treat other cancers and cardiovascular diseases. Reviews of other in vivo delivery systems and improvements using cationic liposomes have been published recently.[7,8]

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