The major limitation of liposomes is that they are not as efficient as viral vectors in transfecting many cell lines. Until recently, their medical utility was limited by their rapid uptake by phagocytic cells. Interest in liposomes as a drug carrier was rejuvenated by two recent technological advances that in essence produced a renaissance in the field. These stealth liposomes are usually composed of lipids containing poly-ethylene glycol. These PEG containing lipids provide a stearic barrier outside the membrane.
Cationic liposomes can condense DNA and increase transfection yields several orders of magnitude.
In the DNA-cationic liposome complex, the nucleic acids or oligonucleotides are not encapsulated but are simply complexed with small unilamellar vesicles by electrostatic interactions. With the liposome complexing with DNA complexes, liposome aggregation and fusion and DNA condensation also occur. Almost all gene therapy experiments with liposomes have been done with these cationic liposomes. A major challenge of gene therapy remains the systemic delivery of transgenes to the tumor or peritumoral area that will effectively decrease the size of the primary tumors and its metastases. There are other non -viral carriers which hold a great deal of promise.