Negatively charged, or classical, liposomes have been used to deliver encapsulated drugs for some time and have also been used as vehicles for gene transfer into cells in culture. Problems with the efficiency of nucleic acid encapsulation, coupled with a requirement to separate the DNA-liposome complexes from "ghost" vesicles has lead to the development of positively charged liposomes. Cationic lipids are able to interact spontaneously with negatively charged DNA to form clusters of aggregated vesicles along the nucleic acid. At a critical liposome density the DNA is condensed and becomes encapsulated within a lipid bilayer, although there is also some evidence that cationic liposomes do not actually encapsulate the DNA, but instead bind along the surface of the DNA, maintaining its original size and shape.

Cationic liposomes are also able to interact with negatively charged cell membranes more readily than classical liposomes. Fusion between cationic vesicles and cell surfaces might result in delivery of the DNA directly across the plasma membrane. This process bypasses the endosomal-lysosomal route which leads to degradation of anionic liposome formulations. Cationic liposomes can be formed from a variety of cationic lipids, and they usually incorporate a neutral lipid such as DOPE (dioleoylphosphatidyl-ethanolamine) into the formulation in order to facilitate membrane fusion. A variety of cationic lipids have been developed to interact with DNA, but perhaps the best known are DOTAP (N-1(-(2,3-dioleoyloxy)propyl)-N,N,N-trimethylammoniumethyl sulphate) and DOTMA (N-(1-(2,3-dioleoyloxy)propyl)-N,N,N-trimethylammonium chloride). These are commercially available lipids that are sold as in vitro transfecting agents, with the latter sold as Lipofectin.

There have been several studies on the in vivo, systemic use of liposome/DNA complexes. The factors controlling the transfection efficiency of liposome/DNA complexes following intravenous administration are still poorly understood. Complexes formed between the cationic lipid DOTMA and DNA are rapidly cleared from the bloodstream and were found to be widely distributed in the body and expression was detected mainly in the lungs but also in the liver, spleen, heart and kidneys. Similar results were found when DOTAP-based liposomes were used and it was found that the main factors controlling transfection efficiency were the structure of the cationic lipid and the ratio of the cationic lipid to DNA. The type of helper lipid used was also important as the addition of DOPE was found to reduce the in vivo transfection efficiency of DOTAP/DNA complexes.

The transfection efficiency of liposome/DNA complexes in vivo has been shown to be relatively low, especially when compared to viral vectors. One study has suggested that the in vivo transfection efficiency of adenoviruses is around 200 times greater than that observed with liposomes. One explanation for the relatively poor transfection efficiency of liposome/DNA complexes is that they are susceptible to disruption by serum proteins. A variety of proteins are known to bind to liposomes in vitro and in vivo and may membrane destabilisation. There are now serious efforts being made to develop liposomal vectors that are resistant to serum disruption. Novel cationic lipids are also being developed to try to improve the transfection efficiency of liposome/DNA complexes. Targeting of the liposomes to specific cell types has also been investigated as a means of improving the transfection efficiency.

There have been several clinical trials of liposome/DNA complexes, although almost all of these have been involved in the treatment of cystic fibrosis. Most protocols involve the use of DC-chol/DOPE liposomes directly instilled onto the nasal epithelium of CF patients. The effect of gene expression on CFTR function was determined and the presence of the gene in the target cells was determined by PCR (polymerase chain reaction) and Southern blot analysis.

Initial clinical studies found no evidence of any safety problems with the use of liposome/DNA complexes. This is surprising as it is well documented that the liposome/DNA complexes used in clinical trials are directly cytotoxic in vitro. Furthermore, studies in mice and macaques have demonstrated that exposure to high doses or to repeat doses of liposome/DNA complexes results in histopathology and gross lung pathology, suggesting that these vectors may not be as clinically safe as previously thought.