Liposomes as carriers for polypeptides

In light of the increasing research focus on intravenous liposomes for altering drug toxicity and efficacy, their use in sustained release applications is overshadowed. For example, though known for more than 12 years, the employment of intramuscular or subcutaneous routes as depot sites for prolonging delivery has only been peripherally examined with few applications to drugs of importance. Furthermore, an even smaller subset of those studies has examined the properties of specific peptide formulations. Likewise, information about use of liposome preparations to enhance peptide absorption from other sites of application is both fleeting and controversial. Nonetheless, enough of a research foundation exists to justify future investigations of these modes. This review has taken a practical approach toward identifying those factors which must be considered to permit development of successful liposomal peptide formulations. The questions addressed are of both a scientific and a commercial nature: Can peptides be slowly delivered in active form given the adjuvant potential of liposomes? How efficient is the delivery? What liposome process methods adapt well to peptide technology? How costly is it to produce a liposome peptide formulation? What parameters of stability must be monitored? What mechanisms govern the release of peptide from the liposome? How important is liposome clearance from the injection site in establishment of blood levels? What biocompatibility issues need be addressed for future clinical applications? What delivery periods should be expected? It is hoped that the answers to these questions plus some newer perspectives will provide a more coherent picture of the past, present, and future uses of liposomes in peptide delivery.