Biological barriers to cellular delivery of lipid-based DNA carriers

Although lipid-based DNA delivery systems are being assessed in gene therap y clinical trials, many investigators in this field are concerned about the inefficiency of lipid-based gene transfer technology, a criticism directed at all formulations used to enhance transfer of plasmid expression vectors . It is important to recognize that many approaches have been taken to impr ove transfection efficiency, however because of the complex nature of the f ormulation technology being developed, it has been extremely difficult to d efine specific carrier attributes that enhance transfection. We believe tha t these optimization processes are flawed for two reasons. First, a very de fined change in formulation components affects the physical and chemical ch aracteristics of the carrier in many ways. As a consequence, it has not bee n possible to define structure/activity relationships. Second, the primary endpoint used to assess plasmid delivery has been transgene expression, an activity that is under the control of cellular processes that have nothing to do with delivery. Gene expression following administration of a plasmid expression vector involves a number of critical steps: (i) DNA protection, (ii) binding to a specific cell population, (iii) DNA transfer across the c ell membrane, (iv) release of DNA into the cytoplasm, (v) transport through the cell and across the nuclear membrane as well as (vi) transcription and translation of the gene. The objective of this review is to describe lipid -based DNA carrier systems and the attributes believed to be important in r egulating the transfection activity of these formulations. Although membran e destabilization activity of the lipid-based carriers plays an important r ole, we suggest here that a critical element required for efficient transfe ction is dissociation of lipids bound to the plasmid expression vector foll owing internalization. (C) 1999 Elsevier Science B.V. All rights reserved.