Dorothy L. Reimer

Biological barriers to cellular delivery of lipid-based DNA carriers.

Although lipid-based DNA delivery systems are being assessed in gene therapy 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 improve transfection efficiency, however because of the complex nature of the formulation technology being developed, it has been extremely difficult to define specific carrier attributes that enhance transfection. We believe that these optimization processes are flawed for two reasons. First, a very defined change in formulation components affects the physical and chemical characteristics of the carrier in many ways. As a consequence, it has not been 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 cell 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 regulating the transfection activity of these formulations. Although membrane destabilization activity of the lipid-based carriers plays an important role, we suggest here that a critical element required for efficient transfection is dissociation of lipids bound to the plasmid expression vector following internalization.

Self-Assembling DNA-Lipid Particles for Gene Transfer

AbstractPurpose. We have demonstrated that a heteromolecular complex consisting of cationic lipids and DNA can be prepared and isolated (1). Cationic lipids bind DNA through electrostatic interactions. However, when sufficient lipids are bound to DNA the physical and chemical properties of the complex are governed by hydrophobic effects. Here we describe an approach where this hydrophobic complex is used as an intermediate in the preparation of lipid-DNA particles (LDPs). Methods. The approach relies on the generation of mixed micelles containing the detergent, n-octyl β-D-glucopyranoside (OGP), the cationic lipid, N-N-dioleoyl-N, N-dimethylammonium chloride (DODAC), and selected zwitterionic lipids, 1,2-dioleoyl-sn-glycero-3 -phosphoethanolamine (DOPE) or egg sphingomyelin (SM). Results. When these micelles were prepared at low detergent concentrations (20 mM OGP) and combined with pCMVβ DNA, LDPs spontaneously formed. The mean diameter of these particles as measured by quasielastic light scattering was 55−70 nm, a result that was confirmed by negative stain electron microscopy. Further characterization of these LDPs showed that DNA within the particles was inaccessible to the small fluorochrome TO-PRO-1 and protected against DNase I degradation. LDPs could also be prepared in high concentrations of OGP (100 mM), however particles formed only after removal of OGP by dialysis. Particles formed in this manner were large (>2000nm) and mediated efficient transfection of Chinese hamster ovary cells. Transfection activity was greater when the lipid composition used consisted of SM/ DODAC. Small particles (<100nm) prepared of SM/DODAC were, however, inefficient transfecting agents. Conclusions. We believe that LDP formation is a consequence of the molecular forces that promote optimal hydrocarbon-hydrocarbon interactions and elimination of the hydrocarbon-water interface.

Analysis of Cationic Liposome-mediated Interactions of Plasmid DNA with Murine and Human Melanoma Cells in Vitro

Lipid-based DNA transfer formulations are typically selected on the basis of in vitro transfection studies where the activity of specific formulations is defined by transgene expression. It is unclear, however, whether expression is directly related to the efficiency of DNA transfer. In an attempt to correlate DNA transfer with transgene expression, we used a simple assay consisting of measuring DNA (3H-plasmid encoding for β-galactosidase) binding to murine (B16/BL6) and human (KZ) melanoma cells in vitro at 4 and 37 °C. The difference in cell association at these temperatures was assumed to be a consequence of DNA uptake, an assumption that was confirmed by protease removal of cell surface-associated DNA. DNA associated with B16/BL6 melanoma cells (up to 30 ng or 12% of the added DNA) following incubation with dioleoyldimethylammonium chloride/dioleoylphosphatidylethanolamine (DOPE) liposome-DNA aggregates was comparable to that achieved with 1,2-dioleoyloxypropyl-3-trimethylammonium bromide/DOPE or dimethyldioctadecylammonium bromide/DOPE liposomes; however, transgene expression was 2- and 5-fold less for the latter two formulations, respectively. Similarly, equivalent amounts of DNA delivery were achieved with B16/BL6 and KZ melanoma cells, yet the level of transgene expression in the KZ cells was undetectable. It was demonstrated that the lack of transgene expression was not a consequence of cell-specific differences in DNA degradation.

Lipid/DNA complexes as an intermediate in the preparation of particles for gene transfer: an alternative to cationic liposome/DNA aggregates

Abstract When cationic liposomes are mixed with plasmid DNA an aggregation reaction occurs. The heterogeneous membrane structures which arise following this reaction are dependent on liposomal lipid composition, liposome/DNA ratio as well as the presence of added salts or proteins. The resulting structures are also unstable, resulting in time dependent changes in the physical attributes of the aggregates. Pharmaceutical development of the liposome/DNA aggregates will be challenging because of these factors. For these reasons we have pursued development of alternative lipid-based systems as a vehicle for gene transfer. The pivotal step that led to development of these novel systems was the identification and isolation of a hydrophobic cationic lipid/DNA complex. This complex can be used as an intermediate in the preparation of well-defined particles. The hydrophobic lipid/DNA complex and the liposome/DNA aggregates are both formed as a consequence of multivalent electrostatic interactions. In contrast to the liposome/DNA aggregates, however, we believe that the particles formed when using lipid/DNA complex intermediates are a consequence of hydrophobic interactions.

15 Targeted Gene Transfer

The overall goal of gene therapy is to cure or stabilize a disease process that results from the production of a mutant protein (for example, the chloride channel protein important in cystic fibrosis) or overproduction of a normal protein (such as the products of certain oncogenes). We can achieve this goal by replacing the defective gene or by reducing the overexpression of the target gene using an antisense strategy, thus reducing the production of the diseasepromoting protein (1,2). For either method, it is critical to transfer DNA into target cells in a concentration high enough to be effective in modifying the disease. DNA must be delivered to the desired cell population in an intact state, whereby it can be efficiently transcribed and ultimately translated. The method of gene transfer must be highly efficient and nontoxic, and the delivery system must be relatively easy to prepare and administer (3). There is a great deal of optimism surrounding the development of gene therapy as an effective strategy for management of many different human diseases. The active agent used to procure gene therapy is likely to consist of oligonucleotides, ribozymes, or a DNA sequence that can be transcribed into a message capable of eliciting a therapeutic response. Unlike conventional small-molecule therapeutics however, gene therapy requires the use of a carrier system to deliver the active agent directly into the target cell population.