Targeting lipopolyplexes using bifunctional peptides incorporating hydrophobic spacer amino acids: synthesis, transfection, and biophysical studies.

We have developed efficient synthetic routes to two hydrophobic amino acids, suitably protected for solid-phase peptide synthesis, and have successfully synthesized peptides containing these or other hydrophobic amino acids as spacers between a Lys16 moiety and an integrin-targeting motif. These peptides have in turn been used to formulate a range of lipopolyplex vectors with Lipofectin and plasmid DNA. The transfection efficiencies of these vectors and their aggregation behavior in buffers and in serum have been studied. We have shown that vectors containing peptides incorporating long linkers that are entirely hydrophobic are less efficient transfection agents. However, linkers of equivalent length that are in part hydrophobic show improved transfection properties, which is probably due to the improved accessibility of the integrin-binding motif.

Biophysical characterization of an integrin-targeted lipopolyplex gene delivery vector.

Nonviral gene delivery vectors now show good therapeutic potential: however, detailed characterization of the composition and macromolecular organization of such particles remains a challenge. This paper describes experiments to elucidate the structure of a ternary, targeted, lipopolyplex synthetic vector, the LID complex. This consists of a lipid component, Lipofectin (L) (1:1 DOTMA:DOPE), plasmid DNA (D), and a dual-function, cationic peptide component (I) containing DNA condensation and integrin-targeting sequences. Fluorophore-labeled lipid, peptide, and DNA components were used to formulate the vector, and the stoichiometry of the particles was established by fluorescence correlation spectroscopy (FCS). The size of the complex was measured by FCS, and the sizes of LID, L, LD, and ID complexes were measured by dynamic light scattering (DLS). Fluorescence quenching experiments and freeze-fracture electron microscopy were then used to demonstrate the arrangement of the lipid, peptide, and DNA components within the complex. These experiments showed that the cationic portion of the peptide, I, interacts with the plasmid DNA, resulting in a tightly condensed DNA-peptide inner core; this is surrounded by a disordered lipid layer, from which the integrin-targeting sequence of the peptide partially protrudes.

Analysis and optimization of the cationic lipid component of a lipid/peptide vector formulation for enhanced transfection in vitro and in vivo.

We have previously described a lipopolyplex formulation comprising a mixture of a cationic peptide with an integrin-targeting motif (K16GACRRETAWACG) and Lipofectin, a liposome consisting of DOTMA and DOPE in a 1:1 ratio. The high transfection efficiency of the mixture involved a synergistic interaction between the lipid/peptide components. The aim of this study was to substitute the lipid component of the lipopolyplex to optimize transfection further and to seek information on the structure-activity relationship of the lipids in the lipopolyplex. Symmetrical cationic lipids with diether linkages that varied in alkyl chain length were formulated into liposomes and then incorporated into a lipopolyplex by mixing with an integrin-targeting peptide and plasmid DNA. Luciferase transfections were performed of airway epithelial cells and fibroblasts in vitro and murine lung airways in vivo. The biophysical properties of lipid structures and liposome formulations and their potential effects on bilayer membrane fluidity were determined by differential scanning calorimetry and calcein-release assays. Shortening the alkyl tail from C18 to C16 or C14 enhanced lipopolyplex and lipoplex transfection in vitro but with differing effects. The addition of DOPE enhanced transfection when formulated into liposomes with saturated lipids but was more variable in its effects with unsaturated lipids. A substantial improvement in transfection efficacy was seen in murine lung transfection with unsaturated lipids with 16 carbon alkyl tails. The optimal liposome components of lipopolyplex and lipoplex vary and represent a likely compromise between their differing structural and functional requirements for complex formation and endosomal membrane destabilization.

The fractal structure of polycation-DNA complexes.

We used static light scattering to obtain new measurements on the internal structure of aggregated non-viral gene-delivery particles in colloidal suspension. The vector particles are prepared by charge neutralization of plasmid DNA either by poly-L-lysine or by a Lipofectin/integrin-targeting peptide. We use established theories of the stability of colloidal particles and fractal concepts to explain the aggregation processes and demonstrate the existence of a new property (fractal dimension) of the aggregated vector particles. Aggregation is shown to produce particles with fractal dimensions in the range between 1.8 and 2.4; the former suggests a loose three-dimensional structure and the latter characterizes an aggregation process that leads to the formation of particles with tightly packed structures. We show that the fractal dimension of the vector particles is sensitive to changes in physicochemical conditions (ionic strength) of the buffer solution and propose that fractal dimension may provide a useful means of monitoring the physical state of non-viral delivery-vector particles during preparation and storage.

Prediction of size distribution of lipid-peptide-DNA vector particles using Monte Carlo simulation techniques.

Concerns with insertional mutagenesis for retrovirus and immunogenicity for adenovirus have motivated research into development of non-viral vectors that can safely deliver desired gene constructs to target cells in tissues and organs. Many non-viral vectors suffer from unacceptably poor in vivo cell transfection and low transgene expression. Evidence suggests that cell transfection is linked to particle size - vector particles below about 200 nm are considered desirable. Experimental measurements indicate, however, that vector particles are susceptible to significant aggregation under most conditions of pH and ionic strength, including physiological conditions, although there are currently no means of predicting the kinetics of aggregation. The present paper addresses this challenge by presenting a mathematical framework based on the Monte Carlo simulation techniques for modelling the dynamics of aggregation. The approach is used to simulate the evolution of particle-size distribution for an integrin-targeting lipid-peptide-DNA vector system in buffers of different pH and ionic strength. The simulations required two input parameters, including the initial-size distribution of the particles and a fitting parameter (alpha). Comparison of simulations with experimental data showed that alpha was closely related to the zeta potential of the particles in the buffer medium, making simulations fully predictive. The modelling approach may be used in other vector systems.