DNA delivery systems based on complexes of DNA with synthetic polycations and their copolymers.

Block and graft copolymers of N-(2-hydroxypropyl)methacrylamide (HPMA) with 2-(trimethylammonio)ethyl methacrylate were synthesised and used for preparation of polyelectrolyte complexes with calf thymus DNA intended for targeted delivery of genes in vivo. In this study the effects of the speed of component mixing, total concentration of polymers, ionic strength of solvents, copolymer structure and content of HPMA in the copolymers on parameters of the polyelectrolyte complexes was investigated. Static and dynamic light scattering methods were used as a main tool for characterising these complexes. The presence of HPMA units in the polycation had no significant effect on its ability to form complexes with DNA, but did affect molecular parameters and aggregation (precipitation) of the complexes. The size of the complexes increases whereas their molecular weight decreases with increasing content of HPMA units. The density of the complexes decreases with increasing HPMA content independently of the copolymer structure. In order to prepare stable DNA complexes containing single DNA molecule, the following rules should be observed: (1) copolymers should have a content of HPMA units higher than 40%; (2) the DNA concentrations in solutions should be kept below 4 x 10(-5) g/ml and (3) both components should be mixed together in deionized water. The stability of the complexes against precipitation in 0.15 M NaCl and the resistance of the complexed DNA to the action of nucleases was also studied. Whereas DNA complexes of all copolymers showed very good nuclease stability, the presence of a sufficiently high content of HPMA is necessary for their good stability in 0.15 M NaCl. The investigation of the stability and the interaction of DNA complexes in aqueous solutions of serum albumin and dilute human blood serum revealed adsorption of biomacromolecules on DNA complexes accompanied by significant changes in the zeta-potential which finally resulted in formation of a "protein layer" and in undesirable precipitation of DNA complexes. In in vitro transfection experiments, the transfection efficiency of DNA complexes with copolymers was always higher than that of the cationic homopolymer slightly increasing with increasing content of HPMA in the copolymers but being about 10-100-times lower than the complexes DNA-poly(L-lysine. In the cytoplasmic injections, it was observed that DNA complexes produced greater gene expression than a direct microinjection of free DNA. The block copolymer complexes were also found to be more efficient than the corresponding simple polycation complexes. In the nuclear microinjection, precisely the opposite behaviour was observed.

Sources of Escherichia coli in a coastal subtropical environment.

Sources of Escherichia coli in a coastal waterway located in Ft. Lauderdale, Fla., were evaluated. The study consisted of an extensive program of field measurements designed to capture spatial and temporal variations in E. coli concentrations as well as experiments conducted under laboratory-controlled conditions. E. coli from environmental samples was enumerated by using a defined substrate technology (Colilert-18). Field sampling tasks included sampling the length of the North Fork to identify the river reach contributing high E. coli levels, autosampler experiments at two locations, and spatially intense sampling efforts at hot spots. Laboratory experiments were designed to simulate tidal conditions within the riverbank soils. The results showed that E. coli entered the river in a large pulse during storm conditions. After the storm, E. coli levels returned to baseline levels and varied in a cyclical pattern which correlated with tidal cycles. The highest concentrations were observed during high tide, whereas the lowest were observed at low tide. This peculiar pattern of E. coli concentrations between storm events was caused by the growth of E. coli within riverbank soils which were subsequently washed in during high tide. Laboratory analysis of soil collected from the riverbanks showed increases of several orders of magnitude in soil E. coli concentrations. The ability of E. coli to multiply in the soil was found to be a function of soil moisture content, presumably due to the ability of E. coli to outcompete predators in relatively dry soil. The importance of soil moisture in regulating the multiplication of E. coli was found to be critical in tidally influenced areas due to periodic wetting and drying of soils in contact with water bodies. Given the potential for growth in such systems, E. coli concentrations can be artificially elevated above that expected from fecal impacts alone. Such results challenge the use of E. coli as a suitable indicator of water quality in tidally influenced areas located within tropical and subtropical environments.

Polyelectrolyte vectors for gene delivery: influence of cationic polymer on biophysical properties of complexes formed with DNA.

Cationic polymer/DNA complexes are widely used for gene delivery, although the influence of the cationic polymer on the biophysical properties of the resulting complex is poorly understood. Here, several series of cationic polymers have been used to evaluate the influence of structural parameters on properties of DNA complexes. Parameters studied included the length of side chain, charge type (primary versus tertiary and quaternary), polymer molecular weight, and charge spacing along the polymer backbone. Cationic polymers with short side chains (such as polyvinylamine) formed small complexes, resistant to destabilization by polyanions, with low surface charge, limited transfection activity, and efficient intranuclear transcription. Conversely, cationic polymers with long side chains (e.g., poly[methacryloyl-Gly-Gly-NH-(CH(2))(6)-NH(2))] showed inefficient complex formation, high positive surface charge, and better transfection activity. The effects of molecular weight varied between polymers, for example, low molecular weight poly(L-lysine) produced relatively small complexes, whereas low molecular weight poly[2-(trimethylammonio)ethyl methacrylate chloride] produced large aggregates. Polymers containing quaternary ammonium groups showed efficient complex formation but poor transfection. Finally, spreading charges widely on the polymer structure inhibited their ability to condense DNA. In summary, to achieve small, stable complexes, the use of cationic polymers with short side chains bearing primary amino groups is suggested.

Factors affecting blood clearance and in vivo distribution of polyelectrolyte complexes for gene delivery.

Self-assembling polycation/DNA complexes represent a promising synthetic vector for gene delivery. However, despite considerable versatility and transfectional activity in vitro, such materials are quickly eliminated from the bloodstream following intravenous injection (plasma alpha half-life typically less than 5 min). For targeted systemic delivery a more prolonged plasma circulation of the vector is essential. Here we have examined factors contributing to rapid elimination of poly(L-lysine) (pLL)/DNA complexes from the bloodstream, and implicate the binding of proteins to the polyelectrolyte complexes as a likely cause for their blood clearance. pLL/DNA complexes reisolated from serum associate with several proteins, depending on their net charge, although the major band on SDS-PAGE co-migrates with albumin. Serum albumin binds to pLL/DNA complexes in vitro, forming a ternary pLL/DNA/albumin complex which regains some ethidium bromide fluorescence and fails to move during agarose electrophoresis. Albumin also causes increased turbidity of complexes, and reduces their zeta potential to the same level (-16 mV) as is measured in serum. We propose that rapid plasma elimination of polycation/DNA complexes results from their binding serum albumin and other proteins, perhaps due to aggregation and phagocytic capture or accumulation of the ternary complexes in fine capillary beds.

Chloroquine and amphipathic peptide helices show synergistic transfection in vitro.

A pH-responsive peptide fragment modelled on the influenza virus haemagglutinin (INF7-SGSC) can promote the transfectional activity of poly(L)-lysine (pLL)/DNA complexes against 293 cells. Chloroquine also promotes transfection, but the combination of INF7-SGSC and chloroquine gives an increased, synergistic, transfectional activity. This was unexpected since the supposed modes of action of these two agents are expected to be incompatible. Microinjection of pLL/DNA complexes into the cytoplasm of Xenopus oocytes produced greater gene expression than microinjection of free DNA, possibly reflecting nuclear-homing or protection from degradation by cytoplasmic nucleases. However, pretreatment of complexes with INF7-SGSC (but not chloroquine) before cytoplasmic microinjection promoted gene expression still further. When pLL/DNA complexes were injected directly into the nucleus, INF7-SGSC again increased gene expression. The mechanism of post-endosomal action of INF7-SGSC is unknown, but could reflect its polyanionic nature, possibly enhancing intranuclear dissociation of the complexes. Whatever the mechanism, it appears that INF7-SGSC mediates two effects-one probably endosomal and the second post-endosomal, the latter showing a synergistic transfection interaction with chloroquine.

Novel vectors for gene delivery formed by self-assembly of DNA with poly(L-lysine) grafted with hydrophilic polymers.

Complexes formed between DNA and cationic polymers are attracting increasing attention as novel synthetic vectors for delivery of genes. We are trying to improve biological properties of such complexes by oriented self-assembly of DNA with cationic-hydrophilic block copolymers, designed to enshroud the complex within a protective hydrophilic polymer corona. Poly(L-lysine) (pLL) grafted with range of hydrophilic polymer blocks, including poly(ethylene glycol) (pEG), dextran and poly[N-(2-hydroxypropyl)methacrylamide] (pHPMA), shows efficient binding to DNA and mediates particle self-assembly and inhibition of ethidium bromide/DNA fluorescence. The complexes formed are discrete and typically about 100 nm diameter, viewed by atomic force microscopy. Surface charges are slightly shielded by the presence of the hydrophilic polymer, and complexes generally show decreased cytotoxicity compared with simple pLL/DNA complexes. pEG-containing complexes show increased transfection activity against cells in vitro. Complexes formed with all polymer conjugates showed greater aqueous solubility than simple pLL/DNA complexes, particularly at charge neutrality. These materials appear to have the ability to regulate the physicochemical and biological properties of polycation/DNA complexes, and should find important applications in packaging of nucleic acids for specific biological applications.

Lipid-mediated enhancement of transfection by a nonviral integrin-targeting vector.

Nonviral vectors consisting of integrin-targeting peptide/DNA (ID) complexes have the potential for widespread application in gene therapy. The transfection efficiency of this vector, however, has been limited by endosomal degradation. We now report that lipofectin (L) incorporated into the ID complexes enhances integrin-mediated transfection, increasing luciferase expression by more than 100-fold. The transfection efficiency of Lipofectin/Integrin-binding peptide/DNA (LID) complexes, assessed by beta-galactosidase reporter gene expression and X-gal staining, was improved from 1% to 10% to over 50% for three different cell lines, and from 0% to approximately 25% in corneal endothelium in vitro. Transfection complexes have been optimized with respect to their transfection efficiency and we have investigated their structure, function, and mode of transfection. Both ID and LID complexes formed particles, unlike the fibrous network formed by lipofectin/DNA complexes (LD). Integrin-mediated transfection by LID complexes was demonstrated by the substantially lower transfection efficiency of LKD complexes in which the integrin-biding peptide was substituted for K16 (K). Furthermore, the transfection efficiency of complexes was shown to be dependent on the amount of integrin-targeting ligand in the complex. Finally, a 34% reduction in integrin-mediated transfection efficiency by LID complexes was achieved with a competing monoclonal antibody. The role of lipofectin in LID complexes appears, therefore, to be that of a co-factor, enhancing the efficiency of integrin-mediated transfection. The mechanism of enhancement is likely to involve a reduction in the extent of endosomal degradation of DNA.

Characterization of vectors for gene therapy formed by self-assembly of DNA with synthetic block co-polymers.

Cationic polymers can self-assemble with DNA to form polyelectrolyte complexes capable of gene delivery, although biocompatibility of the complexes is generally limited. Here we have used A-B type cationic-hydrophilic block co-polymers to introduce a protective surface hydrophilic shielding following oriented self-assembly with DNA. Block co-polymers of poly(ethylene glycol)-poly-L-lysine (pEG-pLL) and poly-N-(2-hydroxypropyl)methacrylamide-poly(trimethylammonioethyl methacrylate chloride) (pHPMA-pTMAEM) both show spontaneous formation of complexes with DNA. Surface charge measured by zeta potential is decreased compared with equivalent polycation-DNA complexes in each case. Atomic force microscopy shows that pHPMA-pTMAEM/DNA complexes are discrete spheres similar to those formed between DNA and simple polycations, whereas pEG-pLL/DNA complexes adopt an extended structure. Biological properties depend on the charge ratio of formation. At optimal charge ratio, pEG-pLL/DNA complexes show efficient transfection of 293 cells in vitro, while pHPMA-pTMAEM/DNA complexes are more inert. Both block co-polymer-DNA complexes show only limited cytotoxicity. Careful selection of block co-polymer structure can influence the physicochemical and biological properties of the complexes and should permit design of materials for specific applications, including targeted delivery of genes in vivo.

Atomic force microscopic analysis of the influence of the molecular weight of poly(L)lysine on the size of polyelectrolyte complexes formed with DNA.

We are developing self-assembling micellar vehicles based on multifunctional block copolymers as well-defined synthetic vehicles suitable for safe in vivo delivery of DNA. As a first stage, DNA expression vectors (6 kb) were condensed with poly(L)lysine of different molecular weights (3970-224 500) to form polyelectrolyte complexes and analysed by atomic force microscopy (AFM). Discrete complexes were formed in every case, although the highest molecular weight poly(L)lysine preparation (224 500) produced large complexes with significant polydispersity (diameters ranging from 120-300 nm), while the smallest poly(L)lysine (3970) produced more homogeneous complexes with diameters ranging from 20-30 nm. Poly (L)lysine preparations of molecular weight 53 700 and 23 800 produced complexes of intermediate size and poly-dispersity. The mean volumes of the complexes formed using poly(L)lysine 224 500 and 3970 were 606 000 nm3 and 3700 nm3, respectively. Polyelectrolyte complexes formed using low molecular weight poly(L)lysine also showed significantly decreased cytotoxicity. Given restrictions of access to many cellular targets and the need for good biocompatibility, synthetic vectors based on DNA condensed with low molecular weight polycations may be more appropriately developed for general use.