Structure-activity relationships of water-soluble cationic methacrylate/methacrylamide polymers for nonviral gene delivery

Synthesis of linear polyethylenimine derivatives for DNA transfection

A series of linear polymers containing varying amounts of ethylenimine or N-propylethylenimine units were synthesized by hydrolysis and/or reduction of polyethyloxazolines. The pK(a)s of the polyamines were determined potentiometrically. Gel mobility shift assay showed that the efficiency of DNA complexation was related to the fraction of amino groups that are protonated at neutral pH. The effects of cationic charge density and molar weight of the polymers on the transfection efficiency were evaluated on HepG2 cells. The results obtained with different copolymers show that the transfection efficiency primarily depends on the fraction of ethylenimine units included in the polymer albeit the molar weight is also of importance. On the basis of the results obtained with poly(N-propylethylenimines), we also demonstrate that the high transfection efficiency of polyethylenimines does not solely rely on their capacity to capture protons which are transferred into the endo-lysosomes during acidification.

Synthesis and characterization of injectable, water-soluble copolymers of tertiary amine methacrylates and poly(ethylene glycol) containing methacrylates

Several homopolymers and copolymers of 2-(diethylamino)ethyl methacrylate (DEAEM) and poly(ethylene glycol) methyl ether methacrylate (PEGMEM) were synthesized using anionic polymerization initiated by potassium t-butoxide. The polymers were characterized by average molecular weight, polydispersity and monomeric unit composition. A very narrow molecular weight distribution was achieved with a well-controlled composition. The glass transition temperatures and compositions of the copolymers followed a Gordon-Taylor relationship. The water solubility and biocompatibility of the copolymers was compared to their parent homopolymers to determine if the addition of a poly(ethylene glycol) group was sufficient to solubilize the polymers in aqueous buffer solutions and to increase the biocompatibility of the polymers. These water-soluble, injectable cationic copolymers have potential applications in gene delivery as well as other biomaterial applications.

Structure-activity relationships of poly(L-lysines): effects of pegylation and molecular shape on physicochemical and biological properties in gene delivery

The influence of shape, molecular weight and pegylation of linear, grafted, dendritic and branched poly-L-lysines on their DNA delivery properties were investigated. DNA binding, condensation, complex size and morphology, cell uptake and transfection efficiency were determined. Most polylysines condense DNA, linear polymers being more efficient than most dendritic ones. At low molecular weights of PLL DNA binding and condensation were less efficient, particularly with dendrimers. Pegylation did not decrease DNA condensation of PLLs at less than 60% (fraction of M(w)) of PEG. Pegylation stabilized the complexes sterically, but did not protect them from interaction with polyanionic chondroitin sulfate. Cell uptake of polylysine/DNA complexes was high and pegylation increased the transfection efficacy. However, overall transfection level of polylysines is low possibly due to inadequate escape of the complexes from endosomes or poor release of DNA from the complexes. Physicochemical and biological structure-property relationships of poly-L-lysines were demonstrated, but no clear correlations between the tested physicochemical determinants (size of complexes, zeta-potentials, condensation of DNA and the shape of complexes) and biological activities were seen. Transfection activity may be ultimately determined by intracellular factors and/or still unknown features of DNA complexation with the carriers.

Influence of polymer architecture on the structure of complexes formed by PEG-tertiary amine methacrylate copolymers and phosphorothioate oligonucleotide

The influence of polymer structure on the characteristics of complexes of a phosphorothioate antisense oligonucleotide (ISIS 5132) was studied, using well-defined cationic copolymers based on 2-(dimethylamino) ethyl methacrylate (DMAEMA) and poly(ethylene glycol) (PEG). The three related copolymer structures were: DMAEMA-PEG (a diblock copolymer) DMAEMA-OEGMA 7 (a brush-type copolymer), DMAEMA-stat-PEGMA (a comb-type copolymer); each of these were examined together with DMAEMA homopolymer, which served as a control. The results revealed that all the polymers exhibited good binding ability with the oligonucleotide (ON). Interestingly, the comb-type polymer DMAEMA-stat-PEGMA demonstrated the highest binding ability and DMAEMA homopolymer the lowest, as judged by a dye displacement assay. DMAEMA homopolymer produced large agglomerates of smaller individual complexes as observed by optical density, photon correlation spectroscopy and transmission electron microscopy studies. In contrast, two PEG-block copolymers, DMAEMA-PEG and DMAEMA-OEGMA 7, formed compact complexes of 80-150 nm which had good long-term colloidal stability. This is attributed to the steric stabilisation effect of the PEG chains on the ON-copolymer complexes. These two copolymers are believed to form complexes with ON that have a micellar structure. Comb-type DMAEMA-stat-PEGMA copolymer formed highly soluble complexes with the ON that did not phase separate from the buffer solution. This study clearly demonstrates that varying the copolymer architecture allows access to a range of ON complexes. In vitro cytotoxicity experiments on HepG2 cells showed that all of the tertiary amine methacrylate copolymers displayed lower cytotoxicity than the control poly(L-lysine).

Intracellular processing and stability of DNA complexed with histidylated polylysine conjugates

BACKGROUND

Glycosylated polylysines and histidylated polylysines complexed with plasmid DNA (pDNA) were proposed to develop polymer-based gene delivery systems. The present work has been undertaken in two steps to study the uptake and the intracellular processing of pDNA, which are still poorly understood in the polyfection pathway.

METHODS AND RESULTS

The kinetics of the uptake and the intracellular processing of pDNA complexed with lactosylated polylysine, histidylated polylysine or histidylated polylysine bearing lactosyl residues (polyplexes) into a CF human airway epithelial cell line were assessed by flow cytometry and confocal microscopy. Complexes formed from histidylated polylysine, even though they were less taken up by cells, show better transfection efficiency with compared with lactosylated complexes. Lactosylated polymers segregated more rapidly when compared with non-lactosylated polymers into compartments different from those containing pDNA on internalization. Intracellular location and pH measurements indicated that polymers ended up in compartments of pH approximately 6.2 while pDNA reached less acidic compartments of pH approximately 6.6. These compartments did not contain the LAMP-1 lysosomal marker.

CONCLUSIONS

The present study exhibits that, upon internalization, pDNA and polylysine conjugates underwent segregation with a rate depending on the polylysine substitution and polymer degradation. The better transfection efficiency of polyplexes with histidylated polylysine can be ascribed to their prolonged stability inside the endocytic vesicles that likely favored the pDNA escape in the cytosol.

Intracellular processing of poly(ethylene imine)/ribozyme complexes can be observed in living cells by using confocal laser scanning microscopy and inhibitor experiments

PURPOSE

Critical steps in the subcellular processing of poly(ethylene imine)/nucleic acid complexes, especially endosomal/lysosomal escape, were visualized by using living cell confocal laser scanning microscopy (CSLM) to obtain an insight into their mechanism.

METHODS

Living cell confocal microscopy was used to examine the intracellular fate of poly(ethylene imine)/ribozyme and poly(L-lysine)/ribozyme complexes over time, in the presence of and without bafilomycin Al, a selective inhibitor of endosomal/lysosomal acidification. The compartment of complex accumulation was identified by confocal microscopy with a fluorescent acidotropic dye. To confirm microscopic data, luciferase reporter gene expression was determined under similar experimental conditions.

RESULTS

Poly(ethylene imine)/ribozyme complexes accumulate in acidic vesicles, most probably lysosomes. Release of complexes occurs in a sudden event, very likely due to bursting of these organelles. After release, poly(ethylene imine) and ribozyme spread throughout the cell, during which slight differences in distribution between cytosol and nucleus are visible. No lysosomal escape was observed with poly(L-lysine)/ribozyme complexes or when poly(ethylene imine)/ ribozyme complexes were applied together with bafilomycin A1. Poly(ethylene imine)/plasmid complexes exhibited a high luciferase expression, which was reduced approximately 200-fold when lysosomal acidification was suppressed with bafilomycin A1.

CONCLUSIONS

Our data provide, for the first time, direct experimental evidence for the escape of poly(ethylene imine)/nucleic acid complexes from the endosomal/lysosomal compartment. CLSM, in conjunction with living cell microscopy, is a promising tool for studying the subcellular fate of polyplexes in nucleic acid/gene delivery.

Histidine-rich peptides and polymers for nucleic acids delivery

Nucleic acids transfer into mammalian cells requires devices to improve their escape from endocytic vesicles where they are mainly confined following cellular uptake. In this review, we describe histidine-rich molecules that enable the transfer of plasmid and oligonucleotides (ODN) in human and non-human cultured cells. An histidine-rich peptide which permeabilizes biological membrane at pH 6.4, favored the transfection mediated by lactosylated polylysine/pDNA complexes. Histidylated polylysine forms cationic particles of 100 nm with a plasmid and yielded a transfection of 3-4.5 orders of magnitude higher than polylysine. The biological activity of antisense ODN was increased more than 20-fold when it was complexed with highly histidylated oligolysine into small cationic spherical particles of 35 nm. Evidence that imidazole protonation mediates the effect of these molecules in endosomes are provided. We also describe a disulfide-containing polylysine conjugate capable of mediating DNA unpackaging in a reductive medium and to increase the transfection efficiency. Overall, these molecules constitute interesting devices for developing non-viral gene delivery systems.

Interactions between oligonucleotides and cationic polymers investigated by fluorescence correlation spectroscopy

PURPOSE

To evaluate whether fluorescence correlation spectroscopy (FCS) can be used to characterize the complexation between oligonucleotides and cationic polymers.

METHODS

The features of the complexes between rhodamine labeled oligonucleotides (Rh-ONs) and poly(2-dimethylamino)ethyl methacrylate (pDMAEMA), poly(ethylene glycol)-poly(ethyleneimine) (pEG-pEI), and diaminobutane-dendrimer-(NH2)64 (DAB64) were characterized by light scattering, electrophoretic mobility, electrophoresis, and FCS.

RESULTS

At low polymer/Rh-ON ratios, a decrease of the fluorescence of the Rh-ONs was observed on binding of the Rh-ONs to all cationic polymers. This was explained by the creation of "multimolecular complexes" in which the Rh-labels quench each other. The multimolecular complexes, which are highly fluorescent as they carry a number of Rh-ONs, resulted in high fluorescence peaks in the fluorescence fluctuation profile as measured by FCS. For pDMAEMA and DAB64, at higher polymer/Rh-ON ratios the fluorescence of the polyplexes increased, caused by the formation of "monomolecular complexes," which consist of only one Rh-ON per polymer. In the case of pEG-pEI, the fluorescence stayed constant when the polymer/Rh-ON ratio increased, so multimolecular polyplexes remained. FCS confirmed these results as the high fluorescence peaks disappeared in case of pDMAEMA/Rh-ON and DAB64/Rh-ON dispersions, but remained present for pEG-pEI/Rh-ON dispersions.

CONCLUSIONS

FCS seems applicable for study of the interactions between ONs and different types of cationic polymers.

Copolymers of amine methacrylate with poly(ethylene glycol) as vectors for gene therapy

A series of structurally related copolymers of tertiary amine methacrylate with poly(ethylene glycol) (PEG) were investigated for their potential to serve as vectors for gene therapy. The effects of copolymer structure on the complexation and transfection ability were assessed. The ability of the PEG-based copolymers and DMAEMA homopolymer to bind and condense DNA was confirmed by gel electrophoresis, ethidium bromide displacement and transmission electron microscopy. The presence of PEG in the copolymers had a beneficial effect on their ability to bind to DNA. Colloidally stable complexes were obtained for all the PEG-copolymer systems as shown by uniformly discrete spherical images from transmission electron microscopy and approximate diameters of 80-100 nm by dynamic light scattering studies. DMAEMA homopolymer, however, produced agglomerated particles, confirming the important role played by the PEG chains in producing compact stable DNA complexes. Assessment of the effect of ionic strength of the buffer on the complexation and dissociation of the complexes indicated the importance of both electrostatic and non-electrostatic interactions in the polymer-DNA complexation. In vitro transfection experiments showed that DMAEMA homopolymer gave the highest level of transfection comparable to a control poly-L-lysine (PLL) system. The PEG-based copolymers gave reduced levels of transfection, most likely due to the steric stabilization effect of a PEG corona.

Non-viral vectors in cancer gene therapy: principles and progress

This review focuses on the use of synthetic (non-viral) delivery systems for cancer gene therapy. Therapeutic strategies such as gene replacement/mutation correction, immune modulation and molecular therapy/'suicide' gene therapy type approaches potentially offer unique and novel ways of fighting cancer, some of which have already shown promise in early clinical trials. However, the specific and efficient delivery of the genetic material to remote tumors/metastases remains a challenge, which is being addressed using a variety of viral and non-viral systems. Each of these disparate systems has distinct advantages and disadvantages, which need to be taken into account when a specific therapeutic gene is being used. The review concentrates on particulate gene delivery systems, which are formed through non-covalent complexation of cationic carrier molecules (e.g. lipids or polymers) and the negatively charged plasmid DNA. Such systems tend to be comparatively less efficient than viral systems, but have the inherent advantage of flexibility and safety. The DNA-carrier complex acts as a protective package, and needs to be inert and stable while in circulation. Once the remote site has been reached the complex needs to efficiently transfect the targeted (tumor) cells. In order to improve overall transfection specificity and efficiency it is necessary to optimize intracellular trafficking of the DNA complex as well as the performance after systemic administration. Common principles and specific advantages or disadvantages of the individual synthetic gene delivery systems are discussed, and their interaction with tumor-specific and generic biological barriers are examined in order to identify potential strategies to overcome them.

Effects of emulsifiers on the controlled release of paclitaxel (Taxol) from nanospheres of biodegradable polymers

Paclitaxel (Taxol) is an antineoplastic drug effective for various cancers especially ovarian and breast cancer. Due to its high hydrophobicity, however, an adjuvant such as Cremophor EL has to be used in its clinical administration, which causes serious side-effects. Nanospheres of biodegradable polymers could be an ideal solution. This study investigates the effects of various emulsifiers on the physical/chemical properties and release kinetics of paclitaxel loaded nanospheres fabricated by the solvent extraction/evaporation technique. It is shown that phospholipids could be a novel type of emulsifiers. The nanospheres manufactured with various emulsifiers were characterized by laser light scattering for their size and size distribution; scanning electron microscopy (SEM) and atomic force microscopy (AFM) for their surface morphology; zeta potential analyser for their surface charge; and, most importantly, X-ray photoelectron spectroscopy (XPS) for their surface chemistry. The encapsulation efficiency and in vitro release profile were measured by high performance liquid chromatography (HPLC). It is found that dipalmitoyl-phosphatidylcholine (DPPC) can provide more complete coating on the surface of the products which thus results in a higher emulsifying efficiency compared with polyvinyl alcohol (PVA). Our result shows that the chain length and unsaturation of the lipids have a significant influence on the emulsifying efficiency. Phospholipids with short and saturated chains have excellent emulsifying effects.

Sterilization of poly(dimethylamino) ethyl methacrylate-based gene transfer complexes

Parental administration of polyplex formulations for gene therapy or genetic vaccination requires sterile preparations. The possibilities and limitations of autoclaving, filtration and a combination of both methods for sterilization of poly(2-(dimethylamino) ethyl methacrylate) (pDMAEMA) based gene transfer complexes were assessed. Agarose gel electrophoresis and circular dichroism spectroscopy showed that sterile filtration of polyplexes did not change the topology and integrity of the DNA. The transfection potential was fully retained in COS-7 and OVCAR-3 cells, although the concentration of DNA was slightly decreased by the filtration process. Pre-coating of the filter with polyplexes reduced the material loss. In contrast, autoclaving dramatically affected physical characteristics of polyplexes, resulting in complete loss of transfection potential. Sterile filtration or autoclaving of polymer alone did not result in material loss, or in decreased transfection potential after complexation with plasmid DNA. 'Naked' DNA could easily be sterilized by filtration as well. In conclusion, sterilization of complexes between pDMAEMA-based cationic polymeric gene transfer agents and DNA plasmid is feasible by filtration. Depending on the filter type used, the filtered volume should be high enough, to prevent substantial material loss. Separate sterilization of the polymer by autoclaving or filtration and DNA by filtration offers a good alternative to filtration of formed polyplexes.

Gene expression control by temperature with thermo-responsive polymeric gene carriers

A thermo-responsive copolymer, poly(N-isopropylacrylamide (IPAAm)-co-2-(dimethylamino)ethyl methacrylate (DMAEMA)-co-butylmethacrylate (BMA)), was synthesized and its in vitro gene transfection efficiency at different incubation temperatures was evaluated. A copolymer containing 8 mol% DMAEMA and 11 mol% BMA (P(IP-8DA-11BM)) had a lower critical solution temperature (LCST) at 21 degrees C, therefore the copolymer was insoluble above 21 degrees C and soluble below 21 degrees C. The LCST of P(IP-8DA-11BM) solution was not affected by the presence of salmon DNA. This copolymer was complexed with plasmid DNA, and the stability of the complex was analyzed by gel electrophoresis. DNA was completely retained in the complex, which was observed in the gel loading slot at 37 degrees C. At 20 degrees C, DNA was found to be partially dissociated from the complex by the appearance of the same band as DNA in the control experiment. These results clearly show that complex formation/dissociation was modulated by temperature alteration. The transfection efficiency of polymer-plasmid complexes was evaluated in COS-1 cells using pCMV-lacZ plasmid, encoding for beta-galactosidase as a reporter gene. The transfection efficiency of PDMAEMA homopolymer incubated at 37 degrees C for 48 h was greater than that incubated at 20 degrees C for 3 h and 37 degrees C for 45 h. In contrast, the transfection efficiency of P(IP-8DA-11BM) incubated at 20 degrees C for 3 h and 37 degrees C for 45 h was much higher than that incubated at 37 degrees C for 48 h. Such an increased transfection efficiency on lowering the temperature is considered to be due to appropriate formation/dissociation control of P(IP-8DA-11BM)-DNA complexes.

Transfection efficiency increases by incorporating hydrophobic monomer units into polymeric gene carriers

The water soluble terpolymer, poly(N-isopropylacrylamide (IPAAm)-co-2-(dimethylamino)ethyl methacrylate (DMAEMA)-co-butylmethacrylate (BMA)) was synthesized, and its efficiency in in vitro gene transfection was evaluated. Copolymers with different compositions were synthesized by radical polymerization. For a series of copolymers containing 60 mol% of DMAEMA, the plasmid bands were retained within the gel loading slot, independent of polymer/plasmid weight ratios or BMA monomer content. In contrast, for a series of copolymers containing 20 mol% DMAEMA, plasmid bands of complexes were retarded with increasing weight ratios. For the copolymer with 10 mol% BMA content, the plasmid was completely retained within the gel loading slot. The transfection efficiency of polymer/plasmid complexes was evaluated in COS-1 cells using a pCMV-lacZ plasmid, encoding for beta-galactosidase as a reporter gene. Transfection efficiency of a series of copolymers containing 20 mol% of DMAEMA varied with BMA content. The transfection efficiency of the copolymers with 0, 2, and 5 mol% of BMA was low. The transfection efficiency of the copolymers with 10 mol% of BMA was about 2-fold higher than that of the PDMAEMA control homopolymer. The transfected cells were observed at a very wide range of polymer/plasmid weight ratios. The transfection efficiency of all copolymers containing 60 mol% of DMAEMA was lower than that of the PDMAEMA homopolymer.

Effects of physicochemical characteristics of poly(2-(dimethylamino)ethyl methacrylate)-based polyplexes on cellular association and internalization

The cationic polymer poly(2-(dimethylamino)ethyl methacrylate) (p(DMAEMA)) is able to efficiently bind and condense DNA and to mediate transfection of a variety of cell types. In this study, fluorescence activated cell sorting (FACS), confocal laser fluorescence microscopy (CSLM) and electron microscopy (EM) techniques were used to investigate in vitro the cellular interaction of p(DMAEMA)-based polyplexes with human ovarian carcinoma cells (OVCAR-3). Cellular association and subsequent internalization only occurred when the polyplexes exhibited a positive zeta potential. Small-sized polyplexes have an advantage over large-sized complexes regarding cellular entry. The effect of the presence of tertiary amine groups versus the presence of quatenary amine groups was evaluated by comparing p(DMAEMA) with its quaternary ammonium analogue poly(2-(trimethylamino)ethyl methacrylate) (p(TMAEMA)). The combined cellular interaction and transfection results suggest that the latter polymer does not have an intrinsic endosomal escape property, in contrast to the 'proton sponge' effect proposed for p(DMAEMA). PEGylation of p(DMAEMA) effectively shielded the surface charge and yielded a notably lower degree of cellular interaction. Data on the effects of the presence of endocytosis inhibitors and an endosome-disruptive peptide in the culture medium on the cellular interaction and transfection activity of p(DMAEMA)-based polyplexes support endocytosis as being the principal pathway for intracellular delivery of plasmid. Both the CLSM and EM studies did not reveal the presence of polyplexes or plasmid outside the endocytic vesicles or within the nucleus, suggesting that intracellular trafficking from the endosomes to the nucleus is a very inefficient process.

Synthetic DNA delivery systems

The ability to safely and efficiently transfer foreign DNA into cells is a fundamental goal in biotechnology. Toward this end, rapid advances have recently been made in our understanding of mechanisms for DNA stability and transport within cells. Current synthetic DNA delivery systems are versatile and safe, but substantially less efficient than viruses. Indeed, most current systems address only one of the obstacles to DNA delivery by enhancing DNA uptake. In fact, the effectiveness of gene expression is also dependent on several additional factors, including the release of intracellular DNA, stability of DNA in the cytoplasm, unpackaging of the DNA-vector complex, and the targeting of DNA to the nucleus. Delivery systems of the future must fully accommodate all these processes to effectively shepherd DNA across the plasma membrane, through the hostile intracellular environment, and into the nucleus.

Association and dissociation characteristics of polymer/DNA complexes used for gene delivery

PURPOSE

The DNA association/dissociation properties of water-soluble cationic methacrylate polymers with closely related structures (poly(2-dimethylamino)ethyl methacrylate) [p(DMAEMA)], poly(2-(trimethylamino)ethyl methacrylate chloride) [p(TMAEMA)]) and the frequently used transfectant poly(L-lysine) were studied to gain a better insight into their transfection characteristics.

METHODS

Association of DNA with different polymers and dissociation of the complexes, achieved by adding an excess of anionic polymers or salt, were studied by using spectroscopic techniques (fluorescence, circular dichroism (CD)), agarose gel electrophoresis and an enzymatic assay (DNase I treatment). The transfection efficiency of the polyplexes was evaluated in tissue culture with OVCAR-3 cells.

RESULTS

Plasmid DNA complexed with either poly(L-lysine) or p(DMAEMA) was protected against digestion by DNase I. Fluorescence and CD spectroscopy as well as gel electrophoresis revealed that p(DMAEMA) with a relatively high molecular weight and poly(L-lysine) have similar DNA association/dissociation characteristics. Therefore, differences in transfection potential of the polyplexes cannot be ascribed to differences in binding characteristics, but are probably caused by other factors. As compared with the other polymers, p(TMAEMA) has a high affinity for DNA as was concluded from the observation that poly(aspartic acid) was unable to fully dissociate complexes containing this polymer. This fact might very well explain the low transfection efficiency of these polyplexes. p(DMAEMA) with a relatively low molecular weight probably has a low affinity for DNA, which might explain both the formation of DNA aggregates (psi-DNA) and the low transfection potential obtained when using this polymer.

CONCLUSIONS

DNA association/dissociation studies shed light on the preferred characteristics of polymeric transfectants.