A novel non-viral vector for DNA delivery based on low molecular weight, branched polyethylenimine: effect of molecular weight on transfection efficiency and cytotoxicity

PURPOSE

Low molecular weight branched polyethylenimine (LMW-PEI) was synthesized and studied as a DNA carrier for gene delivery with regard to physico-chemical properties, cytotoxicity, and transfection efficiency.

METHODS

The architecture of LMW-PEI, synthesized by acid catalyzed ring-opening polymerization of aziridine was characterized by size exclusion chromatography in combination with laser light scattering and 13C-NMR-spectroscopy. In vitro cytotoxic effects were quantified by LDH and MTT assay and visualized by transmission electron microscopy. The potential for transgene expression was monitored in ECV304 cells using luciferase driven by a SV40 promotor as reporter gene system.

RESULTS

LMW-PEI (Mw 11'900 D) with a low degree of branching was synthesized as a DNA carrier for gene delivery. In contrast to high molecular weight polyethylenimines (HMW-PEI; Mw 1'616'000 D), the polymer described here showed a different degree of branching and was less cytotoxic in a broad range of concentrations. As demonstrated by transmission electron microscopy the LMW-PEI formed only small aggregates which were efficiently taken up by different cells in the presence of serum, most likely by an endocytic pathway. LMW-PEI yielded transfection efficiencies measured via expression of the reporter gene luciferase which were up to two orders of magnitude higher than those obtained with HMW-PEI. The reporter gene expression was concentration dependent, but in contrast to lipofection independent of serum addition.

CONCLUSIONS

The LMW-PEI described here is a new, highly efficient, and non-cytotoxic vector with a favorable efficiency/toxicity profile for gene therapeutic applications.

A two-stage poly(ethylenimine)-mediated cytotoxicity: implications for gene transfer/therapy

Poly(ethylenimine) (PEI) is a cationic macromolecule commonly used in gene transfer/therapy protocols with high transfection efficiency both in vitro and in vivo. PEI is also cytotoxic, but the molecular basis of its cytotoxicity is poorly understood. Here, we have demonstrated that branched (25 kDa) and linear (750 kDa) PEI can both induce membrane damage and initiate apoptosis in three clinically relevant human cell lines (Jurkat T cells, umbilical vein endothelial cells, and THLE3 hepatocyte-like cells). We have defined Phase I toxicity as early necrotic-like changes (30 min) resulting from compromised membrane integrity, assessed by considerable lactate dehydrogenase release and phosphatidylserine translocation from the inner plasma membrane to the outer cell surface. Phase II cytotoxicity (24 h) was due to activation of a "mitochondrially mediated apoptotic program," resulting from PEI-induced channel formation in the outer mitochondrial membrane. This led to the release of proapoptotic cytochrome c, subsequent activation of caspase 3, and alteration in mitochondrial membrane potential as a result of caspase translocation into the mitochondria. The reported observations have important implications for the design and execution of gene therapy protocols as well for controlling intracellular distribution of drugs with cationic-based polymer-delivery systems.

Low-molecular-weight polyethylenimine as a non-viral vector for DNA delivery: comparison of physicochemical properties, transfection efficiency and in vivo distribution with high-molecular-weight polyethylenimine

Low-molecular-weight polyethylenimine (LMW-PEI) was synthesized by the acid-catalyzed, ring-opening polymerization of aziridine and compared with commercially available high-molecular-weight PEI (HMW-PEI) of 25 kDa. Molecular weights were determined by size-exclusion chromatography in combination with multi-angle laser light scattering. The weight average molecular weight (M(w)) of synthesized LMW-PEI was determined as 5.4+/-0.5 kDa, whereas commercial HMW-PEI showed a M(w) of 48+/-2 kDa. DNA polyplexes of LMW-PEI and HMW-PEI were characterized with regard to DNA condensation (ethidium bromide fluorescence quenching), size (photon correlation spectroscopy) and surface charge (laser Doppler anemometry). Compared with HMW-PEI, DNA condensation of LMW-PEI was slightly impaired at lower N/P ratios. Complexes with plasmid DNA at a N/P ratio of 6.7 showed significantly increased hydrodynamic diameters (590+/-140 vs. 160+/-10 nm), while the zeta-potential measurements were similar (23+/-2 vs. 30+/-3 mV). The cytotoxicity of LMW-PEI in L929 fibroblasts was reduced by more than one order of magnitude compared with HMW-PEI, as shown by MTT assay. LMW-PEI exhibited increased transfection efficiency in six different cell lines. Reporter gene expression was found to be increased by a factor of 2.1-110. The pharmacokinetics and biodistribution of 125I-PEI in mice were similar for both molecular weights with an AUC of ca. 330+/-100% ID/ml min. Approximately half of the injected dose accumulated in the liver. LMW-PEI proved to be an efficient gene delivery system in a broad range of cell lines. Due to differences in polyplex structure, as well as its relatively low cytotoxicity, which makes the application of high N/P ratios possible, LMW-PEI appears to possess advantageous qualities with regard to transfection efficiency over PEI of higher molecular weight.

Molecular hurdles in polyfectin design and mechanistic background to polycation induced cytotoxicity

Synthetic polymer based Polyfectins (cationic polymer-DNA complex) have received intensive scientific research as they can potentially circumvent problems associated with viral vectors for gene therapy. These cationic macromolecules can readily condense DNA or RNA into stable nanostructures for use in gene delivery. Recently two commonly used polycations, poly(ethylenimine) (PEI) and poly(L-lysine) have demonstrated their ability to induce apoptosis in a range of human cell lines. This may be the explanation for short-term gene transfection observed with polyfectins. It is the aim of this review to discuss these and other factors behind observed toxicities including the inherent polydisperse nature of polymeric macromolecules and their behaviour in vivo. Strategies for reduction of toxicity are included such as new polymeric synthetic technologies and vector pegylation. There is a clear and immediate need for understanding of the mechanisms which cause polyfectin toxicity which will ultimately facilitate improved vector design and safer gene delivery.

Breaking up the correlation between efficacy and toxicity for nonviral gene delivery

Nonviral nucleic acid delivery to cells and tissues is considered a standard tool in life science research. However, although an ideal delivery system should have high efficacy and minimal toxicity, existing materials fall short, most of them being either too toxic or little effective. We hypothesized that disulfide cross-linked low-molecular-weight (MW) linear poly(ethylene imine) (MW<4.6 kDa) would overcome this limitation. Investigations with these materials revealed that the extracellular high MW provided outstandingly high transfection efficacies (up to 69.62+/-4.18% in HEK cells). We confirmed that the intracellular reductive degradation produced mainly nontoxic fragments (cell survival 98.69+/-4.79%). When we compared the polymers in >1,400 individual experiments to seven commercial transfection reagents in seven different cell lines, we found highly superior transfection efficacies and substantially lower toxicities. This renders reductive degradation a highly promising tool for the design of new transfection materials.

A degradable polyethylenimine derivative with low toxicity for highly efficient gene delivery

Routine clinical implementation of human gene therapy awaits safe and efficient gene delivery methods. Polymeric vectors hold promise due to the availability of diverse chemistries, potentially providing targeting, low immunogenicity, nontoxicity, and robustness, but lack sufficient gene delivery efficiency. We have synthesized a versatile group of degradable polycations, through addition of 800-Da polyethylenimine (PEI) to small diacrylate cross-linkers. The degradable polymers reported here are similar in structure, size (14-30 kDa), and DNA-binding properties to commercially available 25-kDa PEI, but mediate gene expression two- to 16-fold more efficiently and are essentially nontoxic. These easily synthesized polymers are some of the most efficient polymeric vectors reported to date and provide a versatile platform for investigation of the effects of polymer structure and degradation rate on gene delivery efficiency.

Long-circulating poly(ethylene glycol)-modified gelatin nanoparticles for intracellular delivery

PURPOSE

The objective of this study was to develop and characterize long-circulating, biodegradable, and biocompatible nanoparticulate formulation as an intracellular delivery vehicle.

METHODS

Poly(ethylene glycol) (PEG)-modified gelatin was synthesized by reacting Type-B gelatin with PEG-epoxide. The nanoparticles, prepared by pH and temperature controlled ethanol-water solvent displacement technique, were characterized for mean size, size distribution, and surface morphology. Electron spectroscopy for chemical analysis (ESCA) was used to confirm the surface presence of PEG chains. In vitro release of tetramethylrhodamine-labeled dextran (TMR-dextran, Mol. wt. 10,000 daltons) from the nanoparticle formulations was examined in PBS, with and without 0.2-mg/ml protease, at 37 degrees C. Relative cytotoxicity profile of control and PEGylated gelatin was evaluated in BT-20 a human breast cancer cell line. The nanoparticles were incubated with BT-20 cells to determine uptake and cellular distribution using confocal microscopy.

RESULTS

Gelatin and PEGylated gelatin nanoparticles were found to be spherical in shape with a smooth surface in a size range of 200-500 nm and a unimodal size distribution. ESCA results showed an increase in the ether carbon (-C-O-) peak in the PEGylated gelatin nanoparticles due to the presence of PEG chains. The presence of PEG chains decreased the percent release of TMR-dextran in the presence of proteolytic enzyme due to steric repulsion. Cytotoxicity assays indicated that both gelatin and PEGylated gelatin were completely non-toxic to the cells. A large fraction of the administered control gelatin and PEGylated gelatin nanoparticles were found to be concentrated in the perinuclear region of the BT-20 cells after 12 hours indicating possible vesicular transport through initial uptake by endocytosis and endosomal processing.

CONCLUSION

The results of this study show that PEGylation of gelatin may prove beneficial as long-circulating delivery system in vivo. Additionally, the nanoparticles could encapsulate hydrophilic macromolecules and are internalized by tumor cells.

A versatile reducible polycation-based system for efficient delivery of a broad range of nucleic acids

Synthetic vectors based on reducible polycations consisting of histidine and polylysine residues (HIS RPCs) were evaluated for their ability to deliver nucleic acids. Initial experiments showed that RPC-based vectors with at least 70% histidine content mediated efficient levels of gene transfer without requirement for the endosomolytic agent chloroquine. Significant gene transfer was observed in a range of cell types achieving up to a 5-fold increase in the percentage of transfected cells compared to 25 kDa PEI, a gold standard synthetic vector. In contrast to 25 kDa PEI, HIS RPCs also mediated efficient transfer of other nucleic acids, including mRNA encoding green fluorescent protein in PC-3 cells and siRNA directed against the neurotrophin receptor p75^NTR in post-mitotic cultures of rat dorsal root ganglion cell neurons. Experiments to elevate intracellular glutathione and linear profiling of cell images captured by multiphoton fluorescent microscopy highlighted that parameters such as the molecular weight and rate of cleavage of HIS RPCs were important factors in determining transfection activity. Altogether, these results demonstrate that HIS RPCs represent a novel and versatile type of vector that can be used for efficient cytoplasmic delivery of a broad range of nucleic acids. This should enable different or a combination of therapeutic strategies to be evaluated using a single type of polycation-based vector.

Gold nanoparticles capped with polyethyleneimine for enhanced siRNA delivery

An efficient and safe delivery system for small interfering RNA (siRNA) is required for clinical application of RNA interfering therapeutics. Polyethyleneimine (PEI)-capped gold nanoparticles (AuNPs) are successfully manufactured using PEI as the reductant and stabilizer, which bind siRNA at an appropriate weight ratio by electrostatic interaction and result in well-dispersed nanoparticles with uniform structure and narrow size distribution. With siRNA binding, PEI-capped AuNPs induce more significant and enhanced reduction in targeted green fluorescent protein expression in MDA-MB-435s cells, though more internalized PEI/siRNA complexes in cells are evidenced by confocal laser scanning microscopy observation and fluorescence-activated cell sorting analyses. PEI-capped AuNPs/siRNA targeting endogenous cell-cycle kinase, an oncogene polo-like kinase 1 (PLK1), display significant gene expression knockdown and induce enhanced cell apoptosis, whereas it is not obvious when the cells are treated with PLK1 siRNA using PEI as the carrier. Without exhibiting cellular toxicity, PEI-capped AuNPs appear to be suitable as a potential carrier for intracellular siRNA delivery.

Partial acetylation of polyethylenimine enhances in vitro gene delivery

PURPOSE

Polyethylenimine (PEI) is a highly effective gene delivery vector, but because it is an off-the shelf material, its properties may not be optimal. To investigate the effects of the protonation properties of the polymer, we generated PEI derivatives by acetylating varying fractions of the primary and secondary amines to form secondary and tertiary amides, respectively.

METHODS

Reaction of PEI with increasing amounts of acetic anhydride at 60 degrees C for 4.5 h yielded polymers with 15%, 27%, and 43% of the primary amines modified with acetyl groups. Polymer-DNA complexes were characterized by dynamic light scattering and zeta potential measurements. Cytotoxicity of the polymers was assessed by XTT assay for metabolic activity, and gene delivery efficiency was determined as the relative expression of a luciferase gene in MDA-MB-231 and C2C12 cell lines.

RESULTS

Acetylation of PEI decreased the "physiological buffering capacity," defined as the moles of protons absorbed per mole of nitrogen on titration from pH 7.5 to 4.5, from 0.29 mol H+/mol N to 0.17 mol H+/mot N, 0.12 mol H+/mol N, and 0.090 mol H+/mol N for PEI-Ac15, PEI-Ac27, and PEI-Ac43, respectively. In addition, acetylation decreased the zeta potential of polyplexes from 14 mV to 8-11 mV and increased the polyplex diameter by two- to threefold. Surprisingly, acetylation had a negligible effect on cytotoxicity of the polymers and increased gene delivery effectiveness by up to 21-fold compared to unmodified PEI, both in the presence and absence of serum.

CONCLUSIONS

Reduction of the buffering capacity of PEI greatly enhanced the gene delivery activity of the polymer. The mechanism is not yet understood, but the enhancement may be caused by more effective polyplex unpackaging, altered endocytic trafficking, and/or increased lipophilicity of acetylated PEI-DNA complexes. Future studies will address these possibilities in more detail.

Multifunctional hybrid nanogel for integration of optical glucose sensing and self-regulated insulin release at physiological pH

Optical detection of glucose, high drug loading capacity, and self-regulated drug delivery are simultaneously possible using a multifunctional hybrid nanogel particle under a rational design in a colloid chemistry method. Such hybrid nanogels are made of Ag nanoparticle (NP) cores covered by a copolymer gel shell of poly(4-vinylphenylboronic acid-co-2-(dimethylamino)ethyl acrylate) [p(VPBA-DMAEA)]. The introduction of the glucose sensitive p(VPBA-DMAEA) gel shell onto Ag NPs makes the polymer-bound Ag NPs responsive to glucose. While the small sized Ag cores (10 +/- 3 nm) provide fluorescence as an optical code, the responsive polymer gel shell can adapt to a surrounding medium of different glucose concentrations over a clinically relevant range (0-30 mM), convert the disruptions in homeostasis of glucose level into optical signals, and regulate release of preloaded insulin. This shows a new proof-of-concept for diabetes treatment that exploits the properties from each building block of a multifunctional nano-object. The highly versatile multifunctional hybrid nanogels could potentially be used for simultaneous optical diagnosis, self-regulated therapy, and monitoring of the response to treatment.

Cytotoxicity of polyethyleneimine (PEI), precursor base layer of polyelectrolyte multilayer films

Polyethyleneimine (PEI) is a synthetic polymer commonly used as precursor base layer in polyelectrolyte multilayer films. However, the biological properties of this cationic macromolecule are poorly understood. The aim of this experimental investigation was to evaluate in vitro the biocompatibility of PEI towards two different human cell lines. The experimental investigation was undertaken on pure titanium (Ti) and nickel-titanium (NiTi) alloy samples with an average surface roughness of Ra=0.3microm. A biological study was undertaken at day 0 (2h after seeding), day 2, day 4 and day 7 to observe the cellular response of fibroblasts and osteoblasts cell lines in terms of morphology, adhesion (as observed by scanning electron microscopy), and viability (Mosmann's test). The results showed that PEI can be successfully deposited onto Ti or NiTi alloy, but generates a detrimental cellular response on both substrates as illustrated by a decrease of both fibroblast and osteoblast adhesion and proliferation over a 7-day culture period. These results suggest that PEI is potentially cytotoxic and may not be biocompatible enough in clinical applications using high molecular weight. As a consequence, polyelectrolyte multilayer films, which are promising in prosthesis and implantology fields, could not be coated with PEI at a high molecular weight. A lower molecular weight should be considered or a more biocompatible molecular base as precursor layer of polyelectrolyte multilayer films would be better to use for a good human bio-integration.

Transfection efficiency and toxicity of polyethylenimine in differentiated Calu-3 and nondifferentiated COS-1 cell cultures

In the present study, we evaluated polyethylenimine (PEI) of different molecular weights (MWs) as a DNA complexing agent for its efficiency in transfecting nondifferentiated COS-1 (green monkey fibroblasts) and well-differentiated human submucosal airway epithelial cells (Calu-3). Studying the effect of particle size, zeta potential, presence of serum proteins or chloroquine, it appeared that transfection efficiency depends on the experimental conditions and not on the MW of the PEI used. Comparing transfection efficiencies in both cell lines, we found that PEI was 3 orders of magnitude more effective in COS-1 than in Calu-3 cells, because Calu-3 cells are differentiated and secrete mucins, which impose an additional barrier to gene delivery. Transfection efficiency was strongly correlated to PEI cytotoxicity. Also, some evidence for PEI-induced apoptosis in both cell lines was found. In conclusion, our results indicate that PEI is a useful vector for nonviral transfection in undifferentiated cell lines. However, results from studies in differentiated bronchial epithelial cells suggest that PEI has yet to be optimized for successful gene therapy of cystic fibrosis (CF).

Linear poly(amido amine)s with secondary and tertiary amino groups and variable amounts of disulfide linkages: Synthesis and in vitro gene transfer properties

A group of novel poly(amido amine) homo- and copolymers (PAAs) containing secondary and tertiary amine groups in their main chain and different structures in the bisacrylamide segments were synthesized and evaluated as non-viral gene delivery vectors. Among these, also the disulfide-containing cystaminebisacrylamide was employed as a (co)monomer, yielding PAAs with variable amounts of bioreducible disulfide linkages in the main chain. Michael addition the trifunctional 1-(2-aminoethyl) piperazine to equimolar amounts of the appropriate bis(acrylamide) yielded linear polymers as was elucidated by their (13)C NMR spectra. The polymers possess buffering capacities between pH 5.1 and pH 7.4 higher than branched polyethylenimine (pEI) and are able to efficiently condense DNA into nanosized (<150 nm) and positively charged complexes. Transfection experiments with COS-7 cells showed that polyplexes from PAAs with disulfide linkages give significant higher transfections than those from PAAs lacking the disulfide linkage, and XTT assays showed that these polymers are essentially non-toxic. Variation of the disulfide content revealed that polyplexes of PAA copolymers with appropriate disulfide content have largely improved biophysical properties, yielding enhanced levels of gene expression along with low toxicity. The results demonstrate that bioreducible poly(amido amine)s are a very promising class of polymers for safe and efficient gene delivery.

Revisit the complexation of PEI and DNA - how to make low cytotoxic and highly efficient PEI gene transfection non-viral vectors with a controllable chain length and structure?

The commercially available branched polyethyleneimine (PEI) with a molar mass of 25 kD (PEI-25K) is an effective in vitro vector to transfer genes, but its cytotoxicity limits its applications in bio-related research. To solve such an efficiency-versus-cytotoxicity catch-22 problem, the disulfide bond has been previously used to link less toxic short PEI chains (2 kD), but previous literature results are controversial. Recently, we found that it is vitally important to remove both carbon dioxide and water in the linking reaction as well as to control the structure of the resultant chains linked by dithiobis(succinimidyl propionate) (DSP). Under a programmable mixing of PEI and DSP, we can use laser light scattering (LLS) to in-situ monitor the linking reaction kinetics in DMSO in terms of the change of the average molar mass (M(w)). Therefore, we were able to withdraw a series of linked PEI chains with different molar masses from one reaction mixture. Two such linked PEI samples (M(w) approximately 7 kD, PEI-7K-L and approximately 400 kD, PEI-400K-L) were used to illustrate the effect of the sample preparation and the chain structure on the in vitro gene transfection and cytotoxicity. Our results reveal that PEI-7K-L is less cytotoxic and more effective in the gene transfection than both PEI-25K and Lipofectamine 2000 in the in vitro gene transfection. However, PEI-400K-L has no gene transfection efficiency even though it is non-toxic.

Optimization of factors influencing the transfection efficiency of folate-PEG-folate-graft-polyethylenimine

Folate-poly(ethylene glycol)-folate-grafted-polyethylenimine (FPF-g-PEI) was synthesized over a range of grafting ratios of folate-poly(ethylene glycol)-folate (FPF) to polyethylenimine (PEI). The conjugation was determined using the absorbance at 363 nm for each polymer. FPF-g-PEIs were determined to have 2.3, 5.2, 9.3 and 20 FPF linear polymers grafted to each PEI. The average molecular weight was calculated to be approximately 34,848, 47,266, 64,823 and 110,640 Da, respectively. The pH profiles of FPF-g-PEIs suggest that the polymers have endosomal disruption capacity, and the gel electrophoretic band retardation showed efficient condensation of DNA. The transfection efficiency, determined using plasmid encoding luciferase, was dependent on the cell type and was different for CT-26 colon adenocarcinoma, KB oral epidermoid, and normal smooth muscle cells (SMC). The relative toxicity of polymer/plasmid complexes was determined using the MTT colorimetric assay. At neutral charge ratio, FPF-g-PEI/pLuc complexes were less toxic to cells and showed higher transfection in cancer cells compared to PEI/pLuc complexes. Smooth muscle cells showed no specificity for FPF-g-PEI/pLuc complexes, whereas PEI/pLuc complexes showed a higher transfection efficiency. The transfection efficiency increased when neutral polymer/DNA complex concentrations increased, but decreased when positively charged polymer/DNA complex concentrations increased. There was little increase in toxicity when FPF-5.2g-PEI/pLuc complex concentrations increased.

Poly(ethylenimine-co-L-lactamide-co-succinamide): a biodegradable polyethylenimine derivative with an advantageous pH-dependent hydrolytic degradation for gene delivery

A biodegradable gene transfer vector has been synthesized by linking several low molecular weight (MW) polyethylenimine (PEI, 1200 Da) blocks using an oligo(L-lactic acid-co-succinic acid) (OLSA, 1000 Da). The resulting copolymer P(EI-co-LSA) (8 kDa) is soluble in water and degrades via base-catalyzed hydrolytic cleavage of amide bonds. With regard to its application as a gene transfer agent, the polymer showed an interesting pH dependency of degradation. At pH 5, when DNases are highly active, the degradation proceeds at a slower rate than at a physiological pH of 7.4. PEI and P(EI-co-LSA) spontaneously formed complexes with plasmid DNA. Whereas the complexes formed with PEI were not stable and aggregated, forming particles of up to 1 microm hydrodynamic diameter, P(EI-co-LSA) formed complexes, which were about 150 nm in size and of narrow size distribution. The latter complexes were stable, due to their high surface charge (zeta-potential + 18 mV). Similar to low MW PEI, the copolymer exhibited a low toxicity profile. At the same time, the copolymer showed a significant enhancement of transfection activity in comparison to the low MW PEI. This makes P(EI-co-LSA) a promising candidate for long-term gene therapy where biocompatibility and biodegradability become increasingly important.

The development and characterization of a glutathione-sensitive cross-linked polyethylenimine gene vector

A glutathione-sensitive cross-linked polyethylenimine gene vector CLPEI(50%) was specially designed via the cross-linking reaction between the low molecular weight polyethylenimine (PEI(1800)) and dimethyl 3.3'-dithiopropionimidate dihydrochloride (DTBP). The acid-base titration test indicated that CLPEI(50%) still possessed efficient proton sponge effect. The property of CLPEI(50%)-DNA complexes were investigated by atomic force microscopy (AFM) and dynamic light scattering (DLS). CLPEI(50%) induced DNA condensation and formed spherical nanoparticles. The diameter of polyplexes prepared at pH value of 6.0 and 7.4 was about 150 and 260 nm, respectively. It was interesting to find the polyplexes were sensitive to the reductive glutathione (GSH). The CLPEI(50%)-DNA polyplexes prepared at N/P ratio of 10 were unpacked at GSH concentration of 3mm, which was comparable to the intracellular environment. The in vitro cytotoxicity of CLPEI(50%) was also significantly reduced comparing with PEI(25k). The biomimetic CLPEI(50%)-DNA polyplexes with the low cytotoxicity and GSH-sensitive property could be a good candidate for gene delivery.

miRNA oligonucleotide and sponge for miRNA-21 inhibition mediated by PEI-PLL in breast cancer therapy

MicroRNA-21 (miR-21) inhibition is a promising biological strategy for breast cancer therapy. However its application is limited by the lack of efficient miRNA inhibitor delivery systems. As a cationic polymer transfection material for nucleic acids, the poly (l-lysine)-modified polyethylenimine (PEI-PLL) copolymer combines the high transfection efficiency of polyethylenimine (PEI) and the good biodegradability of polyllysine (PLL). In this work, PEI-PLL was successfully synthesized and confirmed to transfect plasmid and oligonucleotide more effectively than PEI in MCF-7 cells (human breast cancer cells). In this regard, two kinds of miR-21 inhibitors, miR-21 sponge plasmid DNA (Sponge) and anti-miR-21 oligonucleotide (AMO), were transported into MCF-7 cells by PEI-PLL respectively. The miR-21 expression and the cellular physiology were determined post transfection. Compared with the negative control, PEI-PLL/Sponge or PEI-PLL/AMO groups exhibited lower miR-21 expression and cell viability. The anti-tumor mechanism of PEI-PLL/miR-21 inhibitors was further studied by cell cycle and western blot analyses. The results indicated that the miR-21 inhibition could induce the cell cycle arrest in G1 phase, upregulate the expression of Programmed Cell Death Protein 4 (PDCD4) and thus active the caspase-3 apoptosis pathway. Interestingly, the PEI-PLL/Sponge and PEI-PLL/AMO also sensitized the MCF-7 cells to anti-tumor drugs, doxorubicin (DOX) and cisplatin (CDDP). These results demonstrated that PEI-PLL/Sponge and PEI-PLL/AMO complexes would be two novel and promising gene delivery systems for breast cancer gene therapy based on miR-21 inhibition.

STATEMENT OF SIGNIFICANCE

This work was a combination of the high transfection efficiency of polyethylenimine (PEI), the good biodegradability of polyllysine (PLL) and the breast cancer-killing effect of miR-21 inhibitors. The poly (l-lysine)-modified polyethylenimine (PEI-PLL) copolymer was employed as the vector of miR-21 sponge plasmid DNA (Sponge) or anti-miR-21 oligonucleotide (AMO). PEI-PLL showed more transfection efficiency and lower cytotoxicity in human breast cancer cells than PEI. Moreover, the breast cancer cells exhibited significantly lower miR-21 expression and cell viability post transfection with sponge or AMO. Interestingly, the PEI-PLL/miR-21 inhibitor complexes also sensitized the cancer cells to anti-cancer chemotherapy drugs, doxorubicin (DOX) and cisplatin (CDDP). This synergistic effect provides a good application prospect of co-delivery miR-21 inhibitors and chemical drugs in breast cancer therapy.

A chitosan-modified graphene nanogel for noninvasive controlled drug release

A near infrared (NIR) triggered drug delivery platform based on the chitosan-modified chemically reduced graphene oxide (CRGO) incorporated into a thermosensitive nanogel (CGN) was developed. CGN exhibited an NIR-induced thermal effect similar to that of CRGO, reversible thermo-responsive characteristics at 37-42 °C and high doxorubicin hydrochloride (DOX) loading capacity (48 wt%). The DOX loaded CGN (DOX-CGN) released DOX faster at 42 °C than at 37 °C. The fluorescence images revealed DOX expression in the cytoplasm of cancer cells when incubated with DOX-CGN at 37 °C but in the nucleus at 42 °C. Upon irradiation with NIR light (808 nm), a rapid, repetitive DOX release from the DOX-CGN was observed. Furthermore, the cancer cells incubated with DOX-CGN and irradiated with NIR light displayed significantly greater cytotoxicity than without irradiation owing to NIR-triggered increase in temperature leading to nuclear DOX release. These results demonstrate CGN's promising application for on-demand drug release by NIR light.

FROM THE CLINICAL EDITOR

These investigators report the successful development of a novel near infrared triggered drug delivery platform based on chitosan-modified chemically reduced graphene oxide (CRGO) incorporated into a thermosensitive nanogel (CGN).

Intracellular dynamics of the gene delivery vehicle polyethylenimine during transfection: investigation by two-photon fluorescence correlation spectroscopy

Though polyethylenimine (PEI) is one of the most efficient nonviral vectors, one concern is the significant cytotoxicity of free PEI that represents about 80% of the PEI molecules in PEI/DNA mixtures used for transfection. In this respect, the aim of this work was to further investigate the intracellular fate of PEI during transfection of L929 fibroblasts. To this end, we analyzed by fluorescence correlation spectroscopy (FCS) using two-photon excitation the intracellular concentration and diffusion properties of labeled PEI and PEI/DNA complexes in various compartments of L929 cells. High initial fluorescence intensity, rapid photobleaching and the absence of measurable autocorrelation curves in most selected locations in cytoplasm suggest that PEI/DNA complexes and PEI accumulate (up to 30 times the concentration in the extracellular medium) in late endosomes bound to the inner membrane face. This feature, together with membrane destabilizing properties of PEI, may explain the release of PEI into cytoplasm and subsequent diffusion into the nucleus. In the nucleus, the concentration of PEI was found to be about 2.5- to 3.5-fold higher than the one in the incubation medium. Moreover, autocorrelation curves obtained in the nuclear compartment can be analyzed with either a two-component model (with the major fraction undergoing free Brownian diffusion) or an anomalous diffusion model. Both the endosomal disruption and the large intranuclear PEI concentration may contribute to PEI cytotoxicity.

Optimization of 25kDa linear polyethylenimine for efficient gene delivery

A 25-kDa linear polyethylenimine (25 kDa L-PEI) has proven to be efficient and versatile agent for gene delivery. Therefore, we determined the optimal transfection conditions of 25 kDa L-PEI and examined whether it has comparable transfection efficiency with other commercially available reagents, ExGen 500, LipofectAMINE 2000, and Effectene by using EGFP expression vector in different cell lines. Transfection efficiency and cytotoxicity were measured by flow cytometry. First of all, we determined the optimal ratio of nitrogen to phosphorous (N/P) and DNA concentration. With the increase of N/P ratio and DNA amounts, transfection efficiency increased with a slight variation in cell types. The optimal amounts of 25 kDa L-PEI were determined at N/P ratio 40 and DNA concentration varied among the cell types. In addition, 25 kDa L-PEI worked efficiently and was less toxic than other reagents. However, the efficiency and toxicity of all these reagents varied according to cell types as well as the ratio of DNA to reagents and the amounts of DNA. Our finding illustrates the importance of optimal transfection conditions of 25 kDa L-PEI to obtain maximal transgene expression with less cytotoxicity. Importantly, the optimization of those conditions may make possible to perform transfection cost-effectively and efficiently.

Multimodal analysis of PEI-mediated endocytosis of nanoparticles in neural cells

Polymer nanoparticles are widely used as a highly generalizable tool to entrap a range of different drugs for controlled or site-specific release. However, despite numerous studies examining the kinetics of controlled release, the biological behavior of such nanoparticles remains poorly understood, particularly with respect to endocytosis and intracellular trafficking. We synthesized polyethylenimine-decorated polymer nanospheres (ca. 100-250 nm) of the type commonly used for drug release and used correlated electron microscopy, fluorescence spectroscopy and microscopy, and relaxometry to track endocytosis in neural cells. These capabilities provide insight into how polyethylenimine mediates the entry of nanoparticles into neural cells and show that polymer nanosphere uptake involves three distinct steps, namely, plasma membrane attachment, fluid-phase as well as clathrin- and caveolin-independent endocytosis, and progressive accumulation in membrane-bound intracellular vesicles. These findings provide detailed insight into how the intracellular delivery of nanoparticles is mediated by polyethylenimine, which is presently the most commonly used nonviral gene transfer agent. This fundamental knowledge may also assist in the preparation of next-generation nonviral vectors.

Influence of acyl chain length on transfection mediated by acylated PEI nanoparticles

Polyethylenimine (750 kDa) has been derivatized to influence the proton sponge mechanism and hydrophobic-hydrophilic balance. The polymer was acylated using acid anhydrides of varying carbon chain length, followed by cross-linking with PEG-bis-P to form compact nanoparticles. The chemical linkages in the particles were characterized by FTIR and NMR spectroscopy. The hydrodynamic diameter of nanoparticles was found to be in the range of 83.5-124 nm. AFM imaging of native and DNA-loaded nanoparticles revealed highly compact and spherical shape. The positive surface charge on particles decreased with the increase in percentage of acylation and also on complexing with DNA. The buffering capacity of PEI was reduced considerably on preparing acylated nanoparticles. The nanoparticles formed stable complexes with DNA and higher weight ratios were required for formation of electro-neutral complexes. Further, these nanoparticles were investigated for their gene delivery efficacy on COS-1 cells. It was found that acylated PEI nanoparticles were 5-12-fold more efficient transfecting agents as compared to native PEI (750 kDa) and commercially available transfecting agent lipofectin. The MTT colorimetric assay revealed of considerable reduction in toxicity of acylated PEI nanoparticles as compared PEI. Of all the systems prepared, nanoparticles with 30% acylation using propionic anhydride were found to be the most efficient in in vitro transfection.

Nonviral gene carriers based on diblock copolymers of poly(2-ethyl-2-oxazoline) and linear polyethylenimine

Diblock copolymers that consist of poly(2-ethyl-2-oxazoline) (PEOz) and linear polyethylenimine (LPEI) were prepared for use as nonviral gene carriers. The PEOz-b-LPEI copolymers were synthesized by coupling PEOz with LPEI in a thiol-disulfide exchange reaction between the sulfhydryl and pyridyl disulfide terminal groups. A polymer/DNA weight ratio (P/D) of over 12 was required to enable PEOz-b-LPEI to condense DNA completely. The DNA-condensing capability of the diblock copolymers was increased with increasing the hydrolytic degrees of the LPEI segment. The PEOz-b-LPEI polyplexes were stable in 150 mM NaCl aqueous solution and had a mean diameter around 190 nm, whereas BPEI and LPEI polyplexes formed large aggregates in the range 300-500 nm. In addition, these polyplexes exhibited the sensitivity to solution pH and were dissociated in the acidic buffers (pH < or = 5.5). The results of in vitro cell viability and luciferase assay indicated that PEOz-b-LPEI showed not only low cytotoxicity but also high transfection efficiency in gene expression.