Novel Reduction-Responsive Cross-Linked Polyethylenimine Derivatives by Click Chemistry for Nonviral Gene Delivery

Design of polyethylene glycol-polyethylenimine nanocomplexes as non-viral carriers: mir-150 delivery to chronic myeloid leukemia cells

MicroRNAs (miRNAs) are acknowledged as indispensable regulators relevant in many biological processes, and they have been pioneered as therapeutic targets for curing disease. miRNAs are single-stranded, small (19-22 nt) regulatory non-coding RNAs whose deregulation of expression triggers human cancers, including leukemias, mainly through dysregulation of expression of leukemia genes. miRNAs can function as tumour suppressors (suppressing malignant potential) or oncogenes (activating malignant potential) like actors of complex diseases. To address the issue of overcoming instability and low transfection efficiency in vitro, the polyethylene glycol-polyethyleneimine (PEG-PEI) nanoparticle was used as non-viral vector carrier for miR-150 transfection, which is downregulated in chronic myeloid leukemia. PEG-PEI [PEG(550)3 -g-PEI(1800) ]/miRNA nanocomplexes were synthesised and characterised by particle size distribution (PSD), polydispersity index (PDI) and zeta potential, surface charge, their cytotoxicity, and transfection efficiency. Interaction with human leukemia cells (K-562 and KU812) and control cells NCI-BL2347 with them has been investigated. The transfection efficiency of PEG-PEI/miRNA at N/P 26 rose 6.7-fold above the control by qRT-PCR. The size of homogenous nanocomplexes (PBI < 0.5) was 160.8 ± 11 nm. The data indicate that PEG-PEI may be an encouraging non-viral carrier for altering miRNA expression in the treatment of chronic myeloid leukemia, with many advantages such as relatively high miRNA transfection efficiency and low cytotoxicity.

Reduction biodegradable brushed PDMAEMA derivatives synthesized by atom transfer radical polymerization and click chemistry for gene delivery

Novel reducible and degradable brushed poly(2-(dimethylamino) ethyl methacrylate) (PDMAEMA) derivatives were synthesized and evaluated as non-viral gene delivery vectors. First, alkyne-functionalized poly(aspartic acid) with a disulfide linker between the propargyl group and backbone poly([(propargyl carbamate)-cystamine]-α,β-aspartamide) (P(Asp-SS-AL)) was synthesized. Second, linear low molecular weight (LMW) monoazido-functionalized PDMAEMAs synthesized via atom transfer radical polymerization were conjugated to the polypeptide side-chains of P(Asp-SS-AL) via click chemistry to yield high molecular weight (HMW) polyaspartamide-based disulfide-containing brushed PDMAEMAs (PAPDEs). The PAPDEs were able to condense plasmid DNA to form 100 to 200nm polyplexes with positive ζ-potentials. Moreover, in the presence of dithiothreitol the PAPDEs degraded into LMW PDAMEMA, resulting in disintegration of the PAPDE/DNA polyplexes and subsequent release of plasmid DNA. In vitro experiments revealed that the PAPDEs were less cytotoxic and more effective in gene transfection than control 25kDa poly(ethyleneimine) and HMW linear PDMAEMA. In conclusion, reducible and degradable polycations composed of LMW PDMAEMAs coupled to a polypeptide backbone via reduction-sensitive disulfide bonds are effective gene vectors with an excellent cytocompatibility.

Angelica sinensis polysaccharide nanoparticles as novel non-viral carriers for gene delivery to mesenchymal stem cells

This study centers on the use of a nanoparticle based on the polysaccharide from Angelica sinensis (ASP) as an efficient and safe non-viral gene vector. After modification with branched low molecular weight polyethylenimine (1200 Da), the cationized ASP (cASP) was combined with the plasmid encoding transforming growth factor-beta 1 (TGF-β1) to form a spherical nano-scaled particle (i.e., cASP-pTGF-β1 nanoparticle). This nanoparticle was applied to transfect rat bone marrow mesenchymal stem cells and human umbilical cord mesenchymal stem cells. As a result, nanoparticles (cASP/pDNA weight ratio 10:1) had the greatest transfection efficiency in both cells, which was significantly higher than those of Lipofectamine2000 and PEI (25 kDa). This was in agreement with the findings of the semi-quantitative RT-PCR and live cell imaging. These nanoparticles were also less toxic than Lipofectamine2000 and PEI (25 kDa). Therefore, cASP could be a potential candidate for a novel non-viral gene vector.

FROM THE CLINICAL EDITOR

These authors demonstrate the use of a nanoparticle-based efficient and safe non-viral gene vector delivery system via a spherical nanoparticle based on a polysaccharide from Angelica sinensis, with parameters superior to Lipofectamine2000.

Design and Development of Degradable Polyethylenimines for Delivery of DNA and Small Interfering RNA: An Updated Review

Polyethylenimine (PEI), considered as the most potent and promising alternative carrier to viral vectors, has been studied as the “state of the art” among various polymers for nonviral gene delivery applications for many years. Although PEI-based carrier minimizes the bottlenecks associated with viral vectors such as unwanted immunogenicity and production problems, the toxic side effects of PEI prevent its rapid advancements due to nondegradable nature. In this regard, various degradable cross-linking and/or grafting agents have been linked to synthesize degradable PEIs in order to minimize the toxicity and improve the efficacy of PEI-mediated gene carriers. This paper describes an update on various cross-linkers and grafting agents in the design and development of degradable PEI derivatives and their potential applications for effective delivery of DNA in vitro and in vivo. The molecular weight (MW) of PEI and the structural relationship to its cellular toxicity and transfection ability were also discussed. Finally, the potential applications of various degradable PEIs for small interfering RNA (siRNA)-mediated gene silencing were also covered.

Reversibly crosslinked nanocarriers for on-demand drug delivery in cancer treatment

Polymer micelles have proven to be one of the most versatile nanocarriers for anticancer drug delivery. However, the in vitro and in vivo stability of micelles remains a challenge due to the dynamic nature of these self-assembled systems, which leads to premature drug release and nonspecific biodistribution in vivo. Recently, reversibly crosslinked micelles have been developed to provide solutions to stabilize nanocarriers in blood circulation. Increased stability allows nanoparticles to accumulate at tumor sites efficiently via passive and/or active tumor targeting, while cleavage of the micelle crosslinkages, through internal or external stimuli, facilitates on-demand drug release. In this review, various crosslinking chemistries as well as the choices for reversible linkages in these nanocarriers will be introduced. Then, the development of reversibly crosslinked micelles for on-demand drug release in response to single or dual stimuli in the tumor microenvironment is discussed, for example, acidic pH, reducing microenvironment, enzymatic microenvironment, photoirradiation and the administration of competitive reagents postmicelle delivery.

Bioreducible poly(amido amine)s with different branching degrees as gene delivery vectors

Based on the knowledge that cationic polymers with different topographical structures behave differently in gene transfection process, herein, we synthesized three biodegradable poly(amido amine)s (PAAs) with the same repeating units and molecular weights except for degree of branching: linear PAA (LPAA), low-branched PAA (LBPAA), and high-branched PAA (HBPAA). We found that LBPAA could more effectively compact pDNA into positively charged nanoparticles than both HBPAA and LPAA. LBPAA polyplexes had the highest transfection efficiency among the three PAA polyplexes, and the difference in transfection efficiency is mainly attributed to the endocytosis rate. The cytotoxicity of PAAs was negligible at the transfection doses, probably due to the degradable disulfide bonds. Therefore, we could use branching as a parameter to simply tune a polymer's cellular uptake behavior and transfection efficiency.

Amphiphilic graft copolymer based on poly(styrene-co-maleic anhydride) with low molecular weight polyethylenimine for efficient gene delivery

Background and methods

A new amphiphilic comb-shaped copolymer (SP) was synthesized by conjugating poly(styrene-co-maleic anhydride) with low molecular weight polyethyleneimine for gene delivery. Fourier transform infrared spectrum, ¹H nuclear magnetic resonance, and gel permeation chromatography were used to characterize the graft copolymer.

Results

The buffering capability of SP was similar to that of polyethyleneimine within the endosomal pH range. The copolymer could condense DNA effectively to form complexes with a positive charge (13–30 mV) and a small particle size (130–200 nm) at N/P ratios between 5 and 20, and protect DNA from degradation by DNase I. In addition, SP showed much lower cytotoxicity than polyethyleneimine 25,000. Importantly, the gene transfection activity and cellular uptake of SP-DNA complexes were all markedly higher than that of complexes of polyethyleneimine 25,000 and DNA in MCF-7 and MCF-7/ADR cell lines.

Conclusion

This work highlights the promise of SP as a safe and efficient synthetic vector for DNA delivery.

Development of a reduction-sensitive diselenide-conjugated oligoethylenimine nanoparticulate system as a gene carrier

BACKGROUND

The reduction-sensitive cationic polymer is a promising nonviral carrier for gene delivery. Until now, disulfide bonds have been the only golden standard for its design. The aim of this research was to develop a novel reduction-responsive cationic polymer as a gene carrier.

METHODS

Polycationic carriers were synthesized by addition of branched oligoethylenimine 800 Da (OEI(800)) via an active ester containing diselenide bonds. Disulfide bonds cross-linked with OEI(800)-SS(x) and monoselenide bonds linked with OEI(800)-Se(x) were synthesized and compared. Their molecular weights and degradation properties were determined using gel permeation chromatography. Changes in particle size, morphology, and DNA binding were investigated by dynamic light scattering, transmission electron microscopy, and electrophoresis assay in a reduction environment. Cytotoxicity and transfection in vitro were evaluated in a murine melanoma cell line (B16F10) and a human cervical epithelial carcinoma cell line (HeLa), while intracellular degradation and dissociation with DNA were studied by confocal laser scanning microscopy with FITC-labeled OEI(800) derivatives and Cy5-labeled DNA.

RESULTS

Diselenide-conjugated OEI(800) (OEI(800)-SeSe(x)) polymer carriers of high molecular weight were successfully synthesized. After compacting with DNA, the OEI(800)-SeSe(x) polymers formed nanoparticles with an average size of 140 nm at an adequate C/P ratio. OEI(800)-SeSe(x) showed reduction-responsive degradation properties similar to those of the OEI(800)-SS(x) via gel permeation chromatography, dynamic light scattering, and transmission electron microscopy. OEI(800)-SeSe(x) showed much lower cytotoxicity than PEI(25k), and significantly higher transfection efficiency than OEI(800) in both B16F10 and HeLa cells. Transfection of luciferase in the OEI(800)-SeSe(x) group was comparable with that of standard PEI(25k) and traditional reduction-sensitive polymer OEI(800)-SS(x) groups. Furthermore, intracellular degradation of OEI(800)-SeSe(x) and dissociation with DNA were also confirmed by confocal laser scanning microscopy.

CONCLUSION

The OEI(800)-SeSe(x) obtained was able to bind plasmid DNA efficiently to yield nanosized particles and had reduction sensitivity which is as efficient as that for OEI(800)-SS(x). In vitro experiments confirmed its low cytotoxicity and high transfection ability. Diselenide bonds can be used as effective and novel reduction-sensitive linkages for gene delivery.

Bioreducible polymers for gene silencing and delivery

Polymeric gene delivery vectors show great potential for the construction of the ideal gene delivery system. These systems harness their ability to incorporate versatile functional traits to overcome most impediments encountered in gene delivery: from the initial complexation to their target-specific release of the therapeutic nucleic acids at the cytosol. Among the numerous multifunctional polymers that have been designed and evaluated as gene delivery vectors, polymers with redox-sensitive (or bioreducible) functional domains have gained great attention in terms of their structural and functional traits. The redox environment plays a pivotal role in sustaining cellular homeostasis and natural redox potential gradients exist between extra- and intracellular space and between the exterior and interior of subcellular organelles. In some cases, researchers have designed the polymeric delivery vectors to exploit these gradients. For example, researchers have taken advantage of the high redox potential gradient between oxidizing extracellular space and the reducing environment of cytosolic compartments by integrating disulfide bonds into the polymer structure. Such polymers retain their cargo in the extracellular space but selectively release the therapeutic nucleic acids in the reducing space within the cytosol. Furthermore, bioreducible polymers form stable complex with nucleic acids, and researchers can fabricate these structures to impart several important features such as site-, timing-, and duration period-specific gene expression. Additionally, the introduction of disulfide bonds within these polymers promotes their biodegradability and limits their cytotoxicity. Many approaches have demonstrated the versatility of bioreducible gene delivery, but the underlying biological rationale of these systems remains poorly understood. The process of disulfide reduction depends on multiple variables in the cellular redox environment. Therefore, the quest to unravel various issues such as the site and time of disulfide bond reduction during the cellular uptake and trafficking have stimulated a number of interesting studies which have employed disulfide compounds with a variety of reducible linkers. Such studies help researchers understand not only how modifications made to disulfides can alter their thiol-disulfide exchange characteristics but also to decipher the effect of the induced changes on the dynamics of the redox environment. This Account discusses current research trends and recent progress in the disulfide chemistry enabling novel and versatile designs of reducible polymeric gene delivery systems. We present strategies for the introduction of disulfide bonds into polymers. These representative examples and their respective outcomes elaborate the benefit and efficiency of disulfides at the individual stages of gene delivery.

Disulfide-containing brushed polyethylenimine derivative synthesized by click chemistry for nonviral gene delivery

Polyaspartamide-based disulfide-containing brushed polyethylenimine derivatives P(Asp-Az)X-SS-PEIs were synthesized via click chemistry and evaluated as nonviral gene delivery carrier. First, azide-functional poly(aspartic acid) derivatives with various azide-group densities and monoalkyne-terminated PEI with disulfide linkages were synthesized. Then, click reaction between the azide-functional poly(aspartic acid) derivative as main chain and the monoalkyne-terminated PEI as branched chain resulted in high-molecular-weight disulfide-containing brushed PEI derivative. The structure of obtained polymers was confirmed by (1)H NMR and FTIR. It was shown that the disulfide-containing P(Asp-Az)X-SS-PEIs were able to bind plasmid DNA and condense DNA into small positive nanoparticles. The reduction-sensitivity of the P(Asp-Az)X-SS-PEI/DNA polyplexes was confirmed by gel retardation assay and dynamic light scattering (DLS) in the presence of DTT. In vitro experiments revealed that the reducible P(Asp-Az)X-SS-PEI not only had much lower cytotoxicity, but also posed high transfection activity (both in the presence and absence of serum) as compared to the control nondegradable 25 kDa PEI. This study indicates that a reducibly degradable brushed polymer P(Asp-Az)X-SS-PEI composed of low-molecular-weight (LMW) PEI via a disulfide-containing linkage can be a promising gene delivery carrier.

DEGRADABLE POLYETHYLENIMINE DERIVATIVES AS GENE CARRIERS

Gene therapy is a treatment for inborn and acquired diseases, although the development of safe and effective gene delivery system is a great challenge to make a gene therapy a success. Viral vectors have been used in a majority of clinics because of their high transfection efficiency in vitro and in vivo. However, their use has been limited because of several drawbacks, such as induction of immune response, recombination of wild-type viruses, limitation in the size of inserted gene, and difficulty in large-scale production. Nonviral vectors have been widely proposed safe alternatives to viral vectors because they have low immunogenicity, flexibility in the size of gene to be delivered, cell targetibility, and easy scalability of production, although they have low transfection efficiency compared to viral vectors. Among nonviral vectors, polyethylenimine (PEI) has been widely used as a standard gene carriers due to its high pH-buffering capacity for endosomal escape although high-molecular-weight PEI is too toxic owing to non-degradability. Recently, many types of degradable PEI have been studied due to high transfection efficiency with lower cytotoxicity. This review explains recent progress on the development of degradable PEIs as nonviral vectors. The present paper summarizes the transfection efficiency of DNA or silencing efficiency of small interfering RNA (siRNA) based on the kinds of degradable linkage between low PEI and crosslinkers. Degradable linkages, such as ester, disulfide, imines, carbamate, amide and ketal in the degradable PEIs are covered.

Design of serum compatible tetrary complexes for gene delivery

A novel gene delivery system, called PoSC, consisting of PEI, PSP, and HA is described. In contrast to the DNA/PEI/HA ternary system whose transfection efficiency decreases significantly with increasing serum concentration, PoSC exhibits a high transfection efficiency of about 51 and 87% for NIH3T3 and HCT116 cells, respectively, at 50% serum concentration. Furthermore, PoSC shows no cytotoxic effect at its working concentration. The overall results suggest that HA adsorption on cationic complexes enhances the transfection efficiency, while PSP is essential for high transfection efficiency at higher serum concentration.

Poly(ethylene oxide) grafted with short polyethylenimine gives DNA polyplexes with superior colloidal stability, low cytotoxicity, and potent in vitro gene transfection under serum conditions

Poly(ethylene oxide) grafted with 1.8 kDa branched polyethylenimine (PEO-g-PEI) copolymers with varying compositions, that is, PEO(13k)-g-10PEI, PEO(24k)-g-10PEI, and PEO(13k)-g-22PEI, were prepared and investigated for in vitro nonviral gene transfer. Gel electrophoresis assays showed that PEO(13k)-g-10PEI, PEO(24k)-g-10PEI, and PEO(13k)-g-22PEI could completely inhibit DNA migration at an N/P ratio of 4/1, 4/1, and 3/1, respectively. Dynamic light scattering (DLS) and zeta potential measurements revealed that all three graft copolymers were able to effectively condense DNA into small-sized (80-245 nm) particles with moderate positive surface charges (+7.2 ∼ +24.1 mV) at N/P ratios ranging from 5/1 to 40/1. The polyplex sizes and zeta-potentials intimately depended on PEO molecular weights and PEI graft densities. Notably, unlike 25 kDa PEI control, PEO-g-PEI polyplexes were stable against aggregation under physiological salt as well as 20% serum conditions due to the shielding effect of PEO. MTT assays in 293T cells demonstrated that PEO-g-PEI polyplexes had decreased cytotoxicity with increasing PEO molecular weights and decreasing PEI graft densities, wherein low cytotoxicities (cell viability >80%) were observed for polyplexes of PEO(13k)-g-22PEI, PEO(13k)-g-10PEI, and PEO(24k)-g-10PEI up to an N/P ratio of 20/1, 30/1, and 40/1, respectively. Interestingly, in vitro transfection results showed that PEO(13k)-g-10PEI polyplexes have the best transfection activity. For example, PEO(13k)-g-10PEI polyplexes formed at an N/P ratio of 20/1, which were essentially nontoxic (100% cell viability), displayed over 3- and 4-fold higher transfection efficiencies in 293T cells than 25 kDa PEI standard under serum-free and 10% serum conditions, respectively. Confocal laser scanning microscopy (CLSM) studies using Cy5-labeled DNA confirmed that these PEO-g-PEI copolymers could efficiently deliver DNA into the perinuclei region as well as into nuclei of 293T cells at an N/P ratio of 20/1 following 4 h transfection under 10% serum conditions. PEO-g-PEI polyplexes with superior colloidal stability, low cytotoxicity, and efficient transfection under serum conditions are highly promising for safe and efficient in vitro as well as in vivo gene transfection applications.

Functional properties and biodistribution of poly(triethylenetetramine/cystamine bisacrylamide) and poly(triethylenetetramine/cystamine bisacrylamide)- poly(ethylene glycol) mixtures formed with nucleic acid

The clinical success of non-viral gene delivery reagents is hampered by their inefficient cellular transgene delivery, which is largely influenced by carrier properties that are currently undefined and misunderstood. In an attempt to further define and understand the requirements for a safe and efficient non-viral gene delivery reagent, research labs often engineer and evaluate many putative products with subtle physiochemical differences in order to delineate requirements for improved in vitro and in vivo success. The synthesis of many putative reagents is often time-intensive, laborious and costly. In a previous manuscript published by our lab, different amounts of poly(triethylenetetramine/cystamine bisacrylamide) (p(TETA/CBA) and its pegylated counterpart, poly(triethylenetetramine/cystamine bisacrylamide)- poly(ethylene glycol) (p(TETA/CBA)-g-PEG) were mixed together to easily identify optimal reagent properties and candidates in vitro, while avoiding the synthesis of many putative candidates for study. This report uses the aforementioned facile approach to evaluate reagent properties of products that were obtained via one-pot synthesis, which improved synthetic ease. As such, synthesis time was reduced from 6days to 3days and had comparable or improved transfection and viability compared to previous works. Moreover, this synthesis resulted in higher molecular weight products than were used in the previous study and allow for lower polymer doses to be used for complexation, which is useful for systemic delivery that is used herein. The physiochemical properties of the formulations derived using these novel reagents was studied prior to investigating their in vivo biodistribution profiles in a murine colon adenocarcinoma model. Interestingly, negatively charged complexes exhibited greater passive tumor accumulation compared to positively charged complexes following their systemic administration. These studies warrant further investigation for the use of negatively charged drug and gene delivery reagents for passive tumor targeting, and they substantiate the use of polycation/PEG-polycation mixtures for facile product evaluation in order to elucidate design and formulation mandates for the clinical success of non-viral gene delivery formulations.

Low molecular weight polyethylenimine-graft-Tween 85 for effective gene delivery: synthesis and in vitro characteristics

The development of safe and efficient gene delivery systems is still a challenge for successful gene therapy. In this work, low molecular weight polyethylenimine (PEI 2K) was modified by Tween 85, which bears three oleate chains. Tween 85 modified PEI 2K (TP) could condense DNA efficiently, and TP/DNA complexes (TPCs) showed high resistance to salt-induced aggregation and enzymatic degradation. In addition, TP did not show the obvious cytotoxicity. The introduction of Tween 85 led to a significant increase in the cellular uptake of complexes with higher transfection efficiency, which was strongly inhibited by the addition of free Tween 85 in MCF-7/ADR cells, but not in MCF-7 cells. These results indicated that TP could be a potentially safe and effective copolymer for gene delivery, and TPCs could be taken up mainly by Tween 85-mediated endocytosis in MCF-7/ADR cells.

Non-viral gene transfection in vitro using endosomal pH-sensitive reversibly hydrophobilized polyethylenimine

Reversibly hydrophobilized 10 kDa polyethylenimine (PEI) based on rapidly acid-degradable acetal-containing hydrophobe was designed for nontoxic and highly efficient non-viral gene transfer. Water soluble PEI derivatives with average 5, 9 and 14 units of pH-sensitive 2,4,6-trimethoxybenzylidene-tris(hydroxymethyl)ethane (TMB-THME) hydrophobe per molecule, denoted as PEI-g-(TMB-THME)(n), were readily obtained by treating 10 kDa PEI with varying amounts of TMB-THME-nitrophenyl chloroformate. Gel retardation assays showed that all PEI-g-(TMB-THME)(n) derivatives could effectively condense DNA at an N/P ratio of 5/1. Notably, polyplexes of PEI-g-(TMB-THME)(n) derivatives had smaller sizes (about 100∼170 nm) and higher surface charges (+25 ∼ +43 mV) than the parent 10 kDa PEI at the same N/P ratios ranging from 10/1 to 40/1. MTT assays revealed that these PEI-g-(TMB-THME)(n) derivatives were practically non-toxic at polymer concentrations used in transfection experiments. The acetal degradation of PEI-g-(TMB-THME)(9) was shown to be highly pH dependent in which half lives of 1.3, 2.8 and 11 h were determined for pH 4.0, 5.0 and 6.0, respectively, while negligible hydrolysis (<12%) was observed after 24 h at pH 7.4. Gel electrophoresis, dynamic light scattering (DLS) and zeta potential analyses indicated that polyplexes formed at an N/P ratio of 10/1 were dissociated following 5 h incubation at pH 5.0, highlighting the importance of hydrophobic TMB-THME moieties in DNA condensation and supporting that acetal hydrolysis in endosomes would facilitate DNA release. Notably, in vitro transfection experiments performed at N/P ratios of 10/1 and 20/1 in HeLa, 293T, HepG2 and KB cells using plasmid pGL3 expressing luciferase as the reporter gene showed that reversibly hydrophobilized PEIs had superior transfection activity to 25 kDa PEI control. For example, polyplexes of PEI-g-(TMB-THME)(14) showed about 235-fold and 175-fold higher transfection efficiency as compared to 10 kDa PEI in HeLa cells in serum-free and 10% serum media, respectively, which were approximately 7-fold and 16-fold higher than 25 kDa PEI formulation at its optimal N/P ratio under otherwise the same conditions. Confocal laser scanning microscope (CLSM) studies confirmed that PEI-g-(TMB-THME)(14) efficiently delivered Cy5-labeled DNA to the nuclei of HeLa cells. These endosomal pH-sensitive reversibly hydrophobilized PEIs have great potentials for safe and efficient non-viral gene transfection.

Reducible HPMA-co-oligolysine copolymers for nucleic acid delivery

Biodegradability can be incorporated into cationic polymers via use of disulfide linkages that are degraded in the reducing environment of the cell cytosol. In this work, N-(2-hydroxypropyl)methacrylamide (HPMA) and methacrylamido-functionalized oligo-l-lysine peptide monomers with either a non-reducible 6-aminohexanoic acid (AHX) linker or a reducible 3-[(2-aminoethyl)dithiol] propionic acid (AEDP) linker were copolymerized via reversible addition-fragmentation chain transfer (RAFT) polymerization. Both of the copolymers and a 1:1 (w/w) mixture of copolymers with reducible and non-reducible peptides were complexed with DNA to form polyplexes. The polyplexes were tested for salt stability, transfection efficiency, and cytotoxicity. The HPMA-oligolysine copolymer containing the reducible AEDP linkers was less efficient at transfection than the non-reducible polymer and was prone to flocculation in saline and serum-containing conditions, but was also not cytotoxic at charge ratios tested. Optimal transfection efficiency and toxicity were attained with mixed formulation of copolymers. Flow cytometry uptake studies indicated that blocking extracellular thiols did not restore transfection efficiency and that the decreased transfection of the reducible polyplex is therefore not primarily caused by extracellular polymer reduction by free thiols. The decrease in transfection efficiency of the reducible polymers could be partially mitigated by the addition of low concentrations of EDTA to prevent metal-catalyzed oxidation of reduced polymers.

Linear PEI nanoparticles: efficient pDNA/siRNA carriers in vitro and in vivo

Linear polyethylenimine (lPEI, 25 kDa) nanoparticles' (LPN) series was synthesized by varying percentage of cross-linking with 1,4-butanediol diglycidyl ether (BDE) and their size, surface charge, morphology, pDNA protection/release, cytotoxicity and transfection efficiency were evaluated. Synthesized nanoparticles (NPs) were spherical in shape (size: ∼109 - 235 nm; zeta potential: +38 to +16 mV). These NPs showed increased buffering capacity with increasing percent cross-linking and also exhibited excellent transfection efficiency (i.e., ∼1.3 - 14.7 folds in case of LPN-5) in comparison with lPEI and the commercial transfection agents used in this study. LPN-5 based GFP-specific siRNA delivery resulted in ∼86% suppression of targeted gene expression. These particles were relatively nontoxic in vitro (in cell lines) and in vivo (in Drosophila). In vivo gene expression studies using LPN-5 in Balb/c mice through intravenous injection showed maximum expression of the reporter gene in the spleen. These results together demonstrate the potential of these particles as efficient transfection reagents.

FROM THE CLINICAL EDITOR

The authors demonstrate a novel method of synthesizing linear PEI nanoparticles to utilize these as transfection agents.

Synthesis of a new potential biodegradable disulfide containing poly(ethylene imine)-poly(ethylene glycol) copolymer cross-linked with click cluster for gene delivery

Poly(ethylene glycol)-grafted-polyethylenimine (PEG-PEI) are promising non-viral gene delivery systems. Herein, we aimed to synthesize a biodegradable disulfide containing PEGylated PEI to attempt to reduce its cytotoxicity and enhance the gene transfer activity. Using click chemistry, low Mw PEI (br. 2 kDa) and short chain length PEG (tetraethylene glycol, TEG) were cross-linked to a high Mw PEG-PEI copolymer (∼ 22 kDa). The chemical structure of the copolymer was characterized using (1)H NMR and GPC. The degradation behavior was investigated under in vitro conditions in the presence of 1,4-dithiothreitol (DTT). The gel retardation assay, dynamic light scattering and atomic force microscopy showed good DNA condensation ability by forming polyplexes with small particle size and positive zeta potential. In particular, MTT assay indicated that this PEG-PEI polymer is about 22-fold less toxic than PEI 25k and only 2-fold more toxic than PEI 2k in L929 cell line. After coupling of small PEG chains and cross-linking by disulfide bridges, the transfection efficiency is increased approximately 6-fold in comparison to PEI 2k and still reaches approximately 17% of PEI 25k. Hence, this click cluster cross-linked disulfide containing PEG-PEI copolymer could be an attractive cationic polymer for non-viral gene delivery.

Branched polyethylenimine derivatives with reductively cleavable periphery for safe and efficient in vitro gene transfer

Twenty-five kDa polyethylenimine (PEI) is one of the most efficient nonviral gene transfer agents currently applied as a golden standard for in vitro transfection. In this study, novel 25 kDa PEI derivatives with reductively cleavable cystamine periphery (PEI-Cys) were designed to reduce carrier-associated cytotoxicity and to enhance further the transfection activity. The Michael-type conjugate addition of 25 kDa PEI with N-tert-butoxycarbonyl-N'-acryloyl-cystamine (Ac-Cys-(t)Boc) and N-tert-butoxycarbonyl-N'-methacryloyl-cystamine (MAc-Cys-(t)Boc) followed by deprotection readily afforded PEI-Cys derivatives, denoted as PEI-(Cys)x(Ac) and PEI-(Cys)x(MAc), with degree of substitution (DS) ranging from 14 to 34 and 13 to 38, respectively. All PEI-Cys derivatives had higher buffer capacity than the parent 25 kDa PEI (21.2 to 23.1% versus 15.1%). Gel retardation and ethidium bromide exclusion assays showed that cystamine modification resulted in largely enhanced interactions with DNA. PEI-(Cys)x(Ac) could condense DNA into small-sized particles of 80-90 nm at and above an N/P ratio of 5/1, which were smaller than polyplexes of 25 kDa PEI (100-130 nm). In comparison, PEI-(Cys)x(MAc) condensed DNA into somewhat larger particles (100-180 nm at N/P ratios from 30/1 to 5/1). Gel retardation and dynamic light scattering (DLS) measurements showed that PEI-Cys polyplexes were quickly unpacked to release DNA in response to 10 mM dithiothreitol (DTT). These PEI-Cys derivatives revealed markedly decreased cytotoxicity as compared with 25 kDa PEI with IC(50) values of >100 mg/L and 50-75 mg/L for HeLa and 293T cells, respectively (corresponding IC(50) data of 25 kDa PEI are ca. 11 and 3 mg/L). The in vitro transfection experiments in HeLa and 293T cells using pGL3 as a reporter gene showed that gene transfection activity of PEI-Cys derivatives decreased with increasing DS and PEI-(Cys)x(MAc) exhibited higher transfection activity than PEI-(Cys)x(Ac) at similar DS. Notably, polyplexes of PEI-(Cys)14(Ac) and PEI-(Cys)13(MAc) showed significantly enhanced gene transfection efficiency (up to 4.1-fold) as compared with 25 kDa PEI formulation at an N/P ratio of 10/1 in both serum-free and 10% serum-containing conditions. The modification of PEI with reductively cleavable periphery appears to be a potential approach to develop safer and more efficient nonviral gene vectors.