Poly(4-hydroxy-l-proline ester): Low-Temperature Polycondensation and Plasmid DNA Complexation

Cationic Polymers as Transfection Reagents for Nucleic Acid Delivery

Nucleic acid therapy can achieve lasting and even curative effects through gene augmentation, gene suppression, and genome editing. However, it is difficult for naked nucleic acid molecules to enter cells. As a result, the key to nucleic acid therapy is the introduction of nucleic acid molecules into cells. Cationic polymers are non-viral nucleic acid delivery systems with positively charged groups on their molecules that concentrate nucleic acid molecules to form nanoparticles, which help nucleic acids cross barriers to express proteins in cells or inhibit target gene expression. Cationic polymers are easy to synthesize, modify, and structurally control, making them a promising class of nucleic acid delivery systems. In this manuscript, we describe several representative cationic polymers, especially biodegradable cationic polymers, and provide an outlook on cationic polymers as nucleic acid delivery vehicles.

Charge-Shifting Polycations Based on N,N-(dimethylamino)ethyl Acrylate for Improving Cytocompatibility During DNA Delivery

Synthetic polycations are studied extensively as DNA delivery agents because of their ease of production, good chemical stability, and low cost relative to viral vectors. This report describes the synthesis of charge-shifting polycations based on N,N-(dimethylamino)ethyl acrylate (DMAEA) and 3-aminopropylmethacryamide (APM), called PAD copolymers, and their use for in vitro DNA delivery into HeLa cells. PAD copolymers of varying compositions were prepared by RAFT polymerization to yield polymers of controlled molecular weights with low dispersities. Model hydrolysis studies were carried out to assess the rate of charge-shifting of the polycations by loss of the cationic dimethylaminoethanol side chains. They showed reduction in the net cationic charge by about 10-50% depending on composition after 2 days at pH 7, forming polyampholytes comprising permanent cationic groups, residual DMAEA, as well as anionic acrylic acid groups. HeLa cells exposed for 4 h to PAD copolymers with the greatest charge-shifting ability showed comparable or higher viability at high concentrations, relative to the noncharge shifting polycations PAPM and polyethyleneimine (PEI) 2 days post-exposure. Cell uptake efficiency of PAD/60bp-Cy3 DNA polyplexes at 2.5:1 N/P ratio was very high (>95%) for all compositions, exceeding the uptake efficiency of PEI polyplexes of equivalent composition. These results suggest that these PAD copolymers, and in particular PAD80 containing 80 mol % DMAEA, have suitable rates of charge-shifting hydrolysis for DNA delivery, as PAD80 showed reduced cytotoxicity at high concentrations, while still retaining high uptake efficiencies. In addition, the polyampholytes formed during DMAEA hydrolysis in PAD copolymers can offer enhanced long-term cytocompatibility.

Biodegradable Polymers for Gene-Delivery Applications

Gene-based therapies have emerged as a new modality for combating a myriad of currently incurable diseases. However, the fragile nature of gene therapeutics has significantly hampered their biomedical applications. Correspondingly, the development of gene-delivery vectors is of critical importance for gene-based therapies. To date, a variety of gene-delivery vectors have been created and utilized for gene delivery. In general, they can be categorized into viral- and non-viral vectors. Due to safety issues associated with viral vectors, non-viral vectors have recently attracted much more research focus. Of these non-viral vectors, polymeric vectors, which have been preferred due to their low immunogenicity, ease of production, controlled chemical composition and high chemical versatility, have constituted an ideal alternative to viral vectors. In particular, biodegradable polymers, which possess advantageous biocompatibility and biosafety, have been considered to have great potential in clinical applications. In this context, the aim of this review is to introduce the recent development and progress of biodegradable polymers for gene delivery applications, especially for their chemical structure design, gene delivery capacity and additional biological functions. Accordingly, we first define and categorize biodegradable polymers, followed by describing their corresponding degradation mechanisms. Various types of biodegradable polymers resulting from natural and synthetic polymers will be introduced and their applications in gene delivery will be examined. Finally, a future perspective regarding the development of biodegradable polymer vectors will be given.

Oligo(serine ester) Charge-Altering Releasable Transporters: Organocatalytic Ring-Opening Polymerization and their Use for in Vitro and in Vivo mRNA Delivery

RNA technology is transforming life science research and medicine, but many applications are limited by the accessibility, cost, efficacy, and tolerability of delivery systems. Here we report the first members of a new class of dynamic RNA delivery vectors, oligo(serine ester)-based charge-altering releasable transporters (Ser-CARTs). Composed of lipid-containing oligocarbonates and cationic oligo(serine esters), Ser-CARTs are readily prepared (one flask) by a mild ring-opening polymerization using thiourea anions and, upon simple mixing with mRNA, readily form complexes that degrade to neutral serine-based products, efficiently releasing their mRNA cargo. mRNA/Ser-CART transfection efficiencies of >95% are achieved in vitro. Intramuscular or intravenous (iv) injections of mRNA/Ser-CARTs into living mice result in in vivo expression of a luciferase reporter protein, with spleen localization observed after iv injection.

Biodegradable polymer promotes osteogenic differentiation in immortalized and primary osteoblast-like cells

Biodegradable polymers have been broadly used as agents that can complex with and deliver osteoinductive agents, but osteoinductivity of the polymers themselves has been rarely studied. Here we report the osteoinductivity of poly(4-hydroxy-L-proline ester) (PHPE), a biodegradable cationic polymer with cell penetrating properties. Under physiological conditions, PHPE degrades into trans-4-hydroxy-L-proline (trans-Hyp), a non-coded amino acid with essential functions in collagen fibril formation and fibril stability. Treatment of SaOS-2 osteoblast-like cells and hFOB 1.19 primary osteoblast cells with PHPE promoted earlier collagen nodule formation and mineralization of the extracellular matrix compared to untreated cells, even when mineralization activators were absent in the growth medium. Our results indicate that PHPE is a potential osteoinductive agent in vitro that can favor bone regeneration. Moreover, this osteoinductive property could be partly attributed to the degradation product trans-Hyp, which could recapitulate some, but not all of the osteogenic activity. The primary findings of this study can be summarized as follows: treatment of cells with PHPE led to (1) the induction of COL1A1 expression, collagen synthesis and secretion in osteoblast-like cells, (2) mineralization of the ECM in both SaOS-2 and hFOB 1.19 primary osteoblasts, and (3) induction of BMP2 gene and protein expression in osteoblast-like cells, which can promote mineralization of the ECM and regeneration of the bone tissue. Our results suggest that PHPE is a non-cytotoxic polymer and can be potentially used to overcome collagenopathies such as osteogenesis imperfecta.

Stimuli-Responsive Cationic Hydrogels in Drug Delivery Applications

Stimuli-responsive, smart, intelligent, or environmentally sensitive polymers respond to changes in external stimuli such as pH, temperature, ionic strength, surfactants, pressure, light, biomolecules, and magnetic field. These materials are developed in various network architectures such as block copolymers, crosslinked hydrogels, nanogels, inter-penetrating networks, and dendrimers. Stimuli-responsive cationic polymers and hydrogels are an interesting class of "smart" materials that respond reversibly to changes in external pH. These materials have the ability to swell extensively in solutions of acidic pH and de-swell or shrink in solutions of alkaline pH. This reversible swelling-shrinking property brought about by changes in external pH conditions makes these materials useful in a wide range of applications such as drug delivery systems and chemical sensors. This article focuses mainly on the properties of these interesting materials and their applications in drug delivery systems.

Self-assembly of a novel Cu(ii) coordination complex forms metallo-vesicles that are able to transfect mammalian cells

Coordination-directed self-assembly of a Cu(ii) amphiphilic complex forms homogeneous nanometer-sized metallo-vesicles in water with low toxicity and gene transfection properties.

Synthesis and Characterization of Polyamide of Trans-4-hydroxy-L-proline used as Porogen Filler in Acrylic Bone Cement

The aim of this study was to synthesize on a larger scale, an experimental polyamide based on an amino acid of trans-4-hydroxy-L-proline. The polyamide of trans-4-hydroxy-L-proline has been used as porogen filler (i.e., a hydrophilic pore generating material) in nondegradable acrylic bone cement. In in vitro studies, this hydrophilic filling component has been shown to form porosity within the acrylic bone cement in an aqueous environment. The formation of in situ porosity in the acrylic polymer matrix is believed to improve the fixation between the cement and the living bone. Namely, a porous structure can support bone ingrowth and strengthen the mechanical connection between the acrylic bone cement and the bone. The monomer, trans-4-hydroxy-L-proline methyl ester, was prepared from trans-4-hydroxy-L-proline by means of two steps, and the monomer was then polymerized to polyamide of trans-4-hydroxy-L-proline. The polymerization was carried out using a melt polycondensation method. The molecular weights (Mψ) of the produced polyamides were between 1800 and 3600. The products were characterized by FTIR and 1H-NMR spectroscopy.

Combinatorial Approach to Nanoarchitectonics for Nonviral Delivery of Nucleic Acids

Nanoparticles based on cationic polymers, lipids or lipidoids are of great interest in the field of gene delivery applications. The research on these nanosystems is rapidly growing as they hold promise to treat wide variety of human diseases ranging from viral infections to genetic disorders and cancer. Recently, combinatorial design principles have been adopted for rapid generation of large numbers of chemically diverse polymers and lipids capable of forming multifunctional nanocarriers for the use in gene delivery applications. At the same time, current high-throughput screening systems as well as convenient cell assays and readout techniques allow for fast evaluation of cell transfection efficiencies and toxicities of libraries of novel gene delivery agents. This allows for a rapid evaluation of structure-function relationship as well as identification of novel efficient nanocarriers for cell transfection and gene therapy. Here, the recent contribution of high-throughput synthesis to the development of novel nanocarriers for gene delivery applications is described.

Bio-reducible polycations from ring-opening polymerization as potential gene delivery vehicles

Bio-reducible polycations were prepared via ring-opening polymerization. These materials have relatively low molecular weights and cytotoxicity but have good DNA condensation ability, transfection efficiency and excellent serum tolerance.

Exploring the Solid State Properties of Enzymatic Poly(amine-co-ester) Terpolymers to Expand their Applications in Gene Transfection

Polymers bearing amino functional groups are an important class of materials capable of serving as non-viral carriers for DNA delivery to living cells. In this work biodegradable poly(amine-co-ester) terpolymers were synthesized via ring-opening and polycondensation copolymerization of lactone (ε-caprolactone (CL), ω-dodecalactone, ω-pentadecalactone (PDL), and ω-hexadecalactone) with diethyl sebacate (DES) and N-methyldiethanolamine (MDEA) in diphenyl ether, catalyzed by Candida antarctica lipase B (CALB). All lactone-DES-MDEA terpolymers had random distributions of lactone, sebacate, MDEA repeat units in the polymer chains. PDL-DES-MDEA terpolymers were studied in the composition range from 21 mol% to 90 mol% PDL whereas the terpolymers with other lactones were investigated at a single composition (80 mol% lactone). DSC and WAXS analyses showed that all investigated terpolymers crystallize in their respective homopolylactone crystal lattice. Terpolymers with large lactones and a high lactone content melt well above room temperature and are hard solids, whereas terpolymers with small lactones (e.g. CL) or with a low lactone content melt below/around ambient temperature and are waxy/gluey materials. Given the importance of hydrophobicity in influencing gene delivery, water contact angle measurements were carried out on lactone-DES-MDEA terpolymers showing that it is possible to tune the hydrophilic-to-hydrophobic balance by varying polymer composition and size of lactone units. To demonstrate the feasibility of using solid terpolymers as nanocarriers for DNA delivery, PDL-DES-MDEA copolymers with 65-90% PDL were successfully transformed into free-standing nanoparticles with average particle size ranging from 163 to 175 nm. Our preliminary results showed that LucDNA-loaded nanoparticles of the terpolymer with 65% PDL were effective for luciferase gene transfection of HEK293 cells.

Poly(ethylene glycol)-block-cationic polylactide nanocomplexes of differing charge density for gene delivery

Representing a new type of biodegradable cationic block copolymer, well-defined poly(ethylene glycol)-block-cationic polylactides (PEG-b-CPLAs) with tertiary amine-based cationic groups were synthesized by thiol-ene functionalization of an allyl-functionalized diblock precursor. Subsequently the application of PEG-b-CPLAs as biodegradable vectors for the delivery of plasmid DNAs (pDNAs) was investigated. Via the formation of PEG-b-CPLA:pDNA nanocomplexes by spontaneous electrostatic interaction, pDNAs encoding luciferase or enhanced green fluorescent protein were successfully delivered to four physiologically distinct cell lines (including macrophage, fibroblast, epithelial, and stem cell). Formulated nanocomplexes demonstrated high levels of transfection with low levels of cytotoxicity and hemolysis when compared to a positive control. Biophysical characterization of charge densities of nanocomplexes at various polymer:pDNA weight ratios revealed a positive correlation between surface charge and gene delivery. Nanocomplexes with high surface charge densities were utilized in an in vitro serum gene delivery inhibition assay, and effective gene delivery was observed despite high levels of serum. Overall, these results help to elucidate the influence of charge, size, and PEGylation of nanocomplexes upon the delivery of nucleic acids in physiologically relevant conditions.

Glucose-Based Poly(ester amines): Synthesis, Degradation, and Biological Delivery

Herein, the synthesis and characterization of two glucose-based degradable poly(ester amines), GluN3 and GluN4, are described. Data from gel shift assays, PicoGreen dye exclusion, and dynamic light scattering studies reveals that these polycations can form polyplexes with plasmid DNA at a relatively low N/P ratio and that the polymers degrade rapidly at physiological pH conditions. Assessment of in vitro data via flow cytometry and live cell fluorescence microscopy indicated that GluN3 and GluN4 polyplexes can be internalized by HeLa cells in a highly effective manner with low cytotoxicity profiles.

Well-defined degradable cationic polylactide as nanocarrier for the delivery of siRNA to silence angiogenesis in prostate cancer

Well-defined tertiary amine-functionalized cationic polylactides (CPLAs) are synthesized by thiol-ene click functionalization of an allyl-functionalized polylactide, and utilized for the delivery of interleukin-8 (IL-8) siRNA via CPLA-IL-8 siRNA nanoplexes. The CPLAs possess remarkable hydrolytic degradability, and their cytotoxicity is relatively low. The CPLA-IL-8 siRNA nanoplexes can be readily taken up by prostate cancer cells, resulting in significant IL-8 gene silencing. It is found that the degradability and cytotoxicity of CPLAs, as well as the transfection efficiency of the CPLA-IL-8 siRNA nanoplexes, positively correlate with the amine mol% of CPLAs.

Synthesis and characterization of cationic glycidyl-based poly(aminoester)-folic acid targeting conjugates and study on gene delivery

A new poly(aminoester) (EPAE-FA) containing folic acid and amino groups in the backbone and side chain was synthesized. EPAE-FA self-assembled readily with the plasmid DNA (pCMV-βgal) in HEPES buffer and was characterized by dynamic light scattering, zeta potential, fluorescence images, and XTT cell viability assays. To evaluate the transfection effect of graft ratio of FA on the EPAE system, EPAE-FA polymers with two different graft ratios (EPAE-FA12k and EPAE-FA14k) were also prepared. This study found that all EPAE-FA polymers were able to bind plasmid DNA and yielded positively charged complexes with nano-sized particles (<200 nm). To assess the transfection efficiency mediated by EPAE and EPAE-FA polymers, we performed in vitro transfection activity assays using FR-negative (COS-7) and FR-positive (HeLa) cells. The EPAE-FA12k/DNA and EPAE-FA14k/DNA complexes were able to transfect HeLa cell in vitro with higher transfection efficiency than PEI25k/DNA at the similar weight ratio. These results demonstrated that the introduction of FA into EPAE system had a significant effect on transferring ability for FR-positive cells (HeLa). Examination of the cytotoxicity of PEI25k and EPAE-FA system revealed that EPAE-FA system had lower cytotoxicity. In this paper, EPAE-FA seemed to be a novel cationic poly(aminoester) for gene delivery and an interesting candidate for further study.

Advanced materials for co-delivery of drugs and genes in cancer therapy

With cancer being the major cause of mortality worldwide, the continued development of safe and efficacious treatments is warranted. A better understanding of the molecular mechanism and genetic basis of tumor initiation and progression, coupled with advances in chemistry, molecular biology and engineering have led to discovery of a wide range of therapeutic agents for cancer therapy. However, multidrug-resistance, which is mainly caused by malfunction of genes, has become a major problem in chemotherapy. To overcome this problem, the simultaneous delivery of genes to cancer cells has been proposed to correct the malfunctioned genes to sensitize the cells to chemotherapeutics. This progress report summarizes key advances in drug and gene delivery with focus on the development of polymers, peptides, liposomes and inorganic materials as nanocarriers for co-delivery of small molecular drugs and macromolecular genes or proteins. In addition, challenges and future perspectives in the design of nanocarriers for the co-delivery of therapeutic drugs and genes are discussed.

Polymerized spermine as a novel polycationic nucleic acid carrier system

Spermine, an endogenous amino-group bearing monomer that condenses DNA in sperm, was used as the basic building block to form polycationic nucleic acid carriers via condensation with one of three linker molecules - bischloroformate, succinyl chloride, and glyoxal. The three cationic polymers, polyspermine carbamate (PSP-Carb), polyspermine amide (PSP-Amide) and polyspermine imine (PSP-Imine) were examined for their degradability, cytotoxicity, ability to condense nucleic acids to nanoparticles, and ability to transfect genes or siRNA to cells. PSP-Carb and PSP-Amide exhibited a half-life of more than 2 months when incubated in aqueous buffers at 37°C, while the half-life of PSP-Imine was 11h. Relative cytotoxicity of the polymers, as measured by COS-7 and HepG2 cell viability, was in the order of PSP-Carb>PSP-Amide>PSP-Imine. Each cationic polymer condensed the luciferase plasmid to nanoparticles of 150-200 nm diameters and with a zeta potential of +15-30 mV when the mass ratio of polymer-to-DNA was over 8/1. The three polycationic carriers showed similar luciferase transfection activity in COS-7 cells, while the transfection efficiency of PSP-Carb was significantly higher than that of the other two in HepG2 cells. PSP-Amide exhibited significantly higher gene silencing activity in COS-7 cells, suggesting the linkage structures play an important role in the activity of the polyspermine-based nucleic acid carriers.

Biomedical Applications of Biodegradable Polymers

Utilization of polymers as biomaterials has greatly impacted the advancement of modern medicine. Specifically, polymeric biomaterials that are biodegradable provide the significant advantage of being able to be broken down and removed after they have served their function. Applications are wide ranging with degradable polymers being used clinically as surgical sutures and implants. In order to fit functional demand, materials with desired physical, chemical, biological, biomechanical and degradation properties must be selected. Fortunately, a wide range of natural and synthetic degradable polymers has been investigated for biomedical applications with novel materials constantly being developed to meet new challenges. This review summarizes the most recent advances in the field over the past 4 years, specifically highlighting new and interesting discoveries in tissue engineering and drug delivery applications.

A Biocompatible Arginine-based Polycation

Self assembly between cations and anions is ubiquitous throughout nature. Important biological structures such as chromatin often use polyvalent assembly between a polycation and a polyaninon. Biomedical importance of synthetic polycations arises from their affinity to polyanions such as nucleic acid and heparan sulfate. However, the limited biocompatibility of synthetic polycations hampers the realization of their immense potential. By examining biocompatible cationic peptides, we hypothesize that a biocompatible polycation should be biodegradable and made from endogenous cations. We designed an arginine-based biodegradable polycation and demonstrated that it was orders of magnitude more compatible than conventional polycations in vitro and in vivo. This biocompatibility diminishes when L-arginine is substituted with D-arginine or when the biodegradable ester linker changes to a biostable ether linker. We believe this design can lead to many biocompatible polycations that can significantly advance a wide range of applications including controlled release, tissue engineering, biosensing, and medical devices.

Biocleavable Polycationic Micelles as Highly Efficient Gene Delivery Vectors

An amphiphilic disulfide-containing polyamidoamine was synthesized by Michael-type polyaddition reaction of piperazine to equimolar N, N'-bis(acryloyl)cystamine with 90% yield. The polycationic micelles (198 nm, 32.5 mV), prepared from the amphiphilic polyamidoamine by dialysis method, can condense foreign plasmid DNA to form nanosized polycationic micelles/DNA polyelectrolyte complexes with positive charges, which transfected 293T cells with high efficiency. Under optimized conditions, the transfection efficiencies of polycationic micelles/DNA complexes are comparable to, or even higher than that of commercially available branched PEI (Mw 25 kDa).

Self-Assembled Hydrogel Nanoparticles for Drug Delivery Applications

Hydrogel nanoparticles—also referred to as polymeric nanogels or macromolecular micelles—are emerging as promising drug carriers for therapeutic applications. These nanostructures hold versatility and properties suitable for the delivery of bioactive molecules, namely of biopharmaceuticals. This article reviews the latest developments in the use of self-assembled polymeric nanogels for drug delivery applications, including small molecular weight drugs, proteins, peptides, oligosaccharides, vaccines and nucleic acids. The materials and techniques used in the development of self-assembling nanogels are also described.