Amine-reactive pyridylhydrazone-based PEG reagents for pH-reversible PEI polyplex shielding

Effect of Polyplex Size on Penetration into Tumor Spheroids

Ovarian cancer is one of the most lethal gynecological cancers in the world. In recent years, nucleic acid (NA)-based formulations have been shown to be promising treatments for ovarian cancer, including tumor nodules. However, gene therapy is not that far advanced in clinical reality due to unfavorable physicochemical properties of the NAs, such as high molecular weight, poor cellular uptake, rapid degradation by nucleases, etc. One of the strategies used to overcome these drawbacks is the complexation of anionic NAs via electrostatic interactions with cationic polymers, resulting in the formation of so-called polyplexes. In this work, the role of the size of pDNA and siRNA polyplexes on their penetration into ovarian-cancer-based tumor spheroids was investigated. For this, a methoxypoly(ethylene glycol) poly(2-(dimethylamino)ethyl methacrylate) (mPEG-pDMAEMA) diblock copolymer was synthesized as a polymeric carrier for NA binding and condensation with either plasmid DNA (pDNA) or short interfering RNA (siRNA). When prepared in HEPES buffer (10 mM, pH 7.4) at a nitrogen/phosphate (N/P) charge ratio of 5 and pDNA polyplexes were formed with a size of 162 ± 11 nm, while siRNA-based polyplexes displayed a size of 25 ± 2 nm. The polyplexes had a slightly positive zeta potential of +7-8 mV in the same buffer. SiRNA and pDNA polyplexes were tracked in vitro into tumor spheroids, resembling in vivo avascular ovarian tumor nodules. For this purpose, reproducible spheroids were obtained by coculturing ovarian carcinoma cells with primary mouse embryonic fibroblasts in different ratios (5:2, 1:1, and 2:5). Penetration studies revealed that after 24 h of incubation, siRNA polyplexes were able to penetrate deeper into the homospheroids (composed of only cancer cells) and heterospheroids (cancer cells cocultured with fibroblasts) compared to pDNA polyplexes which were mainly located in the rim. The penetration of the polyplexes was slowed when increasing the fraction of fibroblasts present in the spheroids. Furthermore, in the presence of serum siRNA polyplexes encoding for luciferase showed a high cellular uptake in 2D cells resulting in ∼50% silencing of luciferase expression. Taken together, these findings show that self-assembled small siRNA polyplexes have good potential as a platform to test ovarian tumor nodulus penetration..

pH-responsive polyzwitterion covered nanocarriers for DNA delivery

The success of gene therapy relies on gene nanocarriers to achieve therapeutic effects in vivo. Surface shielding of poly(ethylene glycol) (PEG), known as PEGylation, onto gene delivery carriers is a predominant strategy for extending blood circulation and improving therapeutic outcomes in vivo. Nevertheless, PEGylation frequently compromises the transfection efficiency by decreasing the interactions with the cellular membrane of the targeted cells, thereby preventing the cellular uptake and the subsequent endosomal escape. Herein, we developed a stepwise pH-responsive polyplex micelle for the plasmid DNA delivery with the surface covered by ethylenediamine-based polycarboxybetaines. This polyplex micelle switched its surface charge from neutral at pH 7.4 to positive at tumorous and endo-/lysosomal pH (i.e., pH 6.5 and 5.5, respectively), thus enhancing the cellular uptake and facilitating the endosomal escape toward efficient gene transfection. Additionally, the polyplex micelle demonstrated prolonged blood circulation as well as enhanced tumor accumulation, leading to highly effective tumor growth suppression by delivering an antiangiogenic gene. These results suggest the usefulness of a pH-responsive charge-switchable shell polymer on the surface of the polyplex micelle for the efficient nucleic acid delivery.

Genetic and Chemical Capsid Modifications of Adenovirus Vectors to Modulate Vector-Host Interactions

Adenovirus-based vectors are playing an important role as efficacious genetic vaccines to fight the current COVID-19 pandemic. Furthermore, they have an enormous potential as oncolytic vectors for virotherapy and as vectors for classic gene therapy. However, numerous vector-host interactions on a cellular and noncellular level, including specific components of the immune system, must be modulated in order to generate safe and efficacious vectors for virotherapy or classic gene therapy. Importantly, the current widespread use of Ad vectors as vaccines against COVID-19 will induce antivector immunity in many humans. This requires the development of strategies and techniques to enable Ad-based vectors to evade pre-existing immunity. In this review article, we discuss the current status of genetic and chemical capsid modifications as means to modulate the vector-host interactions of Ad-based vectors.

Polymeric delivery systems for nucleic acid therapeutics: Approaching the clinic

Gene therapy using nucleic acids has many clinical applications for the treatment of diseases with a genetic origin as well as for the development of innovative vaccine formulations. Since nucleic acids in their free form are rapidly degraded by nucleases present in extracellular matrices, have poor pharmacokinetics and hardly pass cellular membranes, carrier systems are required. Suitable carriers that protect the nucleic acid payload against enzymatic attack, prolong circulation time after systemic administration and assist in cellular binding and internalization are needed to develop nucleic acid based drug products. Viral vectors have been investigated and are also clinically used as delivery vehicles. However, some major drawbacks are associated with their use. Therefore there has been substantial attention on the use of non-viral carrier systems based on cationic lipids and polymers. This review focuses on the properties of polymer-based nucleic acid formulations, also referred as polyplexes. Different polymeric systems are summarized, and the cellular barriers polyplexes encounter and ways to tackle these are discussed. Finally attention is given to the clinical status of non-viral nucleic acid formulations.

Optimizing synthetic nucleic acid and protein nanocarriers: The chemical evolution approach

Optimizing synthetic nanocarriers is like searching for a needle in a haystack. How to find the most suitable carrier for intracellular delivery of a specified macromolecular nanoagent for a given disease target location? Here, we review different synthetic 'chemical evolution' strategies that have been pursued. Libraries of nanocarriers have been generated either by unbiased combinatorial chemistry or by variation and novel combination of known functional delivery elements. As in natural evolution, definition of nanocarriers as sequences, as barcode or design principle, may fuel chemical evolution. Screening in appropriate test system may not only provide delivery candidates, but also a refined understanding of cellular delivery including novel, unpredictable mechanisms. Combined with rational design and computational algorithms, candidates can be further optimized in subsequent evolution cycles into nanocarriers with improved safety and efficacy. Optimization of nanocarriers differs for various cargos, as illustrated for plasmid DNA, siRNA, mRNA, proteins, or genome-editing nucleases.

Synthesis of Polyethylenimine-Based Nanocarriers for Systemic Tumor Targeting of Nucleic Acids

Nucleic acid-based therapies offer the option to treat tumors in a highly selective way, while toxicity towards healthy tissue can be avoided when proper delivery vehicles are used. We have recently developed carrier systems based on linear polyethylenimine, which after chemical coupling of protein- or peptide-based ligands can form nanosized polyplexes with plasmid DNA (pDNA) or RNA and deliver their payload into target cells by receptor-mediated endocytosis. This chapter describes the synthesis of LPEI from a precursor polymer and the current coupling techniques and purification procedure for peptide conjugates with linear polyethylenimine. A protocol is also given for the formation and characterization of polyplexes formed with LPEI conjugate and pDNA.

Targeted codelivery of doxorubicin and IL-36γ expression plasmid for an optimal chemo-gene combination therapy against cancer lung metastasis

Cancer metastasis is the main cause for the high mortality in breast cancer patients. In this work we developed a polymer POEG-st-Pmor for targeted co-delivery of IL-36γ expression plasmid and doxorubicin (Dox) to lung metastasis of breast cancer. The polymer readily formed micelles that were effective in loading Dox and simultaneously forming complexes with IL-36γ plasmid. Interestingly, particles co-loaded with Dox and plasmid was significantly smaller and more stable than the particles loaded with Dox only. Gene transfection in both lungs and s.c. tumors was significantly higher with our polymer compared to PEI. In addition, the Dox + IL-36γ/POEG-st-Pmor not only could bring improved anti-metastatic effect but synergistically enhance the type I immune response by increasing the IFN-γ positive CD4⁺ and CD8⁺ T cells and simultaneously decreasing the immunosuppressive myeloid-derived suppressor cells in the lung. POEG-st-Pmor may represent a simple and effective delivery system for an optimal chemo-gene combination therapy.

pH-responsive Drug Delivery Systems

Acidic microenvironments exist in selected organs, tissues, and subcellular compartments, as well as in many dysregulated pathological states. A continuous effort has been made to harness the unique acidic properties of biological tissues for site-specific delivery of drugs. Various pH-responsive drug delivery systems have been designed and developed with improved spatio-temporal control of payload delivery with enhanced efficacy. This chapter will focus on the recent advances in the development of pH-sensitive materials, mechanisms of payload release, and pH-triggered drug targeting in various biomedical applications.

Efficient Shielding of Polyplexes Using Heterotelechelic Polysarcosines

Shielding agents are commonly used to shield polyelectrolyte complexes, e.g., polyplexes, from agglomeration and precipitation in complex media like blood, and thus enhance their in vivo circulation times. Since up to now primarily poly(ethylene glycol) (PEG) has been investigated to shield non-viral carriers for systemic delivery, we report on the use of polysarcosine (pSar) as a potential alternative for steric stabilization. A redox-sensitive, cationizable lipo-oligomer structure (containing two cholanic acids attached via a bioreducible disulfide linker to an oligoaminoamide backbone in T-shape configuration) was equipped with azide-functionality by solid phase supported synthesis. After mixing with small interfering RNA (siRNA), lipopolyplexes formed spontaneously and were further surface-functionalized with polysarcosines. Polysarcosine was synthesized by living controlled ring-opening polymerization using an azide-reactive dibenzo-aza-cyclooctyne-amine as an initiator. The shielding ability of the resulting formulations was investigated with biophysical assays and by near-infrared fluorescence bioimaging in mice. The modification of ~100 nm lipopolyplexes was only slightly increased upon functionalization. Cellular uptake into cells was strongly reduced by the pSar shielding. Moreover, polysarcosine-shielded polyplexes showed enhanced blood circulation times in bioimaging studies compared to unshielded polyplexes and similar to PEG-shielded polyplexes. Therefore, polysarcosine is a promising alternative for the shielding of non-viral, lipo-cationic polyplexes.

Polymers and hydrogels for local nucleic acid delivery

This review focusses on the rational design of materials (from polymers to hydrogel materials) to achieve successful local delivery of therapeutic nucleic acids.

Solid-phase supported design of carriers for therapeutic nucleic acid delivery

Nucleic acid molecules are important therapeutic agents in the field of antisense oligonucleotide, RNA interference, and gene therapies. Since nucleic acids are not able to cross cell membranes and enter efficiently into cells on their own, the development of efficient, safe, and precise delivery systems is the crucial challenge for development of nucleic acid therapeutics. For the delivery of nucleic acids to their intracellular site of action, either the cytosol or the nucleus, several extracellular and intracellular barriers have to be overcome. Multifunctional carriers may handle the different special requirements of each barrier. The complexity of such macromolecules however poses a new hurdle in medical translation, which is the chemical production in reproducible and well-defined form. Solid-phase assisted synthesis (SPS) presents a solution for this challenge. The current review provides an overview on the design and SPS of precise sequence-defined synthetic carriers for nucleic acid cargos.

Self-Assembling Lipid-Peptide Hybrid Nanoparticles of Phospholipid-Nonaarginine Conjugates for Enhanced Delivery of Nucleic Acid Therapeutics

Despite potential applications of nucleic acid therapeutics, the lack of effective delivery systems hinders their clinical application. To overcome the barriers to nucleic acid delivery, we previously reported nanoparticles using phospholipid-polyethylenimine conjugates. However, toxicity of polyethylenimine remains as a problematic issue. Herein, we proposed to substitute the polyethylenimine with arginine-rich peptide to obtain a less-toxic carrier system. Nonaarginine was conjugated to the distal end of phospholipid hydrocarbon chains leading to phospholipid-nonaarginine conjugates (PL9R) and then lipid-peptide hybrid nanoparticles carrying oligonucleotide therapeutics (hNP) were constructed by self-assembly process. The hNP were further modified with cell penetrating Tat peptide (T-hNP) to enhance cellular uptake. The PL9R was less cytotoxic, and the hNP showed high loading capacity and colloidal stability. The T-hNP showed higher cellular uptake and transfection efficiency and effective accumulation to tumor tissue and silencing effect in tumor bearing mice. Altogether, T-hNP could provide a promising nanocarrier for nucleic acid therapeutics.

EGFR Targeting and Shielding of pDNA Lipopolyplexes via Bivalent Attachment of a Sequence-Defined PEG Agent

For successful nonviral gene delivery, cationic polymers are promising DNA carrier, which need to comprise several functionalities. The current work focuses on the postincorporation of epidermal growth factor receptor (EGFR) targeted PEGylation agents onto lipopolyplexes for pDNA delivery. T-shaped lipo-oligomers are previously found to be effective sequence-defined carriers for pDNA and siRNA. Here, the bis-oleoyl-oligoaminoethanamide 454 containing tyrosine trimer-cysteine ends is applied for complex formation with pDNA coding for luciferase or sodium iodide symporter (NIS). In a second step, the lipopolyplexes are modified via disulfide formation with sequence-defined monovalent or bivalent PEGylation agents containing one or two 3-nitro-2-pyridinesulfenyl (NPys)-activated cysteines, respectively. For targeting, the polyethylene glycol (PEG) agents comprise the EGFR targeting peptide GE11. In comparison of all transfection complexes, 454 lipopolyplexes modified with the bidentate PEG-GE11 agent show the best, EGFR-dependent uptake as well as luciferase and NIS gene expression into receptor-positive tumor cells.

A54 peptide-mediated functionalized gold nanocages for targeted delivery of DOX as a combinational photothermal-chemotherapy for liver cancer

The combination of photothermal therapy and chemotherapy (photothermal–chemotherapy) is a promising strategy for cancer therapy. Gold nanocages (AuNCs), with hollow and porous structures and unique optical properties, have become a rising star in the field of drug delivery. Here, we designed a novel targeted drug delivery system based on functionalized AuNCs and evaluated their therapeutic effects in vitro and in vivo. We then loaded doxorubicin into this promising system, designated as DHTPAuNCs consisting of hyaluronic acid-grafted and A54 peptide-targeted PEGylated AuNCs. Its formation was corroborated by ultraviolet–visible spectroscopy, transmission electron microscopy and dynamic light scattering. This delivery platform needed hyaluronidase to release encapsulated drugs, meanwhile the acidic pH and near-infrared irradiation could accelerate the release. In addition, the results of cellular uptake demonstrate that this system could bind specifically with BEL-7402 cells. In vitro, we evaluated therapeutic effects of the DHTPAuNCs in BEL-7402 cells by 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyl-tetrazolium bromide assay. Moreover, in BEL-7402 tumor-bearing nude mice, its therapy effect in vivo was also evaluated. As expected, DHTPAuNCs exhibited excellent therapeutic effect by photothermal–chemotherapy, both in vitro and in vivo. In short, DHTPAuNCs with low toxicity showed great potential as a drug delivery system for cancer therapy.

Polymer nanoassemblies with hydrophobic pendant groups in the core induce false positive siRNA transfection in luciferase reporter assays

Poly(ethylene glycol)-conjugated polyethylenimine (PEG-PEI) is a widely studied cationic polymer used to develop non-viral vectors for siRNA therapy of genetic disorders including cancer. Cell lines stably expressing luciferase reporter protein typically evaluate the transfection efficacy of siRNA/PEG-PEI complexes, however recent findings revealed that PEG-PEI can reduce luciferase expression independent of siRNA. This study elucidates a cause of the false positive effect in luciferase assays by using polymer nanoassemblies (PNAs) made from PEG, PEI, poly-(l-lysine) (PLL), palmitate (PAL), and deoxycholate (DOC): PEG-PEI (2P), PEG-PEI-PAL (3P), PEG-PLL (2P'), PEG-PLL-PAL (3P'), and PEG-PEI-DOC (2PD). In vitro transfection and western blot assays of luciferase using a colorectal cancer cell line expressing luciferase (HT29/LUC) concluded that 2P and 2P' caused no luciferase expression reduction while hydrophobically modified PNAs induced a 35-50% reduction (3P'<2PD<3P). Although cell viability remained stagnant, 3P triggered cellular stress responses including increased membrane porosity and decreased ATP and cellular protein concentrations. Raman spectroscopy suggested that hydrophobic groups influence PNA conformation changes, which may have caused over-ubiquitination and degradation of luciferase in the cells. These results indicate that hydrophobically modified PEG-PEI induces cellular distress causing over-ubiquitination of the luciferase protein, producing false positive siRNA transfection in the luciferase assay.

Influence of Defined Hydrophilic Blocks within Oligoaminoamide Copolymers: Compaction versus Shielding of pDNA Nanoparticles

Cationic polymers are promising components of the versatile platform of non-viral nucleic acid (NA) delivery agents. For a successful gene delivery system, these NA vehicles need to comprise several functionalities. This work focuses on the modification of oligoaminoamide carriers with hydrophilic oligomer blocks mediating nanoparticle shielding potential, which is necessary to prevent aggregation or dissociation of NA polyplexes in vitro, and hinder opsonization with blood components in vivo. Herein, the shielding agent polyethylene glycol (PEG) in three defined lengths (12, 24, or 48 oxyethylene repeats) is compared with two peptidic shielding blocks composed of four or eight repeats of sequential proline-alanine-serine (PAS). With both types of shielding agents, we found opposing effects of the length of hydrophilic segments on shielding and compaction of formed plasmid DNA (pDNA) nanoparticles. Two-arm oligoaminoamides with 37 cationizable nitrogens linked to 12 oxyethylene units or four PAS repeats resulted in very compact 40⁻50 nm pDNA nanoparticles, whereas longer shielding molecules destabilize the investigated polyplexes. Thus, the balance between sufficiently shielded but still compact and stable particles can be considered a critical optimization parameter for non-viral nucleic acid vehicles based on hydrophilic-cationic block oligomers.

History of Polymeric Gene Delivery Systems

As an option for genetic disease treatment and an alternative for traditional cancer chemotherapy, gene therapy achieves significant attention. Nucleic acid delivery, however, remains a main challenge in human gene therapy. Polymer-based delivery systems offer a safer and promising route for therapeutic gene delivery. Over the past five decades, various cationic polymers have been optimized for increasingly effective nucleic acid transfer. This resulted in a chemical evolution of cationic polymers from the first-generation polycations towards bioinspired multifunctional sequence-defined polymers and nanocomposites. With the increasing of knowledge in molecular biological processes and rapid progress of macromolecular chemistry, further improvement of polymeric nucleic acid delivery systems will provide effective tool for gene-based therapy in the near future.

Delivery of nucleic acids for cancer gene therapy: overcoming extra- and intra-cellular barriers

The therapeutic potential of cancer gene therapy has been limited by the difficulty of delivering genetic material to target sites. Various biological and molecular barriers exist which need to be overcome before effective nonviral delivery systems can be applied successfully in oncology. Herein, various barriers are described and strategies to circumvent such obstacles are discussed, considering both the extracellular and intracellular setting. Development of multifunctional delivery systems holds much promise for the progression of gene delivery, and a growing body of evidence supports this approach involving rational design of vectors, with a unique molecular architecture. In addition, the potential application of composite gene delivery platforms is highlighted which may provide an alternative delivery strategy to traditional systemic administration.

How to Tackle the Challenge of siRNA Delivery with Sequence-Defined Oligoamino Amides

RNA interference (RNAi) as a mechanism of gene regulation provides exciting opportunities for medical applications. Synthetic small interfering RNA (siRNA) triggers the knockdown of complementary mRNA sequences in a catalytic fashion and has to be delivered into the cytosol of the targeted cells. The design of adequate carrier systems to overcome multiple extracellular and intracellular roadblocks within the delivery process has utmost importance. Cationic polymers form polyplexes through electrostatic interaction with negatively charged nucleic acids and present a promising class of carriers. Issues of polycations regarding toxicity, heterogeneity, and polydispersity can be overcome by solid-phase-assisted synthesis of sequence-defined cationic oligomers. These medium-sized highly versatile nucleic acid carriers display low cytotoxicity and can be modified and tailored in multiple ways to meet specific requirements of nucleic acid binding, polyplex size, shielding, targeting, and intracellular release of the cargo. In this way, sequence-defined cationic oligomers can mimic the dynamic and bioresponsive behavior of viruses.

PEG stabilized DNA – poly(ferrocenylsilane) polyplexes for gene delivery

Polycationic poly(ferrocenylsilane)s (PFS) with tunable amounts of PEG side chains were used for the condensation of DNA into polyplexes of 110 nm in 5.0 mM HEPES.

Targeted delivery of microRNA-126 to vascular endothelial cells via REDV peptide modified PEG-trimethyl chitosan

Cationic complexes self-assembled from REDV peptide-modified PEG-trimethyl chitosan (TMC-g-PEG-REDV) polyplexes show efficient targeted delivery of miRNA-126 to vascular endothelial cells (VECs).

Heterocyclic amine-modified polyethylenimine as gene carriers for transfection of mammalian cells

Branched polyethylenimine (PEI) is extensively used as a polycationic non-viral vector for gene delivery. Polyplexes formed with PEI are believed to be released from endocytotic vesicles by the osmotic burst mechanism in the rate-limiting step in transfection. Increasing the buffering capacity of PEI derivatives in the endosomal pH range of 4.5-7.5 should enhance transfection efficiency. In this study, PEI was derivatized by covalently attaching heterocyclic amine moieties (piperazine, pyridine and imidazole rings with pKa values from 5.23 to 6.04) through amide bonds. PEI derivatives with 50% of the primary amines on PEI exhibited increased buffering capacity, increased transfection activity and decreased cytotoxicity in murine neuroblastoma (Neuro-2a) cells. The relative effectiveness in enhancing transfection efficiency was piperazine>pyridine>histidine, but each type of amine was the most effective under a particular set of conditions. Modified vectors could significantly improve transfection efficiency in murine mesenchymal stem cells. PEI25 derivatized at 50% with histidine administered as polyplexes in the tail veins of mice resulted in remarkably enhanced luciferase gene expression in the expected organ distribution and much lower toxicity than underivatized PEI25.

Long circulating micelles of an amphiphilic random copolymer bearing cell outer membrane phosphorylcholine zwitterions

Polymeric micelles with cell outer membrane mimetic structure were prepared in water from amphiphilic random copolymers bearing both the hydrophilic phosphorylcholine zwitterions and hydrophobic octadecyl side chains of cell outer membrane. The polymeric micelles showed sizes ranging from 80 nm to 120 nm in hydrodynamic diameter and zeta-potentials from -6.4 mV to -2.4 mV by dynamic light scattering measurements. The micelles loaded with 6-coumarin as a fluorescence probe were stable to investigate their blood circulation and biodistribution. The in vitro phagocytosis results using murine peritoneal macrophages showed 10-fold reduction compared with a reference micelle. The in vivo blood circulation half-life of the polymeric micelles following intravenous administration in New Zealand Rabbits was increased from 0.55 h to 90.5h. More interestingly, tissue distribution results showed that the concentration of the micelles in the kidney is 4-fold higher than that in the liver and other organs 48 h after administration. The results of this work show great promise for designing more effective stealth drug carriers that can minimize reticuloendothelial system clearance and circulate for long time to reach target by using simple cell membrane mimetic random copolymer micelles.

Surface modification of nonviral nanocarriers for enhanced gene delivery

Biomedical nanotechnology has given a new lease of life to gene therapy with the ever-developing and ever-diversifying nonviral gene delivery nanocarriers. These are designed to pass a series of barriers in order to bring their nucleic acid cargo to the right subcellular location of particular cells. For a given application, each barrier has its dedicated strategy, which translates into a physicochemical, biological and temporal identity of the nanocarrier surface. Different strategies have thus been explored to implement adequate surface identities on nanocarriers over time for systemic delivery. In that context, this review will mainly focus on organic nanocarriers, for which these strategies will be described and discussed.

Polymeric Plerixafor: effect of PEGylation on CXCR4 antagonism, cancer cell invasion, and DNA transfection

PURPOSE

To determine the effect of PEG modification on pharmacologic and gene delivery properties of polymeric CXCR4 antagonist based on Plerixafor.

METHODS

Polymeric Plerixafor (PAMD) was synthesized from Plerixafor (AMD3100) and grafted with different amounts of PEG (2 kDa). CXCR4 antagonism of the synthesized polymers was determined using receptor redistribution assay. Inhibition of cancer cell invasion by the polyplexes of the synthesized polymers was assessed using Boyden-chamber method. Transfection activity of DNA polyplexes formed with the synthesized polymers was evaluated in U2OS osteosarcoma and B16F10 melanoma cells.

RESULTS

Our results demonstrate that modification of PAMD with PEG decreased toxicity of the polymers, while preserving their CXCR4 antagonism. Polyplexes prepared with PEG-PAMD inhibited invasion of cancer cells to an extent similar to the commercial CXCR4 antagonist Plerixafor. Negative effect of PEG on transfection activity of PEG-PAMD polyplexes could be overcome by using polyplexes formulated with a mixture of PAMD and PEG-PAMD.

CONCLUSION

Modification of PAMD with PEG is a viable strategy to preserve the desirable CXCR4 antagonism and ability to inhibit cancer cell invasion of PAMD, while improving safety and colloidal stability of the PAMD polyplexes.

Self-assembling micelle-like nanoparticles with detachable envelopes for enhanced delivery of nucleic acid therapeutics

In spite of the great potential of nucleic acids as therapeutic agents, the clinical application of nucleic acid therapeutics requires the development of effective systemic delivery strategies. In an effort to develop effective nucleic acid delivery systems suitable for clinical application, we previously reported a self-assembling micelle-like nanoparticle that was based on phospholipid-polyethylenimine conjugates, i.e., "micelle-like nanoparticles" (MNPs). In this study, we aimed to improve the system by enhancing the efficiency of intracellular delivery of the payload via pH-responsive detachment of the monolayer envelope and release of the nucleic acid therapeutics upon reaching the target tissues with an acidic pH, e.g., tumors. The acid-cleavable phospholipid-polyethylenimine conjugate was synthesized via hydrazone bond, and acid-cleavable MNPs were then prepared and characterized as before. We evaluated the acid-cleavable MNP construct for in vitro and in vivo nucleic acid delivery efficiency using cultured tumor cells and tumor-bearing mice. The acid-cleavable nanocarrier showed an enhanced cellular delivery at pH 6.5 as compared to pH 7.4, whereas the noncleavable nanocarrier did not show any differences. Tail vein injections also led to enhanced intracellular uptake of the acid-cleavable nanocarrier compared to the noncleavable nanocarrier into tumor cells of tumor-bearing mice although no significant difference was observed in total tumor accumulation.

Polymers for nucleic acid transfer-an overview

For the last five decades cationic polymers have been used for nucleic acids transfection. Our understanding of polymer-nucleic acid interactions and their rational use in delivery has continuously increased. The great improvements in macromolecular chemistry and the recognition of distinct biological extra- and intracellular delivery hurdles triggered several breakthrough developments, including the discovery of natural and synthetic polycations for compaction of nucleic acids into stable nanoparticles termed polyplexes; the incorporation of targeting ligands and surface-shielding of polyplexes to enable receptor-mediated gene delivery into defined target tissues; and strongly improved intracellular transfer efficacy by better endosomal escape of vesicle-trapped polyplexes into the cytosol. These experiences triggered the development of second-generation polymers with more dynamic properties, such as endosomal pH-responsive release mechanisms, or biodegradable units for improved biocompatibility and intracellular release of the nucleic acid pay load. Despite a better biological understanding, significant challenges such as efficient nuclear delivery and persistence of gene expression persist. The therapeutic perspectives widened from pDNA-based gene therapy to application of novel therapeutic nucleic acids including mRNA, siRNA, and microRNA. The finding that different therapeutic pay loads require different tailor-made carriers complicates preclinical developments. Convincing evidence of medical efficacy still remains to be demonstrated. Bioinspired multifunctional polyplexes resembling "synthetic viruses" appear as attractive opportunity, but provide additional challenges: how to identify optimum combinations of functional delivery units, and how to prepare such polyplexes reproducibly in precise form? Design of sequence-defined polymers, screening of combinatorial polymer and polyplex libraries are tools for further chemical evolution of polyplexes.

pH-sensitive drug-delivery systems for tumor targeting

Drug-delivery system responses to stimuli have been well investigated recently. As pH decrease is observed in most solid tumors, drug-delivery systems responsive to the slightly acidic extracellular pH environment of solid tumors have been developed as a general strategy for tumor targeting. Drug vehicles that are sensitive to acidic endosome/lysosome pH have been constructed for efficient drug release in tumor cells. This review explains the mechanisms of acidic pH in the tumor microenvironment and endocytic-related organelles, endosomes and lysosomes. Nanoparticle responses to acidic extracellular pH are discussed, along with approaches for improving tumor-specific therapy. Endosome/lysosome pH-triggered vehicles are reviewed, which achieve rapid drug release in tumor cells and overcome multidrug resistance.

PEG length and chemical linkage controls polyacridine peptide DNA polyplex pharmacokinetics, biodistribution, metabolic stability and in vivo gene expression

The pharmacokinetics (PK), biodistribution and metabolism of non-viral gene delivery systems administered systemically are directly related to in vivo efficacy. The magnitude of luciferase expression in the liver of mice following a tail vein dose of a polyplex, composed of 1 μg of pGL3 in complex with a polyethylene glycol (PEG) polyacridine peptide, followed by a delayed hydrodynamic (HD) stimulation (1-9 h), depends on the HD stimulation delay time and the structure of the polyacridine peptide. As demonstrated in the present study, the PEG length and the type of chemical linkage joining PEG to the polyacridine peptide dramatically influence the in vivo gene transfer efficiency. To understand how PEG length, linkage and location influence gene transfer efficiency, detailed PK, biodistribution and HD-stimulated gene expression experiments were performed on polyplexes prepared with an optimized polyacridine peptide modified through a single terminal Cys or Pen (penicillamine) with a PEG chain of average length of 2, 5, 10, 20, or 30 kDa. The chemical linkage was examined by attaching PEG(5 kDa) to the polyacridine peptide through a thiol-thiol (SS), thiol-maleimide (SM), thiol-vinylsulfone (SV), thiol-acetamide (SA), penicillamine-thiol-maleimide (PM) or penicillamine-thiol-thiol (PS). The influence of PEG location was analyzed by attaching PEG(5 kDa) to the polyacridine peptide through a C-terminal, N-terminal, or a middle Cys residue. The results established rapid metabolism of polyplexes containing SV and SA chemical linkages that leads to a decreased polyplex PK half-life and a complete loss of HD-stimulated gene expression at delay times of 5 h. Conversely, polyplexes containing PM, PS, and SM chemical linkages were metabolically stable, allowing robust HD-stimulated expression at delay times up to 5h post-polyplex administration. The location of PEG(₅ kDa) within the polyacridine peptide exerted only a minor influence on the gene transfer of polyplexes. However, varying the PEG length from 2, 5, 10, 20, or 30 kDa dramatically altered polyplex biodistribution, with a 30 kDa PEG maximally blocking liver uptake to 13% of dose, while maintaining the ability to mediate HD-stimulated gene expression. The combination of results establishes important relationships between PEGylated polyacridine peptide structure, physical properties, in vivo metabolism, PK and biodistribution resulting in an optimal PEG length and linkage that leads to a robust HD-stimulated gene expression in mice.

Synthesis of polyethylenimine-based nanocarriers for systemic tumor targeting of nucleic acids

Nucleic acid-based therapies offer the option to treat tumors in a highly selective way, while toxicity towards healthy tissue can be avoided when proper delivery vehicles are used. We have recently developed carrier systems based on linear polyethylenimine, which after chemical coupling of proteinous or peptidic ligands can form nanosized polyplexes with plasmid DNA or RNA and deliver their payload into target cells by receptor-mediated endocytosis. This chapter describes the synthesis of linear PEI (LPEI) from a precursor polymer and the current coupling techniques and purification procedure for peptide conjugates with linear polyethylenimine. A protocol is also given for the formation and characterization of polyplexes formed with LPEI conjugate and plasmid DNA.

Polyethyleneimine/DNA polyplexes with reduction-sensitive hyaluronic acid derivatives shielding for targeted gene delivery

The natural anionic polysaccharide hyaluronic acid (HA) was modified by introducing reduction-sensitive disulfide bond between the carboxyl groups and the backbone of HA (HA-SS-COOH). HA-SS-COOH and its corresponding unmodified stable analog HA were used to shield DNA/PEI polyplexes (DP) to form ternary complexes (DPS and DPH complexes). The shielding/deshielding effect was tested along with size, zeta potential, cell viability and transfection. Both DPS and DPH complexes showed increase in size, decrease in zeta potential and low cytotoxicity in physiological conditions due to the anionic shielding. In the reductive environment, only HA-SS-COOH coated ternary complexes (DPS) demonstrated the size increase and recovered high positive zeta potential. DPS complexes showed an up to 14-fold higher transfection than the stable coated one, indicating the efficiency of the reduction-responsive deshielding design. Moreover, the presence of extra free HA inhibited the transfection of DPS on HepG2 and B16F10 cells with HA receptor expression, while displaying no effect on non-targeted NIH3T3 cells. More rapid cellular association of DPS with HepG2 was observed, thus confirming the targeting reservation of disulfide bond modified HA. Intratumoral injection of DPS complexes resulted in much higher accumulation and luciferase expression in the tumor bearing C57BL/6 mice. Both in vitro and in vivo results demonstrated the successful combination of deshielding and target functions in HA derivatives for gene 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.

Polymers for siRNA delivery: inspired by viruses to be targeted, dynamic, and precise

Synthetic small interfering RNA (siRNA) presents an exciting novel medical opportunity. Although researchers agree that siRNA could have a great therapeutic impact, the required extracellular and intracellular delivery of these molecules into the disease-associated target cells presents the primary roadblock for the broader translation of these molecules into medicines. Thus, the design of adequate delivery technologies has utmost importance. Viruses are natural masterpieces of nucleic acid delivery and present chemists and drug delivery experts with a template for the design of artificial carriers for synthetic nucleic acids such as siRNA. They have been developed into gene vectors and have provided convincing successes in gene therapy. Optimized by biological evolution, viruses are programmed to be dynamic and bioresponsive as they enter living cells, and they carry out their functions in a precisely defined sequence. However, because they are synthesized within living cells and with naturally available nucleotides and amino acids, the chemistry of viruses is limited. With the use of diverse synthetic molecules and macromolecules, chemists can provide delivery solutions beyond the scope of the natural evolution of viruses. This Account describes the design and synthesis of "synthetic siRNA viruses." These structures contain elements that mimic the delivery functions of viral particles and surface domains that shield against undesired biological interactions and enable specific host cell receptor binding through the presentation of multiple targeting ligands. For example, cationic polymers can reversibly package one or more siRNA molecules into nanoparticle cores to protect them against a degradative bioenvironment. After internalization by receptor-mediated endocytosis into the acidifying endosomes of cells, synthetic siRNA can escape from these vesicles through the activation of membrane-disruption domains as viruses do and reach the cytoplasm, the location of RNA interference. This multistep task presents an attractive challenge for chemists. Similar to the design of prodrugs, the functional domains of these systems have to be activated in a dynamic mode, triggered by conformational changes or bond cleavages in the relevant microenvironment such as the acidic endosome or disulfide-reducing cytoplasm. These chemical analogues of viral domains are often synthetically simpler and more easily accessible molecules than viral proteins. Their precise assembly into multifunctional macromolecular and supramolecular structures is facilitated by improved analytical techniques, precise orthogonal conjugation chemistries, and sequence-defined polymer syntheses. The chemical evolution of microdomains using chemical libraries and macromolecular and supramolecular evolution could provide key strategies for optimizing siRNA carriers to selected medical indications.

In vivo gene delivery by nonviral vectors: overcoming hurdles?

The promise of cancer gene therapeutics is hampered by difficulties in the in vivo delivery to the targeted tumor cells, and systemic delivery remains to be the biggest challenge to be overcome. Here, we concentrate on systemic in vivo gene delivery for cancer therapy using nonviral vectors. In this review, we summarize the existing delivery barriers together with the requirements and strategies to overcome these problems. We will also introduce the current progress in the design of nonviral vectors, and briefly discuss their safety issues.

Endosomal escape and transfection efficiency of PEGylated cationic liposome-DNA complexes prepared with an acid-labile PEG-lipid

Cationic liposome-DNA (CL-DNA) complexes are being pursued as nonviral gene delivery systems for use in applications that include clinic trials. However, to compete with viral vectors for systemic delivery in vivo, their efficiencies and pharmacokinetics need to be improved. The addition of poly (ethylene glycol)-lipids (PEGylation) prolongs circulation lifetimes of liposomes, but inhibits cellular uptake and endosomal escape of CL-DNA complexes. We show that this limits their transfection efficiency (TE) in a manner dependent on the amount of PEG-lipid, the lipid/DNA charge ratio, and the lipid membrane charge density. To improve endosomal escape of PEGylated CL-DNA complexes, we prepared an acid-labile PEG-lipid (HPEG2K-lipid, PEG MW 2000) which is designed to lose its PEG chains at the pH of late endosomes. The HPEG2K-lipid and a similar but acid-stable PEG-lipid were used to prepare PEGylated CL-DNA complexes. TLC and dynamic light scattering showed that HPEG2K-CL-DNA complexes are stable at pH 7.4 for more than 24 h, but the PEG chains are cleaved at pH 5 within 1 h, leading to complex aggregation. The acid-labile HPEG2K-CL-DNA complexes showed enhanced TE over complexes stabilized with the acid-stable PEG-lipid. Live-cell imaging showed that both types of complexes were internalized to quantitatively similar particle distributions within the first 2 h of incubation with cells. Thus, we attribute the increased TE of the HPEG2K-CL-DNA complexes to efficient endosomal escape, enabled by the acid-labile HPEG2K-lipid which sheds its PEG chains in the low pH environment of late endosomes, effectively switching on the electrostatic interactions that promote fusion of the membranes of complex and endosome.

Recent advances in stealth coating of nanoparticle drug delivery systems

Modifying surfaces of nanoparticles (NPs) with polyethylene glycol (PEG), the so-called PEGylation, is the most commonly used method for reducing premature clearance of NPs from the circulation. However, several reports point out that PEGylation may negatively influence the performance of NPs as a drug carrier. Alternative surface modification strategies, including substitute polymers, conditional removal of PEG, and biomimetic surface modification, may provide solutions for the limitations of PEG.

A pH-sensitive multifunctional gene carrier assembled via layer-by-layer technique for efficient gene delivery

Background

The success of gene therapy asks for the development of multifunctional vectors that could overcome various gene delivery barriers, such as the cell membrane, endosomal membrane, and nuclear membrane. Layer-by-layer technique is an efficient method with easy operation which can be used for the assembly of multifunctional gene carriers. This work describes a pH-sensitive multifunctional gene vector that offered long circulation property but avoided the inhibition of tumor cellular uptake of gene carriers associated with the use of polyethylene glycol.

Methods

Deoxyribonucleic acid (DNA) was firstly condensed with protamine into a cationic core which was used as assembly template. Then, additional layers of anionic DNA, cationic liposomes, and o-carboxymethyl-chitosan (CMCS) were alternately adsorbed onto the template via layer-by-layer technique and finally the multifunctional vector called CMCS-cationic liposome-coated DNA/protamine/DNA complexes (CLDPD) was constructed. For in vitro test, the cytotoxicity and transfection investigation was carried out on HepG2 cell line. For in vivo evaluation, CMCS-CLDPD was intratumorally injected into tumor-bearing mice and the tumor cells were isolated for fluorescence determination of transfection efficiency.

Results

CMCS-CLDPD had ellipsoidal shapes and showed “core-shell” structure which showed stabilization property in serum and effective protection of DNA from nuclease degradation. In vitro and in vivo transfection results demonstrated that CMCS-CLDPD had pH-sensitivity and the outermost layer of CMCS fell off in the tumor tissue, which could not only protect CMCS- CLDPD from serum interaction but also enhance gene transfection efficiency.

Conclusion

These results demonstrated that multifunctional CMCS-CLDPD had pH- sensitivity, which may provide a new approach for the antitumor gene delivery.