Bioreducible polycations in nucleic acid delivery: past, present, and future trends

Bioreducible Cross-Linked Hyaluronic Acid/Calcium Phosphate Hybrid Nanoparticles for Specific Delivery of siRNA in Melanoma Tumor Therapy

This study introduces a novel cross-linking strategy capable of successfully stabilizing CaP nanoparticles and stimuli-responsive small interfering RNA (siRNA) release. We synthesized a polysaccharide derivative thiolated hyaluronic acid (HA-SH), which was slightly modified but multifunctional and developed a smart redox-responsive delivery system. siRNA was efficaciously condensed by calcium phosphate (CaP) via electrostatic interaction to form a positively charged inner "core". Disulfide cross-linked HA (HA-ss-HA) was formed and played a role as an anionic outer "shell" to stabilize the CaP core. We demonstrated that the nanoparticles were stable both in the storage milieu and systemic circulation, thus overcoming the most serious disadvantage of CaP nanoparticles for gene delivery. Meanwhile, this smart system could selectively release siRNA into the cytosol by both a GSH-triggered disassembly and successful endosomal escape. Therefore, the hybrid delivery system achieved an 80% gene-silencing efficiency in vitro for both luciferase and Bcl2. Silencing of Bcl2 resulted in dramatic apoptosis of B16F10 cells. Besides, equipped with the tumor-targeting component HA, the nanoparticles significantly suppressed the growth of B16F10 xenograft tumor in mice. The anionic HA-ss-HA-equipped nanoparticles showed no apparent toxicity in vitro or in vivo, as well as showed a high transfection efficiency. Taken together, this redox-responsive, tumor-targeting smart anionic nanoparticle holds great promise for exploitation in functionalized siRNA delivery and tumor therapy.

Biochemical evaluation of the anticancer potential of the polyamine-based nanocarrier Nano11047

Synthesizing polycationic polymers directly from existing drugs overcomes the drug-loading limitations often associated with pharmacologically inert nanocarriers. We recently described nanocarriers formed from a first-generation polyamine analogue, bis(ethyl)norspermine (BENSpm), that could simultaneously target polyamine metabolism while delivering therapeutic nucleic acids. In the current study, we describe the synthesis and evaluation of self-immolative nanocarriers derived from the second-generation polyamine analogue PG-11047. Polyamines are absolutely essential for proliferation and their metabolism is frequently dysregulated in cancer. Through its effects on polyamine metabolism, PG-11047 effectively inhibits tumor growth in cancer cell lines of multiple origins as well as in human tumor mouse xenografts. Promising clinical trials have been completed verifying the safety and tolerance of this rotationally restricted polyamine analogue. We therefore used PG-11047 as the basis for Nano11047, a biodegradable, prodrug nanocarrier capable of targeting polyamine metabolism. Following exposure of lung cancer cell lines to Nano11047, uptake and intracellular degradation into the parent compound PG-11047 was observed. The release of PG-11047 highly induced the polyamine catabolic enzyme activities of spermidine/spermine N1-acetyltransferase (SSAT) and spermine oxidase (SMOX). By contrast, the activity of ornithine decarboxylase (ODC), a rate-limiting enzyme in polyamine biosynthesis and a putative oncogene, was decreased. Consequently, intracellular levels of the natural polyamines were depleted concurrent with tumor cell growth inhibition. This availability of Nano11047 as a novel drug form and potential nucleic acid delivery vector will potentially benefit and encourage future clinical studies.

Combining Fluorination and Bioreducibility for Improved siRNA Polyplex Delivery

Polycations are promising vectors for the delivery of siRNA therapeutics but they often suffer from toxicity and low in vivo delivery efficacy. This study tests the hypothesis that combining fluorination and bioreducibility of polycations will overcome problems with both the toxicity and delivery efficacy. To test the hypothesis, we synthesized bioreducible (RHB) and nonreducible (NHB) poly(amido amine)s. The RHB were additionally fluorinated using reaction with heptafluorobutyric anhydride to obtain F-RHB. We found that both RHB and F-RHB showed significantly reduced cytotoxicity compared with NHB, which allowed their safe use in a wider range of doses than NHB. All three synthesized polycations formed polyplexes with siRNA. F-RHB achieved the best siRNA silencing efficacy in multiple cell lines in vitro, which was at least in part because of fluorination-induced enhancement of cellular uptake and improved endosomal escape. Lastly, F-RHB showed greatly improved Luc silencing efficacy in tumors in vivo when compared with polyplexes based on RHB, NHB, as well as control poly(ethylenimine) (PEI). This study suggests that combining fluorination with bioreducibility of polycations is a promising strategy to the design of siRNA delivery vectors with improved toxicity and in vivo activity profiles.

Self-immolative nanoparticles for simultaneous delivery of microRNA and targeting of polyamine metabolism in combination cancer therapy

Combination of anticancer drugs with therapeutic microRNA (miRNA) has emerged as a promising anticancer strategy. However, the promise is hampered by a lack of desirable delivery systems. We report on the development of self-immolative nanoparticles capable of simultaneously delivering miR-34a mimic and targeting dysregulated polyamine metabolism in cancer. The nanoparticles were prepared from a biodegradable polycationic prodrug, named DSS-BEN, which was synthesized from a polyamine analog N1,N11-bisethylnorspermine (BENSpm). The nanoparticles were selectively disassembled in the cytoplasm where they released miRNA. Glutathione (GSH)-induced degradation of self-immolative linkers released BENSpm from the DSS-BEN polymers. MiR-34a mimic was effectively delivered to cancer cells as evidenced by upregulation of intracellular miR-34a and downregulation of Bcl-2 as one of the downstream targets of miR-34a. Intracellular BENSpm generated from the degraded nanoparticles induced the expression of rate-limiting enzymes in polyamine catabolism (SMOX, SSAT) and depleted cellular natural polyamines. Simultaneous regulation of polyamine metabolism and miR-34a expression by DSS-BEN/miR-34a not only enhanced cancer cell killing in cultured human colon cancer cells, but also improved antitumor activity in vivo. The reported findings validate the self-immolative nanoparticles as delivery vectors of therapeutic miRNA capable of simultaneously targeting dysregulated polyamine metabolism in cancer, thereby providing an elegant and efficient approach to combination nanomedicines.

A mannosylated PEI–CPP hybrid for TRAIL gene targeting delivery for colorectal cancer therapy

A mannosylated, bioreducible Man-PEI_5k–CPP/pTRAIL system was developed for treating colon cancer.

Intracellular Delivery and Triggered Release of DNA Using Biodegradable Poly(2-hydroxypropylene imine)s Containing Cystamine Units

Poly(2-hydroxypropylene imine)s containing segments of cystamine (PHPI-CA) are synthesized by polycondensation of 1,3-dibromo-2-propanol with a mixture of 1,3-diamino-2-propanol and cystamine. High molecular weight fractions of these polymers are collected by ultrafiltration and characterized by chemical analysis, ¹ H and ¹³ C-NMR spectroscopy, size-exclusion chromatography with triple detection, and potentiometric titration, and are tested for DNA delivery in vitro. It is shown that PHPI-CA are highly branched polymers containing disulfide linkages. Transfection efficiency of PHPI-CA for DNA gives similar results to that of PHPI with GFP⁺ cell percent reaching 80-90%. Cytotoxicity levels for PHPI-CA are lower than that of PHPI. Novel polymers containing different amounts of disulfide linkages are able to disintegrate and release DNA following the treatment with reducing agent 1,4-dithiothreitol. Downstream application of PHPI-CA transfected cells for RNA purification shows that RNA yield is not affected even after the double transfection suggesting that these polymers could be great candidates for in vitro and in vivo transfection.

The third orthogonal dynamic covalent bond

Orthogonal dynamic covalent bonds are of interest for the construction of functional systems. The orthogonality of disulfide and hydrazone exchange under basic and acidic conditions, respectively, is well established. However, the integration of boronate esters as the third bond has failed so far because they exchanged too easily, especially under hydrazone exchange conditions. In this report, a collection of bioinspired catechols derived from adhesive natural products from cyanobacteria is screened with phenylboronic acids with proximal alcohols (benzoboroxoles), amines and fluorines to identify the least labile boronate esters. Moreover, Kool's 2-aminophenol catalysts are introduced to selectively accelerate hydrazone exchange without disturbing sufficiently inert boronate esters. Based on these results, we identified three different conditions to selectively exchange disulfides, hydrazones and boronate esters, that is to demonstrate the existence of three orthogonal dynamic covalent bonds. Moreover, their compatibility with functional systems is confirmed by successful hydrazone exchange in multicomponent surface architectures in the presence of intact boronate esters and disulfides.

Cell-Targeting Cationic Gene Delivery System Based on a Modular Design Rationale

En route to target cells, a gene carrier faces multiple extra- and intracellular hurdles that would affect delivery efficacy. Although diverse strategies have been proposed to functionalize gene carriers for individually overcoming these barriers, it is challenging to generate a single multifunctional gene carrier capable of surmounting all these barriers. Aiming at this challenge, we have developed a supramolecular modular approach to fabricate a multifunctional cationic gene delivery system. It consists of two prefunctionalized modules: (1) a host module: a polymer (PCD-SS-PDMAEMA) composed of poly(β-cyclodextrin) backbone and disulfide-linked PDMAEMA arms, expectedly acting to compact DNA and release DNA upon cleavage of disulfide linkers in reductive microenvironment; and (2) a guest module: adamantyl and folate terminated PEG (Ad-PEG-FA), expectedly functioning to reduce nonspecific interactions, improve biocompatibility, and provide folate-mediated cellular targeting specificity. Through the host-guest interaction between β-cyclodextrin units of the "host" module and adamantyl groups of the "guest" module, the PCD-SS-PDMAEMA-1 (host) and Ad-PEG-FA (guest) self-assemble forming a supramolecular pseudocopolymer (PCD-SS-PDMAEMA-1/PEG-FA). Our comprehensive analyses demonstrate that the functions preassigned to the two building modules are well realized. The gene carrier effectively compacts DNA into stable nanosized polyplexes resistant to enzymatic digestion, triggers DNA release in reducing environment, possesses significantly improved hemocompatibility, and specifically targets folate-receptor positive cells. Most importantly, endowed with these predesigned functions, the PCD-SS-PDMAEMA-1/PEG-FA supramolecular gene carrier exhibits excellent transfection efficacy for both pDNA and siRNA. Thus, this work represents a proof-of-concept example showing the efficiency and convenience of an adaptable, modular approach for conferring multiple functions to a single supramolecular gene carrier toward effective in vivo delivery of therapeutic nucleic acids.

Cellular uptake: lessons from supramolecular organic chemistry

The objective of this Feature Article is to reflect on the importance of established and emerging principles of supramolecular organic chemistry to address one of the most persistent problems in life sciences. The main topic is dynamic covalent chemistry on cell surfaces, particularly disulfide exchange for thiol-mediated uptake. Examples of boronate and hydrazone exchange are added for contrast, comparison and completion. Of equal importance are the discussions of proximity effects in polyions and counterion hopping, and more recent highlights on ring tension and ion pair-π interactions. These lessons from supramolecular organic chemistry apply to cell-penetrating peptides, particularly the origin of "arginine magic" and the "pyrenebutyrate trick," and the currently emerging complementary "disulfide magic" with cell-penetrating poly(disulfide)s. They further extend to the voltage gating of neuronal potassium channels, gene transfection, and the delivery of siRNA. The collected examples illustrate that the input from conceptually innovative chemistry is essential to address the true challenges in biology beyond incremental progress and random screening.

Dual delivery of biological therapeutics for multimodal and synergistic cancer therapies

Cancer causes >8.2 million deaths annually worldwide; thus, various cancer treatments have been investigated over the past decades. Among them, combination drug therapy has become extremely popular, and treatment with more than one drug is often necessary to achieve appropriate anticancer efficacy. With the development of nanoformulations and nanoparticulate-based drug delivery, researchers have explored the feasibility of dual delivery of biological therapeutics to overcome the current drawbacks of cancer therapy. Compared with the conventional single drug therapy, dual delivery of therapeutics has provided various synergistic effects in addition to offering multimodality to cancer treatment. In this review, we highlight and summarize three aspects of dual-delivery systems for cancer therapy. These include (1) overcoming drug resistance by the dual delivery of chemical drugs with biological therapeutics for synergistic therapy, (2) targeted and controlled drug release by the dual delivery of drugs with stimuli-responsive nanomaterials, and (3) multimodal theranostics by the dual delivery of drugs and molecular imaging probes. Furthermore, recent developments, perspectives, and new challenges regarding dual-delivery systems for cancer therapy are discussed.

Ethynyl benziodoxolones: functional terminators for cell-penetrating poly(disulfide)s

Hypervalent iodine terminators are introduced to secure synthetic access to doubly-labeled cell-penetrating poly(disulfide)s.

Multifunctional cationic polyurethanes designed for non-viral cancer gene therapy

Nano-polyplexes from bioreducible cationic polymers have a massive promise for cancer gene therapy. However, the feasibility of cationic polyurethanes for non-viral gene therapy is so far not well studied. In this work, a linear cationic polyurethane containing disulfide bonds, urethane linkages and protonable tertiary amino groups was successfully generated by stepwise polycondensation reaction between 2,2'-dithiodiethanol bis(p-nitrophenyl carbonate) and 1,4-bis(3-aminopropyl)piperazine (BAP). We confirmed that the cationic polyurethane (denoted as PUBAP) displayed superior gene delivery properties to its cationic polyamide analogue, thus causing higher in vitro transfection efficiency in MCF-7 and SKOV-3 cells. Besides, further folate-PEGylation and hydrophobic deoxycholic acid (DCA) conjugation to amino-containing PUBAP can be conducted to afford multifunctional polyurethane gene delivery system. After optimization, folate-decorated nano-polyplexes from the PUBAP conjugated with 8 folate-PEG chains and 12 DCA residues exhibited superb colloidal stability under physiological conditions, and performed rapid uptake via folate receptor-mediated endocytosis, efficient intracellular gene release and nucleus translocation into SKOV-3 cells in vitro and in vivo. Importantly, PUBAP based polyplexes possess low cytotoxicity as a result of PUBAP biodegradability. Therefore, marked growth inhibition of SKOV-3 tumor xenografted in Balb/c nude mice was achieved with negligible side effects on the mouse health after intravenous administration of PUBAP based polyplexes with a therapeutic plasmid encoding for TNF-related apoptosis-inducing ligand. This work provides a new insight into biomedical application of bio-responsive polyurethanes for cancer therapy.

STATEMENT OF SIGNIFICANCE

In this study, we have confirmed that disulfide-based cationic polyurethane presents a new non-viral vector for gene transfer and cancer gene therapy. The significance of this work includes: (1) design and synthesis of a group of novel disulfide-based cationic polyurethane by non-isocyanate chemistry; (2) comparative study of transfection activity between cationic polyurethanes and cationic polyamides; (3) feasibility of bioreducible cationic polyurethanes for in vivo cancer gene therapy.

A family of cationic polyamides for in vitro and in vivo gene transfection

The purpose of this study is to develop biodegradable cationic polyamides for non-viral gene delivery and elucidate their structural effects on gene transfection activity. To this end, a group of novel cationic polyamides were synthesized by polycondensation reaction between different di-p-nitrophenyl esters and tertiary amine-containing primary diamines. These linear polyamides have flexible alkylene group (ethylene or propylene), protonable amino group and bioreducible disulfide linkage in the polyamide main chain. The alkylene group and disulfide linkage in these polyamides have a distinct effect on their gene delivery properties including buffering capacity, gene binding ability and intracellular gene release profile. Those cationic polyamides containing disulfide linkage and 1,4-bis(3-aminopropyl)piperazine (BAP) residue exhibited high buffering capacity (endosomal escape ability), high gene binding ability, and intracellular gene release ability, thus inducing fast gene nucleus translocation and robust gene transfection in vitro against different cell lines and rat bone marrow mesenchymal stem cells. Moreover, the transfection efficiencies in vitro were comparable or higher than those of 25 kDa branched polyethylenimine and Lipofectamine 2000 transfection agent as positive controls. These cationic polyamides and their polyplexes were of low cytotoxicity when an optimal transfection efficacy was achieved. In vivo transfection tests showed that bioreducible BAP-based polyamides were applicable for intravenous gene delivery in a mouse model, leading to higher level of transgene expression in the liver as compared to 22 kDa linear polyethylenimine as a positive control. These cationic polyamides provide a useful platform to elucidate the relationship between chemical functionalities and gene transfection activity.

Colorful surface architectures with three different types of dynamic covalent bonds: integration of anthocyanins, tritylium ions and flavins

Although they combine the best of covalent and non-covalent bonds, dynamic covalent bonds are usually not used together. Building on pioneering examples for functional systems with two orthogonal dynamic covalent bonds, we herein elaborate on multicomponent surface architectures that operate with three different types of dynamic covalent bonds. Disulfide exchange under basic conditions is used to grow single π stacks directly on oxide surfaces, hydrazone exchange under acidic conditions to add a second string or stack, and boronic-ester exchange under neutral conditions to build the third one. In this study, we show that this synthetic approach to complex systems provides access to emergent properties, as exemplified with ordered stacks of anthocyanins, pyrocatchol violet and riboflavins. The integration of anthocyanins, the central component of the pigments of plant flowers, is interesting to protect the blue flavylium cation against deprotonation, deplanarization and degradation. The integration of pyrocatchol violet is of interest to stabilize the blue, disfavored tritylium cation. The red riboflavin stacks are attractive because they generate high photocurrent. These colorful examples hint at the potential of synthetic methods that use three different types of dynamic covalent bonds in concert to build complex systems with emergent properties.

Self-immolative polycations as gene delivery vectors and prodrugs targeting polyamine metabolism in cancer

Polycations are explored as carriers to deliver therapeutic nucleic acids. Polycations are conventionally pharmacological inert with the sole function of delivering therapeutic cargo. This study reports synthesis of a self-immolative polycation (DSS-BEN) based on a polyamine analogue drug N(1),N(11)-bisethylnorspermine (BENSpm). The polycation was designed to function dually as a gene delivery carrier and a prodrug targeting dysregulated polyamine metabolism in cancer. Using a combination of NMR and HPLC, we confirm that the self-immolative polycation undergoes intracellular degradation into the parent drug BENSpm. The released BENSpm depletes cellular levels of spermidine and spermine and upregulates polyamine catabolic enzymes spermine/spermidine N(1)-acetyltransferase (SSAT) and spermine oxidase (SMO). The synthesized polycations form polyplexes with DNA and facilitate efficient transfection. Taking advantage of the ability of BENSpm to sensitize cancer cells to TNFα-induced apoptosis, we show that DSS-BEN enhances the cell killing activity of TNFα gene therapy. The reported findings validate DSS-BEN as a dual-function delivery system that can deliver a therapeutic gene and improve the outcome of gene therapy as a result of the intracellular degradation of DSS-BEN to BENSpm and the subsequent beneficial effect of BENSpm on dysregulated polyamine metabolism in cancer.

Cell-penetrating poly(disulfide)s: the dependence of activity, depolymerization kinetics and intracellular localization on their length

We report that, with the increasing length, cell-penetrating poly(disulfide)s preferably accumulate in the endosomes, cytosol and then the nucleoli.

Protein delivery with cell-penetrating poly(disulfide)s

The combination of cell-penetrating poly(disulfide)s with biotin–streptavidin biotechnology affords a general method for the delivery of proteins into cells.

Gene therapy and imaging in preclinical and clinical oncology: recent developments in therapy and theranostics

In the case of disseminated cancer, current treatment options reach their limit. Gene theranostics emerge as an innovative route in the treatment and diagnosis of cancer and might pave the way towards development of an efficacious treatment of currently incurable cancer. Various gene vectors have been developed to realize tumor-specific nucleic acid delivery and are considered crucial for the successful application of cancer gene therapy. By adding reporter genes and imaging agents, these systems gain an additional diagnostic function, thereby advancing the theranostic paradigm into cancer gene therapy. Numerous preclinical studies have demonstrated the feasibility of combined tumor gene therapy and diagnostic imaging, and clinical trials in human and veterinary oncology have been executed with partly encouraging results.