Stability of siRNA polyplexes from poly(ethylenimine) and poly(ethylenimine)-g-poly(ethylene glycol) under in vivo conditions: effects on pharmacokinetics and biodistribution measured by Fluorescence Fluctuation Spectroscopy and Single Photon Emission Computed Tomography (SPECT) imaging

The efficacy of nuclease-resistant Chol-siRNA in primary breast tumors following complexation with PLL-PEG(5K)

Modifying the sense strand of nuclease-resistant siRNA with 3'-cholesterol (Chol-*siRNA) increases mRNA suppression after i.v. administration but with relatively low efficacy. We previously found evidence in vitro that suggests complexation of Chol-siRNA with PLL-PEG(5K), a block copolymer of poly-l-lysine and 5 kDa polyethylene glycol, may increase the efficacy of Chol-siRNA in vivo in a PLL block length-dependent manner. In this study, the extent that polyplexes of PLL10-PEG(5K), PLL30-PEG(5K), and PLL50-PEG(5K) protect complexed Chol-siRNA in high concentrations of murine serum and affect the activity of Chol-*siRNA in murine 4T1 breast tumor epithelial cells in vitro and in primary orthotopic tumors of 4T1 was compared. PLL-PEG(5K) required 3'-Chol to protect full-length siRNA from nuclease degradation in 90% (v/v) murine serum and protection was increased by increasing PLL block length and nuclease resistance of Chol-siRNA. Polyplexes of Chol-*siLuc suppressed stably expressed luciferase in 4T1-Luc cells to different levels in vitro where PLL30 > PLL50 > PLL10. In contrast, only polyplexes of Chol-*siLuc and PLL30-PEG(5K) or PLL50-PEG(5K) suppressed high levels of luciferase in primary orthotopic tumors of 4T1-Luc after i.v. administration, whereas polyplexes of Chol-*siLuc and PLL10-PEG(5K), inactive Chol-*siCtrl polyplexes of PLL-PEG(5K), or Chol-*siLuc alone had no detectable activity. As a whole, these results indicate that polyplexes of PLL-PEG(5K) increase the efficacy of nuclease-resistant Chol-siRNA in primary breast tumors after i.v. administration in a PLL block length-dependent manner. Thus, complexation of Chol-siRNA with PLL-PEG(5K) may be a promising approach to increase the efficacy of Chol-siRNA in a wide range of primary tumors, metastases, and other tissues but likely requires a PLL block length that balances polymer-related adverse effects, Chol-siRNA bioavailability, and subsequent activity in the target cell.

Metabolism studies of unformulated internally [3H]-labeled short interfering RNAs in mice

Absorption, distribution, metabolism, and excretion properties of two unformulated model short interfering RNA (siRNAs) were determined using a single internal [(3)H]-radiolabeling procedure, in which the full-length oligonucleotides were radiolabeled by Br/(3)H -exchange. Tissue distribution, excretion, and mass balance of radioactivity were investigated in male CD-1 mice after a single intravenous administration of the [(3)H]siRNAs, at a target dose level of 5 mg/kg. Quantitative whole-body autoradiography and liquid scintillation counting techniques were used to determine tissue distribution. Radiochromatogram profiles were determined in plasma, tissue extracts, and urine. Metabolites were separated by liquid chromatography and identified by radiodetection and high-resolution accurate mass spectrometry. In general, there was little difference in the distribution of total radiolabeled components after administration of the two unformulated [(3)H]siRNAs. The radioactivity was rapidly and widely distributed throughout the body and remained detectable in all tissues investigated at later time points (24 and 48 hours for [(3)H]MRP4 (multidrug resistance protein isoform 4) and [(3)H]SSB (Sjögren Syndrome antigen B) siRNA, respectively). After an initial rapid decrease, concentrations of total radiolabeled components in dried blood decreased at a much slower rate. A nearly complete mass balance was obtained for the [(3)H]SSB siRNA, and renal excretion was the main route of elimination (38%). The metabolism of the two model siRNAs was rapid and extensive. Five minutes after administration, no parent compound could be detected in plasma. Instead, radiolabeled nucleosides resulting from nuclease hydrolysis were observed. In the metabolism profiles obtained from various tissues, only radiolabeled nucleosides were found, suggesting that siRNAs are rapidly metabolized and that the distribution pattern of total radiolabeled components can be ascribed to small molecular weight metabolites.

Surface-modified HK:siRNA nanoplexes with enhanced pharmacokinetics and tumor growth inhibition

We characterized in this study the pharmacokinetics and antitumor efficacy of histidine-lysine (HK):siRNA nanoplexes modified with PEG and a cyclic RGD (cRGD) ligand targeting αvβ3 and αvβ5 integrins. With noninvasive imaging, systemically administered surface-modified HK:siRNA nanoplexes showed nearly 4-fold greater blood levels, 40% higher accumulation in tumor tissue, and 60% lower luciferase activity than unmodified HK:siRNA nanoplexes. We then determined whether the surface-modified HK:siRNA nanoplex carrier was more effective in reducing MDA-MB-435 tumor growth with an siRNA targeting Raf-1. Repeated systemic administration of the selected surface modified HK:siRNA nanoplexes targeting Raf-1 showed 35% greater inhibition of tumor growth than unmodified HK:siRNA nanoplexes and 60% greater inhibition of tumor growth than untreated mice. The improved blood pharmacokinetic results and tumor localization observed with the integrin-targeting surface modification of HK:siRNA nanoplexes correlated with greater tumor growth inhibition. This investigation reveals that through control of targeting ligand surface display in association with a steric PEG layer, modified HK: siRNA nanoplexes show promise to advance RNAi therapeutics in oncology and potentially other critical diseases.

In vivo disassembly of IV administered siRNA matrix nanoparticles at the renal filtration barrier

Intravenous administration of siRNA nanocarriers may provide unique therapeutic opportunities for tissue-specific gene silencing. Although often engineered to overcome the numerous barriers that exist in the systemic circulation, many nanocarriers fail in extending the circulation time of the siRNA. A more detailed assessment of the different clearance mechanisms that are in play after intravenous injection could therefore be of value to improve siRNA nanocarrier design. In this report, the biodistribution in mice of siRNA loaded dextran nanogels was investigated in detail. Both single photon emission computed tomography (SPECT) imaging and fluorescence single particle tracking (fSPT) indicate that the particles are rapidly cleared from the circulation. PEGylation of the nanogels was not able to increase the half-life in the bloodstream. Carrier disassembly in the systemic circulation and phagocytic clearance are known to facilitate the elimination of siRNA nanoparticles. Additionally, it is demonstrated for dextran nanogels that also the kidneys play an important role in their elimination from the bloodstream. SPECT imaging revealed an accumulation of the siRNA loaded dextran nanogels in the kidneys shortly after intravenous injection and a significantly delayed transition of siRNA from kidney to bladder, as opposed to the injection of free siRNA. These data indicate that components of the glomerular filtration barrier may contribute to the dissociation of siRNA from its carrier, as was recently suggested for cationic cyclodextrin siRNA polyplexes. This clearance mechanism should therefore be taken into account when designing siRNA nanocarriers for intravenous administration.

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.

Attacking the genome: emerging siRNA nanocarriers from concept to clinic

Ever since the ground breaking discovery of RNA interference (RNAi), an endogenous mechanism for sequence specific regulation of gene expression via short interfering RNAs (siRNA), researchers and pharmaceutical companies alike have devoted immense time and capital into the design of nanocarriers that can mediate safe and effective delivery. After over a decade of research, a PubMed search for 'siRNA delivery' returns over 2500 references, yet only a handful of delivery systems have been successfully translated to the clinic. This low rate of clinical translation can partly be attributed to the complexity of the barriers that need to be overcome in vivo. Advances in identifying some of these barriers have lead to the development of molecular components that can overcome some of these hurdles. The judicious assembly of these components, as seen in stable nucleic-acid-lipid nanoparticles and the cyclodextrin polymer, will be required for the successful clinical translation of nanoparticle-based siRNA therapeutics.

Enhancing in vivo circulation and siRNA delivery with biodegradable polyethylenimine-graft-polycaprolactone-block-poly(ethylene glycol) copolymers

The purpose of this study was to enhance the in vivo blood circulation time and siRNA delivery efficiency of biodegradable copolymers polyethylenimine-graft-polycaprolactone-block-poly(ethylene glycol) (hy-PEI-g-PCL-b-PEG) by introducing high graft densities of PCL-PEG chains. SYBR(®) Gold and heparin assays indicated improved stability of siRNA/copolymer-complexes with a graft density of 5. At N/P 1, only 40% siRNA condensation was achieved with non-grafted polymer, but 95% siRNA was condensed with copolymer PEI25k-(PCL570-PEG5k)(5). Intracellular uptake studies with confocal laser scanning microscopy and flow cytometry showed that the cellular uptake was increased with graft density, and copolymer PEI25k-(PCL570-PEG5k)(5) was able to deliver siRNA much more efficiently into the cytosol than into the nucleus. The in vitro knockdown effect of siRNA/hyPEI-g-PCL-b-PEG was also significantly improved with increasing graft density, and the most potent copolymer PEI25k-(PCL570-PEG5k)(5) knocked down 84.43% of the GAPDH expression. Complexes of both the copolymers with graft density 3 and 5 circulated much longer than unmodified PEI25 kDa and free siRNA, leading to a longer elimination half-life, a slower clearance and a three- or fourfold increase of the AUC compared to free siRNA, respectively. We demonstrated that the graft density of the amphiphilic chains can enhance the siRNA delivery efficiency and blood circulation, which highlights the development of safe and efficient non-viral polymeric siRNA nanocarriers that are especially stable and provide longer circulation in vivo.

Polycation-siRNA nanoparticles can disassemble at the kidney glomerular basement membrane

Despite being engineered to avoid renal clearance, many cationic polymer (polycation)-based siRNA nanoparticles that are used for systemic delivery are rapidly eliminated from the circulation. Here, we show that a component of the renal filtration barrier--the glomerular basement membrane (GBM)--can disassemble cationic cyclodextrin-containing polymer (CDP)-based siRNA nanoparticles and, thereby, facilitate their rapid elimination from circulation. Using confocal and electron microscopies, positron emission tomography, and compartment modeling, we demonstrate that siRNA nanoparticles, but not free siRNA, accumulate and disassemble in the GBM. We also confirm that the siRNA nanoparticles do not disassemble in blood plasma in vitro and in vivo. This clearance mechanism may affect any nanoparticles that assemble primarily by electrostatic interactions between cationic delivery components and anionic nucleic acids (or other therapeutic entities).

Development of an MRI-visible nonviral vector for siRNA delivery targeting gastric cancer

An antibody-directed nonviral vector, polyethylene glycol-grafted polyethylenimine functionalized with superparamagnetic iron oxide nanoparticles and a gastric cancer-associated CD44v6 single-chain variable fragment (scFv_CD44v6,-PEG-g-PEI-SPION), was constructed as a gastric cancer-targeting and magnetic resonance imaging (MRI)-visible nanocarrier for small interfering RNA (siRNA) delivery. Biophysical characterization of PEG-g-PEI-SPION and scFv_CD44v6-PEG-g-PEI-SPION was carried out, including siRNA condensation capacity, cell viability, and transfection efficiency. Both the targeting and nontargeting nanocarriers were effective for transferring siRNA in vitro. The cellular uptake and distribution of nanoparticles complexed with siRNA was analyzed by fluorescence imaging and immunofluorescent staining. Moreover, the gastric cancer-targeting effect was verified in vivo by MRI and histology analysis. These results indicate that scFv_CD44v6-PEG-g-PEI-SPION is a promising nonviral vector for gastric cancer gene therapy and diagnosis.

Design and biophysical characterization of bioresponsive degradable poly(dimethylaminoethyl methacrylate) based polymers for in vitro DNA transfection

Water-soluble, degradable polymers based on poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) with low cytotoxicity and good p-DNA transfection efficiency are highlighted in this article. To solve the nondegradability issue of PDMAEMA, new polymers based on DMAEMA and 5,6-benzo-2-methylene-1,3-dioxepane (BMDO) for gene transfection were synthesized. A poly(ethylene oxide) (PEO) azo-initiator was used as free-radical initiator. PEGylation was performed to improve water solubility and to reduce cytotoxicity of the polymers. The resulting polymers contain hydrolyzable ester linkages in the backbone and were soluble in water even with very high amounts of ester linkages. These degradable copolymers showed significantly less toxicity with a MTT assay using L929 cell lines and demonstrated promising DNA transfection efficiency when compared with the gold standard poly(ethyleneimine). Bioresponsive properties of the corresponding quaternized DMAEMA based degradable polymers were also studied. Although the quaternized DMAEMA copolymers showed enhanced water solubility, they were inferior in gene transfection and toxicity as compared to the unquaternized copolymers.

Action and reaction: the biological response to siRNA and its delivery vehicles

RNA interference (RNAi)-based therapeutics have significant potential for the treatment of human disease. Safe and effective delivery of RNA to target tissues remains a major barrier to realizing its clinical potential. Several factors can affect the in vivo performance of short interfering RNA (siRNA) delivery formulations, including siRNA sequence, structure, chemical modification, and delivery formulation. This review provides an introduction to the principles underlying the pharmacokinetics and pharmacodynamics of systemically administered siRNA and its delivery formulations, including the factors that lead to its degradation, clearance, and tissue uptake, as well as its potential for immunogenicity, toxicity, and off-target effects within the body.

Microgels and microcapsules in peptide and protein drug delivery

The present review focuses on the interaction of microgels and microcapsules with biological macromolecules, particularly peptides and proteins, as well as drug delivery applications of such systems. Results from recent studies on factors affecting peptide/protein binding to, and release from, microgels and related systems are discussed, including effects of network properties, as well as protein aggregation, peptide length, hydrophobicity and charge (distributions), secondary structure, and cyclization. Effects of ambient conditions (pH, ionic strength, temperature, etc.) are also discussed, all with focus on factors of importance for the performance of microgel and microcapsule delivery systems for biomacromolecular drugs.

Synthesis and characterization of mannosylated pegylated polyethylenimine as a carrier for siRNA

Regulation of gene expression using small interfering RNA (siRNA) is a promising strategy for research and treatment of numerous diseases. In this study, we develop and characterize a delivery system for siRNA composed of polyethylenimine (PEI), polyethylene glycol (PEG), and mannose (Man). Cationic PEI complexes and compacts siRNA, PEG forms a hydrophilic layer outside of the polyplex for steric stabilization, and mannose serves as a cell binding ligand for macrophages. The PEI-PEG-mannose delivery system was constructed in two different ways. In the first approach, mannose and PEG chains are directly conjugated to the PEI backbone. In the second approach, mannose is conjugated to one end of the PEG chain and the other end of the PEG chain is conjugated to the PEI backbone. The PEI-PEG-mannose delivery systems were synthesized with 3.45-13.3 PEG chains and 4.7-3.0 mannose molecules per PEI. The PEI-PEG-Man-siRNA polyplexes displayed a coarse surface in Scanning Electron Microscopy (SEM) images. Polyplex sizes were found to range from 169 to 357 nm. Gel retardation assays showed that the PEI-PEG-mannose polymers are able to efficiently complex with siRNA at low N/P ratios. Confocal microscope images showed that the PEI-PEG-Man-siRNA polyplexes could enter cells and localized in the lysosomes at 2h post-incubation. Pegylation of the PEI reduced toxicity without any adverse reduction in knockdown efficiency relative to PEI alone. Mannosylation of the PEI-PEG could be carried out without any significant reduction in knockdown efficiency relative to PEI alone. Conjugating mannose to PEI via the PEG spacer generated superior toxicity and gene knockdown activity relative to conjugating mannose and PEG directly onto the PEI backbone.

Design of an inhalable dry powder formulation of DOTAP-modified PLGA nanoparticles loaded with siRNA

Matrix systems based on biocompatible and biodegradable polymers like the United States Food and Drug Administration (FDA)-approved polymer poly(DL-lactide-co-glycolide acid) (PLGA) are promising for the delivery of small interfering RNA (siRNA) due to favorable safety profiles, sustained release properties and improved colloidal stability, as compared to polyplexes. The purpose of this study was to design a dry powder formulation based on cationic lipid-modified PLGA nanoparticles intended for treatment of severe lung diseases by pulmonary delivery of siRNA. The cationic lipid dioleoyltrimethylammoniumpropane (DOTAP) was incorporated into the PLGA matrix to potentiate the gene silencing efficiency. The gene knock-down level in vitro was positively correlated to the weight ratio of DOTAP in the particles, and 73% silencing was achieved in the presence of 10% (v/v) serum at 25% (w/w) DOTAP. Optimal properties were found for nanoparticles modified with 15% (w/w) DOTAP, which reduced the gene expression with 54%. This formulation was spray-dried with mannitol into nanocomposite microparticles of an aerodynamic size appropriate for lung deposition. The spray-drying process did not affect the physicochemical properties of the readily re-dispersible nanoparticles, and most importantly, the in vitro gene silencing activity was preserved during spray-drying. The siRNA content in the powder was similar to the theoretical loading and the siRNA was intact, suggesting that the siRNA is preserved during the spray-drying process. Finally, X-ray powder diffraction analysis demonstrated that mannitol remained in a crystalline state upon spray-drying with PLGA nanoparticles suggesting that the sugar excipient might exert its stabilizing effect by sterical inhibition of the interactions between adjacent nanoparticles. This study demonstrates that spray-drying is an excellent technique for engineering dry powder formulations of siRNA nanoparticles, which might enable the local delivery of biologically active siRNA directly to the lung tissue.

Biodistribution of small interfering RNA at the organ and cellular levels after lipid nanoparticle-mediated delivery

Chemically stabilized small interfering RNA (siRNA) can be delivered systemically by intravenous injection of lipid nanoparticles (LNPs) in rodents and primates. The biodistribution and kinetics of LNP-siRNA delivery in mice at organ and cellular resolution have been studied using immunofluorescence (IF) staining and quantitative polymerase chain reaction (qPCR). At 0.5 and 2 hr post tail vein injection of Cy5-labeled siRNA encapsulated in LNP, the organ rank-order of siRNA levels is liver > spleen > kidney, with only negligible accumulation in duodenum, lung, heart, and brain. Similar conclusions were drawn by using qPCR to measure tissue siRNA levels as a secondary end point. siRNA levels in these tissues decreased by more than 10-fold after 24 hr. Within the liver, LNPs delivered siRNA to hepatocytes, Kupffer cells, and sinusoids in a time-dependent manner, as revealed by IF staining and signal quantitation methods established using OPERA/Columbus software. siRNA first accumulated in liver sinusoids and trafficked to hepatocytes by 2 hr post dose, corresponding to the onset of target mRNA silencing. Fluorescence in situ hybridization methods were used to detect both strands of siRNA in fixed tissues. Collectively, the authors have implemented a platform to evaluate biodistribution of siRNA across cell types and across tissues in vivo, with the objective of elucidating the pharmacokinetic and pharmacodynamic relationship to guide optimization of delivery vehicles.

Screening of siRNA nanoparticles for delivery to airway epithelial cells using high-content analysis

Aims: Delivery of siRNA to the lungs via inhalation offers a unique opportunity to develop a new treatment paradigm for a range of respiratory conditions. However, progress has been greatly hindered by safety and delivery issues. This study developed a high-throughput method for screening novel nanotechnologies for pulmonary siRNA delivery. Methodology: Following physicochemical analysis, the ability of PEI–PEG–siRNA nanoparticles to facilitate siRNA delivery was determined using high-content analysis (HCA) in Calu-3 cells. Results obtained from HCA were validated using confocal microscopy. Finally, cytotoxicity of the PEI–PEG–siRNA particles was analyzed by HCA using the Cellomics® multiparameter cytotoxicity assay. Conclusion: PEI–PEG–siRNA nanoparticles facilitated increased siRNA uptake and luciferase knockdown in Calu-3 cells compared with PEI–siRNA.

Biophysical characterization of hyper-branched polyethylenimine-graft-polycaprolactone-block-mono-methoxyl-poly(ethylene glycol) copolymers (hy-PEI-PCL-mPEG) for siRNA delivery

A library of mono-methoxyl-poly(ethylene glycol)-block-poly(ε-caprolactone) (mPEG-PCL) modified hyperbranched PEI copolymers (hy-PEI-PCL-mPEG) was synthesized to establish structure function relationships for siRNA delivery. These amphiphilic block-copolymers were thought to provide improved colloidal stability and endosomal escape of polyplexes containing siRNA. The influence of the mPEG chain length, PCL segment length, hy-PEI molecular weight and the graft density on their biophysical properties was investigated. In particular, buffer capacity, complex formation constants, gene condensation, polyplex stability, polyplex size and zeta-potential were measured. It was found that longer mPEG chains, longer PCL segments and higher graft density beneficially affected the stability and formation of polyplexes and reduced the zeta-potential of siRNA polyplexes. Significant siRNA mediated knockdown was observed for hy-PEI25k-(PCL900-mPEG2k)(1) at N/P 20 and 30, implying that the PCL hydrophobic segment played a very important role in siRNA transfection. These gene delivery systems merit further investigation under in vivo conditions.

In vitro and in vivo complement activation and related anaphylactic effects associated with polyethylenimine and polyethylenimine-graft-poly(ethylene glycol) block copolymers

Complement activation by polymeric gene and drug delivery systems has been overlooked in the past. As more reports appear in the literature concerning immunogenicity of polymers and their impact on gene expression patterns, it is important to address possible immune side effects of polymers, namely complement activation. Therefore, in this study the activity of low and high molecular weight poly(ethylene imine) and two PEGylated derivatives to induce complement activation were investigated in human serum. These in vitro results revealed that PEI 25 kDa caused significant and concentration dependent complement activation, whereas none of the other polymers induced such effects at their IC(50) concentrations determined by MTT-assays. To verify these in vitro results, additionally, studies were carried out in a swine model after intravenous administration, showing complement activation-related pseudoallergy (CARPA), reflected in symptoms of transient cardiopulmonary distress. Injections of PEI 25 kDa or PEI(25k)-PEG(2k)(10) at a dose of 0.05 and 0.1 mg/kg caused strong reactivity, while PEI 5 kDa and with PEI(25k)-PEG(20k)(1) were also reactogenic at 0.1 mg/kg. It was found that PEI 25 kDa caused both self- and cross-tolerance, whereas the PEG-PEIs were neither self- nor cross-reactively tachyphylactic. As a result of this study, it was shown that PEGylation of polycations with PEG of 20 kDa or higher molecular weight may be favorable. However, potential safety concerns in the development of PEI-based polymeric carriers for drugs and nucleic acids and their translation from bench to bedside need to be taken into consideration for human application.

Multifunctional triblock Nanocarrier (PAMAM-PEG-PLL) for the efficient intracellular siRNA delivery and gene silencing

A novel triblock poly(amido amine)-poly(ethylene glycol)-poly-l-lysine (PAMAM-PEG-PLL) nanocarrier was designed, synthesized, and evaluated for the delivery of siRNA. The design of the nanocarrier is unique and provides a solution to most of the common problems associated with the delivery and therapeutic applications of siRNA. Every component in the triblock nanocarrier plays a significant role and performs multiple functions: (1) tertiary amine groups in the PAMAM dendrimer work as a proton sponge and play a vital role in the endosomal escape and cytoplasmic delivery of siRNA; (2) PEG, a linker connecting PLL and PAMAM dendrimers renders nuclease stability and protects siRNA in human plasma; (3) PLL provides primary amines to form polyplexes with siRNA through electrostatic interaction and also acts as penetration enhancer; and (4) conjugation to PEG and PAMAM reduced toxicity of PLL and the entire triblock nanocarrier PAMAM-PEG-PLL. The data obtained show that the polyplexes resulted from the conjugation of siRNA, and the proposed nanocarriers were effectively taken up by cancer cells and induced the knock down of the target BCL2 gene. In addition, triblock nanocarrier/siRNA polyplexes showed excellent stability in human plasma.

Molecular modeling and in vivo imaging can identify successful flexible triazine dendrimer-based siRNA delivery systems

This study aimed to identify suitable siRNA delivery systems based on flexible generation 2-4 triazine dendrimers by correlating physico-chemical and biological in vitro and in vivo properties of the complexes with thermodynamic parameters calculated using molecular modeling. The siRNA binding properties of the dendrimers and PEI 25 kDa were simulated, binding and stability were measured in SYBR Gold assays, and hydrodynamic diameters, zeta potentials, and cytotoxicity were quantified. These parameters were compared with cellular uptake of the complexes and their ability to mediate RNAi. Radiolabeled complexes were administered intravenously, and pharmacokinetic profiles and biodistribution of these polyplexes were assessed both invasively and non-invasively. All flexible triazine dendrimers formed thermodynamically more stable complexes than PEI. While PEI and the generation 4 dendrimer interacted more superficially with siRNA, generation 2 and 3 virtually coalesced with siRNA, forming a tightly intertwined structure. These dendriplexes were therefore more efficiently charge-neutralized than PEI complexes, reducing agglomeration. This behavior was confirmed by results of hydrodynamic diameters (72.0 nm-153.5 nm) and zeta potentials (4.9 mV-21.8 mV in 10 mM HEPES) of the dendriplexes in comparison to PEI complexes (312.8 nm-480.0 nm and 13.7 mV-17.4 mV in 10 mM HEPES). All dendrimers, even generation 3 and 4, were less toxic than PEI. All dendriplexes were efficiently endocytosed and showed significant and specific luciferase knockdown in HeLa/Luc cells. Scintillation counting confirmed that the generation 2 triazine complexes showed more than twofold prolonged circulation times as a result of their good thermodynamic stability. Conversely, generation 3 complexes dissociated in vivo, and generation 4 complexes were captured by the reticulo-endothelial system due to their increased surface charge. Molecular modeling proves very valuable for rationalizing experimental parameters based on the dendrimers' structural properties. Non-invasive molecular imaging predicted the in vivo fate of the complexes. Therefore, both techniques effectively promote the rapid development of safe and efficient siRNA formulations that are stable in vivo.

Oligonucleotide delivery in cancer therapy

IMPORTANCE OF THE FIELD

Cancer is frequently caused by altered protein expression. Oligonucleotides (ONs) are short synthetic nucleic acid fragments, able to selectively correct protein expression into cells by different mechanisms. However, biological barriers hamper the therapeutic use of ONs without suitable delivery strategies.

AREAS COVERED IN THIS REVIEW

This review summarizes the most meaningful non-viral strategies for ON delivery, including the chemical modifications of the ON backbone and non-viral delivery systems.

WHAT THE READER WILL GAIN

The reader will gain an update of the main strategies for ON delivery in cancer. Advantages and limits of each approach are underlined. Emphasis is given to the delivery strategies that contributed to bringing ONs into clinical trials.

TAKE HOME MESSAGE

In the long story of ONs for cancer therapy, the development of delivery strategies has led, in the last few years, to different opportunities to use the high therapeutic potential of these molecules in humans.

Smart siRNA delivery systems based on polymeric nanoassemblies and nanoparticles

RNA interference is a post-transcriptional gene-silencing pathway induced by double-stranded small interfering RNA (siRNA). The potential use of siRNA as a therapeutic agent has attracted great attention as a novel approach to the treatment of several intractable diseases. Despite the rapid progress in the therapeutic use of siRNA, systemic application is still controversial due to the limitations of siRNA, such as low enzymatic tolerability, cellular internalization and body distribution after systemic administration. This review describes the recent progress and strategies of siRNA delivery systems based on polyion complexes. Numerous siRNA-containing polyion complex systems bound together through electrostatic interactions between the negatively charged siRNA and positively charged components, including synthetic polymers, biopolymers and nanoparticles, have been developed for the therapeutic application of siRNA. Additionally, stimulus-sensitive smart siRNA carrier systems, including bioreducible polycations and hydrophilic polymer-siRNA conjugates, have been developed to enhance the gene-silencing efficacy of siRNAs.

Polymer-related off-target effects in non-viral siRNA delivery

Since off-target effects in non-viral siRNA delivery are quite common but not well understood, in this study various polymer-related effects observed in transfection studies were described and their mechanisms of toxicity were investigated. A variety of stably luciferase-expressing cell lines was compared concerning polymer-mediated effects after transfection with polyplexes of siRNA and poly(ethylene imine) (PEI) or poly(ethylene glycol)-grafted PEI (PEG-PEI). Cell viability, LDH release, gene expression profiles of apoptosis-related genes and promoter activation were investigated. Interestingly, PEG-PEI, which is generally better tolerated than PEI, was found to activate apoptosis in a cell line- and concentration-dependent manner. While both polymers showed sigmoidal dose-response of cell viability in L929 cells (IC(50)(PEI) = 6 μg/ml, IC(50)(PEG-PEI) = 11 μg/ml), H1299/Luc cells exhibited biphasic dose-response for PEG-PEI and stronger apoptosis at 2 μg/ml than at 20 μg/ml PEG-PEI, as shown in TUNEL assays. Gene expression profiling confirmed that H1299/Luc cells underwent apoptosis via thousand-fold activation of TNF receptor-associated factors. Additionally, it was demonstrated that NFkB-mediated CMV promoter activation in stably transfected cells can lead to increased target gene levels after transfection instead of siRNA-mediated knockdown. With these results, polymeric vectors were shown not to be inert substances. Therefore, alterations in gene expression caused by the delivery agent must be known to correctly interpret gene-silencing experiments, to understand the mechanisms of off-target effects, and most of all to further develop vectors with reduced side effects. Taking these observations into account, one established cell line was eventually identified to be suitable for RNAi experiments. As shown by these experiments, materials that have been used for many years can elicit unexpected off-target effects. Therefore, non-viral vectors must be screened for several levels of toxicity to make them promising candidates.

Triazine dendrimers as nonviral vectors for in vitro and in vivo RNAi: the effects of peripheral groups and core structure on biological activity

A family of triazine dendrimers, differing in their core flexibility, generation number, and surface functionality, was prepared and evaluated for its ability to accomplish RNAi. The dendriplexes were analyzed with respect to their physicochemical and biological properties, including condensation of siRNA, complex size, surface charge, cellular uptake and subcellular distribution, their potential for reporter gene knockdown in HeLa/Luc cells, and ultimately their stability, biodistribution, pharmacokinetics and intracellular uptake in mice after intravenous (iv) administration. The structure of the backbone was found to significantly influence siRNA transfection efficiency, with rigid, second generation dendrimers displaying higher gene knockdown than the flexible analogues while maintaining less off-target effects than Lipofectamine. Additionally, among the rigid, second generation dendrimers, those with either arginine-like exteriors or peripheries containing hydrophobic functionalities mediated the most effective gene knockdown, thus showing that dendrimer surface groups also affect transfection efficiency. Moreover, these two most effective dendriplexes were stable in circulation upon intravenous administration and showed passive targeting to the lung. Both dendriplex formulations were taken up into the alveolar epithelium, making them promising candidates for RNAi in the lung. The ability to correlate the effects of triazine dendrimer core scaffolds, generation number, and surface functionality with siRNA transfection efficiency yields valuable information for further modifying this nonviral delivery system and stresses the importance of only loosely correlating effective gene delivery vectors with siRNA transfection agents.

The modification of siRNA with 3' cholesterol to increase nuclease protection and suppression of native mRNA by select siRNA polyplexes

Polymer-siRNA complexes (siRNA polyplexes) are being actively developed to improve the therapeutic application of siRNA. A major limitation for many siRNA polyplexes, however, is insufficient mRNA suppression. Given that modifying the sense strand of siRNA with 3' cholesterol (chol-siRNA) increases the activity of free nuclease-resistant siRNA in vitro and in vivo, we hypothesized that complexation of chol-siRNA can increase mRNA suppression by siRNA polyplexes. In this study, the characteristics and siRNA activity of self assembled polyplexes formed with chol-siRNA or unmodified siRNA were compared using three types of conventional, positively charged polymers: (i) biodegradable, cross-linked nanogels (BDNG) (ii) graft copolymers (PEI-PEG), and (iii) linear block copolymers (PLL10-PEG, and PLL50-PEG). Chol-siRNA did not alter complex formation or the resistance of polyplexes to siRNA displacement by heparin but increased nuclease protection by BDNG, PLL10-PEG, and PLL50-PEG polyplexes over polyplexes with unmodified siRNA. Chol-CYPB siRNA increased suppression of native CYPB mRNA in mammary microvascular endothelial cells (MVEC) by BDNG polyplexes (35%) and PLL10-PEG polyplexes (69%) over comparable CYPB siRNA polyplexes but had no effect on PEI-PEG or PLL50-PEG polyplexes. Overall, these results indicate that complexation of chol-siRNA increases nuclease protection and mRNA suppression by select siRNA polyplexes. These results also suggest that polycationic block length is an important factor in increasing mRNA suppression by PLL-PEG chol-siRNA polyplexes in mammary MVEC.

Amine-modified poly(vinyl alcohol)s as non-viral vectors for siRNA delivery: effects of the degree of amine substitution on physicochemical properties and knockdown efficiency

PURPOSE

The objective of this study was to investigate how the degree of amine substitution of amine-modified poly(vinyl alcohol) (PVA) affects complexation of siRNA, protection of siRNA against degrading enzymes, intracellular uptake and gene silencing.

METHODS

A series of DEAPA-PVA polymers with increasing amine density was synthesized by modifying the hydroxyl groups in the PVA backbone with diethylamino propylamine groups using CDI chemistry. These polymers were characterized with regard to their ability to complex and protect siRNA against RNase. Finally, their potential to mediate intracellular uptake and gene silencing in SKOV-luc cells was investigated.

RESULTS

A good correlation between amine density and siRNA complexation as well as protection of siRNA against RNase was found. Consisting solely of tertiary amines, this class of polymer was able to mediate efficient gene silencing when approximately 30% of the hydroxyl groups in the PVA backbone were modified with diethylamino propylamine groups. Polymers with a lower amine density (up to 23%) were inefficient in gene silencing, while increasing the amine density to 48% led to non-specific knockdown effects.

CONCLUSION

DEAPA-PVA polymers were shown to mediate efficient gene silencing and offer a promising platform for further structural modifications.