Enhanced growth inhibition of hepatic multicellular tumor spheroids by lactosylated poly(ethylene glycol)-siRNA conjugate formulated in PEGylated polyplexes

Tumor-Associated Fibroblast-Targeting Nanoparticles for Enhancing Solid Tumor Therapy: Progress and Challenges

Even though nanoparticle drug delivery systems (nanoDDSs) have improved antitumor efficacy by delivering more drugs to tumor sites compared to free and unencapsulated therapeutics, achieving satisfactory distribution and penetration of nanoDDSs inside solid tumors, especially in stromal fibrous tumors, remains challenging. As one of the most common stromal cells in solid tumors, tumor-associated fibroblasts (TAFs) not only promote tumor growth and metastasis but also reduce the drug delivery efficiency of nanoparticles through the tumor's inherent physical and physiological barriers. Thus, TAFs have been emerging as attractive targets, and TAF-targeting nanotherapeutics have been extensively explored to enhance the tumor delivery efficiency and efficacy of various anticancer agents. The purpose of this Review is to opportunely summarize the underlying mechanisms of TAFs on obstructing nanoparticle-mediated drug delivery into tumors and discuss the current advances of a plethora of nanotherapeutic approaches for effectively targeting TAFs.

Bioprinting 3D human cardiac tissue chips using the pin type printer 'microscopic painting device' and analysis for cardiotoxicity

In this study, three-dimensional (3D) cardiac tissue constructed using the pin type bioprinter 'microscopic painting device' and layer-by-layer cell coating technique was confirmed to have drug responsiveness by three different analytical methods for cardiotoxicity assay. Recently, increasing attention has been focused on biofabrication to create biomimetic 3D tissue. Although various tissues can be produced in vitro, there are many issues surrounding the stability and reproducibility of the preparation of 3D tissues. Thus, although many bioprinters have been developed, none can efficiently, reproducibly and precisely produce small 3D tissues (μm-mm order) such as spheroids, which are most commonly used in drug development. The 3D cardiac tissue chips were successfully constructed with a similar number of cells as conventional 2D tissue using a pin type bioprinter, and corresponding drug-induced cardiotoxicities were obtained with known compounds that induce cardiotoxicity. The 3D cardiac tissue chips displayed uniform cell density and completely synchronized electrophysiological properties as compared to 2D tissue. The 3D tissues constructed using a pin type bioprinter as a biofabrication device would be promising tools for cardiotoxicity assay as they are capable of obtaining stable and reproducible data, which cannot be obtained by 2D tissue.

Polymeric vehicles for nucleic acid delivery

Polymeric vehicles are versatile tools for therapeutic gene delivery. Many polymers-when assembled with nucleic acids into vehicles-can protect the cargo from degradation and clearance in vivo, and facilitate its transport into intracellular compartments. Design options in polymer synthesis yield a comprehensive range of molecules and resulting vehicle formulations. These properties can be manipulated to achieve stronger association with nucleic acid cargo and cells, improved endosomal escape, or sustained delivery depending on the application. Here, we describe current approaches for polymer use and related strategies for gene delivery in preclinical and clinical applications. Polymer vehicles delivering genetic material have already achieved significant therapeutic endpoints in vitro and in animal models. From our perspective, with preclincal assays that better mimic the in vivo environment, improved strategies for target specificity, and scalable techniques for polymer synthesis, the impact of this therapeutic approach will continue to expand.

Widespread gene transfer to malignant gliomas with In vitro-to-In vivo correlation

Gene therapy of malignant gliomas has shown a lack of clinical success to date due in part to inability of conventional gene vectors to achieve widespread gene transfer throughout highly disseminated tumor areas within the brain. Here, we demonstrate that newly engineered polymer-based DNA-loaded nanoparticles (DNA-NP) possessing small particle diameters (~50 nm) and non-adhesive surface polyethylene glycol (PEG) coatings efficiently penetrate brain tumor tissue as well as healthy brain parenchyma. Specifically, this brain-penetrating nanoparticle (BPN), following intracranial administration via convection enhanced delivery (CED), provides widespread transgene expression in heathy rodent striatum and an aggressive brain tumor tissue established orthotopically in rats. The ability of BPN to efficiently traverse both tissues is of great importance as the highly invasive glioma cells infiltrated into normal brain tissue are responsible for tumor recurrence. Of note, the transgene expression within the orthotopic tumor tissue occurred preferentially in glioma cells over microglial cells. We also show that three-dimensional (3D) multicellular spheroids established with malignant glioma cells, unlike conventional two-dimensional (2D) cell cultures, serve as an excellent in vitro model reliably predicting gene vector behaviors in vivo. Briefly, DNA-NP possessing greater surface PEG coverage exhibited more uniform and higher-level transgene expression both in the 3D model and in vivo, whereas the trend was opposite in 2D culture. The finding here alerts that gene transfer studies based primarily on 2D cultures should be interpreted with caution and underscores the relevance of 3D models for screening newly engineered gene vectors prior to their in vivo evaluation.

Molecular chaperone prefoldin-assisted biosynthesis of gold nanoparticles with improved size distribution and dispersion

Here we report a novel aspect of molecular chaperone prefoldin (PFD) as a biomaterial in the biocatalytic synthesis of gold nanoparticles (AuNPs) using glycerol dehydrogenase (GLD). We found that PFD could inhibit the aggregation of AuNPs during the biosynthesis, leading to the formation of AuNPs with controlled size distribution.

Optimized delivery of siRNA into 3D tumor spheroid cultures in situ

3D tissue culture provides a physiologically relevant and genetically tractable system for studying normal and malignant human tissues. Despite this, gene-silencing studies using siRNA has proved difficult. In this study, we have identified a cause for why traditional siRNA transfection techniques are ineffective in eliciting gene silencing in situ within 3D cultures and proposed a simple method for significantly enhancing siRNA entry into spheroids/organoids. In 2D cell culture, the efficiency of gene silencing is significantly reduced when siRNA complexes are prepared in the presence of serum. Surprisingly, in both 3D tumour spheroids and primary murine organoids, the presence of serum during siRNA preparation rapidly promotes entry and internalization of Cy3-labelled siRNA in under 2 hours. Conversely, siRNA prepared in traditional low-serum transfection media fails to gain matrigel or spheroid/organoid entry. Direct measurement of CTNNB1 mRNA (encoding β-catenin) from transfected tumour spheroids confirmed a transient but significant knockdown of β-catenin when siRNA:liposome complexes were formed with serum, but not when prepared in the presence of reduced-serum media (Opti-MEM). Our studies suggest a simple modification to standard lipid-based transfection protocols facilitates rapid siRNA entry and transient gene repression, providing a platform for researchers to improve siRNA efficiency in established 3D cultures.

In Situ shRNA Synthesis on DNA-Polylactide Nanoparticles to Treat Multidrug Resistant Breast Cancer

Nanomedicine has shown unprecedented potential for cancer theranostics. Nucleic acid (e.g., DNA and RNA) nanomedicines are of particular interest for combination therapy with chemotherapeutics. However, current nanotechnologies to construct such nucleic acid nanomedicines, which rely on chemical conjugation or physical complexation of nucleic acids with chemotherapeutics, have restrained their clinical translation due to limitations such as low drug loading efficiency and poor biostability. Herein, in situ rolling circle transcription (RCT) is applied to synthesize short hairpin RNA (shRNA) on amphiphilic DNA-polylactide (PLA) micelles. Core-shell PLA@poly-shRNA structures that codeliver a high payload of doxorubicin (Dox) and multidrug resistance protein 1 (MDR1) targeted shRNA for MDR breast cancer (BC) therapy are developed. DNA-PLA conjugates are first synthesized, which then self-assemble into amphiphilic DNA-PLA micelles; next, using the conjugated DNA as a promoter, poly-shRNA is synthesized on DNA-PLA micelles via RCT, generating PLA@poly-shRNA microflowers; and finally, microflowers are electrostatically condensed into nanoparticles using biocompatible and multifunctional poly(ethylene glycol)-grafted polypeptides (PPT-g-PEG). These PLA@poly-shRNA@PPT-g-PEG nanoparticles are efficiently delivered into MDR breast cancer cells and accumulated in xenograft tumors, leading to MDR1 silencing, intracellular Dox accumulation, potentiated apoptosis, and enhanced tumor therapeutic efficacy. Overall, this nanomedicine platform is promising to codeliver anticancer nucleic acid therapeutics and chemotherapeutics.

Multicellular Tumor Spheroids (MCTS) as a 3D In Vitro Evaluation Tool of Nanoparticles

Multicellular tumor spheroid models (MCTS) are often coined as 3D in vitro models that can mimic the microenvironment of tissues. MCTS have gained increasing interest in the nano-biotechnology field as they can provide easily accessible information on the performance of nanoparticles without using animal models. Considering that many countries have put restrictions on animals testing, which will only tighten in the future as seen by the recent developments in the Netherlands, 3D models will become an even more valuable tool. Here, an overview on MCTS is provided, focusing on their use in cancer research as most nanoparticles are tested in MCTS for treatment of primary tumors. Thereafter, various types of nanoparticles-from self-assembled block copolymers to inorganic nanoparticles, are discussed. A range of physicochemical parameters including the size, shape, surface chemistry, ligands attachment, stability, and stiffness are found to influence nanoparticles in MCTS. Some of these studies are complemented by animal studies confirming that lessons from MCTS can in part predict the behaviour in vivo. In summary, MCTS are suitable models to gain additional information on nanoparticles. While not being able to replace in vivo studies, they can bridge the gap between traditional 2D in vitro studies and in vivo models.

Nonviral cancer gene therapy: Delivery cascade and vector nanoproperty integration

Gene therapy represents a promising cancer treatment featuring high efficacy and limited side effects, but it is stymied by a lack of safe and efficient gene-delivery vectors. Cationic polymers and lipid-based nonviral gene vectors have many advantages and have been extensively explored for cancer gene delivery, but their low gene-expression efficiencies relative to viral vectors limit their clinical translations. Great efforts have thus been devoted to developing new carrier materials and fabricating functional vectors aimed at improving gene expression, but the overall efficiencies are still more or less at the same level. This review analyzes the cancer gene-delivery cascade and the barriers, the needed nanoproperties and the current strategies for overcoming these barriers, and outlines PEGylation, surface-charge, size, and stability dilemmas in vector nanoproperties to efficiently accomplish the cancer gene-delivery cascade. Stability, surface, and size transitions (3S Transitions) are proposed to resolve those dilemmas and strategies to realize these transitions are comprehensively summarized. The review concludes with a discussion of the future research directions to design high-performance nonviral gene vectors.

Bulk and nanoscale polypeptide based polyelectrolyte complexes

Polyelectrolyte complexes (PECs) formed using polypeptides have great potential for developing new self-assembled materials, in particular for the development of drug and gene delivery vehicles. This review discusses the latest advancements in PECs formed using polypeptides as the polyanion and/or the polycation in both polyelectrolyte complexes that form bulk materials and block copolymer complexes that form nanoscale assemblies such as PEC micelles and other self-assembled structures. We highlight the importance of secondary structure formation between homogeneous polypeptide complexes, which, unlike PECs formed using other polymers, introduces additional intermolecular interactions in the form of hydrogen bonding, which may influence precipitation over coacervation. However, we still include heterogeneous complexes consisting of polypeptides and other polymers such as nucleic acids, sugars, and other synthetic polyelectrolytes. Special attention is given to complexes formed using nucleic acids as polyanions and polypeptides as polycations and their potential for delivery applications.

Multicellular tumor spheroids: a relevant 3D model for the in vitro preclinical investigation of polymer nanomedicines

Application of 3D multicellular tumor spheroids to the investigation of polymer nanomedicines.

An effective tumor-targeting strategy utilizing hypoxia-sensitive siRNA delivery system for improved anti-tumor outcome

Hypoxia is a feature of most solid tumors, targeting hypoxia is considered as the best validated yet not extensively exploited strategy in cancer therapy. Here, we reported a novel tumor-targeting strategy using a hypoxia-sensitive siRNA delivery system. In the study, 2-nitroimidazole (NI), a hydrophobic component that can be converted to hydrophilic 2-aminoimidazole (AI) through bioreduction under hypoxic conditions, was conjugated to the alkylated polyethyleneimine (bPEI1.8k-C6) to form amphiphilic bPEI1.8k-C6-NI polycations. bPEI1.8k-C6-NI could self-assemble into micelle-like aggregations in aqueous, which contributed to the improved stability of the bPEI1.8k-C6-NI/siRNA polyplexes, resulted in increased cellular uptake. After being transported into the hypoxic tumor cells, the selective nitro-to-amino reduction would cause structural change and elicit a relatively loose structure to facilitate the siRNA dissociation in the cytoplasm, for enhanced gene silencing efficiency ultimately. Therefore, the conflict between the extracellular stability and the intracellular siRNA release ability of the polyplexes was solved by introducing the hypoxia-responsive unit. Consequently, the survivin-targeted siRNA loaded polyplexes shown remarkable anti-tumor effect not only in hypoxic cells, but also in tumor spheroids and tumor-bearing mice, indicating that the hypoxia-sensitive siRNA delivery system had great potential for tumor-targeted therapy.

STATEMENT OF SIGNIFICANCE

Hypoxia is one of the most remarkable features of most solid tumors, and targeting hypoxia is considered as the best validated strategy in cancer therapy. However, in the past decades, there were few reports about using this strategy in the drug delivery system, especially in siRNA delivery system. Therefore, we constructed a hypoxia-sensitive siRNA delivery system utilizing a hypoxia-responsive unit, 2-nitroimidazole, by which the unavoidable conflict between improved extracellular stability and promoted intracellular siRNA release in the same delivery system could be effectively solved, resulting in enhanced siRNA silencing efficiency in tumor cells. To our knowledge, the described work is the first demonstration of a siRNA delivery system using a hypoxia trigger for regulation of siRNA release, which represents a new strategy for tumor-targeted therapy, and it is expected that this meaningful strategy must be widely applied in the future.

Current preclinical small interfering RNA (siRNA)-based conjugate systems for RNA therapeutics

Recent promising clinical results of RNA therapeutics have drawn big attention of academia and industries to RNA therapeutics and their carrier systems. To improve their feasibility in clinics, systemic evaluations of currently available carrier systems under clinical trials and preclinical studies are needed. In this review, we focus on recent noticeable preclinical studies and clinical results regarding siRNA-based conjugates for clinical translations. Advantages and drawbacks of siRNA-based conjugates are discussed, compared to particle-based delivery systems. Then, representative siRNA-based conjugates with aptamers, peptides, carbohydrates, lipids, polymers, and nanostructured materials are introduced. To improve feasibility of siRNA conjugates in preclinical studies, several considerations for the rational design of siRNA conjugates in terms of cleavability, immune responses, multivalent conjugations, and mechanism of action are also presented. Lastly, we discuss lessons from previous preclinical and clinical studies related to siRNA conjugates and perspectives of their clinical applications.

Cancer cell spheroids for screening of chemotherapeutics and drug-delivery systems

Over the last few decades, the most popular platform to perform high-throughput screening for viable anti-neoplastic compounds has been monolayer cell culture. However, cells in monolayer culture lose many of their in vivo characteristics. As a result, this platform provides a limited predictive value in determining the clinical outcome of the compounds of interest. Using a technique known as 3D spheroid culture, may be the answer to this conundrum. Spheroids have been shown to mimic the tissue-like properties of tumors necessary for the proper evaluation of compounds. In this review, production of cancer cell spheroids, utilization of these spheroids in understanding various therapeutic mechanisms and the potential for their use in high-throughput screening of drugs and drug-delivery systems are discussed in detail.

Oligonucleotide conjugates - Candidates for gene silencing therapeutics

The potential therapeutic and diagnostic applications of oligonucleotides (ONs) have attracted great attention in recent years. The capability of ONs to selectively inhibit target genes through antisense and RNA interference mechanisms, without causing un-intended sideeffects has led them to be investigated for various biomedical applications, especially for the treatment of viral diseases and cancer. In recent years, many researchers have focused on enhancing the stability and target specificity of ONs by encapsulating/complexing them with polymers or lipid chains to formulate nanoparticles/nanocomplexes/micelles. Also, chemical modification of nucleic acids has emerged as an alternative to impart stability to ONs against nucleases and other degrading enzymes and proteins found in blood. In addition to chemically modifying the nucleic acids directly, another strategy that has emerged, involves conjugating polymers/peptide/aptamers/antibodies/proteins, preferably to the sense strand (3'end) of siRNAs. Conjugation to the siRNA not only enhances the stability and targeting specificity of the siRNA, but also allows for the development of self-administering siRNA formulations, with a much smaller size than what is usually observed for nanoparticle (∼200nm). This review concentrates mainly on approaches and studies involving ON-conjugates for biomedical applications.

Clustering siRNA conjugates for MMP-responsive therapeutics in chronic wounds of diabetic animals

The MMP-responsive breakdown of siRNA clusters was translated to site-specific gene transfection and enhanced wound healing in diabetic ulcers. MMP-2 siRNA was chemically tethered to the end of multi-armed PEG via MMP-cleavable linkers (4PEG-siRNA) and subsequently clustered into submicron particles complexed with LPEI. 4PEG-siRNA was more tightly complexed with LPEI and the associated cluster showed higher resistance against RNase attack, in comparison to naked siRNA. Because the size of the clusters increased depending on the increase in charge ratio of LPEI to siRNA, cellular uptake of the 4PEG-siRNA/LPEI cluster was significantly attenuated due to the huge size of the cluster. However, upon MMP treatment, the cluster dissociated into smaller particles and was efficiently endocytosed by cells. An in vivo fluorescence resonance energy transfer (FRET) study also revealed that the clusters were effectively dissociated in MMP-rich environments of dorsal wounds in diabetic animals. In addition, diabetic ulcers treated with the clusters showed a faster wound closure rate and the recovered tissue expressed a larger amount of cytokeratin along with a lower expression level of MMP-2 compared to the other groups.

Conjugates of small targeting molecules to non-viral vectors for the mediation of siRNA

To use siRNA (small interfering RNA) for gene therapy, a gene delivery system is often necessary to overcome several challenging requirements including rapid excretion, low stability in blood serum, non-specific accumulation in tissues, poor cellular uptake and inefficient intracellular release. Active and/or passive targeting should help the delivery system to reach the desired tissue or cell, to be internalized, and to deliver siRNA to the cytoplasm so that siRNA can inhibit protein synthesis. This review covers conjugates of small targeting molecules and non-viral delivery systems for the mediation of siRNA, with a focus on their transfection properties in order to help the development of new and efficient siRNA delivery systems, as the therapeutic solutions of tomorrow.

STATEMENT OF SIGNIFICANCE

The delivery of siRNA into cells or tissues remains to be a challenge for its applications, an alternative strategy for siRNA delivery systems is direct conjugation of non-viral vectors with targeting moieties for cellular delivery. In comparison to macromolecules, small targeting molecules have attracted great attention due to their many potential advantages including significant simplicity and ease of production, good repeatability and biodegradability. This review will focus on the most recent advances in the delivery of siRNA using conjugates of small targeting molecules and non-viral delivery systems. Based the editor's suggestions, we hope the revised manuscript could provide more profound understanding to the conjugates of targeting molecules to vectors for mediation of siRNA.

Efficacy of DOPE/DC-cholesterol liposomes and GCPQ micelles as AZD6244 nanocarriers in a 3D colorectal cancer in vitro model

AIM

In this work, we use cationic organic nanocarriers as chemotherapy delivery platforms and test them in a colorectal cancer 3D in vitro model.

MATERIALS & METHODS

We used 3beta-(N-[N',N'-dimethylaminoethane]carbamoyl])cholesterol (DC-chol) and dioleoylphosphatidylethanolamine (DOPE) liposomes and N-palmitoyl-N-monomethyl-N,N-dimethyl-N,N,N-trimethyl-6-O-glycolchitosan (GCPQ) micelles, to deliver AZD6244, a MEK inhibitor, to HCT116 cells cultured as monolayers and in 3D in vitro cancer models (tumoroids).

RESULTS

Nanoparticle-mediated drug delivery was superior to the free drug in monolayer experiments and despite their therapeutic effect being hindered by poor diffusion through the cancer mass, GCPQ micelles were also superior in tumoroids.

CONCLUSION

These results support the role of nanoparticles in improving drug delivery and highlight the need to include 3D cancer models in early phases of drug development.

Evaluation of CdTe/CdS/ZnS core/shell/shell quantum dot toxicity on three-dimensional spheroid cultures

In this work, three-dimensional (3D) spheroid cultures of human adipose-derived mesenchymal stem cells (hAD-MSCs), with tissue-mimetic morphology through well developed cell-cell and cell-matrix interactions and distinct diffusion/transport characteristics, were assessed for dose-dependent toxic effects of red-emitting CdTe/CdS/ZnS quantum dots (Qdots). Morphological investigations and time-resolved microscopy analysis in addition to cell metabolic activity studies revealed that 3D spheroid cultures are more resistant to Qdot-induced cytotoxicity in comparison to conventional 2D cultures. The obtained results suggest the presence of two distinct cell populations in 2D cultures with different sensitivity to Qdots, however that effect wasn't observed in 3D spheroids. Our investigations were aimed to improve the prediction of nanotoxicity of Qdot on tissue-level and provide the essential screening steps prior to any in vivo application. Moreover, penetration ability of highly fluorescent Qdots to densely-packed spheroids will fortify the biological application of developed Qdots in tissue-like structures.

Recent Advances in Targeted, Self-Assembling Nanoparticles to Address Vascular Damage Due to Atherosclerosis

Self-assembling nanoparticles functionalized with targeting moieties have significant potential for atherosclerosis nanomedicine. While self-assembly allows the easy construction (and degradation) of nanoparticles with therapeutic or diagnostic functionality, or both, the targeting agent can direct them to a specific molecular marker within a given stage of the disease. Therefore, supramolecular nanoparticles have been investigated in the last decade as molecular imaging agents or explored as nanocarriers that can decrease the systemic toxicity of drugs by producing accumulation predominantly in specific tissues of interest. In this Progress Report, the pathogenesis of atherosclerosis and the damage caused to vascular tissue are described, as well as the current diagnostic and treatment options. An overview of targeted strategies using self-assembling nanoparticles is provided, including liposomes, high density lipoproteins, protein cages, micelles, proticles, and perfluorocarbon nanoparticles. Finally, an overview is given of current challenges, limitations, and future applications for personalized medicine in the context of atherosclerosis of self-assembling nanoparticles.

Inhibition of atherosclerosis-promoting microRNAs via targeted polyelectrolyte complex micelles

Polyelectrolyte complex micelles have great potential as gene delivery vehicles because of their ability to encapsulate charged nucleic acids forming a core by neutralizing their charge, while simultaneously protecting the nucleic acids from non-specific interactions and enzymatic degradation. Furthermore, to enhance specificity and transfection efficiency, polyelectrolyte complex micelles can be modified to include targeting capabilities. Here, we describe the design of targeted polyelectrolyte complex micelles containing inhibitors against dys-regulated microRNAs (miRNAs) that promote atherosclerosis, a leading cause of human mortality and morbidity. Inhibition of dys-regulated miRNAs in diseased cells associated with atherosclerosis has resulted in therapeutic efficacy in animal models and has been proposed to treat human diseases. However, the non-specific targeting of microRNA inhibitors via systemic delivery has remained an issue that may cause unwanted side effects. For this reason, we incorporated two different peptide sequences to our miRNA inhibitor containing polyelectrolyte complex micelles. One of the peptides (Arginine-Glutamic Acid-Lysine-Alanine or REKA) was used in another micellar system that demonstrated lesion-specific targeting in a mouse model of atherosclerosis. The other peptide (Valine-Histidine-Proline-Lysine-Glutamine-Histidine-Arginine or VHPKQHR) was identified via phage display and targets vascular endothelial cells through the vascular cell adhesion molecule-1 (VCAM-1). In this study we have tested the in vitro efficacy and efficiency of lesion- and cell-specific delivery of microRNA inhibitors to the cells associated with atherosclerotic lesions via peptide-targeted polyelectrolyte complex micelles. Our results show that REKA-containing micelles (fibrin-targeting) and VHPKQHR-containing micelles (VCAM-1 targeting) can be used to carry and deliver microRNA inhibitors into macrophages and human endothelial cells, respectively. Additionally, the functionality of miRNA inhibitors in cells was demonstrated by analyzing miRNA expression as well as the expression or the biological function of its downstream target protein. Our study provides the first demonstration of targeting dys-regulated miRNAs in atherosclerosis using targeted polyelectrolyte complex micelles and holds promising potential for translational applications.

Multicellular tumor spheroids for evaluation of cytotoxicity and tumor growth inhibitory effects of nanomedicines in vitro: a comparison of docetaxel-loaded block copolymer micelles and Taxotere®

While 3-D tissue models have received increasing attention over the past several decades in the development of traditional anti-cancer therapies, their potential application for the evaluation of advanced drug delivery systems such as nanomedicines has been largely overlooked. In particular, new insight into drug resistance associated with the 3-D tumor microenvironment has called into question the validity of 2-D models for prediction of in vivo anti-tumor activity. In this work, a series of complementary assays was established for evaluating the in vitro efficacy of docetaxel (DTX) -loaded block copolymer micelles (BCM+DTX) and Taxotere® in 3-D multicellular tumor spheroid (MCTS) cultures. Spheroids were found to be significantly more resistant to treatment than monolayer cultures in a cell line dependent manner. Limitations in treatment efficacy were attributed to mechanisms of resistance associated with properties of the spheroid microenvironment. DTX-loaded micelles demonstrated greater therapeutic effect in both monolayer and spheroid cultures in comparison to Taxotere®. Overall, this work demonstrates the use of spheroids as a viable platform for the evaluation of nanomedicines in conditions which more closely reflect the in vivo tumor microenvironment relative to traditional monolayer cultures. By adaptation of traditional cell-based assays, spheroids have the potential to serve as intermediaries between traditional in vitro and in vivo models for high-throughput assessment of therapeutic candidates.

Bioresponsive Polyplexes and Micelleplexes

The delivery of nucleic acids (NAs) is hindered by several factors, such as the size of the biomolecule (micron size for plasmid DNA), the presence of different biological barriers or the degradation of NAs. Most of these limitations are avoided by complexation with polycationic species, which collapse NAs into nanometer-sized polyplexes that can be efficiently internalized into the target cells. Because there are subtle changes in physiological conditions, such as the drop in pH at the endosome, or the increase in temperature in tumor tissue, stimuli responsive synthetic polymers are ideal candidates for the synthesis of efficient gene delivery vehicles. In this chapter, representative examples of “smart” polypexes that exploit these changes in physiological environment for the delivery of NAs are described, and the transfection efficiency of pH-, redox-, temperature- and light-responsive polyplexes is analyzed.

Opportunities and challenges for use of tumor spheroids as models to test drug delivery and efficacy

Multicellular spheroids are three dimensional in vitro microscale tissue analogs. The current article examines the suitability of spheroids as an in vitro platform for testing drug delivery systems. Spheroids model critical physiologic parameters present in vivo, including complex multicellular architecture, barriers to mass transport, and extracellular matrix deposition. Relative to two-dimensional cultures, spheroids also provide better target cells for drug testing and are appropriate in vitro models for studies of drug penetration. Key challenges associated with creation of uniformly sized spheroids, spheroids with small number of cells and co-culture spheroids are emphasized in the article. Moreover, the assay techniques required for the characterization of drug delivery and efficacy in spheroids and the challenges associated with such studies are discussed. Examples for the use of spheroids in drug delivery and testing are also emphasized. By addressing these challenges with possible solutions, multicellular spheroids are becoming an increasingly useful in vitro tool for drug screening and delivery to pathological tissues and organs.

Nanoscale drug delivery systems for enhanced drug penetration into solid tumors: current progress and opportunities

Poor penetration of anticancer drags into solid tumors significantly limits their efficacy. This phenomenon has long been observed for small-molecule chemotherapeutics, and it can be even more pronounced for nanoscale therapies. Nanoparticles have enormous potential for the treatment of cancer due to their wide applicability as drug delivery and imaging vehicles and their size-dependent accumulation into solid tumors by the enhanced permeability and retention (EPR) effect. Further, synthetic nanoparticles can be engineered to overcome barriers to drag delivery. Despite their promise for the treatment of cancer, relatively little work has been done to study and improve their ability to diffuse into solid tumors following passive accumulation in the tumor vasculature. In this review, we present the complex issues governing efficient penetration of nanoscale therapies into solid tumors. The current methods available to researchers to study nanoparticle penetration into malignant tumors are described, and the most recent works studying the penetration of nanoscale materials into solid tumors are summarized. We conclude with an overview of the important nanoparticle design parameters governing their tumor penetration, as well as by highlighting critical directions in this field.

Cellular uptake and intracellular trafficking of antisense and siRNA oligonucleotides

Significant progress is being made concerning the development of oligonucleotides as therapeutic agents. Studies with antisense, siRNA, and other forms of oligonucleotides have shown promise in cellular and animal models and in some clinical studies. Nonetheless, our understanding of how oligonucleotides function in cells and tissues is really quite limited. One major issue concerns the modes of uptake and intracellular trafficking of oligonucleotides, whether as "free" molecules or linked to various delivery moieties such as nanoparticles or targeting ligands. In this review, we examine the recent literature on oligonucleotide internalization and subcellular trafficking in the context of current insights into the basic machinery for endocytosis and intracellular vesicular traffic.

Strategies and advances in nanomedicine for targeted siRNA delivery

siRNA are a rapidly emerging class of new therapeutic molecules for the treatment of inherited and acquired diseases. However, poor cellular uptake and instability in physiological conditions limits its therapeutic potential, hence a need to develop a delivery system that can protect and efficiently transport siRNA to the target cells has arisen. Nanoparticles have been proposed as suitable delivery vectors with reduced cytotoxicity and enhanced efficacy. These delivery vectors form condensed complexes with siRNA which, in turn, provides protection to siRNA against enzymatic degradation and further leads to tissue and cellular targeting. Nanoparticles derived from polymers, such as chitosan and polyethylenimine have found numerous applications owing to ease of manipulation, high stability, low cost and high gene carrying capability. This article focuses on various aspects of nanomedicine based siRNA delivery with emphasis on targeted delivery to tumors.

Binding and transport of PAMAM-RGD in a tumor spheroid model: the effect of RGD targeting ligand density

The mechanisms governing the efficient tumor spheroid penetration and transport by poly(amidoamine) (PAMAM) dendrimers displaying varying numbers of cyclic RGD targeting peptides (2, 3, 7, or 10) were evaluated in this work. The cell-free binding affinities and cellular internalization kinetics of PAMAM-RGD conjugates to malignant glioma cells were determined experimentally, and the results were incorporated into a mathematical model to predict the transport of these materials through a multicellular tumor spheroid. The theoretical analysis demonstrated that greater RGD crosslinking may improve transport through tumor spheroids due to their decreased integrin-binding affinity. This study provides evidence that altering the density of tumor-targeting ligands from a drug delivery platform is a feasible way to optimize the tumor-penetration efficiency of an anticancer agent, and provides insight into the physicochemical mechanisms governing the relative effectiveness of these conjugates.

Self-assembled and nanostructured siRNA delivery systems

A wide range of organic and inorganic materials have been used in the development of nano-scale self-assembling gene delivery systems to improve the therapeutic efficacy of nucleic acid drugs. Small interfering RNA (siRNA) has recently been recognized as a promising and potent nucleic acid medicine for the treatment of incurable genetic disorders including cancer; however, siRNA-based therapeutics suffer from the same delivery problems as conventional nucleic acid drugs such as plasmid DNA and antisense oligonucleotides. Many of the delivery strategies developed for nucleic acid drugs have been applied to siRNA therapeutics, but they have not produced satisfactory in vivo gene silencing efficiencies to warrant clinical trials. This review discusses recent progress in the development of self-assembled and nanostructured delivery systems for efficient siRNA-induced gene silencing and their potential application in clinical settings.

Lipid and polymeric carrier-mediated nucleic acid delivery

IMPORTANCE OF THE FIELD

Nucleic acids such as plasmid DNA, antisense oligonucleotide, and RNA interference (RNAi) molecules, have a great potential to be used as therapeutics for the treatment of various genetic and acquired diseases. To design a successful nucleic acid delivery system, the pharmacological effect of nucleic acids, the physiological condition of the subjects or sites, and the physicochemical properties of nucleic acid and carriers have to be thoroughly examined.

AREAS COVERED IN THIS REVIEW

The commonly used lipids, polymers and corresponding delivery systems are reviewed in terms of their characteristics, applications, advantages and limitations.

WHAT THE READER WILL GAIN

This article aims to provide an overview of biological barriers and strategies to overcome these barriers by properly designing effective synthetic carriers for nucleic acid delivery.

TAKE HOME MESSAGE

A thorough understanding of biological barriers and the structure-activity relationship of lipid and polymeric carriers is the key for effective nucleic acid therapy.

Chemical modification of small interfering RNA

Chemically synthesized siRNAs are widely used for gene silencing. For in vitro applications, stability, delivery, and immunological issues are rarely problematic, but for in vivo applications the situation is different. Limited stability, undesirable pharmacokinetic behaviour, and unanticipated side effects from the immune system call for more careful structural siRNA design and inclusion of chemical modifications at selected positions. Also the notion that siRNA induces significant off-target silencing of many non-related genes has promted new effective measures to enhance specificity. The scope of this review is to provide a simple guide to successful chemical and structural modification of siRNAs with improved activity, stability, specificity, and low toxicity.

RNA interference for improving the outcome of islet transplantation

Islet transplantation has the potential to cure type 1 diabetes. Despite recent therapeutic success, it is still not common because a large number of transplanted islets get damaged by multiple challenges including instant blood mediated inflammatory reaction, hypoxia/reperfusion injury, inflammatory cytokines, and immune rejection. RNA interference (RNAi) is a novel strategy to selectively degrade target mRNA. The use of RNAi technologies to downregulate the expression of harmful genes has the potential to improve the outcome of islet transplantation. The aim of this review is to gain a thorough understanding of biological obstacles to islet transplantation and discuss how to overcome these barriers using different RNAi technologies. This eventually will help improve islet survival and function post transplantation. Chemically synthesized small interferring RNA (siRNA), vector based short hairpin RNA (shRNA), and their critical design elements (such as sequences, promoters, and backbone) are discussed. The application of combinatorial RNAi in islet transplantation is also discussed. Last but not the least, several delivery strategies for enhanced gene silencing are discussed, including chemical modification of siRNA, complex formation, bioconjugation, and viral vectors.

The relationship between terminal functionalization and molecular weight of a gene delivery polymer and transfection efficacy in mammary epithelial 2-D cultures and 3-D organotypic cultures

Non-viral gene delivery vectors were developed for efficient gene transfer to hard-to-transfect mouse mammary epithelial cells. Ten modified versions of the same base poly(beta-amino ester), poly(1,4-butanediol diacrylate-co-5-amino-1-pentanol), were tested in both traditional 2-D monolayer and in 3-D organotypic cultures. The polymers self-assembled with plasmid DNA encoding enhanced green fluorescent protein to form nanoparticles (approximately 100 nm) used to transfect the cells. Nanoparticle transfection efficacy was tuned by changes in synthesis and fabrication conditions and the transfection efficacy was analyzed using confocal microscopy and flow cytometry. The best performing polymeric nanoparticles transfected 57 +/- 6% of the cells in 2-D culture and 6 +/- 1% of the cells in 3-D culture. Small modifications to the polymer end-capping molecules and tuning of polymer molecular weight could either significantly enhance the transfection efficacy up to 6-fold or instead abolish efficacy completely. The efficacy of leading polymers was higher than that of the commercial transfection agent FuGENE HD by a factor of 13 in 2-D and 2 in 3-D. These non-viral nanoparticles may be useful as delivery reagents or targeted therapeutics for breast cancer. This gene delivery strategy is also a promising approach for studying the normal development of the mammary gland.

Stimuli-responsive smart nanogels for cancer diagnostics and therapy

This article discusses stimuli-responsive poly(ethylene glycol) (PEG)-coated (PEGylated) nanogels and their biomedical applications. Preparation and characterization of stimuli-responsive PEGylated nanogels composed of a crosslinked poly(2-[N,N-diethylamino]ethyl methacrylate) (PEAMA) core and PEG tethered chains are initially described. Stimuli-responsive PEGylated nanogels show unique properties and functions in synchronizing with the reversible volume phase transition of the PEAMA core in response to the extracellular pH (7-6.5) of a tumor environment as well as endosomal/lysosomal pH (6.5-5.0) and temperature. We list several biomedical applications of stimuli-responsive PEGylated nanogels, including (19)F magnetic resonance spectroscopic imaging (MRS/I) probe to visualize acidosis (tumor tissue), intracellular drug and siRNA delivery, antennas for cancer photothermal therapy and apoptosis probe for monitoring response to cancer therapy. Thus, stimuli-responsive PEGylated nanogels can be utilized as smart nanomedicines for cancer diagnostics and therapy.

The suppression of hypoxia-inducible factor and vascular endothelial growth factor by siRNA does not affect the radiation sensitivity of multicellular tumor spheroids

BACKGROUND

The hypoxic microenvironment is closely associated with the radiation resistance of tumor cells. Hypoxia induces several genes such as hypoxia-inducible factor (HIF-1) and vascular endothelial growth factor (VEGF) to promote tumor cell growth and survival. The up-regulated expression levels of HIF-1 and VEGF in tumor cells also correlate with their resistance to radiation, suggesting that these genes are potential therapeutic targets for strategies designed to enhance radiation effects. To further investigate this possibility, we investigated the effects of suppressing these genes upon the radiation sensitivity of cancer cells. We conducted these experiments using multicellular spheroids as a three-dimensional in vitro tumor model and RNA interference as the method of gene suppression.

MATERIAL AND METHODS

SQ5 human lung carcinoma cells were treated with HIF-1/VEGF siRNA and/or radiation. Reversed transfection methods were employed for the spheroids. Gene expression was analyzed using quantitative RT-PCR and western blotting. Cell toxicity was qualified by colony formation assay.

RESULTS

Compared with monolayer cells, spheroids showed up-regulated expression of HIF-1 and increased radiation resistance. Hypoxic conditions elevated the expression of HIF-1 and VEGF and enhanced the surviving fraction of spheroids after exposure to radiation. However, when the expression of HIF-1 and VEGF was down-regulated by transfection of targeting siRNA, this did not influence the cytotoxic effects of the radiation under either normoxic or hypoxic conditions.

CONCLUSIONS

We have established a method to transfect siRNA into spheroid cells. Our current data indicate that the functions of HIF-1 or VEGF are independent of radiation sensitivity in spheroids under either normoxic or hypoxic conditions.

PAMAM-RGD conjugates enhance siRNA delivery through a multicellular spheroid model of malignant glioma

Generation 5 poly(amidoamine) (PAMAM) dendrimers were modified by the addition of cyclic RGD targeting peptides and were evaluated for their ability to associate with siRNA and mediate siRNA delivery to U87 malignant glioma cells. PAMAM-RGD conjugates were able to complex with siRNA to form complexes of approximately 200 nm in size. Modest siRNA delivery was observed in U87 cells using either PAMAM or PAMAM-RGD conjugates. PAMAM-RGD conjugates prevented the adhesion of U87 cells to fibrinogen-coated plates, in a manner that depends on the number of RGD ligands per dendrimer. The delivery of siRNA through three-dimensional multicellular spheroids of U87 cells was enhanced using PAMAM-RGD conjugates compared to the native PAMAM dendrimers, presumably by interfering with integrin-ECM contacts present in a three-dimensional tumor model.

Nanotechnologies and controlled release systems for the delivery of antisense oligonucleotides and small interfering RNA

Antisense oligonucleotides and small interfering RNA have enormous potential for the treatment of a number of diseases, including cancer. However, several impediments to their widespread use as drugs still have to be overcome: in particular their lack of stability in physiological fluids and their poor penetration into cells. Association with or encapsulation within nano- and microsized drug delivery systems could help to solve these problems. In this review, we describe the progress that has been made using delivery systems composed of natural or synthetic polymers in the form of complexes, nanoparticles or microparticles.

Targeted delivery systems for oligonucleotide therapeutics

Oligonucleotides including antisense oligonucleotides and siRNA are emerging as promising therapeutic agents against a variety of diseases. Effective delivery of these molecules is critical to their successful clinical application. Targeted systems can greatly improve the efficiency and specificity of oligonucleotides delivery. Meanwhile, an effective delivery system must successfully overcome a multitude of biological barriers to enable the oligonucleotides to reach the site of action and access their biological targets. Several delivery strategies based on different platform technologies and different targeting ligands have been developed to achieve these objectives. This review aims at providing a summary and perspective on recent progress in this very active area of research.