The Effects of Spacer Length and Composition on Aptamer-Mediated Cell-Specific Targeting with Nanoscale PEGylated Liposomal Doxorubicin

Effects of PEG-Linker Chain Length of Folate-Linked Liposomal Formulations on Targeting Ability and Antitumor Activity of Encapsulated Drug

Introduction

Ligand-conjugated liposomes are promising for the treatment of specific receptor-overexpressing cancers. However, previous studies have shown inconsistent results because of the varying properties of the ligand, presence of a polyethylene glycol (PEG) coating on the liposome, length of the linker, and density of the ligand.

Methods

Here, we prepared PEGylated liposomes using PEG-linkers of various lengths conjugated with folate and evaluated the effect of the PEG-linker length on the nanoparticle distribution and pharmacological efficacy of the encapsulated drug both in vitro and in vivo.

Results

When folate was conjugated to the liposome surface, the cellular uptake efficiency in folate receptor overexpressed KB cells dramatically increased compared to that of the normal liposome. However, when comparing the effect of the PEG-linker length in vitro, no significant difference between the formulations was observed. In contrast, the level of tumor accumulation of particles in vivo significantly increased when the length of the PEG-linker was increased. The tumor size was reduced by >40% in the Dox/FL-10K-treated group compared to that in the Dox/FL-2K- or 5K-treated groups.

Discussion

Our study suggests that as the length of PEG-linker increases, the tumor-targeting ability can be enhanced under in vivo conditions, which can lead to an increase in the antitumor activity of the encapsulated drug.

Multivalent Aptamer Approach: Designs, Strategies, and Applications

Aptamers are short and single-stranded DNA or RNA molecules with highly programmable structures that give them the ability to interact specifically with a large variety of targets, including proteins, cells, and small molecules. Multivalent aptamers refer to molecular constructs that combine two or more identical or different types of aptamers. Multivalency increases the avidity of aptamers, a particularly advantageous feature that allows for significantly increased binding affinities in comparison with aptamer monomers. Another advantage of multivalency is increased aptamer stabilities that confer improved performances under physiological conditions for various applications in clinical settings. The current study aims to review the most recent developments in multivalent aptamer research. The review will first discuss structures of multivalent aptamers. This is followed by detailed discussions on design strategies of multivalent aptamer approaches. Finally, recent developments of the multivalent aptamer approach in biosensing and biomedical applications are highlighted.

New insight into the aptamer conformation and aptamer/protein interaction by surface-enhanced Raman scattering and multivariate statistical analysis

We study the interaction between one aptamer and its analyte (the MnSOD protein) by the combination of surface-enhanced Raman scattering and multivariate statistical analysis. We observe the aptamer structure and its evolution during the interaction under different experimental conditions (in air or in buffer). Through the spectral treatment by principal component analysis of a large set of SERS data, we were able to probe the aptamer conformations and orientations relative to the surface assuming that the in-plane nucleoside modes are selectively enhanced. We demonstrate that the aptamer orientation and thus its flexibility rely strongly on the presence of a spacer of 15 thymines and on the experimental conditions with the aptamer lying on the surface in air and standing in the buffer. We reveal for the first time that the interaction with MnSOD induces a large loss of flexibility and freezes the aptamer structure in a single conformation.

Lipoplexes to Deliver Oligonucleotides in Gram-Positive and Gram-Negative Bacteria: Towards Treatment of Blood Infections

Bacterial resistance to antibiotics threatens the ability to treat life-threatening bloodstream infections. Oligonucleotides (ONs) composed of nucleic acid mimics (NAMs) able to inhibit essential genes can become an alternative to traditional antibiotics, as long as they are safely transported in human serum upon intravenous administration and they are carried across the multilayered bacterial envelopes, impermeable to ONs. In this study, fusogenic liposomes were considered to transport the ONs and promote their internalization in clinically relevant bacteria. Locked nucleic acids and 2′-OMethyl RNA were evaluated as model NAMs and formulated into DOTAP–DOPE liposomes followed by post-PEGylation. Our data showed a complexation stability between the post-PEGylated liposomes and the ONs of over 82%, during 24 h in native human serum, as determined by fluorescence correlation spectroscopy. Quantification by a lipid-mixing assay showed that liposomes, with and without post-PEGylation, fused with all bacteria tested. Such fusion promoted the delivery of a fraction of the ONs into the bacterial cytosol, as observed by fluorescence in situ hybridization and bacterial fractionation. In short, we demonstrated for the first time that liposomes can safely transport ONs in human serum and intracellularly deliver them in both Gram-negative and -positive bacteria, which holds promise towards the treatment of bloodstream infections.

Anisotropically Functionalized Aptamer-DNA Nanostructures for Enhanced Cell Proliferation and Target-Specific Adhesion in 3D Cell Cultures

The development of supramolecular hydrogel scaffolds for the precise positioning of biochemical cues is paramount for applications such as tissue engineering. Nucleic acid engineering allows fabrication of three-dimensional (3D) nanostructures with high variability and nanoscale precision. In this study, aptamers were anisotropically functionalized onto branched DNA nanostructures to control their cell adhesion capability, and their efficiency as biological signal inducers for 3D cell cultivation was investigated. Each arm of the X-shaped DNA nanostructure (X-DNA) was functionalized with photo-cross-linkable or cell adhesion moieties, and the steric hindrance of the 3D DNA nanostructures on a cell was optimized. X-DNA nanostructures with cell-positioning parameters were rapidly photopolymerized to form hybrid hydrogels, and their effects on cell behaviors and positions were investigated. We observed that aptamer-functionalized X-DNA nanostructures exhibited significantly enhanced cell proliferation and provided homogeneous distribution and target-specific adhesion of encapsulated cells within hydrogel matrices. Overall, the anisotropic functionalization of DNA nanostructures provides a controllable function for the advancement of conventional 3D culture platforms.

The use of aptamers in prostate cancer: A systematic review of theranostic applications

Since prostate cancer (PCa) relies on limited diagnosis and therapies, more effective alternatives are needed. Aptamers are versatile tools that may be applied for better clinical management of PCa patients. This review shows the trends on aptamer-based applications for PCa to understand their future development. We searched articles reporting aptamers applied in PCa on the Pubmed, Scopus and Web of Science databases over the last decade. Almost 80% of the articles used previously selected aptamers in novel approaches. However, cell-SELEX was the most applied technique for the selection of new aptamers allowing their binding to targets in their native configuration. ssDNA aptamers were 24% more common than RNA aptamers. The most studied PCa-specific aptamers were the DNA PSA-specific aptamer PSap4#5 and the PSMA-specific RNA aptamers A10 and A9, being PSA and PSMA the most reported targets. Thus, researchers still prefer the ease of use of DNA aptamers. Blood-based liquid biopsies represented 24% of all samples, being the most promising clinical samples. Especially noteworthy, electro-analytical methods accounted for more than 40% of the diagnostic techniques and treatment approaches with drug delivery systems or transcriptional modifiers were reported in 70% of the articles. Although all these articles showed clinically relevant aptamers for PCa and there are good prospects for their use, the development of all these strategies was in its early stages. Thus, the aptamers are not completely validated and we foresee that the completion of clinical studies will allow the implementation of these aptamer-based technologies in the clinical practice of PCa.

Protein corona: Dr. Jekyll and Mr. Hyde of nanomedicine

Nanomedicine is an interdisciplinary field of research, comprising science, engineering, and medicine. Many are the clinical applications of nanomedicine, such as molecular imaging, medical diagnostics, targeted therapy, and image-guided surgery. Despite major advances during the past 20 years, many efforts must be done to understand the complex behavior of nanoparticles (NPs) under physiological conditions, the kinetic and thermodynamic principles, involved in the rational design of NP. Once administrated in physiological environment, NPs interact with biomolecules and they are surrounded by protein corona (PC) or biocorona. PC can trigger an immune response, affecting NPs toxicity and targeting capacity. This review aims to provide a detailed description of biocorona and of parameters that are able to control PC formation and composition. Indeed, the review provides an overview about the role of PC in the modulation of both cytotoxicity and immune response as well as in the control of targeting capacity.

Study on polyethylene glycol cross-linker in peptide-conjugated antibody on efficiency of cell capture and release

Cell separation is important in cell therapy and disease diagnosis. Therefore, various cell separation methods have been studied, but cellular damage and the need for pretreatment remain substantial problems. Recently, in the diagnostic field, the detachment and recovery of antibody-captured cells was actively studied to obtain more detailed information on cancer cells. Previously, we have developed a highly efficient cell separation method using microfibers. In the present study, the efficiency of cell capture and release was examined by controlling the molecular mobility of an immobilized antibody to efficiently detect cells with low expression of a marker molecule. We found that improvement in molecular mobility of antibodies enhances cell capture efficiency but decreases the detachment effectiveness of the captured cells. Therefore, the molecular mobility of antibodies can be utilized to control cell capture and release according to the level of expression of the marker molecule.

Aptamer-Based Targeted Drug Delivery Systems: Current Potential and Challenges

Aptamers are single-stranded DNA or RNA with 20-100 nucleotides in length that can specifically bind to target molecules via formed three-dimensional structures. These innovative targeting molecules have attracted an increasing interest in the biomedical field. Compared to traditional protein antibodies, aptamers have several advantages, such as small size, high binding affinity, specificity, good biocompatibility, high stability and low immunogenicity, which all contribute to their wide application in the biomedical field. Aptamers can bind to the receptors on the cell membrane and mediate themselves or conjugated nanoparticles to enter into cells. Therefore, aptamers can be served as ideal targeting ligands for drug delivery. Since their excellent properties, different aptamer-mediated drug delivery systems had been developed for cancer therapy. This review provides a brief overview of recent advances in drug delivery systems based on aptamers. The advantages, challenges and future prospectives are also discussed.

Nucleic Acid Aptamer: A Novel Potential Diagnostic and Therapeutic Tool for Leukemia

Leukemia immunotherapy has been dominant via using synthetic antibodies to target cluster of differentiation (CD) molecules, nevertheless inevitable cytotoxicity and immunogenicity would limit its development. Recently, increasing reports have focused on nucleic acid aptamers, a class of high-affinity nucleic acid ligands. Aptamers purportedly serve as “chemical antibodies”, have negligible cytotoxicity and low immunogenicity, and would be widely applied for the therapy and diagnosis of various diseases, especially leukemia. In the preclinical applications, nucleic acid aptamers have displayed the augmented specificity and selectivity via recognizing targets on leukemia cells based on unique three-dimensional conformations. As small molecules with nucleic acid characteristics, aptamers need to be chemically modified to resist nuclease degradation, renal clearance and improve binding affinities. Moreover, aptamers can be linked with neoteric detection techniques to enhance sensitivity and selectivity of diagnosis and therapy. In this review, we summarized aptamers’ preparation, chemical modification and conjugation, and discussed the application of aptamers in diagnosis and treatment of leukemia through highly specifically recognizing target molecules. Significantly, the application prospect of aptamers in fusion genes would be introduced.

Functional DNA Molecules Enable Selective and Stimuli-Responsive Nanoparticles for Biomedical Applications

Nanoparticles (NPs) have enormous potential to improve disease diagnosis and treatment due to their intrinsic electronic, optical, magnetic, mechanical, and physiological properties. To realize their full potential for nanomedicine, NPs must be biocompatible and targetable toward specific biomolecules to ensure selective sensing, imaging, and drug delivery in complex environments such as living cells, tissues, animals, and human bodies. In this Account, we summarize our efforts to impart specific biocompatibility and biorecognition functionality to NPs by developing strategies to integrate inorganic and organic NPs with functional DNA (fDNA), including aptamers, DNAzymes, and aptazymes to create fDNA-NPs. These hybrid NPs take advantage of fDNA's ability to either bind targets or catalyze reactions in the presence of targets selectively and utilize their unique physicochemical properties including small size, low immunogenicity, and ease of synthesis and chemical modification in comparison with other molecules such as antibodies. By integrating inorganic NPs such as gold NPs, quantum dots, and iron oxide nanoparticles with fDNA, we designed stimuli-responsive fDNA-NPs that exhibit target induced assembly and disassembly of NPs, resulting in a variety of colorimetric, fluorescent, and magnetic resonance imaging (MRI)-based sensors for diagnostic of a broad range of analytes. To impart both biocompatibility and selectivity on inorganic NPs for targeted bioimaging, we have demonstrated DNA-mediated surface functionalization, shape-controlled synthesis, and coordinative assembly of such NPs as specific bioprobes. A highlight is provided on the construction of fDNA-based nanoprobes with light-activatable sensing and imaging functions, which provides precise control of recognition properties of fDNA with high spatiotemporal resolution. To explore the potential of organic NPs for biosensing applications, we have developed an enzyme-responsive fDNA-liposome as a universal sensing platform compatible with diverse biological targets as well as different detection methods including fluorescence, MRI, or temperature, making possible point-of-care diagnostics. To expand the application regime of organic NPs, we collaborated with the Zimmerman group to prepare single-chain block copolymer-based NPs and incorporated it with a variety of functions, including monovalent DNA for assembly, tunable surface chemistry for cellular imaging, and coordinative Cu(II) sites for catalyzing intracellular click reactions, demonstrating the potential of using organic NPs to create promising fDNA-NP systems with programmable functionalities. Furthermore, we survey our recent endeavor in integration of cell-specific aptamers with different NPs for targeted drug delivery, showing that introducing stimuli-responsive properties into NPs that target tumor microenvironments would enable safer and more effective therapy for cancers. Finally, current challenges and future perspectives in fDNA-mediated engineering of NPs for biomedical applications are discussed.

Diversity of PEGylation methods of liposomes and their influence on RNA delivery

Gene therapy is a promising approach for personalized medicine, but its application in humans requires development of efficient and safe vehicles. PEGylated liposomes are some of the most suitable delivery systems for nucleic acids because of their stability under physiological conditions and prolonged circulation time, compared to conventional and other types of "stealth" liposomes. In vitro/in vivo activity of PEGylated liposomes is highly dependent on PEG motif abundance. The process of "stealth" coverage formation is a very important parameter for efficient transfection assays and further fate determination of the PEG layer after tissue penetration. In this review, we discuss the latest methods of PEGylated liposome preparation.

Aptamer-functionalized liposomes for targeted cancer therapy

Accumulation of chemotherapeutic agents in the tumor tissue while reducing adverse effects and drug resistance are among the major goals in cancer therapy. Among nanocarriers, liposomes have been found to be more effective in the passive targeting of cancer cells. A promising recent development in targeted drug delivery is the use of aptamer-functionalized liposomes for cancer therapy. Aptamer-targeted liposomes have enhanced uptake in tumor cells as shown in vitro and in vivo. Here, we discuss the aptamer-functionalized liposome platforms and review functionalization approaches as well as the factors affecting antitumor efficiency of aptamer-targeted liposomal systems. Finally, we provide a comprehensive overview of aptamer-targeted liposomes based on the molecular targets on the surface of cancer cells.

Diversity of PEGylation methods of liposomes and their influence on RNA delivery

A brief review and comparison of the methods of PEGylation of liposomal particles and their influence on the delivery of RNA.

Advanced Selection Methodologies for DNAzymes in Sensing and Healthcare Applications

DNAzymes have been widely explored owing to their excellent catalytic activity in a broad range of applications, notably in sensing and biomedical devices. These newly discovered applications have built high hopes for designing novel catalytic DNAzymes. However, the selection of efficient DNAzymes is a challenging process but one that is of crucial importance. Initially, systemic evolution of ligands by exponential enrichment (SELEX) was a labor-intensive and time-consuming process, but recent advances have accelerated the automated generation of DNAzyme molecules. This review summarizes recent advances in SELEX that improve the affinity and specificity of DNAzymes. The thriving generation of new DNAzymes is expected to open the door to several healthcare applications. Therefore, a significant portion of this review is dedicated to various biological applications of DNAzymes, such as sensing, therapeutics, and nanodevices. In addition, discussion is further extended to the barriers encountered for the real-life application of these DNAzymes to provide a foundation for future research.

Current Advances in Aptamers for Cancer Diagnosis and Therapy

Nucleic acid aptamers are single-stranded oligonucleotides that interact with target molecules with high affinity and specificity in unique three-dimensional structures. Aptamers are generally isolated by a simple selection process called systematic evolution of ligands by exponential enrichment (SELEX) and then can be chemically synthesized and modified. Because of their high affinity and specificity, aptamers are promising agents for biomarker discovery, as well as cancer diagnosis and therapy. In this review, we present recent progress and challenges in aptamer and SELEX technology and highlight some representative applications of aptamers in cancer therapy.

Particle Targeting in Complex Biological Media

Over the past few decades, nanoengineered particles have gained increasing interest for applications in the biomedical realm, including diagnosis, imaging, and therapy. When functionalized with targeting ligands, these particles have the potential to interact with specific cells and tissues, and accumulate at desired target sites, reducing side effects and improve overall efficacy in applications such as vaccination and drug delivery. However, when targeted particles enter a complex biological environment, the adsorption of biomolecules and the formation of a surface coating (e.g., a protein corona) changes the properties of the carriers and can render their behavior unpredictable. For this reason, it is of importance to consider the potential challenges imposed by the biological environment at the early stages of particle design. This review describes parameters that affect the targeting ability of particulate drug carriers, with an emphasis on the effect of the protein corona. We highlight strategies for exploiting the protein corona to improve the targeting ability of particles. Finally, we provide suggestions for complementing current in vitro assays used for the evaluation of targeting and carrier efficacy with new and emerging techniques (e.g., 3D models and flow-based technologies) to advance fundamental understanding in bio-nano science and to accelerate the development of targeted particles for biomedical applications.

Aptamer-Decorated Self-Assembled Aggregation-Induced Emission Organic Dots for Cancer Cell Targeting and Imaging

A facile and simple one-step method was developed to fabricate aptamer-decorated self-assembled organic dots with aggregation-induced emission (AIE) characteristics. With integration of the advantages of AIE aggregates with strong emission and the cell-targeting capability of aptamers, the as-prepared Apt-AIE organic nanodots can specifically target to cancer cells with good biocompatibility, high image constrast, and photostability. On the basis of this universal method, a variety of versatile organic fluorescent nanoprobes with high brightness, specific recognition, and clinical-transitional potential could be facilely constructed for biological sensing and imaging applications.

L-Carnitine-conjugated nanoparticles to promote permeation across blood-brain barrier and to target glioma cells for drug delivery via the novel organic cation/carnitine transporter OCTN2

Overcoming blood-brain barrier (BBB) and targeting tumor cells are two key steps for glioma chemotherapy. By taking advantage of the specific expression of Na⁺-coupled carnitine transporter 2 (OCTN2) on both brain capillary endothelial cells and glioma cells, l-carnitine conjugated poly(lactic-co-glycolic acid) nanoparticles (LC-PLGA NPs) were prepared to enable enhanced BBB permeation and glioma-cell targeting. Conjugation of l-carnitine significantly enhanced the uptake of PLGA nanoparticles in the BBB endothelial cell line hCMEC/D3 and the glioma cell line T98G. The uptake was dependent on Na⁺ and inhibited by the excessive free l-carnitine, suggesting involvement of OCTN2 in the process. In vivo mouse studies showed that LC-PLGA NPs resulted in high accumulation in the brain as indicated by the biodistribution and imaging assays. Furthermore, compared to Taxol and paclitaxel-loaded unmodified PLGA NPs, the drug-loaded LC-PLGA NPs showed improved anti-glioma efficacy in both 2D-cell and 3D-spheroid models. The PEG spacer length of the ligand attached to the nanoparticles was optimized, and the formulation with PEG1000 (LC-1000-PLGA NPs) showed the maximum targeting efficiency. We conclude that l-carnitine-mediated cellular recognition and internalization via OCTN2 significantly facilitate the transcytosis of nanoparticles across BBB and the uptake of nanoparticles in glioma cells, resulting in improved anti-glioma efficacy.

Targeting tumor highly-expressed LAT1 transporter with amino acid-modified nanoparticles: Toward a novel active targeting strategy in breast cancer therapy

Designing active targeting nanocarriers with increased cellular accumulation of chemotherapeutic agents is a promising strategy in cancer therapy. Herein, we report a novel active targeting strategy based on the large amino acid transporter 1 (LAT1) overexpressed in a variety of cancers. Glutamate was conjugated to polyoxyethylene stearate as a targeting ligand to achieve LAT1-targeting PLGA nanoparticles. The targeting efficiency of nanoparticles was investigated in HeLa and MCF-7 cells. Significant increase in cellular uptake and cytotoxicity was observed in LAT1-targeting nanoparticles compared to the unmodified ones. More interestingly, the internalized LAT1 together with targeting nanoparticles could recycle back to the cell membrane within 3 h, guaranteeing sufficient transporters on cell membrane for continuous cellular uptake. The LAT1 targeting nanoparticles exhibited better tumor accumulation and antitumor effects. These results suggested that the overexpressed LAT1 on cancer cells holds a great potential to be a high-efficiency target for the rational design of active-targeting nanosystems.

Aptamer-Mediated Targeted Delivery of Therapeutics: An Update

The selective delivery of drugs in a cell- or tissue-specific manner represents the main challenge for medical research; in order to reduce the occurrence of unwanted off-target effects. In this regard, nucleic acid aptamers have emerged as an attractive class of carrier molecules due to their ability to bind with high affinity to specific ligands; their high chemical flexibility; as well as tissue penetration capability. To date, different aptamer-drug systems and aptamer-nanoparticles systems, in which nanoparticles function together with aptamers for the targeted delivery, have been successfully developed for a wide range of therapeutics, including toxins; peptides; chemotherapeutics and oligonucleotides. Therefore, aptamer-mediated drug delivery represents a powerful tool for the safe and effective treatment of different human pathologies, including cancer; neurological diseases; immunological diseases and so on. In this review, we will summarize recent progress in the field of aptamer-mediated drug delivery and we will discuss the advantages, the achieved objectives and the challenges to be still addressed in the near future, in order to improve the effectiveness of therapies.

Recent Developments of Liposomes as Nanocarriers for Theranostic Applications

Liposomes are nanocarriers comprised of lipid bilayers encapsulating an aqueous core. The ability of liposomes to encapsulate a wide variety of diagnostic and therapeutic agents has led to significant interest in utilizing liposomes as nanocarriers for theranostic applications. In this review, we highlight recent progress in developing liposomes as nanocarriers for a) diagnostic applications to detect proteins, DNA, and small molecule targets using fluorescence, magnetic resonance, ultrasound, and nuclear imaging; b) therapeutic applications based on small molecule-based therapy, gene therapy and immunotherapy; and c) theranostic applications for simultaneous detection and treatment of heavy metal toxicity and cancers. In addition, we summarize recent studies towards understanding of interactions between liposomes and biological components. Finally, perspectives on future directions in advancing the field for clinical translations are also discussed.