SDA, a DNA aptamer inhibiting E- and P-selectin mediated adhesion of cancer and leukemia cells, the first and pivotal step in transendothelial migration during metastasis formation

Aptamers Targeting Membrane Proteins for Sensor and Diagnostic Applications

Many biological processes (physiological or pathological) are relevant to membrane proteins (MPs), which account for almost 30% of the total of human proteins. As such, MPs can serve as predictive molecular biomarkers for disease diagnosis and prognosis. Indeed, cell surface MPs are an important class of attractive targets of the currently prescribed therapeutic drugs and diagnostic molecules used in disease detection. The oligonucleotides known as aptamers can be selected against a particular target with high affinity and selectivity by iterative rounds of in vitro library evolution, known as Systematic Evolution of Ligands by EXponential Enrichment (SELEX). As an alternative to antibodies, aptamers offer unique features like thermal stability, low-cost, reuse, ease of chemical modification, and compatibility with various detection techniques. Particularly, immobilized-aptamer sensing platforms have been under investigation for diagnostics and have demonstrated significant value compared to other analytical techniques. These "aptasensors" can be classified into several types based on their working principle, which are commonly electrochemical, optical, or mass-sensitive. In this review, we review the studies on aptamer-based MP-sensing technologies for diagnostic applications and have included new methodological variations undertaken in recent years.

Cell-specific aptamers as potential drugs in therapeutic applications: A review of current progress

Cell-specific aptamers are a promising emerging player in the field of disease therapy. This paper reviews the multidimensional research progress made in terms of their classification, modification, and application. Based on the target location of cell-specific aptamers, it is defined and classified cell-specific aptamers into three groups including aptamers for cell surface markers, aptamers for intracellular components, and aptamers for extracellular components. Moreover, the modification methods of aptamers to achieve improved stability and affinity are concluded. In addition, recent advances in the application of cell-specific aptamers are discussed, mainly focusing on the increasing research attraction of cell state improving helpers and cell recruitment mediators in the improvement of cellular microenvironments to achieve successful disease therapy. This review also highlights 11 types of clinical aptamer drugs. Finally, the challenges and future directions of potential clinical applications are presented. In summary, we believe that cell-specific aptamers are promising drugs in disease therapy.

Future Perspectives of Therapeutic, Diagnostic and Prognostic Aptamers in Eye Pathological Angiogenesis

Aptamers are single-stranded DNA or RNA oligonucleotides that are currently used in clinical trials due to their selectivity and specificity to bind small molecules such as proteins, peptides, viral particles, vitamins, metal ions and even whole cells. Aptamers are highly specific to their targets, they are smaller than antibodies and fragment antibodies, they can be easily conjugated to multiple surfaces and ions and controllable post-production modifications can be performed. Aptamers have been therapeutically used for age-related macular degeneration, cancer, thrombosis and inflammatory diseases. The aim of this review is to highlight the therapeutic, diagnostic and prognostic possibilities associated with aptamers, focusing on eye pathological angiogenesis.

Therapeutic Interventions into Innate Immune Diseases by Means of Aptamers

The immune system plays a crucial role in the body's defense system against various pathogens, such as bacteria, viruses, and parasites, as well as recognizes non-self- and self-molecules. The innate immune system is composed of special receptors known as pattern recognition receptors, which play a crucial role in the identification of pathogen-associated molecular patterns from diverse microorganisms. Any disequilibrium in the activation of a particular pattern recognition receptor leads to various inflammatory, autoimmune, or immunodeficiency diseases. Aptamers are short single-stranded deoxyribonucleic acid or ribonucleic acid molecules, also termed "chemical antibodies," which have tremendous specificity and affinity for their target molecules. Their features, such as stability, low immunogenicity, ease of manufacturing, and facile screening against a target, make them preferable as therapeutics. Immune-system-targeting aptamers have a great potential as a targeted therapeutic strategy against immune diseases. This review summarizes components of the innate immune system, aptamer production, pharmacokinetic characteristics of aptamers, and aptamers related to innate-immune-system diseases.

Selection and characterization of single-stranded DNA aptamers against interleukin-5

Asthma as a chronic inflammatory disorder is associated with many cytokines like interleukin-5 (IL-5) which plays essential role in eosinophil differentiation and maturation. Accordingly, blockage of IL-5 using mepalizumab has been considered as a promising therapeutic approach for asthma. Despite the monocolonal antibody advantages, some restrictions provided an acceptable background for alternative agents like aptamers which could replace with antibodies. In the current study, aptamer isolation against IL-5 molecule was intended, according to the valuable benefits of aptamers over antibodies. HEK-293T/IL-5 cell was constructed to select aptamer using cell-systematic evolution of ligands by exponential enrichment (SELEX) method. Integration of the IL-5 fragment to genome of the HEK-293T was verified by polymerase chain reaction on the genomic DNA of the transfected cells. Moreover, IL-5 protein expression on the cell surface was confirmed using flow cytometry analysis. Then, cell SELEX was carried out in 12 rounds and isolated aptamers were evaluated by flow cytometry analysis. The selected clones were then sequenced and assessed for any possible secondary structure. The results of this study led to the selection of 19 different single-stranded DNA clones after 12 rounds of selection which were clustered to five groups based on common structural motifs. In conclusion, the findings revealed the isolation of IL-5-specific single-stranded DNA aptamers, which can further be substituted with mepolizumab.

Redistribution, homing and organ-invasion of neoplastic stem cells in myeloid neoplasms

The development of a myeloid neoplasm is a step-wise process that originates from leukemic stem cells (LSC) and includes pre-leukemic stages, overt leukemia and a drug-resistant terminal phase. Organ-invasion may occur in any stage, but is usually associated with advanced disease and a poor prognosis. Sometimes, extra-medullary organ invasion shows a metastasis-like or even sarcoma-like destructive growth of neoplastic cells in local tissue sites. Examples are myeloid sarcoma, mast cell sarcoma and localized blast phase of chronic myeloid leukemia. So far, little is known about mechanisms underlying re-distribution and extramedullary dissemination of LSC in myeloid neoplasms. In this article, we discuss mechanisms through which LSC can mobilize out of the bone marrow niche, can transmigrate from the blood stream into extramedullary organs, can invade local tissue sites and can potentially create or support the formation of local stem cell niches. In addition, we discuss strategies to interfere with LSC expansion and organ invasion by targeted drug therapies.

Aptamer Oligonucleotides as Potential Therapeutics in Hematologic Diseases

Aptamers are single-stranded DNA or RNA oligonucleotides generated by a novel in vitro selection technique termed Systematic evolution of ligands by exponential enrichment (SELEX). During the past two decades, various aptamer drugs have been developed and many of them have entered into clinical trials. In the present review, we focus on aptamers as potential therapeutics for hematological diseases, including anemia of chronic inflammation (ACI) and anemia of chronic disease (ACD), hemophilia, thrombotic thrombocytopenic purpura (TTP) or VWD type-2B, and sickle cell disease (SCD), in particular, those that have entered into clinical trials.

Aptamers as the chaperones (Aptachaperones) of drugs-from siRNAs to DNA nanorobots

Aptamers, nucleic acid ligands that are specific against their corresponding targets are increasingly employed in a variety of applications including diagnostics and therapeutics. The specificity of the aptamers against their targets is also used as the basis for the formulation of the aptamer-based drug delivery system. In this review, we aim to provide an overview on the chaperoning roles of aptamers in acting as the cargo or load carriers, delivering contents to the targeted sites via cell surface receptors. Internalization of the aptamer-biomolecule conjugates via receptor-mediated endocytosis and the strategies to augment the rate of endocytosis are underscored. The cargos chaperoned by aptamers, ranging from siRNAs to DNA origami are illuminated. Possible impediments to the aptamer-based drug deliveries such as susceptibility to nuclease resistance, potentiality for immunogenicity activation, tumor heterogeneity are speculated and the corresponding amendment strategies to address these shortcomings are discussed. We prophesy that the future of the aptamer-based drug delivery will take a trajectory towards DNA nanorobot-based assay.

Development of α4 integrin DNA aptamer as a potential therapeutic tool for multiple sclerosis

One of the most important molecules for multiple sclerosis pathogenesis is α4 integrin, which is responsible for autoreactive leukocytes migration into the brain. The monoclonal antibody, natalizumab, was introduced to market for blocking the extravasation of autoreactive leukocytes via inhibition of α4 integrin. However, the disadvantages of antibodies provided a suitable background for other agents to be replaced with antibodies. Considering the profound advantages of aptamers over antibodies, aptamer isolation against α4 integrin was intended in the current study. The α4 integrin-specific aptamers were selected using cell-systematic evolution of ligands by exponential enrichment (SELEX) method with human embryonic kidney (HEK)-293T overexpressing α4 integrin and HEK-293T as target and control cells, respectively. Evaluation of selected aptamer was performed through flow cytometric analysis. The selected clones were then sequenced and analyzed for any possible secondary structure and affinity. The results of this study led to isolation of 13 different single-stranded DNA clones in 11 rounds of selection which were categorized to three clusters based on common structural motifs and the equilibrium dissociation constant (K _d ) of the most stable structure was calculated. The evaluation of SELEX progress showed growth in aptamer affinity with increasing of the number of cycles. Taken together, the findings of this study demonstrated the isolation of α4-specific single-stranded DNA aptamers with suitable affinity for ligand, which can further be replaced with natalizumab.

[Research advances in the role of aptamers in the diagnosis and targeted therapy of pediatric cancer]

Aptamers are single-stranded DNA or RNA which are isolated from synthesized random oligonucleotide library in vitro via systematic evolution of ligands by exponential enrichment (SELEX) and can bind to metal ions, small molecules, carbohydrates, lipids, proteins, and others targets with high affinity and specificity. Aptamers have the advantages of simple preparation, good thermal stability, and low immunogenicity and have great potential in the medical fields such as molecular imaging, biosensing, early diagnosis of diseases, and targeted therapy. Aptamer technology may be useful for early diagnosis and targeted therapy of pediatric cancer, and may avoid the side effects of conventional chemotherapy, such as growth and development disorders and long-term organ dysfunction. This article reviews the latest research advances in the selection and application of aptamers for pediatric cancer.

[Research advances in the role of aptamers in the diagnosis and targeted therapy of pediatric cancer]

Aptamers are single-stranded DNA or RNA which are isolated from synthesized random oligonucleotide library in vitro via systematic evolution of ligands by exponential enrichment (SELEX) and can bind to metal ions, small molecules, carbohydrates, lipids, proteins, and others targets with high affinity and specificity. Aptamers have the advantages of simple preparation, good thermal stability, and low immunogenicity and have great potential in the medical fields such as molecular imaging, biosensing, early diagnosis of diseases, and targeted therapy. Aptamer technology may be useful for early diagnosis and targeted therapy of pediatric cancer, and may avoid the side effects of conventional chemotherapy, such as growth and development disorders and long-term organ dysfunction. This article reviews the latest research advances in the selection and application of aptamers for pediatric cancer.

p38 and JNK pathways control E-selectin-dependent extravasation of colon cancer cells by modulating miR-31 transcription

Extravasation of circulating cancer cells is a key event of metastatic dissemination that is initiated by the adhesion of cancer cells to vascular endothelial cells. It requires the interaction between adhesion receptors such as E-selectin present on endothelial cells and their ligands on cancer cells. Notably, E-selectin influences the metastatic potential of breast, bladder, gastric, pancreatic, and colorectal carcinoma as well as of leukemia and lymphoma. Here, we show that E-selectin expression induced by the pro-inflammatory cytokine IL-1β is directly and negatively regulated by miR-31. The transcription of miR-31 is activated by IL-1β. This activation depends on p38 and JNK MAP kinases, and their downstream transcription factors GATA2, c-Fos and c-Jun. The miR-31-mediated repression of E-selectin impairs the metastatic potential of colon cancer cells by decreasing their adhesion to, and migration through, the endothelium. These results highlight for the first time that microRNA mediates E-selectin-dependent extravasation of colon cancer cells.

p38 activation induces production of miR-146a and miR-31 to repress E-selectin expression and inhibit transendothelial migration of colon cancer cells

Extravasation of circulating cancer cells determines their metastatic potential. This process is initiated by the adhesion of cancer cells to vascular endothelial cells through specific interactions between endothelial adhesion receptors such as E-selectin and their ligands on cancer cells. In the present study, we show that miR-146a and miR-181b impede the expression of E-selectin by repressing the activity of its transcription factor NF-κB, thereby impairing the metastatic potentials of colon cancer cells by decreasing their adhesion to, and migration through, the endothelium. Among the two microRNAs, only miR-146a is activated by IL-1β, through the activation of p38, ERK and JNK MAP kinases, as well as their downstream transcription factors GATA2, c-Fos and c-Jun. Inhibiting p38 MAP kinase increases NF-κB activity, at least partially via miR-146a. Inhibiting p38 also increases the expression of E-selectin at the post-transcriptional level via decreasing miR-31, which targets E-selectin mRNA and also depends on p38 for its expression. In response to IL-1β, p38 MAP kinase hence represses the expression of E-selectin at the transcriptional and the post-transcriptional levels, via miR-146a and miR-31, respectively. These results highlight novel mechanisms by which p38 downregulates the expression of E-selectin through different microRNAs following inflammatory stimuli associated to cancer progression.

SDA and IDA - Two aptamers to inhibit cancer cell adhesion

Aptamers which bind to proteins involved in cell-cell interactions could have significant value to directly affect cancer cell adhesion or for directed cargo delivery. Here, I discuss two aptamers: aptamer SDA which binds to E- and P-selectin, and aptamer IDA which binds to α6β4 integrin. Both aptamers (SDA 91 nt and IDA 77 nt) bind their target proteins with dissociation constants in the 100-150 nM range and substantially inhibit special cellular adhesion, possibly a first and pivotal step in transendothelial migration during metastasis formation. The aptamers' half-lives in cell culture media are between two and six hours. IDA is internalized by integrin presenting cells within minutes thus possibly serving as vehicle for directed cargo delivery.

Selection of Nucleic Acid Aptamers Targeting Tumor Cell-Surface Protein Biomarkers

Aptamers are nucleic acids referred to as chemical antibodies as they bind to their specific targets with high affinity and selectivity. They are selected via an iterative process known as ‘selective evolution of ligands by exponential enrichment’ (SELEX). Aptamers have been developed against numerous cancer targets and among them, many tumor cell-membrane protein biomarkers. The identification of aptamers targeting cell-surface proteins has mainly been performed by two different strategies: protein- and cell-based SELEX, when the targets used for selection were proteins and cells, respectively. This review aims to update the literature on aptamers targeting tumor cell surface protein biomarkers, highlighting potentials, pitfalls of protein- and cell-based selection processes and applications of such selected molecules. Aptamers as promising agents for diagnosis and therapeutic approaches in oncology are documented, as well as aptamers in clinical development.

Therapeutic applications of synthetic nucleic acid aptamers

It is possible to generate oligonucleotide aptamers for a wide variety of target molecules using a process known as Systematic Evolution of Ligands by Exponential Enrichment. Researchers have successfully generated aptamers which recognize specific metal ions, small chemical compounds, peptides, proteins, saccharides, and even whole cells. Aptamers show much promise as future therapeutics and as drug targeting agents. A particularly active area of aptamer research in the past two years was development of aptamer based cancer therapeutics and development of aptamer based cancer drug delivery systems. Aptamers were also used to address inflammatory diseases, infectious diseases, cardiovascular diseases, and eye diseases.

SELEX of Cell-Specific RNA Aptamers

This chapter focuses on the selection of RNA aptamers, which bind to specific cell surface components and thus can be internalized receptor mediated. Such aptamers discriminate between different tissues, e.g., detect malignant cells, and target them or induce apoptosis through drug internalization. However, before starting the selection process the choice of an ideal target can be challenging. To give an example for the selection of cell specific aptamers, we here used the interleukin-6 receptor (IL-6R) as a target, which is presented on hepatocytes, neutrophils, monocytes, and macrophages.

Aptamers: A promising chemical antibody for cancer therapy

Aptamers, also known as chemical antibodies, are single-stranded nucleic acid oligonucleotides which bind to their targets with high specificity and affinity. They are typically selected by repetitive in vitro process termed systematic evolution of ligands by exponential enrichment (SELEX). Owing to their excellent properties compared to conventional antibodies, notably their smaller physical size and lower immunogenicity and toxicity, aptamers have recently emerged as a new class of agents to deliver therapeutic drugs to cancer cells by targeting specific cancer-associated hallmarks. Aptamers can also be structurally modified to make them more flexible in order to conjugate other agents such as nano-materials and therapeutic RNA agents, thus extending their applications for cancer therapy. This review presents the current knowledge on the practical applications of aptamers in the treatment of a variety of cancers.

Structural requirements of mono- and multivalent L-selectin blocking aptamers for enhanced receptor inhibition in vitro and in vivo

L-selectin mediates extravasation of leukocytes from blood into the surrounding tissue during inflammation and is therefore a therapeutical target in certain overwhelming immune reactions. In this study, we characterized an L-selectin specific blocking DNA aptamer with respect to nucleotide composition and target binding. Introduction of deletions and nucleotide exchanges resulted in an optimized DNA sequence but preservation of the IC50 in the low nanomolar range. The inhibitory potential was significantly increased when the aptamer was displayed as a di- and trimer connected via appropriate linker length. Similar to monoclonal antibodies, trimer yielded picomolar IC50 values in a competitive binding assay. In comparison to the monovalent aptamer, the trivalent assembly reduced PBMC interactions to L-selectin ligands 90-fold under shear and exerted superior inhibition of PBMC rolling in vivo. In conclusion, our work demonstrates the feasibility of optimizing aptamer sequences and shows that multivalent ligand presentation enables superior adhesion receptor targeting.

FROM THE CLINICAL EDITOR

During inflammation, leukocytes extravasate from blood vessels under chemotaxic signals. The presence of L-selectin on endothelium acts as a mediator for the extravasation process. In this study, the authors investigated an L-selectin specific blocking DNA aptamer in various forms, as inhibitors to leukocyte binding and extravasation. This new approach confirmed the potential use of aptamers in clinical setting.

Design of liposomal formulations for cell targeting

Liposomes have gained extensive attention as carriers for a wide range of drugs due to being both nontoxic and biodegradable as they are composed of substances naturally occurring in biological membranes. Active targeting for cells has explored specific modification of the liposome surface by functionalizing it with specific targeting ligands in order to increase accumulation and intracellular uptake into target cells. None of the Food and Drug Administration-licensed liposomes or lipid nanoparticles are coated with ligands or target moieties to delivery for homing drugs to target tissues, cells or subcellular organelles. Targeted therapies (with or without controlled drug release) are an emerging and relevant research area. Despite of the numerous liposomes reviews published in the last decades, this area is in constant development. Updates urgently needed to integrate new advances in targeted liposomes research. This review highlights the evolution of liposomes from passive to active targeting and challenges in the development of targeted liposomes for specific therapies.

The cancer glycome: carbohydrates as mediators of metastasis

Glycosylation is a frequent post-translational modification which results in the addition of carbohydrate determinants, "glycans", to cell surface proteins and lipids. These glycan structures form the "glycome" and play an integral role in cell-cell and cell-matrix interactions through modulation of adhesion and cell trafficking. Glycosylation is increasingly recognized as a modulator of the malignant phenotype of cancer cells, where the interaction between cells and the tumor micro-environment is altered to facilitate processes such as drug resistance and metastasis. Changes in glycosylation of cell surface adhesion molecules such as selectin ligands, integrins and mucins have been implicated in the pathogenesis of several solid and hematological malignancies, often with prognostic implications. In this review we focus on the functional significance of alterations in cancer cell glycosylation, in terms of cell adhesion, trafficking and the metastatic cascade and provide insights into the prognostic and therapeutic implications of recent findings in this fast-evolving niche.

Peritoneal tumor carcinomatosis: pharmacological targeting with hyaluronan-based bioconjugates overcomes therapeutic indications of current drugs

Peritoneal carcinomatosis still lacks reliable therapeutic options. We aimed at testing a drug delivery strategy allowing a controlled release of cytotoxic molecules and selective targeting of tumor cells. We comparatively assessed the efficacy of a loco-regional intraperitoneal treatment in immunocompromised mice with bioconjugates formed by chemical linking of paclitaxel or SN-38 to hyaluronan, against three models of peritoneal carcinomatosis derived from human colorectal, gastric and esophageal tumor cell xenografts. In vitro, bioconjugates were selectively internalized through mechanisms largely dependent on interaction with the CD44 receptor and caveolin-mediated endocytosis, which led to accumulation of compounds into lysosomes of tumor cells. Moreover, they inhibited tumor growth comparably to free drugs. In vivo, efficacy of bioconjugates or free drugs against luciferase-transduced tumor cells was assessed by bioluminescence optical imaging, and by recording mice survival. The intraperitoneal administration of bioconjugates in tumor-bearing mice exerted overlapping or improved therapeutic efficacy compared with unconjugated drugs. Overall, drug conjugation to hyaluronan significantly improved the profiles of in vivo tolerability and widened the field of application of existing drugs, over their formal approval or current use. Therefore, this approach can be envisaged as a promising therapeutic strategy for loco-regional treatment of peritoneal carcinomatosis.