Pharmacokinetics of a Cholesterol-conjugated Aptamer Against the Hepatitis C Virus (HCV) NS5B Protein

APTAMERS: USHERING IN NEW HOPES IN TARGETED GLIOBLASTOMA THERAPY

Glioblastoma, a formidable brain cancer, has remained a therapeutic challenge due to its aggressive nature and resistance to conventional treatments. Recent data indicate that aptamers, short synthetic DNA or RNA molecules can be used in anti-cancer therapy due to their better tumor penetration, specific binding affinity, longer retention in tumor sites and their ability to cross the blood-brain barrier. With the ability to modify these oligonucleotides through the selection process, and using rational design to modify them, post-SELEX aptamers offer several advantages in glioblastoma treatment, including precise targeting of cancer cells while sparing healthy tissue. This review discusses the pivotal role of aptamers in glioblastoma therapy and diagnosis, emphasizing their potential to enhance treatment efficacy and also highlights recent advancements in aptamer-based therapies which can transform the landscape of glioblastoma treatment, offering renewed hope to patients and clinicians alike.

Aptamers: Promising Reagents in Biomedicine Application

Nucleic acid aptamers, often termed "chemical antibodies," are short, single-stranded DNA or RNA molecules, which are selected by SELEX. In addition to their high specificity and affinity comparable to traditional antibodies, aptamers have numerous unique advantages such as wider identification of targets, none or low batch-to-batch variations, versatile chemical modifications, rapid mass production, and lack of immunogenicity. These characteristics make aptamers a promising recognition probe for scientific research or even clinical application. Aptamer-functionalized nanomaterials are now emerged as a promising drug delivery system for various diseases with decreased side-effects and improved efficacy. In this review, the technological strategies for generating high-affinity and biostable aptamers are introduced. Moreover, the development of aptamers for their application in biomedicine including aptamer-based biosensors, aptamer-drug conjugates and aptamer functionalized nanomaterials is comprehensively summarized.

The Application of Aptamer and Research Progress in Liver Disease

Aptamers, as a kind of small-molecule nucleic acid, have attracted much attention since their discovery. Compared with biological reagents such as antibodies, aptamers have the advantages of small molecular weight, low immunogenicity, low cost, and easy modification. At present, aptamers are mainly used in disease biomarker discovery, disease diagnosis, treatment, and targeted drug delivery vectors. In the process of screening and optimizing aptamers, it is found that there are still many problems need to be solved such as the design of the library, optimization of screening conditions, the truncation of screened aptamer, and the stability and toxicity of the aptamer. In recent years, the incidence of liver-related diseases is increasing year by year and the treatment measures are relatively lacking, which has attracted the people's attention in the application of aptamers in liver diseases. This article mainly summarizes the research status of aptamers in disease diagnosis and treatment, especially focusing on the application of aptamers in liver diseases, showing the crucial significance of aptamers in the diagnosis and treatment of liver diseases, and the use of Discovery Studio software to find the binding target and sequence of aptamers, and explore their possible interaction sites.

Aptamer Technologies in Neuroscience, Neuro-Diagnostics and Neuro-Medicine Development

Aptamers developed using in vitro Systematic Evolution of Ligands by Exponential Enrichment (SELEX) technology are single-stranded nucleic acids 10-100 nucleotides in length. Their targets, often with specificity and high affinity, range from ions and small molecules to proteins and other biological molecules as well as larger systems, including cells, tissues, and animals. Aptamers often rival conventional antibodies with improved performance, due to aptamers' unique biophysical and biochemical properties, including small size, synthetic accessibility, facile modification, low production cost, and low immunogenicity. Therefore, there is sustained interest in engineering and adapting aptamers for many applications, including diagnostics and therapeutics. Recently, aptamers have shown promise as early diagnostic biomarkers and in precision medicine for neurodegenerative and neurological diseases. Here, we critically review neuro-targeting aptamers and their potential applications in neuroscience research, neuro-diagnostics, and neuro-medicine. We also discuss challenges that must be overcome, including delivery across the blood-brain barrier, increased affinity, and improved in vivo stability and in vivo pharmacokinetic properties.

Types of RNA therapeutics

RNA therapy is one of the new treatments using small RNA molecules to target and regulate gene expression. It involves the application of synthetic or modified RNA molecules to inhibit the expression of disease-causing genes specifically. In other words, it silences genes and suppresses the transcription process. The main theory behind RNA therapy is that RNA molecules can prevent the translation into proteins by binding to specific messenger RNA (mRNA) molecules. By targeting disease-related mRNA molecules, RNA therapy can effectively silence or reduce the development of harmful proteins. There are different types of RNA molecules used in therapy, including small interfering RNAs (siRNAs), microRNAs (miRNAs), aptamer, ribozyme, and antisense oligonucleotides (ASOs). These molecules are designed to complement specific mRNA sequences, allowing them to bind and degrade the targeted mRNA or prevent its translation into protein. Nanotechnology is also highlighted to increase the efficacy of RNA-based drugs. In this chapter, while examining various methods of RNA therapy, we discuss the advantages and challenges of each.

Immune checkpoint inhibitors and cancer immunotherapy by aptamers: an overview

Efforts in cancer immunotherapy aim to counteract evasion mechanisms and stimulate the immune system to recognise and attack cancer cells effectively. Combination therapies that target multiple aspects of immune evasion are being investigated to enhance the overall efficacy of cancer immunotherapy. PD-1 (Programmed Cell Death Protein 1), CTLA-4 (Cytotoxic T-Lymphocyte Antigen 4), LAG-3 (Lymphocyte-Activation Gene 3), and TIM-3 (T Cell Immunoglobulin and Mucin Domain-Containing Protein3) are all immune checkpoint receptors that play crucial roles in regulating the immune response and maintaining self-tolerance often exploited by cancer cells to evade immune surveillance. Antibodies targeted against immune checkpoint inhibitors such as anti-PD-1 antibodies (e.g., pembrolizumab, nivolumab), anti-CTLA-4 antibodies (e.g., Ipilimumab), and experimental drugs targeting LAG-3 and TIM-3, aim to block these interactions and unleash the immune system's ability to recognise and destroy cancer cells. The US FDA has approved different categories of immune checkpoint inhibitors that have been utilised successfully in some patients with metastatic melanoma, renal cell carcinoma, head and neck cancers, and non-small lung cancer. Although several immune checkpoint inhibitor antibodies have been developed, they exhibited immune-related adverse effects, resulting in hypophysitis, diabetes, and neurological issues. These adverse effects of antibodies can be reduced by developing aptamer against the target. Aptamers offer several advantages over traditional antibodies, such as improved specificity, reduced immunogenicity, and flexible design for reduced adverse effects that specifically target and block protein-protein or receptor-ligand interactions involved in immune checkpoint pathways. The current study aims to review the function of particular immune checkpoint inhibitors along with developed aptamer-mediated antitumor cytotoxicity in cancer treatment.

Emerging Progress of RNA-Based Antitumor Therapeutics

RNA-based therapeutics (e.g., mRNAs, siRNAs, microRNAs, ASOs, and saRNAs) have considerable potential for tumor treatment. The development and optimization of RNA modifications and delivery systems enable the stable and efficient delivery of RNA cargos in vivo to elicit an antitumor response. Targeted RNA-based therapeutics with multiple specificities and high efficacies are now available. In this review, we discuss progress in RNA-based antitumor therapeutics, including mRNAs, siRNAs, miRNAs, ASOs, saRNAs, RNA aptamers, and CRISPR-based gene editing. We focus on the immunogenicity, stability, translation efficiency, and delivery of RNA drugs, and summarize their optimization and the development of delivery systems. In addition, we describe the mechanisms by which RNA-based therapeutics induce antitumor responses. Furthermore, we review the merits and limitations of RNA cargos and their therapeutic potential for cancers.

Harnessing aptamers against COVID-19: a therapeutic strategy

The novel coronavirus crisis caused by severe acute respiratory syndrome-coronavirus 2 (SARS-CoV-2) was a global pandemic. Although various therapeutic approaches were developed over the past 2 years, novel strategies with more efficient applicability are required to target new variants. Aptamers are single-stranded (ss)RNA or DNA oligonucleotides capable of folding into unique 3D structures with robust binding affinity to a wide variety of targets following structural recognition. Aptamer-based theranostics have proven excellent capability for diagnosing and treating various viral infections. Herein, we review the current status and future perspective of the potential of aptamers as COVID-19 therapies.

Aptamers in cancer therapy: problems and new breakthroughs

Aptamers, a class of oligonucleotides that can bind with molecular targets with high affinity and specificity, have been widely applied in research fields including biosensing, imaging, diagnosing, and therapy of diseases. However, compared with the rapid development in the research fields, the clinical application of aptamers is progressing at a much slower speed, especially in the therapy of cancer. Obstructions including nuclease degradation, renal clearance, a complex selection process, and potential side effects have inhibited the clinical transformation of aptamer-conjugated drugs. To overcome these problems, taking certain measures to improve the biocompatibility and stability of aptamer-conjugated drugs in vivo is necessary. In this review, the obstructions mentioned above are thoroughly discussed and the methods to overcome these problems are introduced in detail. Furthermore, landmark research works and the most recent studies on aptamer-conjugated drugs for cancer therapy are also listed as examples, and the future directions of research for aptamer clinical transformation are discussed.

DNA Aptamer Targets Mycobacterium tuberculosis DevR/DosR Response Regulator Function by Inhibiting Its Dimerization and DNA Binding Activity

Tuberculosis is recognized as one of the major public health threats worldwide. The DevR-DevS (DosR/DosS) two-component system is considered a novel drug target in Mycobacterium tuberculosis (Mtb), the etiological agent of tuberculosis, owing to its central role in bacterial adaptation and long-term persistence. An increase in DevR levels and the decreased permeability of the mycobacterial cell wall during hypoxia-associated dormancy pose formidable challenges to the development of anti-DevR compounds. Using an in vitro evolution approach of Systematic Evolution of Ligands by EXponential enrichment (SELEX), we developed a panel of single-stranded DNA aptamers that interacted with Mtb DevR protein in solid-phase binding assays. The best-performing aptamer, APT-6, forms a G-quadruplex structure and inhibits DevR-dependent transcription in Mycobacterium smegmatis. Mechanistic studies indicate that APT-6 functions by inhibiting the dimerization and DNA binding activity of DevR protein. In silico studies reveal that APT-6 interacts majorly with C-terminal domain residues that participate in DNA binding and formation of active dimer species of DevR. To the best of our knowledge, this is the first report of a DNA aptamer that inhibits the function of a cytosolic bacterial response regulator. By inhibiting the dimerization of DevR, APT-6 targets an essential step in the DevR activation mechanism, and therefore, it has the potential to universally block the expression of DevR-regulated genes for intercepting dormancy pathways in mycobacteria. These findings also pave the way for exploring aptamer-based approaches to design and develop potent inhibitors against intracellular proteins of various bacterial pathogens of global concern.

Strategies for developing long-lasting therapeutic nucleic acid aptamer targeting circulating protein: The present and the future

Aptamers are short, single-stranded DNA or RNA oligonucleotide sequences that can bind specific targets. The molecular weight of aptamers (<20 kDa) is lower than the renal filtration threshold (30∼50 kDa), resulting in very short half-lives in vivo, which limit their druggability. The development of long-lasting modification approaches for aptamers can help address the druggability bottleneck of aptamers. This review summarized two distinct kinds of long-lasting modification approaches for aptamers, including macromolecular modification and low-molecular-weight modification. Though it is a current approach to extend the half-life of aptamers, the macromolecular modification approach could limit the space for the dosage increases, thus causing potential compliance concerns due to large molecular weight. As for the other modification approach, the low-molecular-weight modification approach, which uses low molecular weight coupling agents (LMWCAs) to modify aptamers, could greatly increase the proportion of aptamer moiety. However, some LMWCAs could bind to other proteins, causing a decrease in the drug amounts in blood circulation. Given these issues, the outlook for the next generation of long-lasting modification approaches was proposed at the end, including improving the administration method to increase dosage for aptamer drugs modified by macromolecule and developing Artificial intelligence (AI)-based strategies for optimization of LMWCAs.

Advances in Aptamer-Based Biosensors and Cell-Internalizing SELEX Technology for Diagnostic and Therapeutic Application

Aptamers are a group of synthetic single-stranded nucleic acids. They are generated from a random library of single-stranded DNA or RNA by a technology named systematic evolution of ligands by exponential enrichment (SELEX). SELEX is a repetitive process to select and identify suitable aptamers that show high affinity and specificity towards target cells. Great strides have been achieved in the design, construction, and use of aptamers up to this point. However, only a small number of aptamer-based applications have achieved widespread commercial and clinical acceptance. Additionally, finding more effective ways to acquire aptamers with high affinity remains a challenge. Therefore, it is crucial to thoroughly examine the existing dearth and advancement in aptamer-related technologies. This review focuses on aptamers that are generated by SELEX to detect pathogenic microorganisms and mammalian cells, as well as in cell-internalizing SELEX for diagnostic and therapeutic purposes. The development of novel aptamer-based biosensors using optical and electrical methods for microbial detection is reported. The applications and limitations of aptamers are also discussed.

Systematic bio-fabrication of aptamers and their applications in engineering biology

Aptamers are single-stranded DNA or RNA molecules that have high affinity and selectivity to bind to specific targets. Compared to antibodies, aptamers are easy to in vitro synthesize with low cost, and exhibit excellent thermal stability and programmability. With these features, aptamers have been widely used in biology and medicine-related fields. In the meantime, a variety of systematic evolution of ligands by exponential enrichment (SELEX) technologies have been developed to screen aptamers for various targets. According to the characteristics of targets, customizing appropriate SELEX technology and post-SELEX optimization helps to obtain ideal aptamers with high affinity and specificity. In this review, we first summarize the latest research on the systematic bio-fabrication of aptamers, including various SELEX technologies, post-SELEX optimization, and aptamer modification technology. These procedures not only help to gain the aptamer sequences but also provide insights into the relationship between structure and function of the aptamers. The latter provides a new perspective for the systems bio-fabrication of aptamers. Furthermore, on this basis, we review the applications of aptamers, particularly in the fields of engineering biology, including industrial biotechnology, medical and health engineering, and environmental and food safety monitoring. And the encountered challenges and prospects are discussed, providing an outlook for the future development of aptamers.

Chemical modifications for a next generation of nucleic acid aptamers

In the past three decades, in vitro systematic evolution of ligands by exponential enrichment (SELEX) has yielded many aptamers for translational applications in both research and clinical settings. Despite their promise as an alternative to antibodies, the low success rate of SELEX (~ 30%) has been a major bottleneck that hampers the further development of aptamers. One hurdle is the lack of chemical diversity in nucleic acids. To address this, the aptamer chemical repertoire has been extended by introducing exotic chemical groups, which provide novel binding functionalities. This review will focus on how modified aptamers can be selected and evolved, with illustration of some successful examples. In particular, unique chemistries are exemplified. Various strategies of incorporating modified building blocks into the standard SELEX protocol are highlighted, with a comparison of the differences between pre-SELEX and post-SELEX modifications. Nucleic acid aptamers with extended functionality evolved from non-natural chemistries will open up new vistas for function and application of nucleic acids.

Novel Complex of PD-L1 Aptamer and Albumin Enhances Antitumor Efficacy In Vivo

The PD-1/PD-L1 pathway blockade can generate a good clinical response by reducing immunosuppression and provoking durable antitumor immunity. In addition to antibodies, aptamers can also block the interaction between PD-1 and PD-L1. For the in vivo application, however, free aptamers are usually too small in size and quickly removed from blood via glomerular filtration. To avoid renal clearance of aptamer, we conjugated the PD-L1 aptamer to albumin to form a larger complex (BSA-Apt) and evaluated whether BSA-Apt would enhance the in vivo antitumor efficacy. The PD-L1 aptamer was thiol-modified and conjugated to the amino group of BSA via a SMCC linker. The average size of BSA-Apt was 11.65 nm, which was above the threshold for renal clearance. Functionally, BSA-Apt retained the capability of the PD-L1 aptamer to bind with PDL1-expressing tumor cells. Moreover, both the free aptamer and BSA-Apt augmented the PBMC-induced antitumor cytotoxicity in vitro. Furthermore, BSA-Apt generated a significantly stronger antitumor efficacy than the free PD-L1 aptamer in vivo without raising systemic toxicity. The results indicate that conjugating the PD-L1 aptamer to albumin may serve as a promising strategy to improve the in vivo functionality of the aptamer and that BSA-Apt may have application potential in cancer immunotherapy.

Aptamer Applications in Neuroscience

Being the predominant cause of disability, neurological diseases have received much attention from the global health community. Over a billion people suffer from one of the following neurological disorders: dementia, epilepsy, stroke, migraine, meningitis, Alzheimer's disease, Parkinson's disease, multiple sclerosis, amyotrophic lateral sclerosis, Huntington's disease, prion disease, or brain tumors. The diagnosis and treatment options are limited for many of these diseases. Aptamers, being small and non-immunogenic nucleic acid molecules that are easy to chemically modify, offer potential diagnostic and theragnostic applications to meet these needs. This review covers pioneering studies in applying aptamers, which shows promise for future diagnostics and treatments of neurological disorders that pose increasingly dire worldwide health challenges.

DNA nanotechnology-facilitated ligand manipulation for targeted therapeutics and diagnostics

Ligands, mostly binding to proteins to form complexes and catalyze chemical reactions, can serve as drug and probe molecules, as well as sensing elements. DNA nanotechnology can integrate the high editability of DNA nanostructures and the biological activity of ligands into functionalized DNA nanostructures in a manner of controlled ligand stoichiometry, type, and arrangement, which provides significant advantages for targeted therapeutics and diagnostics. As therapeutic agents, multiple- and multivalent-ligands functionalized DNA nanostructures increase ligand-receptor affinity and activate multivalent ligand-receptor interactions, enabling improved regulation of cell signaling and enhanced control of cell behavior. As diagnostic agents, multiple ligands interaction via DNA nanostructures endows DNA nanosensors with high sensitivity and excellent signal transduction capability. Herein, we review the principles and advantages of using DNA nanostructures to manipulate ligands for targeted therapeutics and diagnostics and provide future perspectives.

Aptamers in Virology-A Consolidated Review of the Most Recent Advancements in Diagnosis and Therapy

The use of short oligonucleotide or peptide molecules as target-specific aptamers has recently garnered substantial attention in the field of the detection and treatment of viral infections. Based on their high affinity and high specificity to desired targets, their use is on the rise to replace antibodies for the detection of viruses and viral antigens. Furthermore, aptamers inhibit intracellular viral transcription and translation, in addition to restricting viral entry into host cells. This has opened up a plethora of new targets for the research and development of novel vaccines against viruses. Here, we discuss the advances made in aptamer technology for viral diagnosis and therapy in the past decade.

Aptamer-based diagnostic and therapeutic approaches in animals: Current potential and challenges

Fast and precise diagnosis of infectious and non-infectious animal diseases and their targeted treatments are of utmost importance for their clinical management. The existing biochemical, serological and molecular methods of disease diagnosis need improvement in their specificity, sensitivity and cost and, are generally not amenable for being used as points-of-care (POC) device. Further, with dramatic changes in environment and farm management practices, one should also arm ourselves and prepare for emerging and re-emerging animal diseases such as cancer, prion diseases, COVID-19, influenza etc. Aptamer – oligonucleotide or short peptides that can specifically bind to target molecules – have increasingly become popular in developing biosensors for sensitive detection of analytes, pathogens (bacteria, virus, fungus, prions), drug residues, toxins and, cancerous cells. They have also been proven successful in the cellular delivery of drugs and targeted therapy of infectious diseases and physiological disorders. However, the in vivo application of aptamer-mediated biosensing and therapy in animals has been limited. This paper reviews the existing reports on the application of aptamer-based biosensors and targeted therapy in animals. It also dissects the various modifications to aptamers that were found to be successful in in vivo application of the aptamers in diagnostics and therapeutics. Finally, it also highlights major challenges and future directions in the application of aptamers in the field of veterinary medicine.

Systematic Evolution of Ligands by Exponential Enrichment Technologies and Aptamer-Based Applications: Recent Progress and Challenges in Precision Medicine of Infectious Diseases

Infectious diseases are considered as a pressing challenge to global public health. Accurate and rapid diagnostics tools for early recognition of the pathogen, as well as individualized precision therapy are essential for controlling the spread of infectious diseases. Aptamers, which were screened by systematic evolution of ligands by exponential enrichment (SELEX), can bind to targets with high affinity and specificity so that have exciting potential in both diagnosis and treatment of infectious diseases. In this review, we provide a comprehensive overview of the latest development of SELEX technology and focus on the applications of aptamer-based technologies in infectious diseases, such as targeted drug-delivery, treatments and biosensors for diagnosing. The challenges and the future development in this field of clinical application will also be discussed.

A molecular hybrid comprising AS1411 and PDGF-BB aptamer, cholesterol, and doxorubicin for inhibiting proliferation of SW480 cells

Cancer treatment commonly relies on chemotherapy. This treatment faces many challenges, including treatment specificity and undesired side effects. To address these, a Dox-loaded Chol-aptamer molecular hybrid (Dox-CAH) was developed. This multivalent interaction system combines the key function of each integrated species: doxorubicin, cholesterol, and two aptamers binding to nucleolin and platelet-derived growth factor BB (PDGF-BB). The study has four stages: preparation of CAH via oligonucleotide hybridization, intercalation of doxorubicin into CAH, verification of CAH binding on SW480 by fluorescence microscopy and flow cytometry, and investigation of effect of Dox-CAH on SW480 proliferation. CAH was successfully prepared, as confirmed by electrophoresis. Flow cytometry and fluorescence microscopy demonstrated CAH binding to SW480, due to the presence of the AS1411 aptamer. This molecular hybrid exhibited specific binding because it did not bind to CCD 841 CoN. CAH binding to PDGF-BB compromises its function, as shown by enzyme-linked immunosorbent assay (ELISA) and cell assay. The DNA duplex in this molecular hybrid reduces the cytotoxicity of the Dox-CAH. Binding and the reduction of Dox-CAH toxicity may improve treatment specificity and minimize side effects. Dox-CAH is a model for more effective anticancer therapy, allowing incorporation of chemotherapeutic drugs and recognition elements.

Diagnostics and Therapeutics in Targeting HER2 Breast Cancer: A Novel Approach

Breast cancer is one of the most commonly occurring cancers in women globally and is the primary cause of cancer mortality in females. BC is highly heterogeneous with various phenotypic expressions. The overexpression of HER2 is responsible for 15-30% of all invasive BC and is strongly associated with malignant behaviours, poor prognosis and decline in overall survival. Molecular imaging offers advantages over conventional imaging modalities, as it provides more sensitive and specific detection of tumours, as these techniques measure the biological and physiological processes at the cellular level to visualise the disease. Early detection and diagnosis of BC is crucial to improving clinical outcomes and prognosis. While HER2-specific antibodies and nanobodies may improve the sensitivity and specificity of molecular imaging, the radioisotope conjugation process may interfere with and may compromise their binding functionalities. Aptamers are single-stranded oligonucleotides capable of targeting biomarkers with remarkable binding specificity and affinity. Aptamers can be functionalised with radioisotopes without compromising target specificity. The attachment of different radioisotopes can determine the aptamer's functionality in the treatment of HER2(+) BC. Several HER2 aptamers and investigations of them have been described and evaluated in this paper. We also provide recommendations for future studies with HER2 aptamers to target HER2(+) BC.

Aptamers for Anti-Viral Therapeutics and Diagnostics

Viral infections cause a host of fatal diseases and seriously affect every form of life from bacteria to humans. Although most viral infections can receive appropriate treatment thereby limiting damage to life and livelihood with modern medicine and early diagnosis, new types of viral infections are continuously emerging that need to be properly and timely treated. As time is the most important factor in the progress of many deadly viral diseases, early detection becomes of paramount importance for effective treatment. Aptamers are small oligonucleotide molecules made by the systematic evolution of ligands by exponential enrichment (SELEX). Aptamers are characterized by being able to specifically bind to a target, much like antibodies. However, unlike antibodies, aptamers are easily synthesized, modified, and are able to target a wider range of substances, including proteins and carbohydrates. With these advantages in mind, many studies on aptamer-based viral diagnosis and treatments are currently in progress. The use of aptamers for viral diagnosis requires a system that recognizes the binding of viral molecules to aptamers in samples of blood, serum, plasma, or in virus-infected cells. From a therapeutic perspective, aptamers target viral particles or host cell receptors to prevent the interaction between the virus and host cells or target intracellular viral proteins to interrupt the life cycle of the virus within infected cells. In this paper, we review recent attempts to use aptamers for the diagnosis and treatment of various viral infections.

Recent Progress in Aptamer Discoveries and Modifications for Therapeutic Applications

Aptamers are oligonucleotide sequences with a length of about 25-80 bases which have abilities to bind to specific target molecules that rival those of monoclonal antibodies. They are attracting great attention in diverse clinical translations on account of their various advantages, including prolonged storage life, little batch-to-batch differences, very low immunogenicity, and feasibility of chemical modifications for enhancing stability, prolonging the half-life in serum, and targeted delivery. In this Review, we demonstrate the emerging aptamer discovery technologies in developing advanced techniques for producing aptamers with high performance consistently and efficiently as well as requiring less cost and resources but offering a great chance of success. Further, the diverse modifications of aptamers for therapeutic applications including therapeutic agents, aptamer-drug conjugates, and targeted delivery materials are comprehensively summarized.

Recent Progress and Opportunities for Nucleic Acid Aptamers

Coined three decades ago, the term aptamer and directed evolution have now reached their maturity. The concept that nucleic acid could modulate the activity of target protein as ligand emerged from basic science studies of viruses. Aptamers are short nucleic acid sequences capable of specific, high-affinity molecular binding, which allow for therapeutic and diagnostic applications. Compared to traditional antibodies, aptamers have several advantages, including small size, flexible structure, good biocompatibility, and low immunogenicity. In vitro selection method is used to isolate aptamers that are specific for a desired target from a randomized oligonucleotide library. The first aptamer drug, Macugen, was approved by FDA in 2004, which was accompanied by many studies and clinical investigations on various targets and diseases. Despite much promise, most aptamers have failed to meet the requisite safety and efficacy standards in human clinical trials. Amid these setbacks, the emergence of novel technologies and recent advances in aptamer and systematic evolution of ligands by exponential enrichment (SELEX) design are fueling hope in this field. The unique properties of aptamer are gaining renewed interest in an era of COVID-19. The binding performance of an aptamer and reproducibility are still the key issues in tackling current hurdles in clinical translation. A thorough analysis of the aptamer binding under varying conditions and the conformational dynamics is warranted. Here, the challenges and opportunities of aptamers are reviewed with recent progress.

Circular and linear: a tale of aptamer selection for the activation of SIRT1 to induce death in cancer cells

It is a challenge to select the right target to treat conditions without affecting non-diseased cells. Cancer belongs to the top 10 causes of death in the world and it remains difficult to treat. Amongst cancer emerging targets, silent information regulator 1 (SIRT1) - a histone deacetylase - has shown many roles in cancer, ageing and metabolism. Here we report novel SIRT1 ligands that bind and modulate the activity of SIRT1 within cells and enhance its enzymatic activity. We developed a modified aptamer capable of binding to and forming a complex with SIRT1. Our ligands are aptamers, they can be made of DNA or RNA oligonucleotides, their binding domain can recognise a target with very high affinity and specificity. We used the systematic evolution of ligands by exponential enrichment (SELEX) technique to develop circular and linear aptamers selectively binding to SIRT1. Cellular consequences of the interaction were monitored by fluorescence microscopy, cell viability assay, stability and enzymatic assays. Our results indicate that from our pool of aptamers, circular AC3 penetrates cancerous cells and is recruited to modulate the SIRT1 activity. This modulation of SIRT1 resulted in anticancer activity on different cancer cell lines. Furthermore, this modified aptamer showed no toxicity on one non-cancerous cell line and was stable in human plasma. We have demonstrated that aptamers are efficient tools for localisation of internal cell targets, and in this particular case, anticancer activity through modulation of SIRT1.

Novel DEK-Targeting Aptamer Delivered by a Hydrogel Microneedle Attenuates Collagen-Induced Arthritis

DEK protein is critical to the formation of neutrophil extracellular traps (NETs) in rheumatoid arthritis (RA). Blocking DEK using the aptamer DTA via articular injection has been shown to have robust anti-inflammatory efficacy in a previous study. However, DTA is prone to nuclease degradation and renal clearance in vivo. RA is a systemic disease that involves multiple joints, and local injection is impractical in clinical settings. In this study, DTA was modified with methoxy groups on all deoxyribose sugar units and inverted deoxythymidine on the 3' end (DTA4) to enhance its stability against nuclease. DTA4 is stable for 72 h in 90% mouse serum and maintains a high binding affinity to DEK. DTA4 effectively inhibits the formation of NETs and the migration of HUVECs in vitro. DTA4 was then modified with cholesterol on its 5' end to form DTA6. DTA6 dramatically reduces DEK expression in inflammatory RAW264.7 cells. A hydrogel microneedle (hMN) was then fabricated for the transdermal delivery of DTA6. The hMN maintains morphological integrity after absorbing the aptamer solution, effectively pierces the skin, and rapidly releases DTA6 into the dermis. The DTA6-loaded hMN significantly attenuates inflammation and protects joints from cartilage/bone erosion in collagen-induced arthritis (CIA) mice.

Isolation of DNA Aptamer Targeting PD-1 with an Antitumor Immunotherapy Effect

Immune checkpoints play a vital role in regulating T cell responses. Programmed cell death 1 (PD-1), a key inhibitory immune-checkpoint receptor, negatively regulates the human immune response. Anti-PD-1 therapy is an immune-checkpoint inhibition therapy, which is a progressing clinical strategy in treating various human cancers. Aptamers, called "chemical antibodies", have several virtues, including better tissue penetration, lower immunogenicity, and ease of production. Here, after 10 rounds of selection using engineered cells with PD-1 overexpression as target cells, we successfully isolated four anti-PD-1 aptamer candidates using cell-SELEX (systematic evolution of ligands by exponential enrichment) procedure. Among them, the candidate PD4S showed the highest affinity with an equilibrium dissociation constant (K_d) of 10.3 nM and rescued the T cell function suppressed by PD-1/PD-L1. Treatment of PD4S in the CT26 carcinoma model showed an antitumor effect. Together, the anti-PD-1 aptamer PD4S could be applied as an alternative agent in immunotherapy.

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.

The Efficacy of Cholesterol-Based Carriers in Drug Delivery

Several researchers have reported the use of cholesterol-based carriers in drug delivery. The presence of cholesterol in cell membranes and its wide distribution in the body has led to it being used in preparing carriers for the delivery of a variety of therapeutic agents such as anticancer, antimalarials and antivirals. These cholesterol-based carriers were designed as micelles, nanoparticles, copolymers, liposomes, etc. and their routes of administration include oral, intravenous and transdermal. The biocompatibility, good bioavailability and biological activity of cholesterol-based carriers make them potent prodrugs. Several in vitro and in vivo studies revealed cholesterol-based carriers potentials in delivering bioactive agents. In this manuscript, a critical review of the efficacy of cholesterol-based carriers is reported.

Dendrimer- and polymeric nanoparticle-aptamer bioconjugates as nonviral delivery systems: a new approach in medicine

Aptamers are RNA or DNA oligonucleotides interacting to form unique 3D target conformations with high affinity and specificity, and are emerging as a powerful class of ligands for therapeutic applications. In addition, dendrimers are well-defined nano-sized symmetric polymeric molecules. In this review, we provide an analysis of the use of dendrimers modified with aptamers as nonviral vectors to specifically target tumor cells. Various anticancer agents have been encapsulated with dendrimers complexing with aptamers, including epirubicin, camptothecin, Bcl-xL short hairpin (sh)RNA, and 5-fluorouracil rhodamine-labeled dextran. Other types of polymeric nanoparticle (NP)-aptamer bioconjugates have also been developed and loaded with Pt(IV) derivatives, to target specific tumor cells.

Strategies to bioengineer aptamer-driven nanovehicles as exceptional molecular tools for targeted therapeutics: A review

Aptamers are a class of folded nucleic acid strands capable of binding to different target molecules with high affinity and selectivity. Over the years, they have gained a substantial amount of interest as promising molecular tools for numerous medical applications, particularly in targeted therapeutics. However, only the different treatment approaches and current developments of aptamer-drug therapies have been discussed so far, ignoring the crucial technical and functional aspects of constructing a therapeutically effective aptamer-driven drug delivery system that translates to improved in-vivo performance. Hence, this paper provides a comprehensive review of the strategies used to improve the therapeutic performance of aptamer-guided delivery systems. We focus on the different functional features such as drug deployment, payload capacity, in-vivo stability and targeting efficiency to further our knowledge in enhancing the cell-specific delivery of aptamer-drug conjugates. Each reported strategy is critically discussed to emphasize both the benefits provided in comparison with other similar techniques and to outline their potential drawbacks with respect to the molecular properties of the aptamers, the drug and the system to be designed. The molecular architecture and design considerations for an efficient aptamer-based delivery system are also briefly elaborated.

Aptamers as Therapeutic Agents: Has the Initial Euphoria Subsided?

Aptamers are synthetic DNA or RNA oligonucleotide ligands with great potential for therapeutic applications. A vast number of disease-related targets have been used to identify agonistic, antagonistic, or inhibitory aptamers, or aptamer-based targeting ligands. However, only a few aptamers have reached late-stage clinical trials so far and the commercial infrastructure is still far behind that of other therapeutic agents such as monoclonal antibodies. The desirable properties of aptamers such as selectivity, chemical flexibility, or cost-efficiency are faced by challenges, including a short half-life in vivo, immunogenicity, and entrapment in cellular organelles. Aptamer research is still in an early stage, and a deeper understanding of their structure, target interactions, and pharmacokinetics is necessary to catch up to the clinical market. In this review, we will discuss the benefits and limitations in the development of therapeutic aptamers, as well as the advances and future directions of aptamer research. The progress towards effective therapies seems to be slow, but it has not stopped and the best is yet to come.

Aptamer-guided extracellular vesicle theranostics in oncology

In the past decade, the study of exosomes, nanosized vesicles (50-150 nm) released into the extracellular space via the fusion of multivesicular bodies with the plasma membrane, has burgeoned with impressive achievements in theranostics applications. These nanosized vesicles have emerged as key players in homeostasis and in the pathogenesis of diseases owing to the variety of the cargos they can carry, the nature of the molecules packaged inside the vesicles, and the robust interactions between exosomes and target cells or tissues. Accordingly, the development of exosome-based liquid biopsy techniques for early disease detection and for monitoring disease progression marks a new era of precision medicine in the 21st century. Moreover, exosomes possess intrinsic properties - a nanosized structure and unique "homing effects" - that make them outstanding drug delivery vehicles. In addition, targeted exosome-based drug delivery systems can be further optimized using active targeting ligands such as nucleic acid aptamers. Indeed, the aptamers themselves can function as therapeutic and/or diagnostic tools based on their attributes of unique target-binding and non-immunogenicity. This review aims to provide readers with a current picture of the research on exosomes and aptamers and their applications in cancer theranostics, highlighting recent advances in their transition from the bench to the clinic.

Aptamers Chemistry: Chemical Modifications and Conjugation Strategies

Soon after they were first described in 1990, aptamers were largely recognized as a new class of biological ligands that can rival antibodies in various analytical, diagnostic, and therapeutic applications. Aptamers are short single-stranded RNA or DNA oligonucleotides capable of folding into complex 3D structures, enabling them to bind to a large variety of targets ranging from small ions to an entire organism. Their high binding specificity and affinity make them comparable to antibodies, but they are superior regarding a longer shelf life, simple production and chemical modification, in addition to low toxicity and immunogenicity. In the past three decades, aptamers have been used in a plethora of therapeutics and drug delivery systems that involve innovative delivery mechanisms and carrying various types of drug cargos. However, the successful translation of aptamer research from bench to bedside has been challenged by several limitations that slow down the realization of promising aptamer applications as therapeutics at the clinical level. The main limitations include the susceptibility to degradation by nucleases, fast renal clearance, low thermal stability, and the limited functional group diversity. The solution to overcome such limitations lies in the chemistry of aptamers. The current review will focus on the recent arts of aptamer chemistry that have been evolved to refine the pharmacological properties of aptamers. Moreover, this review will analyze the advantages and disadvantages of such chemical modifications and how they impact the pharmacological properties of aptamers. Finally, this review will summarize the conjugation strategies of aptamers to nanocarriers for developing targeted drug delivery systems.

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.

Aptamers: A Review of Their Chemical Properties and Modifications for Therapeutic Application

Aptamers are short, single-stranded oligonucleotides that bind to specific target molecules. The shape-forming feature of single-stranded oligonucleotides provides high affinity and excellent specificity toward targets. Hence, aptamers can be used as analogs of antibodies. In December 2004, the US Food and Drug Administration approved the first aptamer-based therapeutic, pegaptanib (Macugen), targeting vascular endothelial growth factor, for the treatment of age-related macular degeneration. Since then, however, no aptamer medication for public health has appeared. During these relatively silent years, many trials and improvements of aptamer therapeutics have been performed, opening multiple novel directions for the therapeutic application of aptamers. This review summarizes the basic characteristics of aptamers and the chemical modifications available for aptamer therapeutics.

Screening of aptamers and their potential application in targeted diagnosis and therapy of liver cancer

Aptamers are a class of single oligonucleotide molecules (DNA or RNA) that are screened from random DNA or RNA oligonucleotide chain libraries by the systemic evolution of ligands by exponential enrichment technology. The selected aptamers are capable of specifically binding to different targeting molecules, which is achieved by the three-dimensional structure of aptamers. Aptamers are similar in function to monoclonal antibodies, and therefore, they are also referred to as "chemical antibodies". Due to their high affinity and specificity and low immunogenicity, aptamers are topics of intense interest in today's biological targeting research especially in tumor research. They not only have high potential for clinical advances in tumor targeting detection but also are highly promising as targeted tumor drug carriers for use in tumor therapy. Various experimental studies have shown that aptamer-based diagnostic and therapeutic methods for liver cancer have great potential for application. This paper summarizes the structure, characteristics, and screening methods of aptamers and reviews the recent research progress on nucleic acid aptamers in the targeted diagnosis and treatment of liver cancer.

A Mirror Image Fluorogenic Aptamer Sensor for Live-Cell Imaging of MicroRNAs

Development of biocompatible tools for intracellular imaging of RNA expression remains a central challenge. Herein, we report the use of heterochiral strand-displacement to sequence-specifically interface endogenous d-miRNAs with an l-RNA version of the fluorogenic aptamer Mango III, thereby generating a novel class of biocompatible miRNA sensors. Fluorescence activation of the sensor is achieved through the displacement of an achiral blocking strand from the l-Mango aptamer by the d-RNA target. In contrast to d-Mango, we show that the l-Mango sensor retains full functionality in serum, enabling a light-up fluorescence response to the target. Importantly, we employ a self-delivering version of the l-Mango sensor to image the expression of microRNA-155 in living cells, representing the first time l-oligonucleotides have been interfaced with a living system. Overall, this work provides a new paradigm for the development of biocompatible hybridization-based sensors for live-cell imaging of RNAs and greatly expands the utility of fluorogenic aptamers for cellular applications.

Aptamer Chimeras for Therapeutic Delivery: The Challenging Perspectives

Nucleic acid-based aptamers have emerged as efficient delivery carriers of therapeutics. Thanks to their unique features, they can be, to date, considered one of the best targeting moieties, allowing the specific recognition of diseased cells and avoiding unwanted off-target effects on healthy tissues. In this review, we revise the most recent contributes on bispecific and multifunctional aptamer therapeutic chimeras. We will discuss key examples of aptamer-mediated delivery of nucleic acid and peptide-based therapeutics underlying their great potentiality and versatility. Achieved objectives and challenges will be highlighted as well.

Aptamer-iRNAs as Therapeutics for Cancer Treatment

Aptamers are single-stranded oligonucleotides (ssDNA or ssRNA) that bind and recognize their targets with high affinity and specificity due to their complex tertiary structure. Aptamers are selected by a method called SELEX (Systematic Evolution of Ligands by EXponential enrichment). This method has allowed the selection of aptamers to different types of molecules. Since then, many aptamers have been described for the potential treatment of several diseases including cancer. It has been described over the last few years that aptamers represent a very useful tool as therapeutics, especially for cancer therapy. Aptamers, thanks to their intrinsic oligonucleotide nature, present inherent advantages over other molecules, such as cell-based products. Owing to their higher tissue penetrability, safer profile, and targeting capacity, aptamers are likely to become a novel platform for the delivery of many different types of therapeutic cargos. Here we focus the review on interfering RNAs (iRNAs) as aptamer-based targeting delivered agents. We have gathered the most reliable information on aptamers as targeting and carrier agents for the specific delivery of siRNAs, shRNA, microRNAs, and antisense oligonucleotides (ASOs) published in the last few years in the context of cancer therapy.

SELEX methods on the road to protein targeting with nucleic acid aptamers

Systematic evolution of ligand by exponential enrichment (SELEX) is an efficient method used to isolate high-affinity single stranded oligonucleotides from a large random sequence pool. These SELEX-derived oligonucleotides named aptamer, can be selected against a broad spectrum of target molecules including proteins, cells, microorganisms and chemical compounds. Like antibodies, aptamers have a great potential in interacting with and binding to their targets through structural recognition and are therefore called "chemical antibodies". However, aptamers offer advantages over antibodies including smaller size, better tissue penetration, higher thermal stability, lower immunogenicity, easier production, lower cost of synthesis and facilitated conjugation or modification with different functional moieties. Thus, aptamers represent an attractive substitution for protein antibodies in the fields of biomarker discovery, diagnosis, imaging and targeted therapy. Enormous interest in aptamer technology triggered the development of SELEX that has underwent numerous modifications since its introduction in 1990. This review will discuss the recent advances in SELEX methods and their advantages and limitations. Aptamer applications are also briefly outlined in this review.

Aptamer chemistry

Aptamers are single-stranded DNA or RNA molecules capable of tightly binding to specific targets. These functional nucleic acids are obtained by an in vitro Darwinian evolution method coined SELEX (Systematic Evolution of Ligands by EXponential enrichment). Compared to their proteinaceous counterparts, aptamers offer a number of advantages including a low immunogenicity, a relative ease of large-scale synthesis at affordable costs with little or no batch-to-batch variation, physical stability, and facile chemical modification. These alluring properties have propelled aptamers into the forefront of numerous practical applications such as the development of therapeutic and diagnostic agents as well as the construction of biosensing platforms. However, commercial success of aptamers still proceeds at a weak pace. The main factors responsible for this delay are the susceptibility of aptamers to degradation by nucleases, their rapid renal filtration, suboptimal thermal stability, and the lack of functional group diversity. Here, we describe the different chemical methods available to mitigate these shortcomings. Particularly, we describe the chemical post-SELEX processing of aptamers to include functional groups as well as the inclusion of modified nucleoside triphosphates into the SELEX protocol. These methods will be illustrated with successful examples of chemically modified aptamers used as drug delivery systems, in therapeutic applications, and as biosensing devices.

Therapeutic aptamers in discovery, preclinical and clinical stages

The aptamer field witnessed steady growth during the past 28 years as evident from the exponentially increasing number of related publications. The field is "coming of age", but like other biomedical research areas facing a global push towards translational research to carry ideas from bench- to bedside, there is pressure to show impact for aptamers at the clinical end. Being easy-to-make, non-immunogenic, stable and high-affinity nano-ligands, aptamers are perfectly poised to move in this direction. They can specifically bind targets ranging from small molecules to complex multimeric structures, making them potentially useful in a limitless variety of therapeutic approaches. This review will summarize efforts made to accomplish the therapeutic promise of aptamers, with a focus on aptamers directly acting as therapeutic molecules, rather than those used in targeted delivery of other drugs. The review will showcase representative examples at various stages of development, covering different disease categories.

Oligonucleotide aptamers: promising and powerful diagnostic and therapeutic tools for infectious diseases

The entire human population is at risk of infectious diseases worldwide. Thus far, the diagnosis and treatment of human infectious diseases at the molecular and nanoscale levels have been extremely challenging tasks because of the lack of effective probes to identify and recognize biomarkers of pathogens. Oligonucleotide aptamers are a class of small nucleic acid ligands that are composed of single-stranded DNA (ssDNA) or RNA and act as affinity probes or molecular recognition elements for a variety of targets. These aptamers have an exciting potential for diagnose and/or treatment of specific diseases. In this review, we highlight areas where aptamers have been developed as diagnostic and therapeutic agents for both bacterial and viral infectious diseases as well as aptamer-based detection.