Inhibition of von Willebrand factor-mediated platelet activation and thrombosis by the anti-von Willebrand factor A1-domain aptamer ARC1779

Ischaemic Stroke, Thromboembolism and Clot Structure

Ischaemic stroke is a major cause of morbidity and mortality worldwide. Blood clotting and thromboembolism play a central role in the pathogenesis of ischaemic stroke. An increasing number of recent studies indicate changes in blood clot structure and composition in patients with ischaemic stroke. In this review, we aim to summarise and discuss clot structure, function and composition in ischaemic stroke, including its relationships with clinical diagnosis and treatment options such as thrombolysis and thrombectomy. Studies are summarised in which clot structure and composition is analysed both in vitro from patients' plasma samples and ex vivo in thrombi obtained through interventional catheter-mediated thrombectomy. Mechanisms that drive clot composition and architecture such as neutrophil extracellular traps and clot contraction are also discussed. We find that, while in vitro clot structure in plasma samples from ischaemic stroke patients are consistently altered, showing denser clots that are more resistant to fibrinolysis, current data on the composition and architecture of ex vivo clots obtained by thrombectomy are more variable. With the potential of advances in technologies underpinning both the imaging and retrieving of clots, we expect that future studies in this area will generate new data that is of interest for the diagnosis, optimal treatment strategies and clinical management of patients with ischaemic stroke.

Discovery platforms for RNA therapeutics

RNA therapeutics are emerging as a unique opportunity to drug currently "undruggable" molecules and diseases. While their advantages over conventional, small molecule drugs, their therapeutic implications and the tools for their effective in vivo delivery have been extensively reviewed, little attention has been so far paid to the technological platforms exploited for the discovery of RNA therapeutics. Here, we provide an overview of the existing platforms and ex vivo assays for RNA discovery, their advantages and disadvantages, as well as their main fields of application, with specific focus on RNA therapies that have reached either phase 3 or market approval.

Polyamines promote xenobiotic nucleic acid synthesis by modified thermophilic polymerase mutants

The enzymatic synthesis of xenobiotic nucleic acids (XNA), which are artificially sugar-modified nucleic acids, is essential for the preparation of XNA libraries. XNA libraries are used in the in vitro selection of XNA aptamers and enzymes (XNAzymes). Efficient enzymatic synthesis of various XNAs can enable the screening of high-quality XNA aptamers and XNAzymes by expanding the diversity of XNA libraries and adding a variety of properties to XNA aptamers and XNAzymes. However, XNAs that form unstable duplexes with DNA, such as arabino nucleic acid (ANA), may dissociate during enzyme synthesis at temperatures suitable for thermophilic polymerases. Thus, such XNAs are not efficiently synthesised by the thermophilic polymerase mutants at the end of the sequence. This undesirable bias reduces the possibility of generating high-quality XNA aptamers and XNAzymes. Here, we demonstrate that polyamine-induced DNA/ANA duplex stabilisation promotes ANA synthesis that is catalysed by thermophilic polymerase mutants. Several polyamines, including spermine, spermidine, cadaverine, and putrescine promote ANA synthesis. The negative effect of polyamines on the fidelity of ANA synthesis was negligible. We also showed that polyamines promote the synthesis of other XNAs, including 2'-amino-RNA/2'-fluoro-RNA mixture and 2'-O-methyl-RNA. In addition, we found that polyamine promotes DNA synthesis from the 2'-O-methyl-RNA template. Polyamines, with the use of thermophilic polymerase mutants, may allow further development of XNA aptamers and XNAzymes by promoting the transcription and reverse transcription of XNAs.

Aptamers targeting SARS-CoV-2 nucleocapsid protein exhibit potential anti pan-coronavirus activity

Emerging and recurrent infectious diseases caused by human coronaviruses (HCoVs) continue to pose a significant threat to global public health security. In light of this ongoing threat, the development of a broad-spectrum drug to combat HCoVs is an urgently priority. Herein, we report a series of anti-pan-coronavirus ssDNA aptamers screened using Systematic Evolution of Ligands by Exponential Enrichment (SELEX). These aptamers have nanomolar affinity with the nucleocapsid protein (NP) of Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and also show excellent binding efficiency to the N proteins of both SARS, MERS, HCoV-OC43 and -NL63 with affinity KD values of 1.31 to 135.36 nM. Such aptamer-based therapeutics exhibited potent antiviral activity against both the authentic SARS-CoV-2 prototype strain and the Omicron variant (BA.5) with EC50 values at 2.00 nM and 41.08 nM, respectively. The protein docking analysis also evidenced that these aptamers exhibit strong affinities for N proteins of pan-coronavirus and other HCoVs (-229E and -HKU1). In conclusion, we have identified six aptamers with a high pan-coronavirus antiviral activity, which could potentially serve as an effective strategy for preventing infections by unknown coronaviruses and addressing the ongoing global health threat.

Novel strategies in antithrombotic therapy: targeting thrombosis while preserving hemostasis

Antithrombotic therapy is a delicate balance between the benefits of preventing a thrombotic event and the risks of inducing a major bleed. Traditional approaches have included antiplatelet and anticoagulant medications, require careful dosing and monitoring, and all carry some risk of bleeding. In recent years, several new targets have been identified, both in the platelet and coagulation systems, which may mitigate this bleeding risk. In this review, we briefly describe the current state of antithrombotic therapy, and then present a detailed discussion of the new generation of drugs that are being developed to target more safely existing or newly identified pathways, alongside the strategies to reverse direct oral anticoagulants, showcasing the breadth of approaches. Combined, these exciting advances in antithrombotic therapy bring us closer than we have ever been to the "holy grail" of the field, a treatment that separates the hemostatic and thrombotic systems, preventing clots without any concurrent bleeding risk.

Success probability of high-affinity DNA aptamer generation by genetic alphabet expansion

Nucleic acid aptamers as antibody alternatives bind specifically to target molecules. These aptamers are generated by isolating candidates from libraries with random sequence fragments, through an evolutionary engineering system. We recently reported a high-affinity DNA aptamer generation method that introduces unnatural bases (UBs) as a fifth letter into the library, by genetic alphabet expansion. By incorporating hydrophobic UBs, the affinities of DNA aptamers to target proteins are increased over 100-fold, as compared with those of conventional aptamers with only the natural four letters. However, there is still plenty of room for improvement of the methods for routinely generating high-affinity UB-containing DNA (UB-DNA) aptamers. The success probabilities of the high-affinity aptamer generation depend on the existence of the aptamer candidate sequences in the initial library. We estimated the success probabilities by analysing several UB-DNA aptamers that we generated, as examples. In addition, we investigated the possible improvement of conventional aptamer affinities by introducing one UB at specific positions. Our data revealed that UB-DNA aptamers adopt specific tertiary structures, in which many bases including UBs interact with target proteins for high affinity, suggesting the importance of the UB-DNA library design. This article is part of the theme issue 'Reactivity and mechanism in chemical and synthetic biology'.

bioTCIs: Middle-to-Macro Biomolecular Targeted Covalent Inhibitors Possessing Both Semi-Permanent Drug Action and Stringent Target Specificity as Potential Antibody Replacements

Monoclonal antibody therapies targeting immuno-modulatory targets such as checkpoint proteins, chemokines, and cytokines have made significant impact in several areas, including cancer, inflammatory disease, and infection. However, antibodies are complex biologics with well-known limitations, including high cost for development and production, immunogenicity, a limited shelf-life because of aggregation, denaturation, and fragmentation of the large protein. Drug modalities such as peptides and nucleic acid aptamers showing high-affinity and highly selective interaction with the target protein have been proposed alternatives to therapeutic antibodies. The fundamental limitation of short in vivo half-life has prevented the wide acceptance of these alternatives. Covalent drugs, also known as targeted covalent inhibitors (TCIs), form permanent bonds to target proteins and, in theory, eternally exert the drug action, circumventing the pharmacokinetic limitation of other antibody alternatives. The TCI drug platform, too, has been slow in gaining acceptance because of its potential prolonged side-effect from off-target covalent binding. To avoid the potential risks of irreversible adverse drug effects from off-target conjugation, the TCI modality is broadening from the conventional small molecules to larger biomolecules possessing desirable properties (e.g., hydrolysis resistance, drug-action reversal, unique pharmacokinetics, stringent target specificity, and inhibition of protein-protein interactions). Here, we review the historical development of the TCI made of bio-oligomers/polymers (i.e., peptide-, protein-, or nucleic-acid-type) obtained by rational design and combinatorial screening. The structural optimization of the reactive warheads and incorporation into the targeted biomolecules enabling a highly selective covalent interaction between the TCI and the target protein is discussed. Through this review, we hope to highlight the middle to macro-molecular TCI platform as a realistic replacement for the antibody.

Selection and identification of a novel ssDNA aptamer targeting human skeletal muscle

Skeletal muscle disorders have posed great threats to health. Selective delivery of drugs and oligonucleotides to skeletal muscle is challenging. Aptamers can improve targeting efficacy. In this study, for the first time, the human skeletal muscle-specific ssDNA aptamers (HSM01, etc.) were selected and identified with Systematic Evolution of Ligands by Exponential Enrichment (SELEX). The HSM01 ssDNA aptamer preferentially interacted with human skeletal muscle cells in vitro. The in vivo study using tree shrews showed that the HSM01 ssDNA aptamer specifically targeted human skeletal muscle cells. Furthermore, the ability of HSM01 ssDNA aptamer to target skeletal muscle cells was not affected by the formation of a disulfide bond with nanoliposomes in vitro or in vivo, suggesting a potential new approach for targeted drug delivery to skeletal muscles via liposomes. Therefore, this newly identified ssDNA aptamer and nanoliposome modification could be used for the treatment of human skeletal muscle diseases.

Targeting treatment of bladder cancer using PTK7 aptamer-gemcitabine conjugate

BACKGROUND

Gemcitabine (GEM) is one of the first-line chemotherapies for bladder cancer (BC), but the GEMs cannot recognize cancer cells and have a low long-term response rate and high recurrence rate with side effects during the treatment of BC. Targeted transport of GEMs to mediate cytotoxicity to tumor and avoid the systemic side effects remains a challenge in the treatment of BC.

METHODS

Based on a firstly confirmed biomarker in BC-protein tyrosine kinase 7 (PTK7), which is overexpressed on the cell membrane surface in BC cells, a novel targeting system protein tyrosine kinase 7 aptamer-Gemcitabine conjugate (PTK7-GEMs) was designed and synthesized using a specific PTK7 aptamer and GEM through auto-synthesis method to deliver GEM against BC. In addition, the antitumor effects and safety evaluation of PTK7-GEMs was assessed with a series of in vitro and in vivo assays.

RESULTS

PTK7-GEMs can specifically bind and enter to BC cells dependent on the expression levels of PTK7 and via the macropinocytosis pathway, which induced cytotoxicity after GEM cleavage from PTK7-GEMs respond to the intracellular phosphatase. Moreover, PTK7-GEMs showed stronger anti-tumor efficacy and excellent biosafety in three types of tumor xenograft mice models.

CONCLUSION

These results demonstrated that PTK7-GEMs is a successful targeted aptamer-drug conjugates strategy (APDCs) to treat BC, which will provide new directions for the precision treatment of BC in the field of biomarker-oriented tumor targeted therapy.

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.

Factors Associated with Platelet Activation-Recent Pharmaceutical Approaches

Platelets are at the forefront of human health and disease following the advances in their research presented in past decades. Platelet activation, their most crucial function, although beneficial in the case of vascular injury, may represent the initial step for thrombotic complications characterizing various pathologic states, primarily atherosclerotic cardiovascular diseases. In this review, we initially summarize the structural and functional characteristics of platelets. Next, we focus on the process of platelet activation and its associated factors, indicating the potential molecular mechanisms involving inflammation, endothelial dysfunction, and miRs. Finally, an overview of the available antiplatelet agents is being portrayed, together with agents possessing off-set platelet-inhibitory actions, while an extensive presentation of drugs under investigation is being given.

Aptamers: Potential Diagnostic and Therapeutic Agents for Blood Diseases

Aptamers are RNA/DNA oligonucleotide molecules that specifically bind to a targeted complementary molecule. As potential recognition elements with promising diagnostic and therapeutic applications, aptamers, such as monoclonal antibodies, could provide many treatment and diagnostic options for blood diseases. Aptamers present several superior features over antibodies, including a simple in vitro selection and production, ease of modification and conjugation, high stability, and low immunogenicity. Emerging as promising alternatives to antibodies, aptamers could overcome the present limitations of monoclonal antibody therapy to provide novel diagnostic, therapeutic, and preventive treatments for blood diseases. Researchers in several biomedical areas, such as biomarker detection, diagnosis, imaging, and targeted therapy, have widely investigated aptamers, and several aptamers have been developed over the past two decades. One of these is the pegaptanib sodium injection, an aptamer-based therapeutic that functions as an anti-angiogenic medicine, and it is the first aptamer approved by the U.S. Food and Drug Administration (FDA) for therapeutic use. Several other aptamers are now in clinical trials. In this review, we highlight the current state of aptamers in the clinical trial program and introduce some promising aptamers currently in pre-clinical development for blood diseases.

Single-molecule imaging of von Willebrand factor reveals tension-dependent self-association

von Willebrand factor (VWF) is an ultralong concatemeric protein important in hemostasis and thrombosis. VWF molecules can associate with other VWF molecules, but little is known about the mechanism. Hydrodynamic drag exerts tensile force on surface-tethered VWF that extends it and is maximal at the tether point and declines linearly to 0 at the downstream free end. Using single-molecule fluorescence microscopy, we directly visualized the kinetics of binding of free VWF in flow to surface-tethered single VWF molecules. We showed that self-association requires elongation of tethered VWF and that association increases with tension in tethered VWF, reaches half maximum at a characteristic tension of ∼10 pN, and plateaus above ∼25 pN. Association is reversible and hence noncovalent; a sharp decrease in shear flow results in rapid dissociation of bound VWF. Tethered primary VWF molecules can recruit more than their own mass of secondary VWF molecules from the flow stream. Kinetics show that instead of accelerating, the rate of accumulation decreases with time, revealing an inherently self-limiting self-association mechanism. We propose that this may occur because multiple tether points between secondary and primary VWF result in lower tension on the secondary VWF, which shields more highly tensioned primary VWF from further association. Glycoprotein Ibα (GPIbα) binding and VWF self-association occur in the same region of high tension in tethered VWF concatemers; however, the half-maximal tension required for activation of GPIbα is higher, suggesting differences in molecular mechanisms. These results have important implications for the mechanism of platelet plug formation in hemostasis and thrombosis.

Oligonucleotide aptamers: Recent advances in their screening, molecular conformation and therapeutic applications

Aptamers are single stranded oligonucleotides with specific recognition and binding ability to target molecules, which can be obtained by Systematic Evolution of Ligands by Exponential Enrichment (SELEX). Aptamers have the advantages of low molecular weight, low immunogenicity, easy modification and high stability. They play promising role in promoting food safety, monitoring the environment and basic research, especially in clinical diagnosis and therapeutic drugs. To date, great achievements regarding the selection, modifications and application of aptamers have been made. However, since it is still a challenge to obtain aptamers with high affinity in a more effective way, few aptamer-based products have already successfully entered into clinical use. This review aims to provide a thorough overview of the latest advances in this rapidly developing field, focusing on aptamer screening methods for different targets, the structure of the interaction between aptamers and target substances, and the challenges and potential of current therapeutic aptamers.

Von Willebrand factor: A key glycoprotein involved in thrombo-inflammatory complications of COVID-19

COVID-19 is an ongoing public health emergency that has affected millions of people worldwide and is still a threat to many more. One of the pathophysiological features of COVID-19 is associated with the activation of vascular endothelial cells (ECs) leading to the disruption of vascular integrity, coagulation and inflammation. An interlink mechanism between coagulation and inflammatory pathways has been reported in COVID-19. Multiple components are involved in these pathological pathways. Out of all, Von Willebrand Factor (VWF) is one of the primary components of coagulation pathway and also a mediator of vascular inflammation that plays an important role in thrombo-inflammation that further leads to acute respiratory distress syndrome (ARDS). The thrombo-inflammatory co-morbidities such as hyper-coagulation, thrombosis, ARDS etc. have become the major cause of mortality in the patients of COVID-19 admitted to the ICU. Thus, VWF can be explored as a potential target to manage COVID-19 associated co-morbidities. Supporting this hypothesis, there are literature reports which disclose previous attempts to target VWF for the management of thrombo-inflammation in other pathological conditions. The current report summarizes emerging insights into the pathophysiology, mechanism(s), diagnosis, management and foundations for research on this less explored clinically relevant glycoprotein as coagulation biomarker in COVID-19.

Targeting von Willebrand factor in liver diseases: A novel therapeutic strategy?

Acute and chronic liver disease are associated with substantial alterations in the hemostatic system. Evidence from both experimental and clinical studies suggests that anticoagulants slow the progression of liver disease. Efficacy of those anticoagulant drugs is, in part, attributed to a reduction of microthrombi formation within the liver. Although anticoagulant drugs show promising results, bleeding risk associated with these drugs is an obvious drawback, particularly in patients with a complex coagulopathy driven by decreased liver function. Identifying therapies that reduce intrahepatic thrombosis with minimal bleeding risk would significantly advance the field. Among the hemostatic alterations observed in patients are substantially increased levels of the platelet-adhesive protein von Willebrand factor (VWF). In contrast, levels of A Disintegrin and Metalloproteinase with Thrombospondin motifs, the enzyme that regulates VWF activity, are significantly reduced in patients with liver disease. Highly elevated VWF levels are proposed to accelerate intrahepatic thrombus formation and thus be a driver of disease progression. Strong clinical evidence suggesting a link between liver disease and changes in VWF is now being matched by emerging mechanistic data showing a detrimental role for VWF in the progression of liver disease. This review focuses on clinical and experimental evidence supporting a connection between VWF function and the progression of acute and chronic liver diseases. Furthermore, with the recent anticipated approval of several novel therapies targeting VWF, we discuss potential strategies and benefits of targeting VWF as an innovative therapy for patients with liver disease.

Structural Biology for the Molecular Insight between Aptamers and Target Proteins

Aptamers are promising therapeutic and diagnostic agents for various diseases due to their high affinity and specificity against target proteins. Structural determination in combination with multiple biochemical and biophysical methods could help to explore the interacting mechanism between aptamers and their targets. Regrettably, structural studies for aptamer–target interactions are still the bottleneck in this field, which are facing various difficulties. In this review, we first reviewed the methods for resolving structures of aptamer–protein complexes and for analyzing the interactions between aptamers and target proteins. We summarized the general features of the interacting nucleotides and residues involved in the interactions between aptamers and proteins. Challenges and perspectives in current methodologies were discussed. Approaches for determining the binding affinity between aptamers and target proteins as well as modification strategies for stabilizing the binding affinity of aptamers to target proteins were also reviewed. The review could help to understand how aptamers interact with their targets and how alterations such as chemical modifications in the structures affect the affinity and function of aptamers, which could facilitate the optimization and translation of aptamers-based theranostics.

Overview of the Therapeutic Potential of Aptamers Targeting Coagulation Factors

Aptamers are single-stranded DNA or RNA sequences that bind target molecules with high specificity and affinity. Aptamers exhibit several notable advantages over protein-based therapeutics. Aptamers are non-immunogenic, easier to synthesize and modify, and can bind targets with greater affinity. Due to these benefits, aptamers are considered a promising therapeutic candidate to treat various conditions, including hematological disorders and cancer. An active area of research involves developing aptamers to target blood coagulation factors. These aptamers have the potential to treat cardiovascular diseases, blood disorders, and cancers. Although no aptamers targeting blood coagulation factors have been approved for clinical use, several aptamers have been evaluated in clinical trials and many more have demonstrated encouraging preclinical results. This review summarized our knowledge of the aptamers targeting proteins involved in coagulation, anticoagulation, fibrinolysis, their extensive applications as therapeutics and diagnostics tools, and the challenges they face for advancing to clinical use.

Aptamers used for biosensors and targeted therapy

Aptamers are single-stranded nucleic acid sequences that can bind to target molecules with high selectivity and affinity. Most aptamers are screened in vitro by a combinatorial biology technique called systematic evolution of ligands by exponential enrichment (SELEX). Since aptamers were discovered in the 1990s, they have attracted considerable attention and have been widely used in many fields owing to their unique advantages. In this review, we present an overview of the advancements made in aptamers used for biosensors and targeted therapy. For the former, we will discuss multiple aptamer-based biosensors with different principles detected by various signaling methods. For the latter, we will focus on aptamer-based targeted therapy using aptamers as both biotechnological tools for targeted drug delivery and as targeted therapeutic agents. Finally, challenges and new perspectives associated with these two regions were further discussed. We hope that this review will help researchers interested in aptamer-related biosensing and targeted therapy research.

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.

Novel aptamer to von Willebrand factor A1 domain (TAGX-0004) shows total inhibition of thrombus formation superior to ARC1779 and comparable to caplacizumab

von Willebrand factor (VWF) is a blood glycoprotein that plays an important role in platelet thrombus formation through interaction between its A1 domain and platelet glycoprotein Ib. ARC1779, an aptamer to the VWF A1 domain, was evaluated in a clinical trial for acquired thrombotic thrombocytopenic purpura (aTTP). Subsequently, caplacizumab, an anti-VWF A1 domain nanobody, was approved for aTTP in Europe and the United States. We recently developed a novel DNA aptamer, TAGX-0004, to the VWF A1 domain; it contains an artificial base and demonstrates high affinity for VWF. To compare the effects of these three agents on VWF A1, their ability to inhibit ristocetin- or botrocetin-induced platelet aggregation under static conditions was analyzed, and the inhibition of thrombus formation under high shear stress was investigated in a microchip flow chamber system. In both assays, TAGX-0004 showed stronger inhibition than ARC1779, and had comparable inhibitory effects to caplacizumab. The binding sites of TAGX-0004 and ARC1779 were analyzed with surface plasmon resonance performed using alanine scanning mutagenesis of the VWF A1 domain. An electrophoretic mobility shift assay showed that R1395 and R1399 in the A1 domain bound to both aptamers. R1287, K1362, and R1392 contributed to ARC1779 binding, and F1366 was essential for TAGX-0004 binding. Surface plasmon resonance analysis of the binding sites of caplacizumab identified five amino acids in the VWF A1 domain (K1362, R1392, R1395, R1399, and K1406). These results suggested that TAGX-0004 possessed better pharmacological properties than caplacizumab in vitro and might be similarly promising for aTTP treatment.

Nucleic acid therapeutics: a focus on the development of aptamers

INTRODUCTION

Aptamers provide exciting opportunities for the development of specific and targeted therapeutic approaches.

AREAS COVERED

In this review, the authors discuss different therapeutic options available with nucleic acids, including aptamers, focussing on similarities and differences between them. The authors concentrate on case studies with specific aptamers, which exemplify their distinct advantages. The reasons for failure, wherever available, are deliberated upon. Attempts to accelerate the in vitro selection process have been discussed. Challenges with aptamers in terms of their specificity and targeted delivery and strategies to overcome these are described. Examples of precise regulation of systemic half-life of aptamers using antidotes are discussed.

EXPERT OPINION

Despite their nontoxic nature, a variety of reasons limit the therapeutic potential of aptamers in the clinic. The analysis of adverse effects observed with the pegnivacogin/anivamersen pair has highlighted the need to screen for preexisting PEG antibodies in any clinical trial involving pegylated molecules. Surprisingly, and promisingly, the ability of nucleic acid therapeutics to breach the blood brain barrier seems achievable. The recognition of specific motifs, e.g. G-quadruplex in thrombin-binding aptamers, or a 'nucleation' zone while designing aptamer-antidote pairs, is likely to accelerate the discovery of therapeutically efficacious molecules.

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.

Potent and rapid reversal of the von Willebrand factor inhibitor aptamer BT200

BACKGROUND

BT200, a pegylated form of the aptamer BT100, inhibits binding of von Willebrand factor (VWF) to platelet glycoprotein GPIb, preventing arterial thrombosis in cynomolgus monkeys. It is being developed for secondary prevention of arterial thrombosis such as stroke or myocardial infarction. Inhibition of thrombogenesis by BT200 is expected to provide a therapeutic benefit. However, there may be unexpected bleeding (eg, incidental trauma) in which a reversal agent is required. To address this need, BT101, a complementary aptamer, has been developed to specifically inhibit BT100 and BT200 function.

OBJECTIVES

To characterize the effects of BT101 both in vitro and in vivo.

METHODS

The direct interaction between BT101 and the core aptamer BT100 was evaluated using polyacrylamide gel electrophoresis. The binding of BT200 to purified human VWF and inhibition of VWF activity was further characterized using enzyme-linked immunosorbent assay. VWF-dependent platelet function was measured by the platelet function analyzer and aggregometry in whole blood. In addition, both the in vivo pharmacokinetic profile of BT101 as well as its ability to reverse BT200 activity, were evaluated in cynomolgus monkeys.

RESULTS

BT101 bound to the core aptamer BT100 at a 1:1 ratio, inhibited BT200 binding to purified human VWF, and reversed BT200-induced inhibition of both VWF activity and VWF-dependent platelet function in vitro. After intravenous injection to monkeys, BT101 reversed BT200-induced effects on VWF activity and platelet function within minutes, without causing any adverse effects.

CONCLUSIONS

The results of this study demonstrate that BT101 is an effective reversal agent for BT200.

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.

The development and characterization of a long acting anti-thrombotic von Willebrand factor (VWF) aptamer

BACKGROUND

Thrombus formation involves coagulation proteins and platelets. The latter, referred to as platelet-mediated thrombogenesis, is predominant in arterial circulation. Platelet thrombogenesis follows vascular injury when extracellular von Willebrand factor (VWF) binds via its A3 domain to exposed collagen, and the free VWF A1 domain binds to platelet glycoprotein Ib (GPIb).

OBJECTIVES

To characterize the antiplatelet/antithrombotic activity of the pegylated VWF antagonist aptamer BT200 and identify the aptamer VWF binding site.

METHODS

BT100 is an optimized aptamer synthesized by solid-phase chemistry and pegylated (BT200) by standard conjugation chemistry. The affinity of BT200 for purified human VWF was evaluated as was VWF inhibition in monkey and human plasma. Efficacy of BT200 was assessed in the monkey FeCl3 femoral artery thrombosis model.

RESULTS

BT200 bound human VWF at an EC50 of 5.0 nmol/L and inhibited VWF A1 domain activity in monkey and human plasma with mean IC50 values of 183 and 70 nmol/L. BT200 administration to cynomolgus monkeys caused a time-dependent and dose-dependent effect on VWF A1 domain activity and inhibited platelet function as measured by collagen adenosine diphosphate closure time in the platelet function analyzer. BT200 demonstrated a bioavailability of ≥77% and exhibited a half-life of >100 hours after subcutaneous injection. The treatment effectively prevented arterial occlusion in an FeCl3 -induced thrombosis model in monkeys.

CONCLUSIONS

BT200 has shown promising inhibition of human VWF in vitro and prevented arterial occlusion in non-human primates. These data including a long half-life after subcutaneous injections provide a strong rationale for ongoing clinical development of BT200.

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.

Novel Antiplatelet Agents in Cardiovascular Disease

Antiplatelet therapy reduces atherothrombotic risk and has therefore become a cornerstone in the treatment of cardiovascular disease. Aspirin, adenosine diphosphate P2Y12 receptor antagonists, glycoprotein IIb/IIIa inhibitors, and the thrombin receptor blocker vorapaxar are effective antiplatelet agents but significantly increase the risk of bleeding. Moreover, atherothrombotic events still impair the prognosis of many patients with cardiovascular disease despite established antiplatelet therapy. Over the last years, advances in the understanding of thrombus formation and hemostasis led to the discovery of various new receptors and signaling pathways of platelet activation. As a consequence, many new antiplatelet agents with high antithrombotic efficacy and supposedly only moderate effects on regular hemostasis have been developed and yielded promising results in preclinical and early clinical studies. Although their long journey from animal studies to randomized clinical trials and finally administration in daily clinical routine has just begun, some of the new agents may in the future become meaningful additions to the pharmacological armamentarium in cardiovascular disease.

Shear-Dependent Platelet Aggregation: Mechanisms and Therapeutic Opportunities

Cardiovascular diseases (CVD) are the number one cause of morbidity and death worldwide. As estimated by the WHO, the global death rate from CVD is 31% wherein, a staggering 85% results from stroke and myocardial infarction. Platelets, one of the key components of thrombi, have been well-investigated over decades for their pivotal role in thrombus development in healthy as well as diseased blood vessels. In hemostasis, when a vascular injury occurs, circulating platelets are arrested at the site of damage, where they are activated and aggregate to form hemostatic thrombi, thus preventing further bleeding. However, in thrombosis, pathological activation of platelets occurs, leading to uncontrolled growth of a thrombus, which in turn can occlude the blood vessel or embolize, causing downstream ischemic events. The molecular processes causing pathological thrombus development are in large similar to the processes controlling physiological thrombus formation. The biggest challenge of anti-thrombotics and anti-platelet therapeutics has been to decouple the pathological platelet response from the physiological one. Currently, marketed anti-platelet drugs are associated with major bleeding complications for this exact reason; they are not effective in targeting pathological thrombi without interfering with normal hemostasis. Recent studies have emphasized the importance of shear forces generated from blood flow, that primarily drive platelet activation and aggregation in thrombosis. Local shear stresses in obstructed blood vessels can be higher by up to two orders of magnitude as compared to healthy vessels. Leveraging abnormal shear forces in the thrombus microenvironment may allow to differentiate between thrombosis and hemostasis and develop shear-selective anti-platelet therapies. In this review, we discuss the influence of shear forces on thrombosis and the underlying mechanisms of shear-induced platelet activation. Later, we summarize the therapeutic approaches to target shear-sensitive platelet activation and pathological thrombus growth, with a particular focus on the shear-sensitive protein von Willebrand Factor (VWF). Inhibition of shear-specific platelet aggregation and targeted drug delivery may prove to be much safer and efficacious approaches over current state-of-the-art antithrombotic drugs in the treatment of cardiovascular diseases.

Humanized GPIbα-von Willebrand factor interaction in the mouse

The interaction of platelet glycoprotein Ibα (GPIbα) with von Willebrand factor (VWF) initiates hemostasis after vascular injury and also contributes to pathological thrombosis. GPIbα binding to the VWF A1 domain (VWFA1) is a target for antithrombotic intervention, but attempts to develop pharmacologic inhibitors have been hindered by the lack of animal models because of the species specificity of the interaction. To address this problem, we generated a knockin mouse with Vwf exon 28-encoding domains A1 and A2 replaced by the human homolog (VWF^h28). VWF^h28 mice (M1HA) were crossbred with a transgenic mouse strain expressing human GPIbα on platelets (mGPIbα^null;hGPIbα^Tg; H1MA) to generate a new strain (H1HA) with humanized GPIbα-VWFA1 binding. Plasma VWF levels in the latter 3 strains were similar to those of wild-type mice (M1MA). Compared with the strains that had homospecific GPIbα-VWF pairing (M1MA and H1HA), M1HA mice of those with heterospecific pairing had a markedly greater prolongation of tail bleeding time and attenuation of thrombogenesis after injury to the carotid artery than H1MA mice. Measurements of GPIbα-VWFA1 binding affinity by surface plasmon resonance agreed with the extent of observed functional defects. Ristocetin-induced platelet aggregation was similar in H1HA mouse and human platelet-rich plasma, and it was comparably inhibited by monoclonal antibody NMC-4, which is known to block human GPIbα-VWFA1 binding, which also inhibited FeCl₃-induced mouse carotid artery thrombosis. Thus, the H1HA mouse strain is a fully humanized model of platelet GPIbα-VWFA1 binding that provides mechanistic and pharmacologic information relevant to human hemostatic and thrombotic disorders.

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.

Rapid selection of single-stranded DNA aptamers binding Staphylococcus epidermidis in platelet concentrates

Staphylococcus epidermidis is the most common transfusion-associated pathogen contaminating platelet concentrates. Methods to reduce or eliminate contaminating bacteria from platelet units are critical for improving the safety of blood transfusions. We used rapid isolation of DNA aptamers (RIDA) to identify single-stranded (ss)DNA aptamers as ligands that specifically bind to S. epidermidis. Five target-specific ssDNA aptamers (76 mer) were obtained under stringent selection conditions. Aptamer SE43 demonstrated higher binding affinity compared with scrambled control. Furthermore, when binding assays were conducted in platelet concentrate, there was a twofold increase in binding affinity compared with the SE43 binding in buffer alone. Our data identified an aptamer that may be useful as a ligand to capture, detect or remove S. epidermidis contaminant from platelet concentrates.

A Mini-Review: Clinical Development and Potential of Aptamers for Thrombotic Events Treatment and Monitoring

The unique opportunity for aptamer uses in thrombotic events has sparked a considerable amount of research in the area. The short half-lives of unmodified aptamers in vivo remain one of the major challenges in therapeutic aptamers. Much of the incremental successful therapeutic aptamer stories were due to modifications in the aptamer bases. This mini-review briefly summarizes the successes and challenges in the clinical development of aptamers for thrombotic events, and highlights some of the most recent developments in using aptamers for anticoagulation monitoring.

Short-Acting Anti-VWF (von Willebrand Factor) Aptamer Improves the Recovery, Survival, and Hemostatic Functions of Refrigerated Platelets

OBJECTIVE

Refrigeration-induced binding of VWF (von Willebrand factor) to platelets contributes to the rapid clearance of refrigerated platelets. In this study, we investigate whether inhibiting VWF binding by a DNA-based aptamer ameliorates the clearance of refrigerated platelets without significantly impeding hemostatic functions. Approach and Results: Platelets were refrigerated with or without aptamer ARC1779 for 48 hours. VWF binding, the effective lifetime of ARC1779, platelet post-transfusion recovery and survival, and the hemostatic function were measured. ARC1779 treatment during refrigeration inhibited the platelet-VWF interaction. ARC1779-treated refrigerated murine platelets exhibited increased post-transfusion recovery and survival than untreated ones (recovery of ARC1779-treated platelets: 76.7±5.5%; untreated: 63.7±0.8%; P<0.01. Half-life: 31.4±2.36 hours versus 28.1±0.86 hours; P<0.05). A similar increase was observed for refrigerated human platelets (recovery: 49.4±4.4% versus 36.8±2.1%, P<0.01; half-life: 9.2±1.5 hours versus 8.7±0.9 hours, ns). The effective lifetime of ARC1779 in mice was 2 hours. Additionally, ARC1779 improved the long-term (2 hours after transfusion) hemostatic function of refrigerated platelets (tail bleeding time of mice transfused with ARC1779-treated refrigerated platelets: 160±65 seconds; untreated: 373±96 seconds; P<0.01). The addition of an ARC1779 antidote before transfusion improved the immediate (15 minutes after transfusion) hemostatic function (bleeding time of treated platelets: 149±21 seconds; untreated: 320±36 seconds; P<0.01).

CONCLUSIONS

ARC1779 improves the post-transfusion recovery of refrigerated platelets and preserves the long-term hemostatic function of refrigerated platelets. These results suggest that a short-acting inhibitor of the platelet-VWF interaction may be a potential therapeutic option to improve refrigeration of platelets for transfusion treatment.

Nucleic Acid Therapies for Ischemic Stroke

Stroke remains a leading cause of disability and death worldwide despite significant scientific and therapeutic advances. Therefore, there is a critical need to improve stroke prevention and treatment. In this review, we describe several examples that leverage nucleic acid therapeutics to improve stroke care through prevention, acute treatment, and recovery. Aptamer systems are under development to increase the safety and efficacy of antithrombotic and thrombolytic treatment, which represent the mainstay of medical stroke therapy. Antisense oligonucleotide therapy has shown some promise in treating stroke causes that are genetically determined and resistant to classic prevention approaches such as elevated lipoprotein (a) and cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL). Targeting microRNAs may be attractive because they regulate factors involved in neuronal cell death and reperfusion-associated injury, as well as neurorestorative pathways. Lastly, microRNAs may aid reliable etiologic classification of stroke subtypes, which is important for effective secondary stroke prevention.

Preclinical Development of a vWF Aptamer to Limit Thrombosis and Engender Arterial Recanalization of Occluded Vessels

Endothelial surface and circulating glycoprotein von Willebrand factor (vWF) regulates platelet adhesion and is associated with thrombotic diseases, including ischemic stroke, myocardial infarction, and peripheral vascular disease. Thrombosis, as manifested in these diseases, is the leading cause of disability and death in the western world. Current parenteral antithrombotic and thrombolytic agents used to treat these conditions are limited by a short therapeutic window, irreversibility, and major risk of hemorrhage. To overcome these limitations, we developed a novel anti-vWF aptamer, called DTRI-031, that selectively binds and inhibits vWF-mediated platelet adhesion and arterial thrombosis while enabling rapid reversal of this antiplatelet activity by an antidote oligonucleotide (AO). Aptamer DTRI-031 exerts dose-dependent inhibition of platelet aggregation and thrombosis in whole blood and mice, respectively. Moreover, DTRI-031 can achieve potent vascular recanalization of platelet-rich thrombotic occlusions in murine and canine carotid arteries. Finally, DTRI-031 activity is rapidly (<5 min) and completely reversed by AO administration in a murine saphenous vein hemorrhage model, and murine toxicology studies indicate the aptamer is well tolerated. These findings suggest that targeting vWF with an antidote-controllable aptamer potentially represents an effective and safer treatment for thrombosis patients having platelet-rich arterial occlusions in the brain, heart, or periphery.

Targeting hormone refractory prostate cancer by in vivo selected DNA libraries in an orthotopic xenograft mouse model

The targeting of specific tissue is a major challenge for the effective use of therapeutics and agents mediating this targeting are strongly demanded. We report here on an in vivo selection technology that enables the de novo identification of pegylated DNA aptamers pursuing tissue sites harbouring a hormone refractory prostate tumour. To this end, two libraries, one of which bearing an 11 kDa polyethylene glycol (PEG) modification, were used in an orthotopic xenograft prostate tumour mouse model for the selection process. Next-generation sequencing revealed an in vivo enriched pegylated but not a naïve DNA aptamer recognising prostate cancer tissue implanted either subcutaneous or orthotopically in mice. This aptamer represents a valuable and cost-effective tool for the development of targeted therapies for prostate cancer. The described selection strategy and its analysis is not limited to prostate cancer but will be adaptable to various tissues, tumours, and metastases. This opens the path towards DNA aptamers being experimentally and clinically engaged as molecules for developing targeted therapy strategies.

Cardiovascular therapies utilizing targeted delivery of nanomedicines and aptamers

Cardiovascular ailments are the foremost trigger of death in the world today, including myocardial infarction and ischemic heart diseases. To date, extraordinary measures have been prescribed, from the perspectives of both conventional medical therapies and surgeries, to enforce cardiac cell regeneration post cardiac traumas, albeit with limited long-term success. The prospects of successful heart transplants are also grim, considering exorbitant costs and unavailability of suitable donors in most cases. From the perspective of cardiac revascularization, use of nanoparticles and nanoparticle mediated targeted drug delivery have garnered substantial attention, attributing to both active and passive heart targeting, with enhanced target specificity and sensitivity. This review focuses on this aspect, while outlining the progress in targeted delivery of nanomedicines in the prognosis and subsequent therapy of cardiovascular disorders, and recapitulating the benefits and intrinsic challenges associated with the incorporation of nanoparticles. This article categorically provides an overview of nanoparticle-mediated targeted delivery systems and their implications in handling cardiovascular diseases, including their intrinsic benefits and encountered procedural trials and challenges. Additionally, the solicitations of aptamers in targeted drug delivery with identical objectives, are presented. This includes a detailed appraisal on various aptamer-navigated nanoparticle targeted delivery platforms in the diagnosis and treatment of cardiovascular maladies. Despite a few impending challenges, subject to additional investigations, both nanoparticles as well as aptamers show a high degree of promise, and pose as the next generation of drug delivery vehicles, in targeted cardiovascular therapy.

Investigations on the interface of nucleic acid aptamers and binding targets

Nucleic acid aptamers are single-stranded DNA or RNA of 20-100 nucleotides in length that have attracted substantial scientific interest due to their ability to specifically bind to target molecules via the formation of three-dimensional structures. Compared to traditional protein antibodies, aptamers have several advantages, such as their small size, high binding affinity, specificity, flexible structure, being chemical synthesizable and modifiable, good biocompatibility, high stability and low immunogenicity, which all contribute to their widely applications in the biomedical field. To date, much progress has been made in the study and applications of aptamers, however, detailed information on how aptamers bind to their targets is still scarce. Over the past few decades, many methods have been introduced to investigate the aptamer-target binding process, such as measuring the main kinetic or thermodynamic parameters, detecting the structural changes of the binding complexes, etc. Apart from traditional physicochemical methods, various types of molecular docking programs have been applied to simulate the aptamer-target interactions, while these simulations also have limitations. To facilitate the further research on the interactions, herein, we provide a brief review to illustrate the recent advances in the study of aptamer-target interactions. We summarize the binding targets of aptamers, such as small molecules, macromolecules, and even cells. Their binding constants (KD) are also summarized. Methods to probe the aptamer-target binding process, such as surface plasmon resonance (SPR), circular dichroism spectroscopy (CD), isothermal titration calorimetry (ITC), footprinting assay, truncation and mutation assay, nuclear magnetic resonance spectroscopy (NMR), X-ray crystallography and molecular docking simulation are indicated. The binding forces mediating the aptamer-target interactions, such as hydrogen bonding, electrostatic interaction, the hydrophobic effect, π-π stacking and van der Waals forces are summarized. The challenges and future perspectives are also discussed.

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.

Aptamers in the Therapeutics and Diagnostics Pipelines

Aptamers are short single-stranded DNA or RNA oligonucleotides that can selectively bind to small molecular ligands or protein targets with high affinity and specificity, by acquiring unique three-dimensional structures. Aptamers have the advantage of being highly specific, relatively small in size, non-immunogenic and can be easily stabilized by chemical modifications, thus allowing expansion of their diagnostic and therapeutic potential. Since the invention of aptamers in the early 1990s, great efforts have been made to make them clinically relevant for diseases like macular degeneration, cancer, thrombosis and inflammatory diseases. Furthermore, owing to the aforementioned advantages and unique adaptability of aptamers to point-of-care platforms, aptamer technology has created a stable niche in the field of in vitro diagnostics by enhancing the speed and accuracy of diagnoses. The aim of this review is to give an overview on aptamers, highlight the inherent therapeutic and diagnostic opportunities and challenges associated with them and present various aptamers that have reached therapeutic clinical trials, diagnostic markets or that have immediate translational potential for therapeutics and diagnostics applications.

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.

Probing and characterizing the high specific sequences of ssDNA aptamer against SGIV-infected cells

As the major viral pathogen of grouper aquaculture, Singapore grouper iridovirus (SGIV) has caused great economic losses in China and Southeast Asia. In the previous study, we have generated highly specific ssDNA aptamers against SGIV-infected grouper spleen cells (GS) by Systematic Evolution of Ligands by Exponential Enrichment technology (SELEX), in which Q2 had the highest binding affinity of 16.43 nM. In this study, we would try to identify the specific sequences in the aptamer Q2 that exhibited the high binding affinity to SGIV-infected cells by truncating the original Q2 into some different specific segments. We first evaluated the specificity and binding affinity of these truncated aptamers to SGIV-infected cells by flow cytometry, fluorescent imaging of cells and aptamer-based enzyme-linked apta-sorbent assay (ELASA). We then performed cytotoxicity analysis, assessment of the inhibitory effects upon SGIV infection and the celluar internalization kinetics of each truncated aptamer. Compared to the initial Q2, one of the truncated aptamer Q2-C5 showed a 3-fold increase in the binding affinity for SGIV-infected cells, and held more effective inhibitory effects, higher internalization kinetics and stability. Hence, the aptamer's truncated methods could be applied in the research of identifying aptamer's key sequences. The shorter, structure optimizing aptamer showed more excellent performance over the originally selected aptamer, which could potentially be applied in developing commercial detection probes for the early and rapid diagnosis of SGIV infection, and highly specific therapeutic drugs against SGIV infection.