Aptamers are short RNA/DNA oligonucleotide molecules that bind specifically to complementary target molecules. As potential recognition elements, aptamers show promising applications in both diagnosis and treatment. They offer diverse therapeutic and diagnostic options for various blood-related diseases such as hematologic malignancies and hemophilia. In comparison to antibodies, aptamers possess several advantages, including a simple in vitro screening and production process, ease of modification and conjugation, high stability, and low immunogenicity.
Positioned as promising alternatives to antibodies, aptamers have the potential to overcome the limitations of current monoclonal antibody therapies, providing new avenues for the diagnosis, treatment, and preventive care of blood-related diseases. Researchers in the biomedical field, focusing on biomarker detection, diagnostics, imaging, and targeted therapy, have extensively studied aptamers. Over the past two decades, several aptamers have been developed, with Pegaptanib being one notable example. Pegaptanib, based on an aptamer, is a therapeutic drug with anti-angiogenic properties and is the first aptamer approved by the U.S. Food and Drug Administration (FDA) for treatment.
Nucleic acid aptamers are a unique class of synthetically engineered polymers or oligomers based on single-stranded DNA or RNA molecules. They can form secondary and/or tertiary structures to specifically bind to various targets such as proteins, peptides, small molecules, metal ions, bacteria, viruses, and entire living cells. The development of aptamers began in 1990 through the Systematic Evolution of Ligands by Exponential Enrichment (SELEX) process, where Tuerk and Gold pioneered the selection of aptamers in an enriched ligand system. SELEX involves cycles of target binding, selection, and amplification from a large library of random nucleic acid sequences.
Considered promising molecules and chemical equivalents to antibodies, aptamers share comparability with monoclonal antibodies, but they offer certain superior features. Aptamers are cost-effective to produce on a large scale while maintaining high reproducibility and reliability. They are produced using animal-free techniques, providing an alternative to monoclonal antibodies. Aptamers can bind to toxic or non-immunogenic antigens, a feature challenging to achieve with animal-based monoclonal antibody production methods. Their size, ranging between antibodies (150 kDa) and small peptides (1-5 kDa), is approximately 1/20th of an antibody’s volume, allowing better tissue penetration and applicability in solid tumor therapy.
Pegaptanib, developed from the NX1838 aptamer, functions as an anti-angiogenic agent by selectively binding and inhibiting Vascular Endothelial Growth Factor (VEGF). It is the first FDA-approved aptamer for the treatment of ocular vascular diseases. The success of Pegaptanib provides compelling evidence of the potential application of aptamers as a new therapeutic approach.
Aptamer Services at BOC Sciences
| Services | Description |
| Aptamer-Drug Conjugates | Aptamer-drug conjugates (ApDCs) are composed of aptamers, therapeutic agents (e.g. cytotoxic agents, small-molecule degraders, therapeutic proteins, oligonucleotides), and suitable linkers that provide a certain distance and are capable of releasing the payloads at the right place and time, which represent an efficient, targeted delivery system. |
| Aptamer-siRNA conjugates | Aptamer-siRNA conjugates represent a combination of two types of molecules: aptamers and small interfering RNAs (siRNAs). |
| Custom Aptamer Synthesis | BOC Sciences offers aptamer customization services to generate high-quality aptamers tailored to your goals, delivering excellent results even for the most difficult target molecules. |
Aptamer Screening Techniques
The SELEX process is an in vitro selection method used to isolate DNA or RNA sequences that bind specifically to a particular target. Although SELEX is a complex process that typically takes several weeks to complete, recent advancements have enhanced and accelerated aptamer selection, reducing the workload, time, and cost associated with the process. Strategies such as Counting SELEX, Capillary Electrophoresis SELEX, and Electrochemical SELEX have been introduced to modify and optimize SELEX methods, improving the success rate of aptamer selection.
Aptamer Optimization and Modification
Advancements in aptamer development and nanotechnology over the past decades have made aptamers an attractive tool for biomedical applications. Aptamers can be easily chemically modified without compromising their interaction with the target. Such modifications enhance binding affinity, stability, resistance to nucleases, and in vivo stability in the biological environment.
Various polymerases are available to produce more stable aptamer libraries, preventing nucleolytic degradation. Additionally, introducing non-natural nucleotides, such as 2’-fluoro ribose, 2’-amino ribose, 2’-O-methyl ribose, and locked nucleic acids (LNA), enhances stability. SomaLogic’s technology incorporates chemical modifications to aptamers, improving their stability, structural diversity, and robust target-binding capabilities. Modified aptamers, known as SOMAmers, exhibit optimal biological functionality in terms of stability and affinity. Researchers have explored diverse modifications, including heterocycles, hydrophobic moieties, phenyl groups, large naphthyl groups, and the more complex indole, to replace dT bases in DNA libraries, creating shape isomers. Different array technologies based on SOMAmers, such as SOMAscan and SOMApanel, have been utilized in clinical applications.
Clinical Progress in Aptamers for Hematologic Diseases
AS1411, the first aptamer-based anticancer drug, is a unique DNA aptamer composed of a 26-base G-rich oligonucleotide using a novel method to target nucleic acids. AS1411, in combination with high-dose cytarabine, has been used as a cancer-targeting drug. It has undergone phase II clinical trials (NCT00512083 and NCT01034410) for refractory and relapsed acute myeloid leukemia (AML), showing promising results. This combination exhibits a synergistic effect in inhibiting cancer cell growth with acceptable safety, usually associated with cytarabine treatment.
CD33, a transmembrane protein expressed in mature myeloid cells, AML progenitor cells, and normal myeloid precursor cells, has been a significant target for anti-AML therapy. CD33-specific aptamers demonstrate properties comparable to anti-CD33 antibodies in binding and internalization into CD33-positive myeloid cells. These aptamers also show the potential to carry chemotherapy drugs to CD33-positive cells in AML patients. A CD-33 aptamer conjugated with doxorubicin (Dox) forms a Dox-aptamer complex, inhibiting acute myeloid leukemia with CD33 positivity.
The CXCR4/CXCL12 axis plays a crucial role in the homing of multiple myeloma (MM) cells to the bone marrow and the interaction between the bone marrow microenvironment and MM cells. NOX-A12 (olaptesed pegol), a pegylated L-stereoisomer RNA aptamer, binds and neutralizes CXCL12. Anti-CXCL12 aptamers can inhibit tumor-supporting pathways and mobilize CLL cells from their protective microenvironment, inducing apoptosis and chemosensitization in these leukemia cells. In a phase II pilot study, the combination of NOX-A12 with bortezomib and dexamethasone in relapsed or refractory MM patients showed good safety, tolerability, and effective mobilization of MM cells, making it a promising strategy.
Clinical Progress in Aptamers for Other Blood Diseases
ARC1779 is the pegylated form of ARC1172, and pegylation prevents nuclease degradation, stabilizes affinity for the von Willebrand factor (VWF) A1 domain, and inhibits platelet adhesion, aggregation, and thrombus formation. ARC1779 is used to treat VWF-related platelet dysfunction in thrombotic thrombocytopenic purpura (TTP) and patients with VWF-related platelet dysfunction in hemophilia 2B. Clinical trials evaluating the safety, pharmacokinetics, and pharmacodynamics of ARC1779 in patients with VWF-related platelet dysfunction have demonstrated its ability to inhibit platelet aggregation without significant bleeding. A phase II clinical trial investigated the impact of ARC1779 on brain microembolism immediately after carotid endarterectomy; however, the study had to be paused due to insufficient patient recruitment. Nevertheless, crucial observations in acute TTP patients included with plasma exchange therapy suggested that blocking the VWF A1 domain might increase platelet counts.
Tissue factor (TF) is crucial in hemostasis, playing a key role in the initiation of the extrinsic pathway of the coagulation cascade, regulated by tissue factor pathway inhibitor (TFPI). BAX 499, an anti-TFPI aptamer, binds to multiple structural domains of TFPI, presenting a new therapeutic strategy for hemophilia. The first human in vivo and mechanistic validation study of the anti-TFPI aptamer, BAX 499, was conducted in hemophilia patients to test its safety and tolerability.
Sickle cell disease (SCD) is the most common inherited blood disorder, leading to severe complications such as hemolytic anemia, intermittent vascular occlusion, and progressive multi-organ damage. P-selectin is a cell adhesion molecule expressed in activated endothelial cells and platelets. One new therapeutic strategy for managing SCD involves targeting major complications by inhibiting interactions with endothelial cell adhesion. A P-selectin inhibitor based on an aptamer is being researched, demonstrating the potential to inhibit adhesion of sickle red cells and leukocytes to endothelial cells by 90% and 80%, respectively, showing promise as a new treatment for SCD.
The complement system plays a crucial role in several pathophysiological processes, and extreme complement activation is a key factor in the pathogenesis of various diseases, including paroxysmal nocturnal hemoglobinuria (PNH) and atypical hemolytic uremic syndrome (aHUS). Researchers have developed a specific aptamer for human complement C5 using SELEX methods. Currently, ARC1905 (anti-C5 aptamer) is undergoing phase II clinical trials (NCT02686658) for the treatment of AMD patients.
Conclusion
Due to their high binding specificity and affinity, coupled with advantages over antibodies, aptamers have become excellent alternatives in the diagnosis and treatment of blood-related diseases. Aptamers offer a new set of tools for diagnosing, drug delivery, and treating various diseases. The focus on therapeutic aptamers is steadily increasing, with numerous aptamer drugs undergoing conceptual validation studies and different stages of clinical trials, showing tremendous potential in treating diseases of the hematologic system.