Current Topics in Medicinal Chemistry - Volume 15, Issue 12, 2015

Aptamers- short nucleic acid or peptide ligands- attracted considerable attention across various disciplines as target specific therapeutic, delivery vehicle and diagnostic tools. The applicability of aptamers - often termed as ‘chemical antibodies’ – continues to grow and possess certain edge over the antibody counterpart. In this special issue, we have invited seven contributors to highlight the chemical biology, therapeutic, biosensor and imaging applications of aptamers. Furthermore, the issue end with an outlook of aptamer technology after 25 years of history.

Apatamer technology has been around for a quarter of a century and the field had matured enough to start seeing real applications, especially in the medical field. Since their discovery, aptamers rapidly emerged as key players in many fields, such as diagnostics, drug discovery, food science, drug delivery and therapeutics. Because of their synthetic nature, aptamers are evolving at an exponential rate gaining from the newest advances in chemistry, nanotechnology, biology and medicine. This review is meant to give an overview of the aptamer field, by including general aspects of aptamer identification and applications as well as highlighting certain features that contribute to their quick deployment in the biomedical field.

Peptide aptamers are small combinatorial proteins that are selected to bind to specific sites on their target molecules. Peptide aptamers consist of short, 5-20 amino acid residues long sequences, typically embedded as a loop within a stable protein scaffold. Various peptide aptamer scaffolds and in vitro and in vivo selection techniques are reviewed with emphasis on specific biomedical, bioimaging, and bioanalytical applications.

The use of aptamers as targeting molecules has revolutionized nanomedicine. Aptamers are short oligonucleotides or peptides that can be used as targeting ligands against numerous targets. Aptamers are versatile moieties that can be used both in therapeutic and targeting applications. Aptamers bind to the target in a process that is similar to the lock and key mechanism. There are numerous reports on the use of aptamers as targeting moieties for imaging and drug delivery applications. In this review, we would like to give a comprehensive report on aptamers, its properties and the developments in aptamer mediated drug delivery in the recent years.

Aptamers represent the novel class of oligonucleotides holding multiple applications in the area of biomedicine. The advancements introduced with the Systematic Evolution of Ligands by EXponential enrichment (SELEX) approach further eased the scope of producing modified aptamers within a short span yet retaining the properties of stability and applicability. In the recent times, aptamers were identified to have the potential for penetrating into the deep human crevices and thus can be utilized in addressing the issues of complex neurological disorders. Considering the specificity and stability enhancement by chemical modifications, aptamer-based nanotechnologies may have great potential for future therapeutics and diagnostics (theranostics). The research community has already witnessed success with the approval of macugen (an anti-vascular endothelial growth factor aptamer) for treating degenerating eye disease, and hopefully those that are in the clinical trials will soon be translated for human application. Herein, we have summarized the aptamer chemistry, aptamer-nanoconjugates and their applications against neurological diseases.

Aptamers have been increasingly applied in biomedical field as a class of biorecognition elements that possess many advantages such as high specificity and binding affinity, easy synthesis, easy modification, small size, non-toxicity and good stability. Many diseases like cancer exhibit cellular aberrations at morphological and molecular levels. Medical diagnosis based on molecular features can be highly specific and extremely sensitive when proper recognition molecule and an efficient signal transduction system are employed. However, bioanalysis of human diseases at the molecular level is an extremely challenging field because effective probes to identify and recognize biomarkers of diseases are not readily available. Traditional bio-recognition molecule, antibody has been exploited to develop excellent diagnosis assays in many formats, but antibodies are insufficient to match the requirements of fast and portable biosensors for point-of-care applications, which are at high demand in pathogenic bacteria detection as well as other diseases like cancer. Aptamers are short single-stranded oligonucleotides, which can be selected from random combinatorial library by SELEX in vitro. This relatively new biorecognition agent has superior intrinsic characteristics for biosensor development. In this review, we first present major aptamer selection technologies and the main formats of biosensors, which were frequently employed in aptasensor development. Then, the current state of aptamers as applied to medical diagnosis was discussed for specifically cancer and pathogen diagnosis. Finally, an overview of aptamer-nanomaterials conjugates was presented in many applications such as diagnosis, bioimaging, and theranostics.

Aptamers are single-stranded oligonucleotides with high affinity and specificity to the target molecules or cells, thus they can serve as an important category of molecular targeting ligand. Since their discovery, aptamers have been rapidly translated into clinical practice. The strong target affinity/selectivity, cost-effectivity, chemical versatility and safety of aptamers are superior to traditional peptides- or proteins-based ligands which make them unique choices for molecular imaging. Therefore, aptamers are considered to be extremely useful to guide various imaging contrast agents to the target tissues or cells for optical, magnetic resonance, nuclear, computed tomography, ultrasound and multimodality imaging. This review aims to provide an overview of aptamers' advantages as targeting ligands and their application in targeted imaging. Further research in synthesis of new types of aptamers and their conjugation with new categories of contrast agents is required to develop clinically translatable aptamer-based imaging agents which will eventually result in improved patient care.

Twenty-five years after the emergence of powerful molecular recognition elements known as aptamers, these synthetic receptors are finding applications in therapeutics, imaging, diagnostics and biosensors. Comparisons have been drawn between aptamers and antibodies since their earliest days. This review will highlight how some of the most promising and successful aptamer-based applications have exploited the advantages aptamers have over their antibody counterparts and will point to future applications of this enabling technology.