Aptamers are a class of functional oligonucleotides that have been engineered by artificial combinatorial methodologies of in vitro selection or systematic evolution of ligands by exponential enrichment that can bind a wide variety of specific targets with affinities and specificities that rival antibodies (Keefe et al., 2010).
From: Biomedical Applications of Functionalized Nanomaterials, 2018
Brendan O’Farrell, in The Immunoassay Handbook (Fourth Edition), 2013
Alternative capture reagents for lateral flow applications – aptamer technology
One of the critical elements to be addressed in the design and development of rapid assays for POC is the issue of thermal stability. The majority of LFIAs require controlled storage conditions for optimal stability. Certain assays are completely intolerant of higher temperatures. This is largely a function of the stability of the binding reagents used in the system. This requirement for temperature control, even if the assays do not necessarily have to be kept at low temperatures, imposes cost on the supply chain and also may prevent the use of the test in high-temperature field environments. This is a major issue for diagnostics in developing world applications or in, for example, biowarfare, agricultural, or veterinary testing applications in the developed world.
The use of aptamers represents an alternative to antibodies for assays that require high thermal stability.
Aptamers are oligonucleic acid or peptide molecules that bind to a specific target molecule. Aptamers are usually created by selecting them from a large random sequence pool. More specifically, aptamers can be classified as
• DNA or RNA aptamers. They consist of (usually short) strands of oligonucleotides.
• Peptide aptamers. They consist of a short variable peptide domain, attached at both ends to a protein scaffold.
Nucleic acid aptamers are nucleic acid species that have been engineered through repeated rounds of in vitro selection. Aptamers are useful in biotechnological and therapeutic applications as they offer molecular recognition properties that rival that of antibodies. In addition to their discriminate recognition, aptamers offer advantages over antibodies as they can be engineered completely in a test tube, are readily produced by chemical synthesis, possess desirable storage properties, and elicit little or no immunogenicity in therapeutic applications.
In 1990, two labs independently developed the technique of selection: the Gold lab, using the term systematic evolution of ligands by exponential enrichment (SELEX) for their process of selecting RNA ligands against T4 DNA polymerase and the Szostak lab, coining the term in vitro selection, selecting RNA ligands against various organic dyes. The Szostak lab also coined the term aptamer (from the Latin, aptus, meaning “to fit”) for these nucleic acid-based ligands. Two years later, the Szostak lab and Gilead Sciences, independent of one another, used in vitro selection schemes to evolve single stranded DNA ligands for organic dyes and human coagulant, thrombin, respectively. There do not appear to be any systematic differences between RNA and DNA aptamers, save the greater intrinsic chemical stability of DNA.
Since the discovery of aptamers, many researchers have used aptamer selection as a means for application and discovery. In 2001, the process of in vitro selection was automated by the Ellington lab at the University of Texas at Austin and at SomaLogic, Inc. (Boulder, CO), reducing the duration of a selection experiment from 6 weeks to 3 days.
The SELEX process has been most commonly applied in commercial applications. However, alternative selection methods are coming to market now that increase the efficiency of selection. Companies such as Base Pair Biotechnologies (Houston, TX, USA) and BioAptus (Belo Horizonte, Brazil) have developed methods for rapidly screening aptamer libraries and generating highly purified, high-affinity aptamers in fast turnaround times, greatly decreasing the cost of the reagent.
The primary advantages of aptamers for use in rapid diagnostic applications include
• high thermal stability
• tailored specificity and affinity
• ability to generate aptamers to almost any epitope, even nonimmunogenic epitopes
• scalability of production—once selected, the molecules are simply sequenced and produced.
The use of aptamers in lateral flow technology is under investigation at the time of writing. Commercialization of assays based on aptamers has been limited to date, however, the technology shows promise based on the advantages listed above.