Aptamers: novelty tools for cancer biology

Abstract

Although the term 'cancer' was still over two thousand years away of being coined, the first known cases of the disease date back to about 3000BC, in ancient Egypt. Five thousand years later, still lacking a cure, it has become one of the leading causes of death, killing over half a dozen million people yearly. So far, monoclonal antibodies are the most successful immune-therapy tools when it comes to fighting cancer. The number of clinical trials that use them has been increasing steadily during the past few years, especially since the Food and Drug Administration greenlit the use of the first immune-checkpoint blockade antibodies. However, albeit successful, this approach does come with the cost of auto-inflammatory toxicity. Taking this into account, the development of new therapeutic reagents with low toxicity becomes evident, particularly ones acting in tandem with the tools currently at our disposal. Ever since its discovery in the early nineties, aptamer technology has been used for a wide range of diagnostic and therapeutic applications. With similar properties to those of monoclonal antibodies, such as high-specificity of recognition and high-affinity binding, and the advantages of being developed using in vitro selection procedures, aptamers quickly became convenient building blocks for the generation of multifunctional constructs. In this review, we discuss the steps involved in the in vitro selection process that leads to functional aptamers - known as Systematic Evolution of Ligands by Exponential Enrichment - as well as the most recent applications of this technology in diagnostic and treatment of oncological illnesses. Moreover, we also suggest ways to improve such use.

Keywords: Cell-SELEX; aptamers; cancer; clinical application.

Figure 1. Depiction of the aptamer structure and its interaction with the target

(A) The ssDNA is subjected to a set of conditions that enable it to fold and adopt a secondary structure. (B) The ssDNA base pairs in the linear parts of the molecule interact via hydrogen bonds, (C) in the loops, however, the bases are free to interact with the targets and do so via hydrogen bonds and/or dipolar interactions.

Figure 2. Schematic representation of the SELEX process

(A) Not only is the process iterative, but also (B) with each round, the stringency is increased, thus resulting in higher affinity and specificity. This increase is achieved by increasing both the number of washes and non-specific competitors, and decreasing the number of target molecules within every cycle.

Figure 3. Strategies to enhance bio-therapeutic therapies in cancer using aptamers

Aptamer-siRNA chimeras can be used to stimulate receptor internalization, which leads to siRNA delivery within the cytoplasm. Aptamers can also be used to induce cellular uptake of therapeutic agents (drugs, other aptamers, nanoparticles, polymers) which can act in different ways by apoptosis induction, lower RNA expression, tumor regression, and other mechanisms.