Aptamers – the right fit for sensors and therapy

Janice Limson

Sensor technology is getting a lot smarter - not just identifying precisely a certain molecule which can be an indicator of disease, but also delivering a drug at the same time. Aptamers are a new class of biorecognition agents which scientists believe can act as the ultimate courier.

Following on the global success of the glucose sensor that is now used worldwide, electrochemical biosensor technologies have emerged as a promising tool for health diagnostics. The promise is of low-cost, sensitive, accurate and portable devices which can bring rapid answers when they are needed – on-site and without lengthy delays awaiting results from laboratories.

Sensors most people would be familiar with are the so-called lateral flow sensors, devices which for the most part provide a yes-or-no answer. For example, urine strips for pregnancy tests provide a positive result when a specific hormone is present (in sufficient quantities) and interacts with an antibody coated on the strip to give a coloured line. HIV testing in clinics uses a similar approach to search for specific antibodies in blood.

But for those with diabetes, a yes-or-no result is not sufficient – the amount or concentration of glucose in blood is the critical matter. This is where another technology called an electrochemical sensor is so important and why several sensor designs for glucose employ this particular method. While other methods can be used by scientists and engineers to design these sensors to provide actual measurements of concentration of the target molecule, the real attraction of electrochemical sensors is their sensitivity and low-cost.

Few electrochemical sensor technologies have emerged that can target a specific molecule and those which have, such as the glucose sensor, have been hugely successful.

While there are several factors critical to their design, one of the biggest of these delaying the translation of laboratory research (for developing sensors for a wide range of targets) is really centred on the biorecognition agent. This is simply the biological component of these sensors, which can be an enzyme (as in the case with glucose sensors) capable of specifically detecting its target. For a sensor with real marketability, the device needs to be able to withstand reasonable fluctuations in temperature and to retain its activity for long periods. That means that the biological component, if an enzyme is used for example, needs to be stable and inexpensive.

Imagine simple sensors that can detect diseases such as heart disease or cancers  early enough to initiate treatment. Or a sensor that can monitor any deficiencies in levels of antioxidants in your body or perhaps sensors which can detect pathogens in your drinking water … or a milk bottle that can tell whether your milk really is going off. The list is endless. Laboratory-based methods are already well suited to providing many of these answers but when time is a factor or when routine monitoring is needed, rapid, cheap tests can save lives or improve quality of life.

With a rather long list the issue then is not just whether the biorecognition agent is stable and cost-effective, but whether a biorecognition agent can be found that can specifically target just one specific molecule or virus or protein. Importantly, the inclusion of biorecognition agents is also what limits sensors in what they can detect – if you’re trying to monitor a compound, you need a biological molecule that can interact with it in such a way that a signal is generated. These molecules are often very difficult, if not impossible, to either find in nature, or make in a lab for all the things that we’d like to detect. It’s what has restricted these kind of sensors to detect very simple compounds.

By changing the biorecognition agent, the technology has vast applications in the medical, agricultural, environmental, food, and even military markets, besides industrial bioprocess monitoring and control. The demand however for medical sensors outstrips that of any diagnostic sector.

Aptamers

Crystallographic image showing an RNA aptamer (green) binding to its target molecule, AMP. Accession number: 1RAW.pdb

Many scientists believe that molecules known as aptamers can play a lasting role. The word aptamer comes

from the Latin apto meaning “to fit”. Molecules designed to “fit” a specific analyte or target.

There are two main classes of aptamers – nucleic (DNA and RNA) aptamers and peptide aptamers. DNA and RNA aptamers typically consist of between 20 - 80 building blocks termed nucleotides or bases. Nucleotides are the building blocks of DNA and RNA. The beauty of these aptamers is that they can be engineered in a process called SELEX such that they can bind to a wide array of different targets. These targets range from simple molecules to larger proteins and even whole cells. The SELEX process was published in the 1990s and stands for Systematic Evolution of Ligands by Exponential Enrichment. Scientists have also called the process “in vitro selection”.

While the process for generating these aptamers is fascinating in itself, the key point about the aptamers is that they can bind their target specifically, making them particularly attractive for use in biosensors. The scope for generating these and the benefits to biotechnology are simply vast. What has emerged though is that while they tick the box on specificity, unlike antibodies, they also tend to be far more stable, robust and nuch cheaper to produce.

Aptamers are now being routinely researched in the development of cost-effective biosensors for a wide range of different targets. Worldwide the patented database of aptamers is rapidly growing – from  small molecules (such as mycotoxins), proteins (thrombin), viruses (HIV) and human cancers (prostate-specific antigen (PSA), breast cancer), and more effectively opening a wide range of opportunities for the development of new classes of rather smart, selective and effective biosensors.

Several South African research groups (including groups at the Council for Scientific and Industrial Research, Rhodes University, University of the Western Cape and University Cape Town) are actively engaged in designing and researching aptamers for cancers, TB and HIV. The CSIR aptamer group headed by Dr Makobetsa Khati has paved the way nationally with several patents generated in a large study of this field.

But there is more. The ability to specifically bind to a specific target extends the biotechnology application of aptamers from sensors to therapy.

Aptamers in therapy

If bound to drugs, aptamers could serve as a courier, carrying the drug to the right address: a specific target in the body. Several studies in SA and worldwide are examining this approach for cancer treatment and a range of aptamers are currently in various phases of clinical trials for cancer therapeutics.

Another way in which aptamers can work in the field of therapy is as drugs themselves. The first such aptamer-based drug has been licensed for use in treating age-related macular degeneration. Branded as Macugen, the nucleic acid aptamer works by binding and inhibiting vascular endothelial growth factor (VEGF), a protein responsible for the damage to eyesight associated with this illness.

Developments in aptamers for sensors and therapy comes at a time when there is also a parallel revolution in terms of nanotechnology for applications in enhancing drug delivery. One of the further advantages of aptamers is that they can readily be coupled to other compounds (besides drugs) such as nanomaterials which can serve as beacons to help in imaging of diseases once they have reached their target.

Enter Nanotechnology

Nanomaterials can also aid in drug delivery of aptamers and increase the amount of drugs which can be delivered to a diseased site in the body. For direct biosensors, the application of nanomaterials to increase the sensitivity of biosensors is well researched and in South Africa, the DST Mintek Nanotechnology Innovation Centres are driving a comprehensive programme in collaboration with several South African universities to unlock their potential for low cost sensor devices.

The twin benefits of a designer molecule such as an aptamer that can selectively bind to a target coupled with the benefits of nanotechnology is helping to pave the way for new approaches to drug treatment and diagnostics, heralding some exciting times ahead with future benefits to human health.

* Janice Limson is Editor-in-chief of Science in Africa Magazine