Researchers believe that clinical laboratory assays that use aptamers would have multiple advantages when compared to diagnostic tests utilizing anti-bodies
New diagnostic technology has been developed that has the potential to accurately detect such diseases as cancer and diabetes, even when the patient is pre-symptomatic. Not only would medical laboratory tests using this technology be low cost and portable, but some experts think that diagnostic assays using this technology could make it through the regulatory process and be cleared for clinical use in just five years or less.
This highly-sensitive diagnostic technology is able to detect specific proteins in human blood. It was developed by a research team at the University of Toledo in Ohio. Last fall, they published their findings in the Optical Society’s (OSA) open-access journal, Biomedical Optics Express.
These researchers were seeking a way to improve upon diagnostic tests that incorporate anti-bodies. As pathologists and clinical laboratory managers know, the use of antibodies in medical laboratory assays presents certain problems, ranging from cost of reagents to cross-reactivity. The University of Toledo researchers decided to base their diagnostic assays on aptamers, because aptamers connect to only one type of molecule in the body.
There are numerous advantages to using aptamers, when compared with anti-body-based assays. According to Brent D. Cameron, Ph.D., a member of the University of Toledo’s Department of Bioengineering and one of the paper’s authors, clinical laboratory assays incorporating aptamers could provide functionality similar to that of the very large clinical laboratory analyzers currently now used to identify and quantify blood proteins—but at a significantly reduced cost. Aptamers can also be engineered solely in a test tube.
Cost savings, however, are only one of the reasons that the University of Toledo’s findings were cause for excitement for the researchers. In clinical testing, aptamers:
- can tolerate a wide range of pH, both acid and base environments, and salt concentrations;
- have high heat stability;
- are easily synthesized, and, again;
- are cost efficient.
Aptamers also have been found to:
- be readily produced by chemical synthesis;
- possess desirable storage properties, and;
- elicit little or no immunogenicity in therapeutic applications efficient.
Since aptamers connect with free-floating proteins in the blood, it means that specific aptamers can be used to search for target compounds. These can range from small molecules—such as drugs and dyes—to complex biological molecules such as enzymes, peptides, and proteins.
In the next phase of development the researchers plan to pursue commercial use in medical diagnostics. They estimate a timeline of three to five years will be necessary to meet U.S. Food and Drug Administration procedures and filings.
“The time frame is very dependent on the target application area,” stated Cameron. “We are currently determining suitable aptamers for a range of target proteins for both diabetic and cancer-related applications.”
SPR helps bind proteins to aptamers for a fast response time and high sensitivity
In demonstrating their findings, the researchers chose a naturally occurring protein in humans that plays a role in clotting—thrombin and thrombin-binding aptamers. They affixed the aptamers to a glass slide—a sensor surface—coated with a nanoscale layer of gold. When the blood sample gets applied to this testing surface, the aptamers and their corresponding proteins latch together. (Aptamer sensors are also capable of being reversibly denatured: They can easily release their target molecules, making them perfect receptors for biosensing applications.)
To actually determine if the pairings were successful, the researchers used a real-time optical sensing technique known as surface plasmon resonance, or SPR. The surface plasmon is a so-called virtual particle formed by the wave motion of electrons on the sensor’s surface.
If the protein is present and has bound to the aptamer, conditions for which resonance will occur at the gold layer will change. That change is detected with a simple reflectance technique that is coupled to a linear detector. This surface plasmon sensor enabled the research group to demonstrate low sample consumption, high sensitivity, and fast response time.
The direct detection of blood proteins using less-bulky optics is the key to the portability aspect of the design. “By monitoring certain conditions, we can quantify the amount of the target protein that is present, even at very low concentrations,” noted Cameron. “This approach is very robust in that unique aptamers for almost any given protein can be identified. This makes the technique very specific and adaptable for any given application.”
Aptamers have been around for a while. In the early 1990s, several research laboratories, including the Szostak Laboratory, were independently experimenting with what Jack W. Szostak’s group termed “in vitro selection.”
It had affixed the word “aptamers”—Latin for aptus, meaning “to fit”—to describe molecular recognition properties for what were nucleic acid-based ligands. But naming aptamers was not nearly as interesting as discovering that their properties compete quite well with those of antibodies, which are the most commonly used biomolecule.
Flash forward to February 2011: According to a report from Research Australia—a national not-for-profit alliance of organizations and companies engaged in health research— aptamer-related discoveries were published in the international cancer research journal Cancer Science.
In this approach, researchers from Australia’s Deakin University worked with scientists in India and Australia to create the world’s first RNA aptamer. The RNA aptamer is described as a chemical antibody that acts like a guided missile to seek out and bind only to cancer stem cells.
Director of Deakin Medical School’s Nanomedicine Program, Professor Wei Duan, said, “What we have created is the ‘guided missile’ part of a ‘smart bomb’.” He explained how the medical “smart bomb” can target a tumor and bind to the root of the cancer.
The researchers now plan to combine the aptamer with a microscopic fat particle—the medical “smart bomb”— in order to send anti-cancer drugs or diagnostic imaging agents directly to the cancer stem cells for a more effective cancer imaging system for early cancer detection. Duan said, “The medical ‘smart bomb’ opens up exciting possibilities for detection, diagnosis, and treatment of cancer.”
The separate research initiatives at the University of Toledo and Deakin University demonstrate that aptamers may soon be developed into an important diagnostic technology. It is important to note that assays incorporating aptamers are demonstrating significant accuracy and sensitivity to the degree that these researchers are willing to predict that clinical laboratory tests incorporating this technology will be able to detect cancer and other diseases in patients who show no symptoms.
It this were to come to pass, it would allow clinical laboratories and pathology groups to contribute greater value to patients, their physicians, and payers. In a healthcare system now evolving toward value-based reimbursement, this would be a most auspicious development.
—By Carren Bersch
By Larry Gold, Nebojsa Janjic, Thale Jarvis, et al.