Antibodies aren’t just a critical part of the body’s defense against disease: they’re also the gold standard for biosensing. These natural proteins are widely used in diagnostic tests for cancer and other diseases because they recognize and bind so efficiently to disease markers, which include bacterial and viral proteins and protein fragments. However, protein antibodies are expensive to synthesize in the lab and only last a few months. Now biochemists have developed a method to make artificial antibodies that may be just as effective as the real thing.
Researchers have been working on synthetic antibodies for about twenty years. Artificial antibodies, made from polymers rather than proteins, promise to be cheap and long lasting, says Kenneth Shea, a professor of chemistry at the University of California, Irvine, who led development of the new method. However, no one has been able to make artificial antibodies that bind their targets as tightly and as specifically as natural antibodies do.
To make the new and improved artificial antibodies, Shea and his collaborators at the Tokyo Institute of Technology refined a technique called molecular imprinting. This involves taking a target molecule and placing it in a solution containing the building blocks of a polymer antibody. The polymer then grows around its target, conforming to its shape; once it’s done, the target molecule is rinsed away. Then, when the artificial antibody next meets the target molecule, they fit together like a key in a lock. “You can make a mold around almost any molecule,” explains Klaus Mosbach, founder of the Center for Molecular Imprinting at the Center for Chemistry and Chemical Engineering, in Lund, Sweden, who pioneered the technique.
Molecular imprinting is “a beautiful idea and a great technology,” says Vincent Rotello, a professor of chemistry at the University of Massachusetts Amherst. Plastic antibodies have found some industrial applications, mostly for separating small molecules out of solutions. But they have not been suitable for therapeutic, diagnostic, or biosensing applications because none that have been made work as well as natural antibodies in water-based solutions such as blood. And they could not be made to effectively target large molecules such as proteins.
The researchers developed much better plastic antibodies by enhancing the imprinting process in a novel way. Shea and his collaborators started with a target protein, melittin, and screened libraries of polymer building blocks for those that would bind to it most effectively. “Some effort went into determining the composition that would minimize interaction with [nontarget] garden-variety proteins,” says Shea. His artificial antibody has a high affinity for its target protein, comparable in strength to that of a natural antibody, and also works in water solutions. The synthesis is described in a paper in the Journal of the American Chemical Society.
Shea says that better synthetic antibodies could be used therapeutically in parts of the body where normal antibodies break down quickly, such as the digestive tract. They could also be used in portable devices designed to detect traces of chemical weapons.
The new method for making the artificial antibodies is “a dramatic breakthrough,” says Ken Shimizu, a chemist at the University of South Carolina. “People always say their polymers work a lot like antibodies, but in truth, they never did.” He adds that Shea’s work comes close to living up to this promise. “This is the first time you could imagine these [synthetics] would rival antibodies.”
David Spivak, a chemistry professor at Louisiana State University, agrees that Shea’s work is “the first example of an imprinted polymer acting like an antibody.” Spivak expects that Shea’s method will work with other target molecules, including proteins besides melittin. Indeed, Shea says that he is working on targeting about five more complex proteins.
However, some chemists are skeptical that Shea’s artificial protein is targeting melittin like a real antibody. They suggest that melittin, which has positively charged regions, may simply be attracted to negative charge on the polymer. But Shea says he selected polymer building blocks based on their ability to bind regions of melittin, and that he has performed controlled experiments to demonstrate that the binding is indeed specific. Further, he says that he has unpublished work showing that his polymer binds to melittin in cell cultures and in small animals, suppressing its toxic activity.