US scientists have clicked together synthetic antibodies using the enzymes they want to target as a template. These synthetic antibodies can then be used, in turn, to bind to the enzyme templates they were cast from, which could open up a whole new field of therapeutic molecules. 

Ordinary antibodies can be raised against enzymes and so are used to detect their presence in diagnostic tests. However, there are a few drawbacks to using antibodies, not least their cost. 

'The antibody costs just kill us, and on top of that you have these antibodies that are typically not that stable,' says Jim Heath at Caltech, California, US. 'It's the major cost associated with doing these diagnostic measurements.' So the answer was to find an alternative. 

Heath knew that Barry Sharpless, down the road at the Scripps Research Institute, was using click chemistry to build small molecule inhibitors for drugs. 'We thought we could hijack pieces of that approach and turn it into a more general way to make protein capture agents,' says Heath. 

Heath's team adapted Sharpless's chemistry to create anchor peptides for an enzyme, in this case the kinase Akt1 which has been implicated in many different forms of cancer. 

Akt1 was incubated with a library of solid phase pentamers with an N-terminal azido-amino acid, and then with another pentamer library. The ligands that bind to domains on Akt1 are then close enough for an in situ click reaction to bind them together, making best fit capture agents. In effect, the enzyme acts as the template to manufacture its own synthetic antibody. 

Tom Kodadek, a chemical biologist at Scripps, who was not involved with the work, says the idea is 'very important'. Antibody use in diagnostics is limited by their cost and sensitivity to heat and non-ideal conditions, Kodadek says, and 'binding agents that are selective for a particular post-translationally modified form of a protein, like a phosphoprotein, can often be difficult to come by'. He says he thinks this is where Heath's approach will be especially useful. 

But although it's diagnostics that motivated Heath, the new approach also has therapeutic applications. 'We've found there're all sorts of things we can do with this technique,' adds Heath. 'It's launched a whole area in my group that's not just about capture agents anymore; it's capture agents plus novel inhibitor mechanisms.' 

Heath explains that kinase inhibitors are often not very specific, and now some of his group are working on inhibiting kinase activity in different and unique ways. He does acknowledge, however, that there's a way to go before there will be any commercial therapeutic applications. 'There are therapeutic applications for sure,' he says. 'But the diagnostic applications really fit into a pretty significant, unmet need.' 

Laura Howes

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