When Craig Crews first managed to make proteins disappear on command with a bizarre new compound, the biochemist says that he considered it a “parlour trick”, a “cute chemical curiosity”.
Today, that cute trick is driving billions of US dollars in investment from pharmaceutical companies such as Roche, Pfizer, Merck, Novartis and GlaxoSmithKline. “I think you can infer that pretty much every company has programmes in this area,” says Raymond Deshaies, senior vice-president of global research at Amgen in Thousand Oaks, California, and one of Crews’s early collaborators.
The drug strategy, called targeted protein degradation, capitalizes on the cell’s natural system for clearing unwanted or damaged proteins. These protein degraders take many forms, but the type that is heading for clinical trials this year is one that Crews, based at Yale University in New Haven, Connecticut, has spent more than 20 years developing: proteolysis-targeting chimaeras, or PROTACs.
Large and unwieldy, PROTACs defy conventional wisdom on what a drug should be. But they also raise the possibility of tackling some of the most indomitable diseases around. Because they destroy rather than inhibit proteins, and can bind to them where other drugs can’t, protein degraders could conceivably be used to go after targets that drug developers have long considered ‘undruggable’: cancer-fuelling villains such as the protein MYC, or the tau protein that tangles up in Alzheimer’s disease.
“This is new territory,” says Alessio Ciulli, a biochemist at the University of Dundee, UK. “We’re breaking the rules of what we thought would be druggable.”
The field has reason to be optimistic. In 2014, scientists discovered that the myeloma treatment lenalidomide (Revlimid), one of the world’s best-selling drugs, works in a similar way to protein degraders to chew up two formerly untouchable proteins1,2.
Yet the field lacks published data confirming that PROTACs and other emerging compounds can make undruggable proteins disappear. And there are questions about where and how these odd-looking molecules will work in the body.
For now, all eyes are on Arvinas, a biotech company in New Haven, Connecticut, founded by Crews. It’s scheduled to begin testing a PROTAC for prostate cancer, albeit attacking a protein that’s been targeted successfully by other drugs. “We’re on the cusp of proving these PROTACs can be drugs,” says Ian Taylor, senior vice-president of biology at Arvinas. “Right behind that will be: can we do this with an undruggable?”