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Peptide screening reveals irreversible inhibitors for cancer's 'undruggable' cJun protein

For the first time, scientists have identified promising drug candidates that bind irreversibly with a notoriously undruggable cancer protein target, permanently blocking it.

Transcription factors are proteins that act as master switches of gene activity and play a key role in cancer development. Attempts over the years to design small molecule drugs that block them have been largely unsuccessful, so in recent years scientists have explored using peptides—small protein fragments—to block these undruggable targets.

Now, researchers from the University of Bath have for the first time detailed an approach to discover peptides that bind selectively and irreversibly within cells, permanently blocking a transcription factor that drives cancer known as cJun.

The team, publishing in the journal , used a new drug discovery screening platform technology, called the Transcription Block Survival (TBS) assay, which tests a huge number of peptides to switch off transcription factors that drive cancer.

Their previous work identified reversible inhibitors of cJun, but this latest work builds on that by discovering peptides that bind selectively and irreversibly within cells, permanently blocking cJun action.

The transcription factor cJun has two identical halves, which bind on either side of the DNA strand to alter gene expression.

It can become overactive in cancer, driving uncontrolled cell growth, so the researchers designed a peptide inhibitor that binds to one half of cJun, stopping it from forming pairs and attaching to the DNA.

Once they had made a peptide that bound to the transcription factor, the researchers modified it to bind irreversibly.

Dr. Andy Brennan, first author of the study and Research Fellow at the University of Bath's Department of Life Sciences, said, "The inhibitor works a bit like a harpoon that fires across to the target and won't let go—it grips the cJun tightly and stops it from binding to the DNA.

"We'd previously identified reversible inhibitors but this is the first time we've managed to block a transcription factor irreversibly with a peptide inhibitor."

For the Transcription Block Survival assay, researchers inserted binding sites for cJun into an essential gene in cells grown in the lab. As cJun binds to the gene, it prevents it working and the cell dies. In contrast, if cJun is blocked by the peptide inhibitor, the gene activity is restored and the cell survives.

Jody Mason, CSO of Revolver Therapeutics and Professor of Biochemistry in the University of Bath's Department of Life Sciences, said, "Many drug candidates that are effective in vitro turn out to be toxic or don't penetrate cancer cells at all.

"However, our platform screens for peptide activity directly in the cell, overcoming many common challenges faced by drugs based on small molecules or antibodies.

"The screen checks the activity of the inhibitor in a real cell environment which includes proteases and other proteins that can sometimes interfere with peptide activity, while also checking toxicity.

"We hope this technology can in the future uncover other promising drug candidates for previously 'undruggable' targets."

Having proven cell permeability and activity in cancer cells, as well as target selectivity, the researchers now need to show the inhibitors work in preclinical cancer models.