Taking Away Cancer's Bite

Theodorescu Targets a Protein Before It Gets Active

By Mark Couch

(May 2015) Dan Theodorescu was driving home from work one evening several years ago, going with the flow of traffic across a quiet country road in Virginia, when the idea came to him.

For years Theodorescu, MD, PhD, and his colleagues had been puzzling with how to stall a protein called Ral that is central to cancerous cell growth and a close relative of Ras, one of the most common oncogenes in human cancer. Everybody was looking for a way to hang a compound on Ral that would take away its power. But nothing was working.

“We were doing the same thing over and over again, trying to get a different result,” Theodorescu says. “I thought, ‘Why don’t we try to stop Ral before it gets started?’”

So, Theodorescu, who became director of the CU Cancer Center in 2010, began chasing Ral down a different path. Rather than look for compounds that disrupt the protein after it starts its deadly growth pattern, he started looking for a way to keep it from revving up in the first place.

Theodorescu and his research team examined the structure of the Ral protein in its “inactive” form, looking specifically for changes in its structure as it became “active” and they found that the inactive Ral protein had a cavity that disappears when the protein becomes active.

The trick now was to find a compound that could fit inside that cavity and not slip, slide, or get squeezed out.

Easy right?

Not so fast.

That task requires massive computing power to simulate how each compound interacts with Ral’s transformation from its apparently harmless inactive state into a ravenous agent with a dangerous bite.

Ral-dependent cancers are common. RalA and RalB “are important drivers of the proliferation, survival and metastasis of several human cancers, including skin, lung, pancreatic, colon, prostate, and bladder cancers,” Theodorescu and his colleagues wrote in an article published last September in the journal Nature.

Obviously a breakthrough identifying how to stop Ral in its tracks would be profound. Not only were the researchers looking for specific compounds that would perform the task, they were also proving “the utility of structure-based discovery for the development of therapeutics for Ral-dependent cancers,” they wrote.

Theodorescu and his colleagues tested 500,000 compounds to discover 88 possible candidates that might bind to Ral and prevent its activation.

And that’s just the beginning of the journey. At this point, the team had created a map that still had many possible routes.

The next round of research would move from computer models to testing cells in the lab. They evaluated the compounds for their ability to slow the growth of human cancer cells in suspension, which is a proxy for metastasis. The researchers found one molecule that was most successful and from there, they synthesized derivatives of that molecule to find a compound that was effective.

Next, they tested the compound in mice models to determine whether what worked in the dish would work in an animal. In those tests, they discovered that the compound entered the tumor tissue and slowed the tussue growth. The compound stopped the activation of Ral in treated tumors.

Theodorescu describes the molecule as a stick in the mouth of an alligator, preventing Ral from taking its first bite. Obviously that needs to be a very strong stick, so more tests are needed.

“We still need to optimize these compounds and then characterize these agents for toxicity in several animal species and determine their optimal route of delivery, such as oral or intravenous before moving to the clinic,” Theodorescu says.

“But we see this work as a valuable first step in development of a novel class of therapeutic agents directed at Ral. The concept of targeting sites on proteins that collapse upon activation and whose collapse is required for activation could in principle be used discover drugs aimed at other proteins driving human diseases as well.”