Photo by Sam Ogden
A chemical strategy may allow researchers to target “undruggable” proteins and overcome resistance to current targeted therapies, according to a report published in Science.
The strategy uses tumor cells’ own protein-elimination system to break down and dispose of the proteins that drive cancer growth.
When tested in vitro and in vivo, the approach caused leukemia cells to die more quickly than they do with conventional targeted
therapies.
“One of the reasons [treatment] resistance occurs is that cancer-related proteins often have multiple functions within the cell, and conventional targeted therapies inhibit just one or a few of those functions,” said study author James Bradner, MD, of the Dana-Farber Cancer Institute in Boston, Massachusetts.
“Conventional drugs allow the targeted protein to adapt to the drug, and the cell finds alternate routes for its growth signals. We began designing approaches that cause the target protein to disintegrate, rather than merely be inhibited. It would be very powerful if we could chemically convert an inhibitor drug into a degrader drug.”
With this in mind, Dr Bradner’s team designed a chemical adapter that attaches to a targeted drug molecule. The adapter enables the drug to tow the cell’s protein-degradation machinery directly to the protein of interest. Once bound to the protein, the combination drug-and-protein-degrader essentially demolishes it.
The investigators tested the technology in leukemia cells. They built an adapter out of phthalimide, a chemical derivative of the drug thalidomide, and attached it to the BRD4 inhibitor JQ1. The phthalimide was designed to “hijack” the cereblon E3 ubiquitin ligase complex.
When the researchers treated the leukemia cells with a JQ1-phthalimide conjugate called dBET1, the BRD4 protein within the cells was degraded in less than an hour. The team said such rapid and extensive degradation suggests conjugates may be able to prevent or hinder cancer cells from developing resistance to targeted therapies.
“The potency, selectivity, and rapidity of this approach—namely, the ability to home in specifically on BRD4—are unprecedented in clinical approaches to protein degradation,” Dr Bradner said.
To determine how selective dBET1 actually is, the investigators measured the levels of all proteins in leukemia cells at 1 hour and 2 hours after treatment.
“We were stunned to find that only 3 proteins of more than 7000 in the entire cell were degraded: BRD2, 3, and 4, an exceptional degree of selectivity guided by the intended targets of JQ1,” Dr Bradner said. “It’s as though dBET1 is laser-guided to deliver protein-degrading machinery to targeted proteins.”
The researchers then tested dBET1 in mice bearing leukemia. As in the cell samples, there was a rapid degradation of BRD4 in the tumor cells and a potent anti-leukemic effect, with few noticeable side effects.
To see if compounds other than JQ1 can be used as a guidance system for a conjugate, the investigators created a set of molecules that lock the protein-degradation machinery onto a compound called SLF, which targets the protein FKBP12.
When they treated cancer cells with SLF, the team found it degraded the vast majority of FKBP12 in the cells within a few hours.
Buoyed by these results, the researchers are working to create a derivative of dBET1 that can be used as a drug in humans and to extend the conjugate strategy for the treatment of other diseases.
“The dBET1 and the dFKBP12 compounds are presently in a late stage of lead optimization for therapeutic development in both cancer and non-malignant diseases,” said Prem Das, PhD, chief research business development officer at Dana-Farber.
“Composition-of-matter and method-of-use patent applications have been filed on these and other additional targeted agents, as well as on the chemistry platform. They will be licensed for commercialization to an appropriate company according to standard Dana-Farber practice.”
