Study identifies first potential targeted drug for triple-negative breast cancer and uncovers drug-resistance mechanism

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Anticipating cancer’s ability to bounce back from some therapies, scientists at Dana-Farber Cancer Institute have not only shown the promise of a new drug for a form of breast cancer, but have also uncovered a mechanism by which the cancer can outmaneuver the drug.

The findings, reported in a study posted online today by the journal Nature, may lead to a more farsighted strategy for the treatment of breast cancers classified as “triple-negative” – one that uses drug combinations to simultaneously arrest the disease and prevent it from becoming resistant to front-line therapies. The dual approach could significantly extend patient survival times, the authors say.

“We found that a class of agents known as BET bromodomain inhibitors significantly impeded the growth of triple-negative breast cancer cells in laboratory as well as animal-model tests,” says the study’s senior author, Kornelia Polyak, MD, PhD, of Dana-Farber. “On the basis of these results, such inhibitors will be tested in patients with triple-negative breast cancer [TNBC] in a phase 2 study and they are also included in ongoing phase 1 trials.

“Even if these drugs prove successful, we know that cancer often devises a way to circumvent therapies and resume its growth,” she continues. “By understanding the series of steps that allows TNBC cells to become resistant to BET inhibitors, we can devise approaches that use combinations of therapies to slow or prevent resistance.”

Triple-negative breast cancer – named for the absence of three key receptors on its cells – accounts for 15-20 percent of breast cancer cases, according to the American Cancer Society. It’s often aggressive and tends to have a poorer prognosis than breast cancers fueled by estrogen, particularly in the first five years after diagnosis.

In the new study, Polyak and her colleagues tested BET inhibitors in different types of breast cancer cells, including triple-negative. Such inhibitors work by dislodging a protein known as BRD4 and other bromodomain proteins from sections of chromatin, the fiber that makes up chromosomes. The bromodomain proteins form part of the cellular machinery that switches genes on. By supplanting BRD4 and its kin in the chromatin, BET inhibitors can cause genes to become less active.

When investigators tested such inhibitors in different varieties of breast cancer cells, the best results – the biggest reduction in cell growth – occurred in the triple-negative cells. The research team repeated the experiment in animal models of various types of breast cancer and achieved the same results.

“We reported these results to pharmaceutical companies that are supporting clinical trials of BET inhibitors in patients, and the companies agreed to expand the trials to include patients with triple-negative breast cancer,” Polyak remarks. “Those phase 1 trials are now underway. If these agents prove successful in phase 2 and 3 trials, they will be the first targeted therapies shown to be effective in TNBC.”

The researchers set out to determine how TNBC can become resistant to BET inhibitors. “Knowing the resistance mechanism could not only help us develop better therapies for the disease, but also could tell us a great deal about how the bromodomain proteins function in this and other types of cancer,” Polyak explains.

When they analyzed TNBC cells that were resistant to BET-inhibiting drugs, the investigators didn’t find any significant gene mutations that might be responsible for the resistance. Nor had the cells acquired the ability to pump the drugs out of their interior (a common tactic of drug-resistant cancer cells).

They found, instead, that the cells remained dependent on the BRD4 protein for their tumultuous growth, but that a different section of the protein had bound to the chromatin. Not only that, but a protein called MED1 allowed BRD4 to bind especially tightly; and BRD4 was heavily “phosphorylated” – mottled with clumps of phosphorous and oxygen. The glut of phosphoryl groups, in turn, was due to a drop in the activity of an enzyme known as PP2A.

Each of these abnormalities – the outsized role of MED1, the hyper-phosphorylation of BRD4, the decline in PP2A activity – represents a potential target for future therapies, Polyak says. “Each represents a vulnerability that it may be possible to exploit – potentially enabling us to slow or prevent the development of resistance to BET-inhibiting drugs.”

Funding for the study was provided by the National Institutes of Health (grants CA168504, CA080111, and CA103867); the Susan G. Komen Foundation; the Cancer Prevention Research Institute of Texas; the Welch Foundation; the U.S. Department of Defense; the Princess Margaret Center Foundation; the Canada Foundation for Innovation and Ontario Research Fund; the Natural Sciences and Engineering Research Council of Canada; and the Harvard Ludwig Center for Cancer Research.

The lead authors of the study are Shaokun Shu, PhD, Charles Y. Lin, PhD, and Robert M. Witwicki, PhD, of Dana-Farber and Brigham and Women’s Hospital; and Housheng Hansen He, PhD, of Dana-Farber, Brigham and Women’s, Princess Margaret Cancer Center/University Health Network, Toronto, Ontario, and the University of Toronto, with Polyak and James E. Bradner, MD, also of Dana-Farber, Brigham and Women’s and the Broad Institute; as co-senior authors. Co-authors are Doris P. Tabassum, Justin M. Roberts, Prakash K. Rao, PhD, Melissa Duarte, and Henry Long, PhD, of Dana-Farber; Michalina Janiszewska, PhD, Sung Jin Huh, Jeremy Ryan, SM, Ernest Doherty, Hao Guo, Daniel G. Stover, MD, Muhammad B. Ekram, PhD, Guillermo Peluffo, PhD, Jonathan Brown, Ian E. Krop, MD, PhD, Deborah Dillon, MD, Michael McKeown, Christopher Ott, PhD, Jun Qi, PhD, Min Ni, Eric P. Winer, MD, Antony Letai, MD, PhD, William T. Barry, PhD, Myles Brown, MD, and Clifford A. Meyer, PhD, of Dana-Farber and Brigham and Women’s; X. Shirley Liu, PhD, of Dana-Farber and the Broad Institute of MIT and Harvard; Yi Liang, PhD, of Princess Margaret Cancer Center; Hisham Mohammed, Clive D’Santos, and Jason S. Carroll, of Cancer Research UK, Cambridge Institute, University of Cambridge, United Kingdom; Shwu-Yuan Wu, PhD, and Cheng-Ming Chiang, PhD, of University of Texas Southwestern Medical Center; and Lars Anders, PhD, and Richard A. Young, PhD, of the Whitehead Institute for Biomedical Research.

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