'Magic Bullet' Drug Delays Breast Cancer Worsening

T-DM1 Spares Women From Some, But Not All, Serious Side Effects

By Charlene Laino
WebMD Health News

Reviewed by Laura J. Martin, MD

June 4, 2012 (Chicago) -- A new targeted cancer drug delayed the worsening of metastatic breast cancer, possibly with fewer side effects than traditional treatments, according to results of a late-stage study.

Called T-DM1, the new drug combines Herceptin with a potent chemotherapy drug. It's been called a "magic bullet": Herceptin homes in on cancer cells and delivers the cancer-killing agent directly to its target without damaging nearby cells.

The study involved nearly 1,000 women with metastatic breast cancer that was continuing to get worse despite treatment with Herceptin and a common chemotherapy drug.

Half the women got T-DM1 and the other half got Tykerb and Xeloda, a standard treatment for women who aren't helped by Herceptin.

T-DM1 delayed the time to the disease progressing by about three months. Among women who received T-DM1, the average time before the disease got worse was about nine-and-a-half months vs. about six-and-a-half months for those getting the standard drugs.

Although a few months might not sound like much, it can be a huge gain for seriously ill patients who are running out of options, say doctors studying the drug.

It is still too early to know for sure that T-DM1 will prolong lives, but the early data suggest that it will, says researcher Kimberly Blackwell, MD, of Duke University.

For the most part, women taking T-DM1 did not suffer the often debilitating side effects associated with chemotherapy, such as diarrhea, nausea, vomiting, and hair loss, she tells WebMD.

Still, the drug is not without toxicity: About 41% of women on T-DM1 suffered a serious side effect vs. 57% of those getting standard treatment.

About 13% of women getting T-DM1 had lowered blood platelet counts, which can increase the risk of bleeding. But there were very few cases of bleeding.

The findings were presented here at the annual meeting of the American Society of Clinical Oncology.

How T-DM1 Works

About 20% of breast cancer patients have an aggressive form of the disease known as HER2-positive tumors. Herceptin, a man-made antibody, binds to and blocks part of the HER2 protein that appears on the surface of some breast cancer cells. It revolutionized the way this type of tumor was treated when it was approved in the late 1990s.

But many metastatic breast cancers eventually become resistant to Herceptin. So researchers have been searching for new drugs that target HER2.

T-DM1 is such a drug. The "T" stands for trastuzumab, the scientific name for Herceptin. The DM1 is derived from an old chemotherapy drug called maytansine that was abandoned several decades ago when it was found to be too toxic for patients.

Because Herceptin only zeroes in on cancer cells that express HER2, DM1 is delivered only to those cells, Blackwell says.

"I think this will offer a very important therapeutic option for patients with HER2-positive metastatic breast cancer," she says. "I think it's the first of many antibody-drug [combinations] to follow that will link a potent anti-cancer agent to a targeted delivery system with an antibody."

About two dozen similar drugs, collectively known as antibody-drug conjugates, are in various stages of development, according to Louis M. Weiner, MD, director of the Lombardi Comprehensive Cancer Center at Georgetown University.

T-DM1 is a "magic bullet," Weiner says. He likens it to a Trojan horse that is welcomed into the cell while hiding something that is toxic to the cell.

The drug is made by Genentech, which funded the trial. The company plans to file for FDA approval later this year.

No cost has been set, but targeted cancer drugs often cost tens of thousands of dollars a month.

These findings were presented at a medical conference. They should be considered preliminary, as they have not yet undergone the "peer review" process, in which outside experts scrutinize the data prior to publication in a medical journal.

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SOURCES: 48th Annual Meeting of the American Society of Clinical Oncology, Chicago, June 1-5, 2012. Kimberly Blackwell, MD, Duke University, Durham, N.C. Louis M. Weiner, MD, director, Lombardi Comprehensive Cancer Center, Georgetown University, Washington, D.C.

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