'Magic Bullet' Drug Delays Breast Cancer Worsening

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.