GlaxoSmithKline (NYSE:GSK) has halted part of a phase 3 breast cancer clinical study after an independent committee concluded that the GSK drug administered alone is unlikely to work better than a rival cancer drug from Roche.

But three other arms of the trial studying the GSK drug lapatinib, also called Tykerb, will continue. Those studies are evaluating Tykberb in some combination with Roche blockbuster drug trastuzumab, which is marketed as Herceptin.

“Patients assigned to the lapatinib alone arm of the trial will discontinue lapatinib and discuss treatment options with their study physician,” GSK said in the clinical trial update.

London-based GSK, which has its U.S. headquarters in Research Triangle Park, is studying Tykerb as an adjuvant therapy for patients with HER2 positive primary breast cancer, a particularly aggressive form of the disease. Adjuvant therapy is additional cancer treatment given after the primary treatment and surgery. It is given to reduce the risk that cancer will return. While Tykerb is approved in more than 100 countries, including the United States, as a cancer treatment it is not yet approved as an adjuvant therapy anywhere in the world.

The U.S. Food and Drug Administration approved Tykerb in 2007 to be used in combination with another Roche product, the chemotherapy drug Xeloda, to treat patients with advanced or metastatic breast cancer. Tykerb’s sales have been growing and it generated more than $360 million revenue in 2010. U.S. Tykerb sales topped $154 million last year. But Tykerb has a long way to go to approach Herceptin’s more than $6 billion in 2010 sales. To expand Tykerb’s sales potential to blockbuster levels, GSK is studying the drug for additional treatments.

The phase 3 “ALTTO” trial (Adjuvant Lapatinib and/or Trastuzumab Treatment Optimization) randomized patients to receive lapatinib, trastuzumab, trastuzumab followed by lapatinib or the combination of the two drugs. The study aims to find out whether a combination of the drugs works better than the Roche drug alone.

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