Amgen Inc. shares fell more than 4 percent Friday, the day after the biotech giant said it had cancelled a Vectibix clinical trial following signs that using the drug in combination with a popular cancer treatment might worsened the condition of some patients.

Amgen was comparing the use of Vectibix, its first direct cancer drug, in combination with chemotherapy and Genentech Inc.'s Avastin. Vectibix is already approved to treat colon cancer in combination with chemotherapy alone.

The trial enrolled 1,054 patients at 240 sites in the U.S. over the past two years. As part of the study, the Thousand Oaks-based biotech reviewed interim data after 231 patients either died or their disease got worse. A preliminary analysis appears to show that the patients who did best took Avastin with chemotherapy but did not use Vectibix. Amgen said it has discontinued the trial while it analyzes these early findings.

"Patient safety is Amgen's top priority," said Roger Perlmutter, president of research and development, in a statement. The company is continuing studies of other treatment uses of the drug.

This is the second set-back in recent months for the company's efforts to expand the potential markets for a promising treatment. Amgen has had to scale back expectations for its second-generation anemia drug Aranesp after studies indicated that what are now off-label uses of the drug may endanger patients.

Shares in Amgen closed down $2.45, or 4 percent to $58.02 on the New York Stock Exchange.

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