Roche Holding AG is working with U.S. authorities to determine the source of counterfeit Avastin, purchased by at least 19 medical practices in the U.S. Roche and the Food and Drug Administration are seeking to prevent the fake cancer drug, which doesn't include the active ingredient bevacizumab, from being distributed further, the company's Genentech unit said in an e-mail late yesterday. The FDA notified the company about the issue in December, said Edward Lang, a spokesman for Roche's South San Francisco, California-based unit.