Amgen Inc., the world's largest biotechnology company, agreed to buy Immunomedics Inc.'s epratuzumab, an experimental antibody treatment for non-Hodgkin's lymphoma, to build up its pipeline of cancer drugs. Thousand Oaks-based Amgen will pay Immunomedics, which develops cancer treatments, $18 million plus royalties and clinical-research milestone payments potentially worth $65 million. Amgen also will make one-time milestone payments totaling up to $225 million if the drug has sales of as much as $1 billion. Amgen will also pay Immunomedics for each second-generation product Amgen develops that is based on epratuzumab. Epratuzumab is being studied in patients who don't respond to Rituxan, the current antibody for non-Hodgkin's lymphoma. In Nasdaq trading, Amgen shares rose $3.88 to close at $63.81, and Immunomedics gained $1.94 to close at $22.44.