The investment bank Cowen and Co. launched coverage of Immunogen Inc. with a “buy” rating. The Waltham company has no drugs on the market, but analyst Simos Simeonidis said he expects its first drug, a potential treatment for breast cancer, to be approved by mid-2013, Mass High Tech reported. The drug, T-DM1, combines Herceptin, from Genentech, with a cell-killing agent made by Immunogen. It targets HER2+ breast cancer, one of the deadliest forms.