Genentech Inc. has accused Samsung Bioepis Co. in Delaware federal court of infringing 21 patents for its breast cancer drug Herceptin.

In the lawsuit, filed Tuesday in the U.S. District Court for the District of Delaware, Genentech aims to stave off the latest threat of biosimilar competition for the drug, which is used to treat about 20 percent of the 2.8 million women who suffer from a form of breast cancer known as HER2-positive.