American Pharmaceutical Partners Inc. said Monday it is buying its largest shareholder, privately held American BioScience Inc., for stock worth about $4.1 billion, creating a biopharmaceutical company with annual revenue of more than $500 million.

The new company will be named Abraxis BioScience and will have worldwide rights to cancer treatment Abraxane, which is marketed in the United States for metastatic breast cancer and is being developed for other breast, lung, ovarian, prostate, melanoma and head and neck cancers in more than 74 clinical trials, including seven Phase III studies.