Repligen Corp. said Friday it has joined the University of Michigan in a lawsuit alleging a newly approved rheumatoid arthritis drug from Bristol-Myers Squibb Corp. violates a patent.

New York City-based Bristol-Myers Squibb won federal approval last month to market a drug under the trade name Orencia as a treatment for the chronic disease, which affects more than 2 million people in the United States. Analysts have estimated the drug could eventually reach $1 billion in annual sales.