Mylan NV and Apicore LLC are facing a challenge to their dominance in the market for isosulfan blue, a diagnostic imaging agent used to locate cancer, from San Diego-based Beloteca Inc.

Beloteca is seeking to get into the market with a generic version of the product. It filed a complaint in the U.S. District Court for the Northern District of Illinois seeking a ruling that patents owned by Mylan Institutional LLC and Apicore US LLC are either invalid or wouldn’t...