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 be infringed by its generic isosulfan blue for injection product.

Mylan is essentially the only provider of isosulfan blue in...