The transaction would take place in stages and depend on the development of an asthma treatment by Actimis called AP768, the companies said in a joint statement.
If AP768, currently at phase one of the development process, reached phase three, Boehringer Ingelheim would acquire 100 percent of Actimis, it added. The German group was ready to pay $515 million in that case.
The treatment would provide "an innovative addition" to Boehringer Ingelheim's portfolio of potential treatments for respiratory diseases, board vice president Andreas Barner was quoted as saying.
For its part, Actimis acting chief executive Peter McWilliams said the deal would help get AP768 "through the critical and demanding further steps in development."
Boehringer Ingelheim is present in 47 countries around the world, employs 39,800 workers, and posted net sales of €10.9 billion in 2007.
Actimis is a privately held bio-pharmaceutical research and development company based in San Diego, California that was spun off from the German pharmaceutical giant Bayer in November 2004. Its investors include Sanderling Ventures and Mitsui and Co. Venture Partners.