Onyx Pharmaceuticals Inc said on Sunday it sued Bayer AG over its rights to an anti-cancer compound that it says it discovered during joint research with the German drugmaker.

Onyx's lawsuit, filed in the U.S. District Court for the Northern District of California, seeks a declaration that fluoro-sorafenib is a jointly owned collaboration compound under an agreement between the two companies.

Onyx said it learned that the compound is a variant of Nexavar tablets, which Onyx and Bayer co-develop and market.

The company said fluoro-sorafenib has the same chemical structure as Nexavar except that fluoro-sorafenib contains a fluorine atom instead of a hydrogen atom.

The new molecule had been identified in 1998 during the research collaboration period by the companies' joint research teams, Onyx said in a statement. Discussions with Bayer regarding Onyx's rights to fluoro-sorafenib under the companies' 1994 collaboration agreement were not productive.

Doctors use Nexavar to treat liver cancer and advanced kidney cancer. Onyx and Bayer co-developed Nexavar and have marketed the drug in the United States and other countries.

Bayer said it disagrees with Onyx.

We plan to defend ourselves against the complaint as we believe that Onyx has no rights to the substance, Bayer said in an e-mailed statement.

A hearing has not been scheduled yet, an Onyx spokeswoman said.

Onyx's revenue from its agreement rose 10 percent to $53.7 million in the first quarter of this year, it said on May 6.

On May 7, Britain's healthcare cost-effectiveness watchdog group said it recommended that Nexavar should not be given by the state health service to patients with liver cancer. Britain's National Institute of Health and Clinical Excellence already has turned it down as a treatment for kidney cancer.

Nexavar has been one of Bayer's top new drug hopes. The company believes it can get peak worldwide sales of 2 billion euros ($2.7 billion) a year from the drug.

It is seeking approvals for its use against lung and breast tumors. It failed to prove effective against melanoma in a recent trial.

(Additional reporting by Peter Dinkloh in Frankfurt; Editing by Jan Paschal)