Onyx Pharmaceuticals is suing partner Bayer over its rights to a mid-stage anti-cancer compound that it says is a variant of the big-selling oncology drug Nexavar, which the companies co-developed and market.

The lawsuit, which has been filed in a California court, declares that Onyx is asserting its rights to the Phase II drug fluoro-sorafenib. The US firm claims that the latter is “a jointly-owned collaboration compound under the Bayer/Onyx collaboration agreement, together with other remedies”.

Onyx says it was “recently advised” that the compound is a variant of Nexavar (sorafenib), arguing that it has the same chemical structure 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 says, but discussions with Bayer regarding rights to fluoro-sorafenib under the companies' 1994 collaboration agreement “were not productive”.

Greg Giotta, chief legal counsel for Onyx, said that the firms “have had a long-standing and successful collaboration for more than 15 years. Against that background, we are disappointed that we could not resolve this matter”. He claimed that “this complaint is necessary to protect our rights and the rights of our shareholders under the collaboration agreement."

Bayer believes that Onyx has no rights to the substance and intends to vigorously defend its rights to fluro-sorafenib. The company noted that along with Onyx, it will be presenting more than 65 studies evaluating the use of Nexavar at the upcoming American Society of Clinical Oncology meeting.

Nexavar is currently approved in more than 70 countries for liver cancer and in more than 80 countries for advanced kidney cancer. The drug, which is also being investigated in a wide range of cancers such as breast, lung and ovarian, had first-quarter sales of 137 million euros, up 35.6%.