Pfizer said Friday it’s suing Novo Nordisk and obesity biotech Metsera, a day after Novo set off a bidding war against Pfizer to acquire Metsera.

A month ago, Pfizer said that it would acquire Metsera for $4.9 billion. But Novo on Thursday launched a takeover bid valuing the biotech at around $6 billion, triggering a deadline of four business days for Pfizer to renegotiate its offer. Pfizer is now seeking a temporary restraining order to block Metsera from terminating its merger agreement.

The lawsuit, filed in the Delaware Court of Chancery, alleges that Novo’s offer cannot qualify as a superior proposal since it’s not reasonably likely to be completed due to its significant regulatory risk. It “is an illegal attempt by a company with a dominant market position to suppress competition and uses an unprecedented structure designed to deliberately evade antitrust review,” Pfizer said in a statement.