Metsera, a biotechnology company specialising in obesity treatments, said Novo Nordisk’s fresh offer is about US$10 billion, compared to the latest Pfizer bid of about US$8.1 billion.

Metsera’s statement triggers a two-day period when Pfizer could further boost its bid for Metsera.

If no new offer is made in two days, Metsera “would be entitled to terminate the Pfizer Merger Agreement,” the company said.

The latest back-and-forth further raises the stakes as Pfizer spars with Novo Nordisk, the maker of the popular weight-loss drugs Ozempic and Wegovy. Control of Metsera includes access to Metsera’s most advanced treatment, which is currently in phase 2 clinical trials.

In September, Pfizer unveiled a US$4.9 billion deal to acquire Metsera.

But on Oct 30, Novo Nordisk announced an unsolicited US$6 billion bid for Metsera.

Pfizer had four days to respond to that offer and ended up raising its bid to US$8.1 billion. But that spurred Novo Nordisk to lift its price again.

Pfizer filed a lawsuit on Friday in Delaware Court of Chancery against Metsera and Novo Nordisk over breach of contract.

On Monday, Pfizer filed a second lawsuit in US District Court against Metsera and Novo Nordisk based around federal antitrust claims.

Pfizer chief executive Albert Bourla called the Novo Nordisk bid “illusory” because “it violates antitrust laws, and there is a higher risk it will never be” completed, he told analysts on an earnings conference call.

Shares of Metsera surged 21.3% in afternoon trading, while Pfizer dropped 1.6%.