(Reuters) -Shares of Metsera fell over 15% in premarket trading on Monday, after the weight-loss drug developer accepted a sweetened offer from Pfizer to end a fierce bidding war between the pharma giant and Danish rival Novo Nordisk.

U.S. drugmaker Pfizer said late on Friday it had clinched a $10 billion deal for Metsera, in a blow to Novo as the Danish group tries to claw back lost ground against U.S. rival Eli Lilly.

Metsera accepted Pfizer's offer late on Friday, citing U.S. antitrust risks in Novo's bid that it had previously called superior. The Danish obesity drug behemoth said on Saturday it would exit the race.

Under the terms of the final deal, Pfizer has agreed to pay $65.60 per share upfront and up to $20.65 per share contingent on the success of its pipeline of drugs, valuing Metsera at up to $10 billion.

Metsera shares were down nearly 15.5% at $70.33.

As of last close, Metsera shares have surged nearly 150% since Pfizer first said it would acquire the biotech firm in a deal valued at up to $7.3 billion.

The win hands Pfizer a way into the lucrative obesity drug market, even if Metsera's treatments remain years from hitting the market.

"Given the potential for Metsera's portfolio of obesity/metabolic assets, we think the deal terms fairly values the assets and gives Pfizer a leg up in breaking into the lucrative market," BMO Capital Market analyst Evan Seigerman wrote in a note on Sunday.

Metsera's experimental obesity drugs, currently in early-to-mid-stage development, include MET-097i, a GLP-1 therapy designed for a once-monthly injection, compared with similar treatments from Lilly and Novo, which require weekly injections. It is also developing MET-233i, which mimics the pancreatic hormone amylin.