Novo Nordisk on Thursday launched an unsolicited bid valued at up to $9 billion for obesity-drug developer Metsera, hoping to top the deal Pfizer reached last month and teeing up a potential legal battle for the biotech company.

"We asked them to become more aggressive and ambitious," said Mikael Bak, head of the Danish Shareholders' Association which has around 17,000 members, most of them investors in Novo.