Novo Nordisk stock was dropping on Wednesday after Roche said it had licensed a promising obesity drug in a deal that could cost it up to $5.3 billion.U.S.-listed shares of Novo Nordisk were down 5.2%.

Roche moved to enter the booming weight-loss drug market now dominated by Novo Nordisk and Eli Lilly. The Swiss pharmaceutical firm signed an exclusive agreement with Zealand Pharma to produce treatments using Zealand's experimental petrelintide injectable.

Over the past few days, as the rest of the stock market melted down, a revolution has been quietly brewing in the market for GLP-1 weight-loss drugs. Current runners-up Viking Therapeutics ( VKTX 10.22%) and Roche Holdings ( RHHBY 3.62%), up 11.3% and 4%, respectively, through 10:05 a.m. ET today, are behind it.