** Shares of weight-loss drug developer Metsera MTSR.O fall 5% to $69.70

** U.S. antitrust regulators have raised concerns about Novo Nordisk's NOVOb.CO proposed acquisition of the company, saying the deal may violate federal merger review rules, a letter showed

** Could be seen as an attempt to avoid the mandatory premerger review required under the Hart-Scott-Rodino Act, Federal Trade Commission says

** Both Novo and Metsera did not immediately respond to Reuters' request for comment

** Letter comes as Novo and Pfizer PFE.N are engaged in a heated bidding war for Metsera with each raising its bid this week

** Metsera's board has called Novo's offer "superior," while Pfizer has filed lawsuits accusing Novo of trying to block competition and evade antitrust review

** MTSR has more than doubled YTD

(Reporting by Kamal Choudhury in Bengaluru)

((Kamal.Choudhury@thomsonreuters.com;))