Genzyme Corp investors' confidence that the U.S. biotech is close to an agreement to be taken over by Sanofi-Aventis could be shaken if the French drugmaker extends its tender offer a third time next week, some investors said.

Sanofi's current $69 per share, or $18.5 billion, cash tender offer to buy Genzyme will expire Feb. 15, and Sanofi has said it may extend the offer again if needed as it pores over the U.S. biotech's financial records and operations.

Yet investors who remained confident in a deal getting done even as Sanofi's earnings - seen by some as a key date - passed this week without one, could become more anxious if Sanofi formally seeks another extension.

Sanofi Chief Executive Chris Viehbacher would only say this week that talks with Genzyme were progressing and that his company was conducting a careful due diligence process.

Investors seem to willing to believe that, up to a point.

"After the 15th, investors will be wondering whether due diligence is truly that complex or whether there's something bad that Sanofi has uncovered," said one arbitrageur who declined to be named because he was not authorized to speak to the media.

Another trader with an arbitrage fund -- which tries to make money by taking positions in stocks that are takeover targets --expressed similar sentiments.

Sanofi disclosed on Jan. 31 that it was beginning two weeks of due diligence on Genzyme, a period that would expire on Monday.

Sanofi "did say two weeks of due diligence, so to get nervous before that is premature," the first arbitrageur said.

Genzyme had rejected the $69-per-share offer as too low, and the two sides have spent recent weeks in talks over a higher valuation.

Several sources have expressed confidence that a deal is likely to get done at close to $74 per share, plus the right to future payments based on the success of Genzyme's experimental multiple sclerosis drug Lemtrada.

"We expect this deal is destined to get done ... which may remove at least a partial overhang on Sanofi shares," Bernstein Research analyst Tim Anderson said in a research note.

Sanofi shares fell 1.5 percent to €49.65 on Friday, while shares Genzyme of were roughly flat at $72.90.

Any review is complicated by the fact that Genzyme is on the mend from a manufacturing crisis that led to shortages of two of its most important drugs.

"There's a lot to get through. There are rooms full of boxes of paper and lawyers and people poring through them. So you can understand why timelines are uncertain," said Atlantic Securities analyst Ben Yeoh. "I wouldn't be surprised either way, if a deal were announced before or after the (tender offer) deadline."

Bionest consultant Claude Allary said he doubted Sanofi would find any major outstanding manufacturing problems at Genzyme, since it is already under close supervision by the U.S Food and Drug Administration and a third party.

"Surely the letter of intent (to agree on a takeover) has been written several times already, it's there with the lawyers," Bionest's Allary said. "As soon as they can they will announce the deal. I imagine it can happen any day and I'd be very surprised if it didn't go through."

He expects any extension to be short, about ten days, and said a prolongation of weeks would not be a good sign.