Sanofi-Aventis, led by former GlaxoSmithKline executive Chris Viehbacher, has extended its unsolicited $18.5 billion takeover offer for U.S.-based biotech company Genzyme.

Sanofi’s previous tender offer for Genzyme shares expired just before midnight Friday, and now the French company says it has extended the $69-per-share offer until 11:59 p.m. ET on January 21.

In October, Viehbacher, a long-time Triangle resident who lost out in a battle to be CEO at GSK, told analysts and reporters during conference calls he went straight to shareholders because Genzyme management “refused to engage in constructive discussions” despite several attempts by his company.

The acquisition could “unlock value for both sets of shareholders, so I think it would be a real shame to walk away,” Viehbacher said, “but again all options are on the table.”

“Right now, the position of Genzyme is to (have us) put more money on the table, and given the absence of another bidder we’re not” doing that, he said.

Sanofi said in a statement before the open of the Paris stock exchange on Monday that 2.21 million Genzyme shares were tendered by Friday — or about 0.9 percent of the U.S. company’s outstanding shares.

Sanofi spokesman Jean-Marc Podvin said such a low percentage of acceptance was “typical” in such tender offers, adding that executives at the French company “are not surprised.”

“The key message is that we want to engage in a constructive dialogue with Genzyme to reach a transaction,” said Podvin, insisting on the “strategic fit” that it would offer.

Shares of Sanofi were down 0.3 percent to euro49.16 in late morning trading in Paris. Genzyme shares closed Friday at $69.82 on the Nasdaq exchange.

Massachusetts-based Genzyme, which specializes in drugs for rare diseases, has indicated it would fight Sanofi’s hostile takeover effort and last month estimated its own shares are worth $89 apiece.

Sanofi is the world’s fourth-biggest drugs maker.

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