French drug giant Sanofi-Aventis publicly disclosed its $18.5 billion bid for Genzyme on Sunday, intensifying pressure on the U.S. biotechnology company to negotiate a sale.

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French drug giant Sanofi-Aventis publicly disclosed its $18.5 billion bid for Genzyme on Sunday, intensifying pressure on the U.S. biotechnology company to negotiate a sale.

Sanofi approached Genzyme in June, and the two companies were engaged in friendly merger talks. But, according to Sanofi CEO Christopher Viehbacher, Genzyme’s management stifled discussions.

“We are disappointed that you rejected our proposal on Aug. 11 without discussing its substance with us,” he wrote in a letter to Genzyme CEO Henri Termeer. “Our financial advisers finally met briefly on Aug. 24, but the meeting simply served as further confirmation that as throughout you remain unwilling to have constructive discussions.”

Genzyme’s director of corporate communications, Bo Piela, said Sunday the Cambridge, Mass., company would have no comment on Sanofi’s statements.

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In its bid, Sanofi is offering $69 a share in cash. That represents a 38 percent premium over the share price before speculation hit the market about a possible deal. Shares of Genzyme closed at $67.62 Friday.

People briefed on the talks said Termeer and Genzyme’s board had pressed Sanofi for a higher bid before they would be willing to engage in meaningful negotiations and open up the company’s books to its rival.

Sanofi, meanwhile, faces no apparent bidding competition, and its public statement seemed aimed at raising shareholder pressure on Genzyme to sell.

A recent survey by Reuters of European and U.S. analysts showed an average forecast of a final deal price at $77.90 a share, with expectations ranging from $73 to $83.

Sanofi’s move Sunday falls short of going hostile, in which the company would appeal directly to Genzyme’s shareholders with its offer.

The deal would rank as the 10th-largest merger in the health-care industry in the United States, according to Capital IQ, which tracks data on deals.

Genzyme is considered attractive because it has promising drugs for high cholesterol and other disorders in late development, and it already sells some lucrative drugs for rare genetic disorders. That’s a hot niche as big drugmakers diversify beyond blockbuster pills that get slammed by cheaper generic rivals after several years.

Sanofi, for instance, has patent protection on its blood thinner Plavix, the world’s second-best-selling drug, only until 2012.

Genzyme won U.S. approval in May for a new drug for the metabolic disorder Pompe disease, and its experimental biologic drug for multiple sclerosis is getting expedited review by the Food and Drug Administration.

Analyst Steve Brozak of WBB Securities said Sunday that Sanofi’s bid is “just an opening salvo,” and the deal isn’t going to “happen overnight.”

Termeer “isn’t going to go quietly,” he added.

Material from The Associated Press was included in this report.