French pharmaceutical Sanofi Aventis shares slumped near 6 percent on Friday after a generic company in Europe announced it was likely to gain approval for a generic version of Plavix in Germany and Luxembourg.

A Swiss generic developer Schweizerhall announced today it was close to obtaining approval to launch a generic version of one of Sanofi's best selling products - the blood-thinning Plavix.

But Sanofi Aventis said it will defend vigorously its intellectual property rights and evaluate its legal and regulatory options.

Sanofi-Aventis holds patent of the product until 2013 but the competitor stated it is launching a generic version before June and is expecting to have sales in the current quarter.

Schweizerhall said the European market for the clopidogrel - Plavix's generic name - is valued in $2 billion euros, the equivalent to $3.09 billion.

According to analysts, an approval favoring Schweizerhall could slash Sanofi-Aventis sales of more than $500 million of the product in Germany.

Analysts speculate the Swiss group obtained the patent by developing a different version of clopidogrel based on an alternative salt form meaning its product is not exactly the same as Plavix.

Shares of Sanofi-Aventis plunged 5.96 percent to $36.45 on the New York stock Exchange at the closing trade on Friday.