Nov. 6 (Bloomberg) -- Momenta Pharmaceuticals Inc. fell the most ever in New York trading after U.S. regulators rejected its generic copy of an anti-clotting drug with more than $3 billion in sales, Sanofi-Aventis SA's Lovenox.

Momenta dropped $7.71, or 58 percent, to $5.67 in Nasdaq Stock Market composite trading at 4 p.m. New York time. The decline was the most in a day since the company first sold shares to the public, in June 2004.

The U.S. Food and Drug Administration may require clinical trials of the medicine, analysts said. The regulator said the application didn't adequately address concerns that the drug may provoke an unwanted immune response. Momenta is developing its version of Lovenox with Novartis AG of Switzerland.

``Most of the company's value is centered around this drug, which is why you're seeing the reaction in their share price,'' said Biren Amin, an analyst at the Stanford Group in Boca Raton, Florida. ``The FDA is leaning towards human clinical data to ensure that the generic versions are equivalent to the brand.''

Amin reiterated his ``hold'' rating on the stock and removed his $12 price target. The fair value for Momenta shares is $6 to $7 if the drug is launched in 2011, two years later than he had originally estimated, he said in a phone interview.

Lovenox generated 2.4 billion euros ($3.5 billion) in sales for Paris-based Sanofi last year. Momenta, based in Cambridge, Massachusetts, had reported in June that FDA consideration would be delayed.

The decision ``removes the key valuation driver, generic Lovenox, for the foreseeable future and raises concerns as to whether there is a path forward for the drug,'' Sapna Srivastava, an analyst at Morgan Stanley, said today in a note to clients. It raises questions whether Momenta's ``sugar characterization technology be sufficient for approval of any generic biologics without conducting human trials.''

Srivastava downgraded Momenta to ``underweight'' from ``overweight'' and cut the price target to $6 from $30.

Momenta and Sandoz, the generics division of Novartis, ``are working together to address the FDA's questions and determine the information necessary to obtain approval,'' according to today's filing.

Lovenox and Momenta's similar version, Menoxaparin, are complex ``pseudo-biologics'' derived from carbohydrates. This category ``isn't clearly defined by the FDA as far as generic approval processes go,'' Amin said.

To contact the reporter on this story: Beth Jinks in New York at bjinks1@bloomberg.net

Last Updated: November 6, 2007 16:19 EST