Shares of Momenta Pharmaceuticals Inc. tanked Tuesday after the FDA rejected an abbreviated new drug application for M-Enoxaparin, a generic version of the Sanofi-Aventis's blood thinner Lovenox.

In its letter, the agency said Momenta and collaborator Sandoz Inc., the generics division of Basel, Switzerland-based Novartis AG, failed to adequately address M-Enoxaparin's potential for immunogenicity. The companies are working to determine what additional information might be required, though Momenta executives said they believe that those issues can be resolved based on "detailed characterization of enoxaparin and on current medical and scientific literature."

Nevertheless, investors bailed on the stock all day. Momenta's shares (NASDAQ:MNTA), which were trading at more than 40 times their average volume, fell $7.71, or 58 percent, to close at $5.67.

Cambridge, Mass.-based Momenta and Sandoz filed the ANDA in August 2005 for M-Enoxaparin, a version of low-molecular-weight heparin for treating deep-vein thrombosis and cardiovascular conditions.

The two companies have been working together on the product since 2003, and later expanded their deal to include development and commercialization in the European Union as well.

Momenta and Sandoz also are working on a four-product biogenerics collaboration signed in July 2006 to create generic and follow-on versions of protein drugs and biologics. That deal involved an initial investment in Momenta of $75 million, and the firm could receive an additional $188 million in milestones.

Away from its generics pipeline, Momenta is developing a next-generation heparin product for acute coronary syndromes. Last month, it started a Phase II study of M118, an intravenous anticoagulant, in patients with stable coronary artery disease who are undergoing a planned percutaneous coronary intervention.