The good news is that, after 26 months of having an abbreviated new drug application (ANDA) sitting in the FDA's Office of Generic Drugs, Momenta Pharmaceuticals (NASDAQ:MNTA) and Novartis (NYSE:NVS) finally have a decision on their generic version of Sanofi-Aventis' (NYSE:SNY) Lovenox. The bad news was that it was in the form of a not-approvable letter.

With no prior warning, the FDA shot down the application because the agency is worried that the drug might cause an immune reaction. Without a mechanism in place to hand out an approvable letter like it would with an NDA, the office had no other choice but to issue the not-approvable letter. It's possible that the duo could amend its ANDA, so it won't have to reset the review clock.

Unless they were smart enough to address the issue in their application, Teva Pharmaceuticals (NASDAQ:TEVA) and Amphastar should expect a not-approvable letter in their mailboxes soon, too. Momenta reports that the FDA told it that all companies developing generic versions of Lovenox will have to address the potential for immunogenicity of the drug.

At this point, Momenta doesn't have much information about what the FDA will require to prove the lack of immunogenicity. The possibilities range from a PubMed search of the literature (a delay of a few weeks) to animal studies (a delay of a month or two) to clinical trials (a delay of six months or more). Until the companies meet with the FDA, everyone -- sans the agency -- is in the dark as to exactly what will be expected. Looks like Newton's first law of motion will apply and Momenta will remain at rest until it receives some momentum from the FDA.

That lack of information is what spurred the more than 50% drop in Momenta's stock price yesterday. There's clearly upside potential at this price if the delay is minor, but without more knowledge, buying the stock at this point is more akin to gambling than investing.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. The Fool has a disclosure policy.