Earlier today, the FDA stunned Momenta Pharmaceuticals by rejecting its abbreviated NDA for a generic version of the Lovenox anti-clotting drug. The agency decided the application doesn’t address the potential for the drug’s immunogenicity, which means there is concern the drug will provoke an unwanted immune response.

The move hammered Momenta stock, which fell 57 percent, and left the biotech and its development partner, the Sandoz generic unit at Novartis, searching for ways to calm investors. “We believe that we can address what we anticipate to be the FDA’s concerns, based on our detailed characterization of enoxaparin and on the current medical and scientific literature,” Craig Wheeler, Momenta’s ceo and president, in a statement. “Our path forward will be determined in conjunction with Sandoz and the FDA.”

In an investor note tonight, Michael King, an analyst at Rodman & Renshaw raised a more intriguing question: Is the FDA’s request real or a veiled policy decision? He’s referring to the unsettled issues surrounding FDA approval of biosimilars, or biogenerics, as some call them. Without an approved pathway, King suggests the agency may be signaling extreme caution. And while he doesn’t attempt to answer his own question, he implies the hurdles will be large and lengthy.

“While we continue to believe in Momenta’s unparalleled ability to characterize complex biomolecules, this protracted setback is surprising. We anticipate that MNTA will need to take a number of long steps before gaining US approval; namely, communication with the FDA, characterization of the molecule’s immunogenicity, and design, execution, write-up and submission of the results,” he writes. “Our best estimate is that this process will take approximately 18 months to complete.”

“The FDA is leaning towards human clinical data to ensure that the generic versions are equivalent to the brand,” Biren Amin, an analyst at The Stanford Group, told Bloomberg News earlier today. This category “isn’t clearly defined by the FDA as far as generic approval processes go.”