The Food and Drug Administration seems less than enthralled with Eli Lilly's (NYSE: LLY) florbetapir, an imaging agent used to detect amyloid plaques in Alzheimer's patients. In addition to being worried that the scans are being interpreted differently by various readers, the agency isn't necessarily convinced that knowing that a patient has amyloid plaques is useful.

The FDA's opinion was released in documents ahead of tomorrow's advisory panel, where outside experts will make recommendations on whether florbetapir should be approved.

In the big picture, whether Lilly gets florbetapir approved doesn't matter all that much. Sure, if the FDA asks for more data, the delay might help General Electric (NYSE: GE) and Bayer, which are also developing amyloid imaging reagents, to catch up. But it's not like florbetapir is expected to be an instant blockbuster. Lilly only paid $800 million to gain access to the agent when it acquired Avid Radiopharmaceuticals a few months ago, and a majority of the payment -- $500 million -- was tied to regulatory or commercial milestones.

What investors should listen for is what the panel members think of amyloid as both a disease marker and a cause of the disease. Except for the patient representative, everyone on the panel has an M.D. or a Ph.D. after their names -- and some have both. These are influential people in the Alzheimer's disease community, and their view shouldn't be taken lightly by anyone investing in companies developing Alzheimer's disease drugs.

Both Lilly's solanezumab and rival bapineuzumab -- which is being developed by Elan (NYSE: ELN), Johnson & Johnson (NYSE: JNJ), and Pfizer (NYSE: PFE) -- attack amyloid plaques directly, so if researchers aren't convinced that the plaque is the cause of Alzheimer's disease, it may be a bad sign for these development-stage drugs.

If the researchers are convinced that amyloid plaque is just a sign that the patient has Alzheimer's, but is not necessarily the cause, that could still be good news for current drugs like Aricept, which Pfizer sells with Eisai, and Forest Labs' (NYSE: FRX) Namenda. Florebetapir could be used to diagnose patients earlier, which could mean increased revenue for these companies.

The FDA is offering a webcast -- details in this PDF -- so if you're interested in this high-risk, high-reward space, don't forget to watch.

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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.