This moonshot is exciting -- but it isn't a compelling reason to buy Biogen's shares right now.

Biogen's (NASDAQ:BIIB) plaque-busting Alzheimer's disease (AD) drug candidate, aducanumab, could represent a jaw-dropping $20 billion peak commercial opportunity for the company and its shareholders. Putting this monstrous figure into context, this experimental medicine has the potential to even surpass AbbVie's anti-inflammatory drug, Humira, which is on track to generate upwards of $16 billion in global sales by 2018 and is currently the world's best-selling drug.

Although this $20 billion peak sales figure may sound ridiculous at first, there are some solid reasons to believe that it's not completely far-fetched. First off, there are presently no disease-modifying treatments available for this devastating neurodegenerative disorder, reflecting the astronomically high failure rate of experimental AD drugs. Second, the incidence rates of AD have been growing by leaps and bounds largely due to an aging global population.

That's why J.P. Morgan's analysts, among others, have suggested that any disease-modifying AD drug that somehow makes it all the way to market would quickly rack up over $10 billion in sales. And without a major competitor to divvy up the market, that figure could balloon to over $20 billion by 2030.

Sounds good, but should investors believe the hype?

Biogen's AD drug candidate belongs to a class of experimental therapies designed to reduce beta-amyloid plaque levels within the brain, which theoretically should slow disease progression -- that is, if these plaques really are the root cause of AD. Unfortunately, the clinical data thus far have revealed a perplexing disconnect between plaque levels and underlying disease pathology.

As an example, Eli Lilly's (NYSE:LLY) solanezumab -- a drug not unlike aducanumab -- failed to show an improvement in cognitive ability in patients with mild to moderate Alzheimer's disease across two large late-stage studies. Making matters worse, several other anti-amyloid therapies have also failed to improve cognitive ability and/or daily function in AD patients, casting doubt on the amyloid plaque hypothesis.

More broadly, the picture that's slowly emerging is that the complex biology underpinning AD may simply make it impossible to develop a "one-size-fits-all" type of medication. Anti-amyloid therapies -- like those being developed by Biogen and Lilly -- may ultimately end up being used as part of a combination therapy as a result.

Given the disappointing clinical history of anti-amyloid therapies and the fact that researchers still don't fully understand the underlying causes of AD, investors should probably view this $20 billion peak sales estimate purely as a blue sky scenario. In other words, it's not impossible, but it's certainly not probable.

What's next?
Biogen is in the midst of carrying out two phase 3 trials for aducanumab, dubbed "Engage" and "Emerge," respectively. Taken together, these two trials are expected to enroll 2,700 patients worldwide, which should provide ample power to assess the drug's impact on cognitive ability and the daily lives of AD patients.