The road to FDA approval is littered with failed drug candidates. For every 10 that head into clinical trials with high hopes, one will get to market. That's the bad news. And when it comes to a disease like Alzheimer's, the numbers are even worse, because despite all the research that has been done so far, scientists aren't yet sure quite what causes it, nor what exactly causes the symptoms we have all come to associate with it.

In this segment from Motley Fool Money, host Chris Hill and senior analyst Jim Mueller talk about the disappointing news that Biogen (BIIB -2.85%) has pulled the plug on its trials for aducanumab, which targets amyloid plaque, as well as where drug developers might look next, and whether the company's shares are a buy after their steep pullback.

A full transcript follows the video.

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This video was recorded on March 22, 2019.

Chris Hill: Shares of Biogen fell more than 30% this week after the company ended the trial for an Alzheimer's drug that it was developing. Biogen has been around since the 1970s. Jim, this is still a $43 billion company, but this was a gut-punch, not just for shareholders but for the patients and families who are dealing with Alzheimer's and were hoping for some type of treatment.

Jim Mueller: It's bad news all around. Investors got hurt when they saw the dollar signs, $10 billion to $12 billion of expected annual revenue if this drug came through. They failed to keep the risks in mind. Science, to put it truthfully, and as a former scientist myself, science is hard, especially when you're dealing with a disease such as Alzheimer's where the cause of it is not quite really known yet. The drug, I know I'm going to mess this up, aducanumab --

Hill: You notice, I didn't even attempt to say the name of the drug.

Mueller: No, you left that for me. That drug is going after one potential cause, the amyloid plaques. It's the latest in several drugs that have been halted that do not show the efficacy needed. Pfizer and Johnson & Johnson stopped trials on their drug a while back. AstraZeneca, Eli Lilly just a couple of years ago, they stopped. This might turn out to be a failure, but a good kind of failure, in the sense that science needs failures in order to learn what doesn't work so that we can figure out what does work. If amyloid plaques are not the way to treat Alzheimer's then drug companies will hopefully rotate to something else.

Biogen does have three drugs working on another potential issue with Alzheimer's, the tau protein. Those might do better. We'll just have to wait and see.

Hill: Normally, when we see a stock drop this much in a single week, it's natural to ask, "is this a buying opportunity?" Do you think that's the case with Biogen? Or do you want to see what their next move is?

Mueller: I would want to see what their next move is or wait for it to drop further, I think.

Check out the latest earnings call transcript for Biogen.