A trial snapshot from Biogen (NASDAQ:BIIB) is giving the Alzheimer's disease community a reason to cheer just weeks after a similar candidate from Eli Lilly failed miserably and gave investors a reason to be concerned about similarities between the two programs. Let's see if this changes the investing thesis for the blue-chip biotech.

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What happened?

Biogen stock popped about 5% on Friday's opening bell over the previous day's close after the company presented encouraging data for its Alzheimer's disease candidate, aducanumab. Long-term results from a phase 1b trial pitting the amyloid-clearing antibody against a placebo alleviated concerns that cropped up during a previous readout. 

Last year, patients treated with a dose of 10 mg per kg of body weight showed a significant cognitive benefit, but there were also instances of brain swelling in this group that could make aducanumab an unviable long-term preventative treatment for otherwise healthy patients. The 3 mg group exhibited a significant benefit without swelling, but those in the 6 mg group didn't.

The recently released data includes a group given a small dosage that was slowly increased to 10 mg over 12 months. Incidence of brain swelling was about 35% in this group, which was far lower than groups receiving 10 mg, or even 6 mg, from the outset.

Does it matter? 

It sure does. The 197-patient Prime study figures are incredibly encouraging because they help alleviate safety and efficacy concerns and increase the odds that aducanumab's enormous pivotal trials will succeed.

In 2015, Biogen initiated a massive, expensive phase 3 program expected to enroll about 2,700 patients and measure their rate of cognitive decline over 78 weeks. To be considered successful, patients need to show a significant slowing of cognitive decline over 78 weeks compared to those receiving a placebo as measured by the clinical dementia rating sum of boxes (CDR-SB) test. In the 196-patient prime study, CDR-SB scores in patients receiving 10 mg from the outset or slowly increased to 10 mg over 12 months worsened at less than half the rate of those receiving a placebo.

Biogen isn't completely out of the woods yet. Knowing a slowly increasing dosage is effective and leads to fewer side effects helps a great deal. Whether it will be considered safe enough by the FDA, though, is still uncertain.

Biogen stock is a stronger buy now than it was ahead of the recent data, but there are no guarantees aducanumab will succeed. If it does, it could eventually contribute around $8 billion or more each year to the company's top line. Its chances look better than ever, but I wouldn't bet the farm on it.