What happened

Biogen (NASDAQ:BIIB), a leader in the treatment of neurological and neurodegenerative diseases, saw its shares sink by a noteworthy 16.5% last month, according to S&P Global Market Intelligence.

The culprit? Biogen's shares slumped in Feb. in part because of the downturn across the broader markets. However, the main reason was a key change to the company's ongoing Alzheimer's disease trial for the monoclonal antibody aducanumab.

A human neuron transmitting a nerve impulse.

Image source: Getty Images.

So what

In a presentation to investors on Feb. 14, Biogen's chief medical officer, Alfred Sandrock, revealed that the company was planning to enroll an additional 510 patients in the ongoing pivotal stage trial. Sandrock said the change was needed due to "greater-than-expected variability on the primary endpoint." Although the option to expand the trial was already planned, investors appear to be concerned that aducanumab may not be performing as expected in the clinic.   

Now what

Unfortunately, Biogen doesn't have a particularly strong pipeline outside of aducanumab, which is somewhat surprising given the 99% failure rate for clinical-stage Alzheimer's disease drugs over the past twenty odd years. In fact, Merck just threw in the towel on its BACE1 inhibitor, verubecestat, as a potential treatment for Alzheimer's disease, and pharma giant Pfizer decided to shutter its Alzheimer's disease research unit altogether earlier this year due to this painfully high failure rate.  

The point is that Biogen needs to make a big splash on the M&A scene soon in the likely event that aducanumab does miss the mark. This experimental Alzheimer's disease drug, after all, has always been an extreme long-shot. Complicating matters further, the company is staring down a number of novel competitive threats to its core multiple sclerosis franchise -- putting even more pressure on the biotech to pursue a bolt-on acquisition sooner rather than later. 

George Budwell owns shares of Pfizer. The Motley Fool recommends Biogen. The Motley Fool has a disclosure policy.