Biogen's (NASDAQ:BIIB) stock caught fire yesterday. Although the biotech posted stellar third-quarter results that surpassed analysts' expectations ahead of the opening bell on Tuesday, the real market-mover proved to be the unexpected news about its Alzheimer's disease candidate, aducanumab.

Specifically, the company performed a new analysis on a larger data set that reportedly revealed a significant clinical benefit for at least some Alzheimer's disease patients who were administered a high dosage of the drug in the study. Biogen said that it now plans on filing for the drug's approval in the U.S. in early 2020, following encouraging discussions with the Food and Drug Administration.

An elderly person assembling a puzzle of a human head at a wooden table.

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What's key to understand is that this clinical update significantly tilted the biotech's underlying risk-to-reward ratio -- arguably for the better. So here's why risk-tolerant investors may want to pounce on this potentially once-in-a-lifetime opportunity right now. 

Biogen's outlook now favors the bold

When aducanumab's twin late-stage trials were halted after a futility analysis earlier this year, the biotech's stock lost over a third of its value in the blink of an eye. This dramatic sell-off was unquestionably warranted at the time -- the company's core value proposition, after all, was intimately tied to aducanumab.

Fleshing out this point, Biogen's core multiple sclerosis franchise has started to taper off, and its other main growth driver, Spinraza, is probably close to hitting a commercial plateau as well. Aducanumab, in turn, was widely viewed among analysts as the company's saving grace. Underscoring this point, EvaluatePharma, a top biopharmaceutical market research firm, estimated that aducanumab's sales would exceed a whopping $7.4 billion per year early in the next decade -- that is, before its late-stage miss.

So this new statistical analysis could prove to be a game-changer for Biogen and its shareholders. Aducanumab, after all, has a real shot at generating well over $100 billion in cumulative sales over its entire life cycle. That's an enormous revenue stream for a company with a market cap of $52 billion right now -- and even that stately revenue figure might be wildly underestimating the drug's true commercial potential. Some analysts, in fact, have suggested that aducanumab might go on to become the best-selling drug of all time.

Is Biogen's stock now a buy?

The answer to this all-important question is arguably a yes -- at least for investors comfortable with high levels of volatility and risk. The crux of the situation is that Biogen's shares will quickly give back all of yesterday's gains if this proposed regulatory filing turns into a dead end. On the flip side, Biogen's shares could realistically quadruple in value over the next five years in the event the company can somehow pull off this miracle.

All told, Biogen's chances of pushing aducanumab over the finish line remain extremely slim -- even with the FDA's apparent blessing in regards to this long-shot regulatory filing. Investors shouldn't lose sight of that basic fact. That said, this large-cap biotech stock is arguably worth the risk in the wake of this nearly unprecedented turn of events. Biogen's upside potential, after all, now far outweighs its downside risk with aducanumab seemingly back from the dead.