Multinational biotech Biogen (NASDAQ:BIIB) is facing two important decisions -- one that could open the door to a vast new market (Alzheimer's disease treatment) and another that could challenge the dominance of its top-selling multiple sclerosis (MS) drug. It's the perfect moment to keep an eye on the share price, but with those two unknowns hovering, it may be too early for biotech investors to expect steady gains or a lasting rebound.

Bottles of prescription medications.

SOURCE: GETTY IMAGES.

Biogen's shares are up about 35% since the company said last month that it would apply to the FDA for the approval of its Alzheimer's treatment. Aducanumab earlier was thought to be inefficacious, but a look at broader data from Phase 3 studies showed otherwise. The drug reduced clinical decline in patients with early stage Alzheimer's.

There are currently no drugs on the market that slow or reverse the disease, so an approval could be big for Biogen. Today, more than 5 million Americans suffer from Alzheimer's disease, and by 2050, that number may reach 16 million. The therapeutics market for the disease is expected to total more than $12.4 billion by 2026, according to a report by Reports and Data.

For further clues about the importance of this drug, mark your calendars for Dec. 4-7. Biogen will present its report on aducanumab at the Clinical Trials on Alzheimer's Disease annual meeting in San Diego. Even the best news there won't guarantee an FDA approval, but it should offer investors a bit more clarity.

The waiting has just begun

Biogen is targeting an FDA submission early in the new year, and approval time varies. The agency reaches decisions on fast-track applications in about eight months, while standard applications require at least a year.

So now we switch gears in our waiting game, focusing instead on the company's multiple sclerosis business. Mylan is challenging a patent relating to MS treatment Tecfidera, Biogen's top-selling drug. Tecfidera generated $1.1 billion in the third quarter, for a 3% year-over-year gain. Mylan aims to bring a biosimilar, or less expensive, version of the drug to market. A decision from the U.S. Patent and Trademark Office is expected early next year. A Tecfidera patent expiration in 2020 would represent 10% to 15% of Biogen's value, according to an article in FiercePharma.

Bottles of prescription medications.

SOURCE: GETTY IMAGES.

European rivals in MS

On top of those uncertainties, growth has slowed for Biogen's MS drugs as the company faces competition from European rivals Novartis (NYSE:NVS), Sanofi (NASDAQ:SNY), and Roche (OTC:RHHBY) in the space. At Biogen, MS drugs represented $2.3 billion in third-quarter revenue, a 2% increase year over year. The company's total interferon sales declined 10 percent, to $530 million, in the same time period. Interferon is a type of protein found in the body, and it can be used to reduce inflammation and reactions of the immune system. Biogen's interferon drugs, used to treat MS, are made from the same amino acids as the interferon naturally found in the body. 

Although share gains have lifted Biogen's valuation in recent weeks, the stock still trades at a discount when compared with competitors in the MS market, and that alone might convince some investors to bet on Biogen today. The company trades at about 11 times earnings, compared with 18 for Novartis, 24 for Sanofi, and 25 for Roche.  

Though Biogen stock has wowed the stock market in recent weeks, it had a tough year prior to that, losing about 30% through its lowest point in October. Given recent share gains, investors may be wondering whether it's too late to play the recovery story. In reality, it may be too early. Any positive details from the Clinical Trials on Alzheimer's Disease meeting next month could be a short-term catalyst for further gains, but the slowing growth in MS drugs and the uncertainty surrounding the FDA and Tecfidera patent decisions may lead to volatility. A positive decision on one or both of those decisions may be the remedy Biogen needs for a lasting rebound.