Biogen (NASDAQ: BIIB) once stood out as one of the most successful biotech companies. It's portfolio of blockbuster multiple sclerosis drugs grew revenue into the billions of dollars. But as the company's top drugs faced losses of exclusivity, Biogen started to face one big problem: growth.

The biotech needed a new product or products to drive revenue as sales of older drugs declined. Biogen turned its attention to Alzheimer's disease. Along with partner Eisai, Biogen advanced Leqembi -- and the companies just reached a major milestone. The U.S. Food and Drug Administration (FDA) granted Leqembi traditional approval. So, why isn't Biogen's stock rising? Let's take a closer look.

First, let's back up a bit and consider Biogen's path up until this moment. The company's first attempt at conquering the Alzheimer's market failed. That was with monoclonal antibody treatment Aduhelm. Clinical trial results didn't win over the medical community, so Biogen cut investments in that program and put its efforts into another monoclonal antibody candidate -- today known as Leqembi.

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Source Fool.com