Alzheimer's disease affects over 5 million Americans. It's the sixth-highest cause of death in the U.S. There's such an intense demand for effective treatments that research and consulting firm GlobalData projects the market for Alzheimer's disease drugs could top $13 billion by 2023.

Five companies appear to be poised to potentially dominate this market: AstraZeneca (NYSE:AZN), Biogen (NASDAQ:BIIB), Lilly (NYSE:LLY), Merck (NYSE:MRK), and Roche (NASDAQOTH:RHHBY). Here's how they plan to win.

Pad that says Alzheimers Disease surrounded by pills, syringes, a pen, and a stethoscope.

Image source: Getty Images.

The BACE race

One of the approaches being researched to combat Alzheimer's disease focuses on beta secretase cleaving enzyme (BACE) inhibitors. BACE is an enzyme associated with the development of amyloid beta, which are pieces of protein that clump together and gradually build up into amyloid plaque in the brain. The thought is that inhibiting BACE could prevent the buildup of this plaque and slow progression of Alzheimer's disease.

Three companies stand out as leaders in the race to develop BACE inhibitors for treating Alzheimer's. AstraZeneca and Lilly are working together on AZD3293, which is currently in two late-stage clinical studies. Merck also has a couple of late-stage studies in progress for its experimental Alzheimer's disease drug verubecestat.   

AstraZeneca and Lilly recently won Fast Track designation from the U.S. Food and Drug Administration for AZD3293. This means that the FDA will work more closely with the drugmakers in the approval process and could pave the way for a quicker approval. 

Merck expects results from the first late-stage study of verubecestat to be available in late 2017. The company recently announced positive findings from an early-stage study of the drug, with patients at the highest dosage of verubecestat experiencing an 84% reduction in amyloid beta.

Another approach

The other primary approach for treating Alzheimer's disease being researched involves use of monoclonal antibodies. Like BACE inhibitors, monoclonal antibodies can potentially prevent the buildup of amyloid plaque.

Lilly is a major player in this alternative Alzheimer's approach, also. The company's solanezumab is in a late-stage clinical study. Mid-stage studies of the drug didn't show significant improvement versus placebo. However, Lilly moved forward with solanezumab anyway, changing the late-stage study in March to only have one primary endpoint: improvement in cognition.

Biogen has its own late-stage monoclonal antibody, aducanumab. The biotech won Fast Track designation for aducanumab in September. Earlier studies showed significant potential for the drug. There's still a long way to go, though. The primary outcome results for the late-stage studies of aducanumab won't be available until 2020.

Biogen and Lilly aren't the only companies in the hunt for a monoclonal antibody that could potentially be a game changer for Alzheimer's. Roche has two monoclonal antibodies targeting the disease -- gantenerumab and crenezumab. Both are in late-stage clinical studies. Neither drug, however, met primary endpoints in mid-stage studies.Gantenerumab also flopped in a previous late-stage study targeting Alzheimer's disease.

Risky business

While there's a lot of potential, developing Alzheimer's disease treatments has proven to be a very risky business. Several of the experimental drugs in development have not proven to be effective in earlier studies. Historically, Alzheimer's has one of the worst drug failure rates of any disease. Between 2002 and 2012, only one out of 244 Alzheimer's drug candidates won approval -- a dismal 99.6% failure rate.

It's entirely possible that none of the five companies mentioned succeed. Even Lilly's straddling the fence with a BACE inhibitor candidate and a monoclonal antibody candidate doesn't necessarily give the drugmaker an upper hand. 

Still, though, each of these companies has a shot at claiming a stake in what could become a big market. Based on the data available so far, I suspect that Biogen and the AstraZeneca/Lilly partnership have the best chances for success. It's too soon to know for sure, however. Alzheimer's disease is just that tough of an opponent. 

Keith Speights has no position in any stocks mentioned. The Motley Fool owns shares of and recommends Biogen. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.