Archery Micolo J

Source: Flickr user Micolo J.

The Holy Grail of biotech companies is a billion-dollar blockbuster therapy, but more often than not, efforts to invent game-changing medicines that can eclipse the billion-dollar threshold fall short.

That's particularly true for biopharma companies targeting tough-to-treat diseases such as Alzheimer's disease and Parkinson's disease. Unfortunately, efforts to revolutionize treatment for those indications have mostly failed.

Despite the high failure rate, companies continue to plow big money into researching potential medicines like the following three long shots, all of which could have big-time market potential if they succeed.

No. 1: Axovant Sciences Ltd. (NASDAQ:AXON): RVT-101
At this time last year, Axovant Sciences didn't even exist. Instead, it was created by upstart private biotech company Roivant Sciences to develop RVT-101, an Alzheimer's disease drug originally created by GlaxoSmithKline.

Following mid-phase trial disappointments, GlaxoSmithKline punted RVT-101 in December to Roivant in exchange for $5 million up front, milestones, and double-digit royalties on any eventual sales.

Axovant Sciences has lined up an impressive group of scientists to usher RVT-101 through phase 3 trials, including Lawrence Friedhoff, who led the team responsible for creating the blockbuster Alzheimer's disease drug Aricept, and Lawrence Olanoff, the person behind the blockbuster Alzheimer's disease drug Namenda.

We won't know if the two Lawrences will succeed again with RVT-101 until 2017, when phase 3 trial data should be available, but if they do, RVT-101 could have blockbuster potential.

An increasingly larger and longer-living population is expected to increase the number of Americans suffering from Alzheimer's disease from 5 million last year to 13.8 million in 2050.

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Source: Biogen.

No. 2: Biogen (NASDAQ:BIIB): Aducanumab
Biogen has long been the dominant player in multiple sclerosis treatment, but the company is funneling hundreds of millions of dollars into research programs designed to diversify its business across other indications, including Alzheimer's disease.

In December, Biogen reported early-stage trial results for its most promising Alzheimer's disease drug, aducanumab, which impressed industry watchers.

In that trial, aducanumab successfully reduced amyloid plaque build-ups thought to be responsible for Alzheimer's disease's progression. The results were encouraging enough that Biogen has decided to skip straight over mid-stage trials to phase 3 studies, which began in June.

If aducanumab's phase 3 trials confirm the early-stage results, Biogen could have a big-time seller on its hands. Today, Americans are already spending more than $200 billion annually caring for Alzheimer's disease patients, so the market demand for therapies that can ease that burden could be massive.

No 3: Prothena Corporation PLC (NASDAQ:PRTA): PRX-002
Unlike the first two drugs, Prothena's PRX-002 is taking on Parkinson's disease, the second most common neurological disease behind Alzheimer's disease.

Prothena was spun off by Elan plc, the co-developer of the MS drug Tysabri, in 2012, and in late 2013, Roche Holdings partnered up with Prothena to develop monoclonal antibodies in an attempt to reduce an unnatural build-up of the neural protein, alpha-synuclein, associated with Parkinson's disease.

In phase 1 trials, patients taking PRX-002 saw their alpha synuclein levels drop by as much as 96%, which was significantly better than placebo. Although those are early-stage results, and later-stage trials may not confirm them, the significant reduction does help validate Prothena's approach.

If PRX-002's future trials pan out, Roche has agreed to pay Prothena up to an additional $555 million in milestones, and 30% royalties on any eventual U.S. sales.

Tying it together
Roughly 90% of drugs entering phase 1 clinical trials fail to make it to market, and that win rate is even less encouraging when we consider that these companies are developing drugs for some of the most notoriously hard-to-treat indications.

Regardless, the unmet need for new Alzheimer's disease and Parkinson's disease therapies means the potential opportunity associated with these drugs could be huge, and for that reason, all three of these long shots are worth keeping on your radar.

Todd Campbell owns shares of Prothena. Todd owns E.B. Capital Markets, LLC. E.B. Capital's clients may have positions in the companies mentioned. The Motley Fool recommends GlaxoSmithKline. 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.