Photo by Images_of_Money / Flickr Creative Commons

Every investor who puts some money to work in the biotech or pharmaceutical space dreams about owning a company with a drug that goes on to generate billions in worldwide sales, as companies that do so tend to become great long-term investments that generate market-smashing returns for their investors. However, drug candidates that hold the potential to deliver billions in profits don't come along every day, so if you can find one that holds huge revenue potential it can be well worth your time to follow the drug as it moves through clinical trials.

Knowing that, we asked our team of Motley Fool contributors to share a company that has a drug in the experimental stage that they believe could offer more than $5 billion in annual sales down the road. Read below to hear about the drugs that have them excited.

George Budwell: Drugs that can rack up more than $5 billion in annual sales are exceedingly rare. In 2013, for instance, we saw a grand total of 12 drugs achieve this lofty feat, according to a report by Chemical & Engineering News. So, when a developmental-stage company appears to have one of these gems on its hands, it's fairly common that it attracts a significant amount of interest from potential buyers.

And that was the case recently with Receptos. Celgene (NASDAQ:CELG) paid $7.2 billion for Receptos earlier this year mainly for its experimental immune-inflammatory drug candidate Ozanimod. 

Ozanimod is presently in late-stage studies for relapsing multiple sclerosis (RMS) and moderate to severe ulcerative colitis (UC). And based on the clinical data so far, Celgene believes that Ozanimod could be on the market as a best-in-class treatment for RMS by 2018 and for UC by 2019.

What's truly noteworthy about this clinical-stage product candidate, however, is that management thinks Ozanimod could eventually generate peak sales of nearly $6 billion. If true, that would turn this drug candidate into Celgene's flagship product by perhaps the mid-2020s. 

Spicing things up further, Celgene thinks Ozanimod might also become an important new therapy for Crohn's disease down the road -- a market that the Decision Resources Group expects to grow to $5.6 billion by 2021. As such, investors will definitely want to keep a close watch on the clinical progress of this potential megablockbuster.   

Todd Campbell: Trying to pick drugs that may or may not become blockbusters is risky business, especially when it comes to early- and mid-stage drugs. Plenty of once-promising drugs have failed to live up to efficacy and safety expectations in large, late-stage trials, and even if they do pass muster in those trials, there's no guarantee of commercial success.
 
But just because there are risks doesn't mean that investors should ignore the potential for game-changing experimental medicine such as Novo Nordisk's (NYSE:NVO) oral diabetes drugs.  
 
Novo Nordisk is working on tablet formulations of multibillion-dollar blockbuster therapies, including GLP-1 drugs and insulin, and earlier this year, Novo Nordisk reported positive phase 2 trial results for its tablet formulation of its GLP-1 drug Victoza, clearing the way for phase 3 trials. The company also has two tablet versions of insulin analogues in trials that, if successful, could revolutionize patient treatment by eliminating the need for insulin injections.
 
Admittedly, challenges exist to developing oral insulin therapies, including the ability to create formulations that won't be broken down during digestion and that can maintain the right levels of blood sugar in the body for the right period of time, and that means that Novo Nordisk has its work cut out for it. But if Novo Nordisk can overcome these hurdles, then the market opportunity tied to oral diabetes drugs could be massive. 
 
In the first six months of this year, Novo Nordisk reported diabetes-related sales in excess of $6 billion, and according to projections by the International Diabetes Federation, diabetes medication is only going to climb because the number of people diagnosed with diabetes globally is expected to increase to 592 million in 2035 from 387 million today. Given that backdrop, the sales potential associated with oral diabetes drugs shouldn't be underestimated.

Brian Feroldi: Diseases of the brain like dementia, Alzheimer's, and schizophrenia affect millions of patients around the world and many of the currently available treatment options are inadequate, so companies with a promising drug candidate that could help treat these diseases certainly holds huge sales potential.

One company in the space that I have my eye on is Intra-Cellular Therapies (NASDAQ:ITCI), as the company just reported top-line phase 3 results from a study of its lead product candidate, ITI-007, which is being evaluated as a treatment option for schizophrenia. The study showed that ITI-007 was effective in reducing the negative symptoms associated with the disease, as it improved patients social functioning significantly and did so while maintaining placebo-like safety. These early results have certainly got its investors excited as they have been bidding up its stock ever since the data was released.

Schizophrenia affects roughly 1% of the world's adult population, which puts its addressable market opportunity in the U.S. alone at over 2.5 million adults, so the sales potential for ITI-007 could be huge. Worldwide peak sales estimates for ITI-007 are currently north of $6 billion for the drug's schizophrenia opportunity alone, and that number could easily soar higher over time as ITI-007 is also being evaluated as a potential treatment for other diseases of the central nervous system like bipolar disorder, dementia, depression, and more.

While it's still too early to know if ITI-007 is the real deal, early results look promising, so this may be a great stock to take a small position in now and add to over time if clinical studies continue to look good.

Brian Feroldi has no position in any stocks mentioned. George Budwell owns shares of Celgene. Todd Campbell owns shares of Celgene. The Motley Fool owns shares of and recommends Celgene. The Motley Fool recommends Novo Nordisk. 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.