A slew of new clinical trial data has recently emerged regarding unorthodox applications for some of Pfizer's (NYSE:PFE) lead drugs, including Celebrex and Viagra. While the results have been mixed at best, they do highlight the potentially vast and divergent uses for some of the pharmaceutical industry's blockbuster drugs.

Pfizer's Celebrex is a COX-2 inhibitor, used primarily as a treatment for inflammatory-pain-related indications like arthritis. A search on clinicaltrials.gov shows that researchers have also been testing it for indications ranging from Crohn's disease to numerous types of cancer. Earlier this week, The Wall Street Journal's health blog cited initial results from an apparently failed study of the drug as a treatment to help prevent Alzheimer's disease.

The Celebrex data is not even a minor loss for Pfizer; it's more like a missed Hail Mary attempt by non-Pfizer researchers. Earlier in the year, Pfizer's blockbuster cholesterol drug Lipitor similarly failed to reduce Alzheimer's symptoms in a large study.

However, non-Pfizer researchers this week did report some initial success in using Pfizer's Viagra as a treatment for muscular dystrophy. Viagra is approved as a treatment for erectile dysfunction, but the active ingredient is also sold as Revatio, a treatment for pulmonary hypertension.

Besides Viagra, numerous well-known drugs are increasingly used in a broad range of indications. Compounds like Eli Lilly's (NYSE:LLY) Cymbalta or Cephalon's (NASDAQ:CEPH) Provigil have found on- and off-label uses to treat multiple seemingly unrelated conditions.

Drugmakers and unaffiliated researchers generally don't test drug candidates in random indications, hoping that something will stick. Their tests generally spring from a scientific hypothesis about why a compound might work in a potential label-expanding indication.

Even while these label-expanding studies are going on, many doctors aren't waiting for the results. A 2006 study published in the Archives of Internal Medicine estimated that 21% of prescriptions for commonly used drugs involve off-label usage.

If a drugmaker can better define when this usage should take place by producing positive clinical trial data, it can actually market the drugs for these indications, growing their sales. Of course, those plans don't always pan out -- but that's the ever-present risk of drug development.

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