The problem with investing in biotechs that carry high expectations? Sometimes they don't live up to those expecations. Regeneron Pharmaceuticals (Nasdaq: REGN) is down about 4.5% today after its IL-1inhibitor, Arcalyst, passed only one of its two phase 3 trials.

Regeneron is testing Arcalyst as a treatment for gout, a type of arthritis that results in painful flare-ups. In one trial, Arcalyst failed to show a decrease in pain associated with the flare-ups compared to a standard painkiller when given during an attack. In another trial, the drug decreased the number of flare-ups by 80% at the highest dose when given weekly over 16 weeks.

On the surface, if only one of them was going to turn up positive, it seems the right one passed. Given the choice of marketing an acute treatment or a maintenance treatment, most drugmakers would choose the one that had to be taken over a long period of time. Of the 10 drugs expected to sell the best this year, eight of them have to be taken regularly. It's easy for drugs such as Pfizer's (NYSE: PFE) Lipitor and sanofi-aventis (NYSE: SNY) and Bristol-Myers Squibb's (NYSE: BMY) Plavix to rack up sales since they don't really cure the patient. On the flip side, Merck's (NYSE: MRK) Gardasil, which gets injected three times and then protects the patient for life, doesn't exactly use the best business model.

Dig a little deeper into the data, though, and you'll see that gout flare-ups are generally spaced far apart; during the 16-week trial, patients taking placebo averaged only one flare-up per person. Patients may not be interested in injecting themselves weekly to avoid relatively infrequent attacks. Of course, there will be some patients with more frequent attacks who will probably take the drug, but that market may be fairly small.

Regeneron is still testing the drug in two additional trials, which will need to come back positive before the company can apply for approval in the middle of next year. In addition, it'll need a hit from its cancer drug Aflibercept, which is also in phase 3 trials, if it wants to support its lofty valuation.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. Pfizer is a recommendation of the Inside Value newsletter service. The Fool has a disclosure policy.