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An experimental pill developed by BioCryst Pharmaceuticals (BCRX) - Get BioCryst Pharmaceuticals, Inc. Report prevented significantly more swelling attacks than placebo in patients with hereditary angioedema, but investors expected stronger results from the mid-stage study released Monday, causing the company's stock price to fall.

Hereditary angioedema is a rare, genetic blood disorder that causes episodes of swelling in various body parts. HAE can be life threatening when the swelling affects a patient's airway.

Current drugs approved to prevent and reduce the frequency of HAE swelling attacks require injections, so BioCryst's once-daily pill, known as BCX7353, has the potential to be more convenient if efficacy and safety are comparable.

Monday's interim results from a mid-stage study, however, raise concerns that BCX7353 may not be as effective as competing HAE drugs.

BioCryst shares are down 12% to $4.81 in Monday trading, after the BCX7353 study results were announced.

In the phase II study of 28 patients, treatment with BCX7353 reduced the rate of angioedema attacks by 52% compared to a placebo. When four patients lacking a confirmed HAE diagnosis were excluded from the analysis, Biocryst's drug reduced the rate of angioedema attacks by 63% compared to a placebo.

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Across both analyses, the reduction in the frequency of swelling episodes was statistically significant favoring BCX7353 over placebo, BioCryst said.

Dr. Emel Aygoren-Pursun, an HAE specialist in Germany and the principal investigator in the BioCryst study described the BCX7353 results as "extraordinarily encouraging," in a statement.

However, investors were hoping to see the BioCryst drug achieve HAE attack reductions in the 70% range to be competitive against injectable drugs, including a product under development by Shire (SHPG) - Get Shire PLC Sponsored ADR Report , that have shown 90% attack reductions.

BioCryst conducted a pre-planned analysis of the BCX7353 study showing an 88% reduction in so-called "peripheral attacks" that are considered more serious and life threatening to patients than "abdominal attacks."

The overall results may have been lower than expected because some patients in the study could have told doctors they were experiencing an abdominal attack caused by HAE when what they were actually feeling was a transient abdominal pain side effect of BCX7353, the company said.

BioCryst said Monday that the interim results support continued development of BCX7353 as a potential, once-daily oral treatment to prevent HAE attacks. The company is enrolling more patients into the phase II study and adding a lower dose of the drug.

Adam Feuerstein writes regularly for TheStreet. In keeping with company editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.