Updated from 9:37 a.m. EST



experimental drug maribavir failed to reduce viral infections in patients undergoing bone marrow transplants, according to results of a phase III study announced Monday.

In the phase III study, there was no difference between maribavir or a placebo in reducing the rate of cytomegalovirus (CMV) disease in patients undergoing bone marrow transplants, the company said.

"We are extremely disappointed by the outcome of this pivotal study," said Vincent Milano, ViroPharma's president and chief executive officer. "We just received these data and there are far more questions than answers; we still have a significant amount of work to do to fully understand this outcome and its impact on the overall program."

Viropharma trading was halted pending the announcement. Upon resuming, the stock was plunging more than 50% to $5.65.

CMV is a member of the herpes virus group, which includes the viruses that cause chicken pox, mononucleosis, cold sores, and genital herpes. CMV can remain dormant in the body for long periods of time and it's thought that between 50-85% of U.S. adults are infected, although the virus causes little or no illness in healthy people.

However, people with weakened immune systems, including patients undergoing bone marrow transplants, are susceptible to CMV-related disease, which can include inflammation of the lungs, or rejection of transplanted organs.

Current anti-CMV drugs are effective against the virus, but their use is limited by toxicities, most notably bone marrow suppression and kidney impairment.

Viropharma is developing maribavir as a preventative treatment for CMV with fewer toxicities.

The primary endpoint of the phase 3 study was the incidence of CMV disease, confirmed by an independent endpoint committee, within 6 months after bone marrow transplant. The incidence of CMV disease within 6 months was 4.4% for maribavir compared to 4.8% for placebo, the company said.



and AstraZeneca

(AZN) - Get Report

are also developing vaccines against CMV.

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