* Virus undetectable in 75 pct of treated patients
* Best response seen for any drug in mid-stage trials
* But anemia seen in half of treated patients
* 39 to 51 pct of treated patients needed EPO for anemia
NEW YORK, April 23 - A Schering-Plough Corp drug knocked the hepatitis C virus down to undetectable levels in three-fourths of patients in a mid-stage study, twice the effectiveness seen with standard treatments, researchers said on Thursday.
But half the patients taking the boceprevir experimental medicine developed anemia -- a potential commercial disadvantage to a similar pill called telaprevir that Vertex Pharmaceutical Inc is developing.
Both drugs work through a new mechanism -- by blocking a protein called protease that the virus needs to replicate -- and are considered potential big-selling products. As many as 4 million Americans are believed to be infected with the virus, the leading reason for liver transplants.
Results of the Phase II boceprevir trial, involving 595 patients who were infected with the virus but had not previously been treated, were presented in Copenhagen at the annual meeting of the European Association for the Study of the Liver. Patients had genotype 1, the most common form of the virus.
During the first month of the study, all patients received the two standard hepatitis C treatments sold by Schering-Plough -- a long-acting form of interferon called Pegintron and the anti-viral pill ribavirin.
One group of patients then added boceprevir to Pegintron and ribavirin for 44 weeks, while another group took only Pegintron and ribavirin over the same period.
After the 48-week trial concluded, 75 percent of those in the boceprevir group had a sustained virologic response (SVR) -- meaning undetectable levels of virus.
"This is the highest sustained virologic response reported for any Phase II study of patients with genotype 1," said Dr. Paul Kwo, an associate professor at Indiana University School of Medicine, the study's chief investigator.
By contrast, only 38 percent of patients who did not get boceprevir drove the virus down to undetectable levels in the same 48-week period.
Researchers also determined that after only 28 weeks of treatment with boceprevir, Pegintron and ribavirin, 56 percent of patients achieved SVR.
Anemia was seen in about half of patients taking boceprevir, and in a third of patients taking only Pegintron and ribavirin.
Erythropoietin (EPO), a widely used anemia treatment, was used by 39 percent to 51 percent of patients taking boceprevir, and by 26 percent of those taking only Pegintron and ribavirin.
"Management of anemia with EPO is a common practice in hepatitis C treatment today," said Kwo, who noted that ribavirin and protease inhibitors both increase the risk of anemia.
A lesser incidence of anemia was reported last year for Vertex' rival telaprevir in mid-stage trials -- ranging from 29 to 37 percent of patients. Telaprevir knocked the virus to undetectable levels in about two thirds of patients.
Kwo, who has also tested the Vertex product, said data from late-stage trials of boceprevir and telaprevir will provide a clearer indication of relative anemia risk for the two new protease inhibitors.
Geoffrey Porges, a biotech analyst for Sanford Bernstein who is attending the Copenhagen meeting, said the need to take EPO to control anemia will hurt boceprevir.
"It would be very unusual for a drug to go forward and more or less require the use of another drug that's not even approved for that indication," Porges said.
Boceprevir is considered one of the most important of Schering-Plough's experimental drugs. Merck & Co will inherit the pill when it completes its planned purchase of Schering later this year.