MARLEY SEAMAN AP Health Writer NEW YORK (AP) — Schering-Plough Corp. said its experimental hepatitis C drug boceprevir helped eliminate the virus in about half of patients who did not respond to standard treatments. Boceprevir is considered one of the most important products being developed by the Kenilworth, N.J., drugmaker, which is being acquired by Merck & Co. for $41.1 billion. Initial results from the late-stage study, called Sprint-1, were reported earlier this year, but on Sunday, Schering-Plough reported the results of patients who had no response to other drugs, a hard-to-treat group called "null responders." A total of 50 patients were found to be "null responders" after four weeks of standard treatment. After 28 weeks of treatment, Schering-Plough said 25 percent of the null responders had a "sustained virologic response," meaning their virus levels were too low to measure. After 48 weeks of treatment, 55 percent of patients had a sustained virologic response. "When you add boceprevir to these individuals, these null responders, you can get a sustained response, a cure of up to 55 percent, in the most difficult-to-treat individuals," said Paul Quo, the principal investigator in the study. The result was similar to data reported by Vertex Pharmaceuticals Inc. on Wednesday. Vertex said 57 percent of null responders had a sustained response after 48 weeks of treatment with the company's drug telaprevir. The results were presented in Boston at a meeting of the American Associated for the Study of Liver Diseases. The most common side effects in the Schering study were fatigue, anemia, nausea and headache. About half of the boceprevir patients developed anemia, a frequent side effect of hepatitis C drugs, compared to a third of the patients who did not receive the drug. Patients who took boceprevir were more likely to stop treatment than those who did not, but they had fewer relapses. Schering-Plough is running a larger late-stage clinical trial of boceprevir on null responders, and a separate study on patients with untreated hepatitis C.