Drug developer Schering-Plough Corp. said an ongoing study of its hepatitis C treatment showed sustained viral suppression in 56 percent of patients after nearly a year on the drug.

The company's statement comes on the heels of an announcement by possible future competitor Idenix Pharmaceuticals, touting the effectiveness of its hepatitis C treatment in a clinical trial. Both companies are presenting data at the annual meeting of the American Association of the Study of Liver Diseases in Boston.

The 48-week study involves more than 2,200 patients receiving Peg-Intron and Rebetol, following failed treatment with other combination therapies. Both drugs are already approved by the Food and Drug Administration.

Shares of Schering fell 40 cents to $22.38 on the New York Stock Exchange in morning trading. Shares of Idenix rose 9 cents to $9.97 on the Nasdaq.