(Corrects third paragraph to show hold on U.S. trial, not European study)
July 25 (Reuters) - Vertex Pharmaceutical Inc said U.S. health regulators placed a partial clinical hold on its mid-stage study of an experimental oral hepatitis C treatment because of potential liver problems, sending its shares sharply lower on Thursday.
Vertex said the U.S. Food and Drug Administration took the action on the Phase II study of its VX-135 in combination with the standard hepatitis drug ribavirin after elevated liver enzymes were observed in three patients taking the 400 milligram dose of its drug.
As a result of the hold on the U.S. clinical trial, the Massachusetts-based biotechnology company said it was unable to begin dosing patients in a planned U.S. study aimed at testing VX-135 at 200 mg.
Vertex shares fell 11 percent to $78 in after hours trading from their Nasdaq close at $87.62. (Reporting by Bill Berkrot; Editing by Grant McCool)