CAMBRIDGE, Mass. (AP) — Vertex Pharmaceuticals Inc. said today that a study showed extended treatment with its hepatitis C drug candidate telaprevir led to better results for some patients.

Vertex said patients who responded quickly to treatment with telaprevir had better results after 24 weeks of therapy than after 12. However it said there was no benefit to extending therapy to 48 weeks.

The company said 92 percent of patients who responded well to telaprevir had had undetectable levels of the hepatitis C virus after 24 weeks of treatment. After 48 weeks, 88 percent of patients had undetectable virus levels. Patients were chosen to receive extended treatment if their virus levels was undetectable after 4 weeks and 12 weeks of treatment in an earlier study.

The company plans to file for Food and Drug Administration approval of telaprevir in the fourth quarter. The drug is seen as a potential billion-seller for Vertex.

The clinical trial was called ILLUMINATE, and it was a follow up to a previous study called ADVANCE. Patients who were responded well to treatment with telaprevir were then enrolled in the longer ILLUMINATE study, and their treatment was extended to 24 or 48 weeks. Patients who did not have a quick response to telaprevir were given standard hepatitis C drugs.

The most common side effects of telaprevir treatment were fatigue, itching, nausea, anemia, rash, and headache. Vertex said most of those side effects were mild or moderate.