Vertex Pharmaceuticals said yesterday its rheumatoid arthritis drug VX-702 showed efficacy in a Phase II study, although its share price fell on the announcement.

The 315-patient study of VX-702, in a new class of RA drugs known as p38 MAP kinase inhibitors, suggested that the drug was well-tolerated when given as a once-daily, oral dose and had significant effects on the signs and symptoms of arthritis.

But Vertex’ shares fell more than 7% in after-hours trading, suggesting that investors were hoping that the data on VX-702 would be stronger.

The results indicated that 40% of patients on a 10mg daily dose of VX-702 achieved a 20% improvement in symptoms according to American College Rheumatology criteria (ACR20). This level of response was seen in 38% of those on a lower, 5mg dose, and 30% of patients receiving placebo after 12 weeks’ treatment.

Vertex said it plans to start combination studies of VX-702 and methotrexate – a standard treatment for RA – by mid-2006. The drug also has potential in the treatment of other inflammatory diseases, including multiple sclerosis, psoriasis and Crohn’s.

The company’s chief executive, Joshua Boger, said last month he believed VX-702 could surpass the $2.6 billion annual sales of Amgen’s injectable RA drug Enbrel (etanercept).

Other companies developing p38 MAP kinase inhibitors include Scios, with SCIO-469 in Phase II testing in RA, and Amgen.