Chelsea Therapeutics International has halted development of an investigational rheumatoid arthritis after it failed in a mid-stage study.

The US firm has presented data from a preliminary analysis of a Phase II trial of CH-4051, a non-metabolised antifolate, in patients RA who experience an inadequate response to methotrexate. The results indicates that the drug did not demonstrate superior efficacy to methotrexate in the dose range evaluated and Chelsea has decided to end development.

Chief executive Simon Pedder said the results "provide evidence of the clinical activity of CH-4051, in a dose dependent manner, across multiple RA assessment criteria". However, the study was "confounded by the unexpectedly robust response reported by patients treated with methotrexate".

He added that Chelsea believes higher doses of CH-4051 "could provide enhanced therapeutic benefit in RA" and that the compound could be developed for other anti-inflammatory and autoimmune indications. However, "we believe our current resources would be better allocated toward the planned completion of our Northera (droxidopa) development programme in neurogenic orthostatic hypotension".

However, Northera has hit some regulatory barriers. In February, the US.Food and Drug Administration in February voted 7-4 to back the approval of Northera, but a month later, it asked for more efficacy data.

News that the RA project has been dropped has unnerved investors and Chelsea's stock sank 31.3% to $1.34.