US biotechnology firm Repligen has filed a lawsuit claiming that Bristol-Myers Squibb has infringed a patent in developing its just-approved rheumatoid arthritis drug Orencia (abatacept).

B-MS won US approval to market Orencia at the end of 2005. It has been tipped as a potential blockbuster treatment as it is the first in a new class of RA treatments that could quickly find a use in treating the third of all patients who do not respond to current treatments, including the anti-tumor necrosis factor (TNF) biologics - such as Johnson & Johnson's Remicade (infliximab), Wyeth's Enbrel (etanercept) and Abbott Laboratories' Humira (adalimumab) - which make up the bulk of the $5.5 billion market for RA treatments.

Orencia is a T cell co-stimulation modulator, controlling T cell function by binding to the cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) cell–surface molecule. Repligen insists that Orencia contravenes a patent (No 6,685,941) held jointly by the company and the University of Michigan, which covers methods of using CTLA4-Ig to treat rheumatoid arthritis, as well as other therapeutic methods. This patent is valid out to 2021, according to Repligen.

Repligen is evaluating a CTLA4-Ig product. RG2077, in a Phase I clinical trial in patients with multiple sclerosis.

Walter Herlihy, Repligen’s CEO, said: "We intend to fully protect Repligen's …patent rights, by seeking a royalty bearing license agreement with B-MS whether through litigation or negotiation." B-MS has said it plans to launch Orencia in the USA in February.