Genentech and Biogen Idec won approval to market their cancer drug Rituxan as a treatment for moderately-to-severely active rheumatoid arthritis yesterday, an indication that analysts suggest could add another $1 billion to the product’s peak sales.

Rituxan (rituximab) is already a blockbuster drug in its current indication of non-Hodgkin’s lymphoma, and is expected to make strong headway in rheumatoid arthritis because it works by a different mechanism of action to all the available drugs for the disease.

The drug, which is sold by Roche in Europe as MabThera, has been approved for use alongside standard rheumatoid arthritis treatment methotrexate in patients who have not responded to biologic therapy based on tumour necrosis factor blockers, namely Centocor’s Remicade (infliximab), Amgen/Wyeth’s Enbrel (etanercept) and Abbott Laboratories’ Humira (adalimumab).

The biologic anti-TNF drugs have combined annual sales of around $4.6 billion out of a total RA market estimated at around $5.5 billion. Between 15% and 25% of patients taking anti-TNF drugs fail to respond to therapy, although studies have suggested that they may be helped by switching from one anti-TNF therapy to another.

Roche filed a regulatory submission for MabThera in rheumatoid arthritis to the European Medicines Agency (EMEA) in September 2005.

Rituxan will compete with a recently-approved drug from Bristol-Myers Squibb, Orencia (abatacept), that is also indicated for second-line treatment of rheumatoid arthritis after TNF inhibitors. Orencia has also been tipped as a billion-dollar seller by analysts.

Rituxan is expected to do well because it requires only two injections a year, while most of the existing products need injections at least monthly. This low dosing frequency could also give the drug a pricing advantage.