Rituxan is already one of the most successful biotech-derived cancer drugs ever created. Now, early research data suggest the drug also might prove to be effective as a treatment for rheumatoid arthritis.

If the early data pan out, it would be welcome news for biotech firms

Genentech

(DNA)

and

Idec Pharmaceuticals

(IDPH)

, as well as Swiss drug company

Roche

, which currently co-market Rituxan as a treatment for non-Hodgkin's lymphoma, which attacks the lymphatic system.

Rituxan is expected to post U.S. sales of about $1 billion this year as a treatment for non-Hodgkin's lymphoma. But while approximately 300,000 Americans are diagnosed with this form of cancer, there are about 2 million rheumatoid arthritis sufferers in the U.S.

In recent Wednesday trading, shares of Genentech were up 94 cents, or just under 3%, to $34.68; Idec was up $1.04, or just under 3%, to $37.31. The Amex Biotechnology Index was down 1%.

Wednesday, the three companies announced "positive preliminary results" from a phase II clinical trial that evaluated Rituxan alone or in combination with other drugs in patients with rheumatoid arthritis. Roche conducted the study.

Genentech spokesman Neil Cohen said actual results from the study were not released so that they could be presented later this year at a major medical meeting. Cohen declined to name the meeting, but it's believed to be the annual meeting of the American College of Rheumatology, scheduled for October.

Cohen added that the three companies are gearing up to plan additional phase II and phase III studies that will seek to validate Rituxan's role in treating rheumatoid arthritis, and eventually, get the drug approved for the disease.

That goal is likely still at least two years away, but it does put another potentially potent rival on the radar screens of

Immunex

(IMNX)

(soon to be acquired by

Amgen

(AMGN) - Get Report

) and

Johnson & Johnson

(JNJ) - Get Report

, which sell Enbrel and Remicade, respectively, as rheumatoid arthritis drugs.

Abbott Labs

(ABT) - Get Report

is expected to get its new rheumatoid arthritis drug, dubbed D2E7, approved in the U.S. in the middle of next year.

Rituxan works in cancer by targeting B cell lymphocytes, a type of immunity-boosting white blood cell. Genentech, Idec and Roche are now investigating whether this same B-cell mechanism is effective in preventing a patient's immune system from attacking joints, the primary symptom of rheumatoid arthritis that leads to swelling and pain.

By comparison, Enbrel, Remicade and D2E7 work by counteracting the debilitating effects of tumor necrosis factor, a regulatory protein produced by T cells, another form of white blood cell.

Scientists have been working on the theory that B cells play a role in rheumatoid arthritis for at least a couple of years. In 2000, researchers conducted a small pilot study that treated five patients with advanced rheumatoid arthritis with a four-week course of Rituxan and two other drugs.

The results were impressive: At six months, two patients achieved an ACR 50, while the other three achieved ACR 70. The ACR score is a measure of symptom improvement in rheumatoid arthritis patients; drugs are approved if they can help a meaningful number of patients achieve a score of ACR 20 and above.