UPDATE 2-U.S. approves Roche's Actemra arthritis drug

* U.S. FDA clears drug for use in rheumatoid arthritis

* Roche sees therapy as potential blockbuster (Adds background)

WASHINGTON Jan 08 (Reuters) - The U.S. Food and Drug Administration has approved Roche Holding AG's (ROG.VX) Actemra to treat rheumatoid arthritis, the company said on Friday.

The drug, made by Roche's Genentech unit, is already approved for use in Europe and Japan and is expected by the company to become a blockbuster product.

The FDA approved the drug for treatment of adults with moderate to severe rheumatoid arthritis who do not respond well enough to another class of biotech drugs designed to block an inflammatory protein called tumor necrosis factor (TNF).

Actemra is an anti-interleukin-6 receptor antibody and works differently from TNF blockers such as Humira, sold by Abbott Laboratories (ABT.N), and Remicade, sold by Johnson & Johnson (JNJ.N).

Also known by its chemical name tocilizumab, Actemra will compete against Roche's own anti-TNF therapy Rituxan and is seen by the company as a way to help diversify its business.

Actemra may be used alone or in combination with methotrexate or other disease modifying anti-rheumatic drugs.

Patients with rheumatoid arthritis experience inflamed membranes that line joints throughout the body, causing pain, stiffness and disability. The condition is estimated to affect more than 20 million people worldwide.

(Reporting by Susan Heavey and Deena Beasley; Editing Bernard Orr)