UPDATE 2-Roche arthritis drug Actemra backed by fifth study

(Adds comment and stock)

ZURICH, May 9 (Reuters) - Swiss drugmaker Roche (ROG.VX) said on Friday its Actemra drug for rheumatoid arthritis had proved effective in a fifth late-stage study that boosts its chance of bumper sales.

The medicine, which Roche Holding AG sees as potential blockbuster to reduce its reliance on cancer medicines, was approved in Japan for treating rheumatoid arthritis last month. It has yet to reach the market in Europe or the United States.

Results from the latest Phase-III clinical trial, known as LITHE, showed Actemra could significantly inhibit structural damage to joints. It also improved patients' physical function after one year's use, leading to a better quality of life.

"As all the other trials were positive, it is not a huge surprise that this trial was successful as well," said Landsbanki Kepler analyst Denise Anderson. "However, it is still good news as the depth of the clinical programme is likely to be a big plus in selling the drug."

Deutsche Bank analysts pointed out the trial was important because it measured joint damage -- a clear indication of progression of the disease -- and quality of life.

Roche stock fell 2.6 percent to 164.40 Swiss francs by 1012 GMT, compared to a 1.8 percent drop in the European pharmaceuticals sector .SXDP.

Actemra is the first of a new class of drug designed to suppress the action of interleukin-6, a protein that acts as a trigger for the body's inflammatory process.

Rheumatoid arthritis -- characterised by inflammation of the membrane lining joints throughout the body, which can cause pain, stiffness and swelling and ultimately disability -- is thought to affect 21 million people worldwide.

(Reporting by Ben Hirschler and Sam Cage; Editing by David Holmes and Jason Neely)