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Sanofi, Regeneron arthritis drug meets goals in Phase 3 trial

* Trial combined sarilumab and methotrexate

* Sarilumab met all three co-primary endpoints

* Most reported adverse events were infections

PARIS, Nov 22 (Reuters) - An experimental drug for rheumatoid arthritis developed by French drugmaker Sanofi and Regeneron, when combined with methotrexate, improved symptoms and physical function and slowed progression of the disease in a late-stage clinical trial.

Rheumatoid arthritis is an autoimmune disease in which the body's immune system mistakenly attacks healthy tissue, causing inflammation and pain in the joints.

Sanofi and Regeneron's drug, called sarilumab, is an injectable antibody that works by blocking an inflammation-causing protein called interleukin 6. It is similar to Actemra, Roche's fast-growing treatment approved in 2010.

The success of the trial pushes the new drug one step closer to the production line, although it still to pass further long-term trials and the approval process in particular markets.

The 52 week SARIL-RA-MOBILITY Phase 3 trial enrolled some 1,200 patients with active, moderate-to-severe rheumatoid arthritis, who have not benefited from or been able to tolerate the standard oral treatment, methotrexate, whose side effects can include nausea and liver damage.

Patients given a 200 mg dose of sarilumab every other week on top of methotrexate saw a 66 percent improvement in signs and symptoms of rheumatoid arthritis after six months, Sanofi and Regeneron said in a statement on Friday.

Those given a 150 mg dose saw a 58 percent improvement, while those given a placebo alongside methotrexate saw a 33 percent improvement.

Sarilumab met the other two primary endpoints of the study, improving physical function at week 16 and inhibiting progression of joint damage after one year, the companies said.

Infections were the most frequently reported adverse side effects, as well as increases in "bad" LDL cholesterol and transaminases, they added.

Sarilumab, alongside cholesterol drug alirocumab, is one of the promising products Sanofi is developing under its partnership with U.S. biotech Regeneron to offset the loss of patents on once top-selling drugs like blood thinner Plavix.