Pfizer drug effective in rheumatoid arthritis study

* Oral RA drug significantly better than placebo

* ACR 20 tops placebo at all tested doses

* 5 mg and 10 mg to move on to Phase III testing

NEW YORK, June 9 (Reuters) - An experimental oral rheumatoid arthritis drug being developed by Pfizer Inc (PFE.N) was significantly more effective than a placebo in a mid-stage clinical trial, according to data to be presented at a medical meeting in Denmark on Thursday.

The drug, given twice daily, was tested at strengths of 3 milligrams, 5 mg, 10 mg and 15 mg and demonstrated a statistically significant response at all doses compared with a placebo, the company said.

Data from a 12-week interim analysis of the six-month study of the drug, CP-690,550, has been used to help select the 5 mg and 10 mg doses for larger Phase III clinical trials, Pfizer said. Phase III is typically the final stage of testing before a new drug is submitted to the U.S. Food and Drug Administration for an approval decision.

The primary goal of the study was an ACR 20 response rate, defined as 20 percent improvement in tender and swollen joints.

At the two highest doses, 75.4 percent of patients achieved ACR 20 response rates compared with 28.8 percent of those in the placebo group. The ACR 20 response was 49 percent at 3 mg and 63.3 percent at 5 mg, according to data to be presented at the European League Against Rheumatism meeting in Copenhagen.

The differences in response rates were seen as early as two weeks into the study, researchers said.