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.

The study involved 384 patients with active rheumatoid arthritis, who had not responded to treatment with another anti-rheumatic drug, such as methotrexate.

CP-690,550 is a so-called JAK-3 inhibitor that works by blocking enzymes involved in inflammatory and autoimmune diseases.

In addition to the ACR 20 primary goal, there were statistically significant ACR 50 responses -- 50 percent improvement -- for the drug at 5 mg, 10 mg and 15 mg, and ACR 70 responses at the two highest doses, compared with placebo, researchers said.

The most common adverse events were mild to moderate urinary tract infection, diarrhea, bronchitis and headaches.

Significant dose-dependent decreases in white blood cells and increases in both good and bad cholesterol were consistent with previous studies of CP-690,550, Pfizer said. (Reporting by Bill Berkrot)