BOULDER, Colo. (AP) — Shares of Array BioPharma Inc. tumbled in premarket trading Friday after the drug maker said its experimental rheumatoid arthritis treatment didn't prove more effective than a placebo in reducing symptoms in a midstage study. The stock fell 86 cents, or 23 percent, to $2.92. Late Thursday, Array reported data from a 12-week, Phase II clinical trial involving 201 patients with active rheumatoid arthritis (RA) that was not completely responsive to the drug methotrexate, the standard treatment for the illness for many years. RA is a debilitating autoimmune disease that affects more than two million Americans, and about 1 percent of the global population. The immune system attacks joint tissue, causing painful chronic inflammation, often resulting in irreversible destruction of cartilage, tendons and bones. The study included a placebo group and three different dose groups of Array's drug candidate ARRY-162. None of the treatment groups saw a statistically significant response compared with the placebo group at 12 weeks. Array said placebo response rates in the study were higher than expected and showed regional differences, with patients in South America having much higher placebo response rates than those in Eastern Europe. The most common side effects were skin-related rash and diarrhea. There were no serious drug-related adverse events. "While we are disappointed in the overall efficacy outcome, we were pleased with the confirmation of the favorable safety profile and are continuing to evaluate the regional results," said Kevin Koch, president and chief scientific officer, in a statement. In a note to clients, Rodman & Renshaw analyst Simos Simeonidis said the results were "definitely a disappointment." However Simeonidis said Array is testing several other promising drugs and is steadily creating new ones, and he believes a sharp drop in the stock price would create a good opportunity to buy Array shares. The company said it will present complete results from the study at a medical conference in 2010. It is also studying ARRY-162 as a cancer treatment.