The preliminary analysis of the Phase IIb results showed that TRU-015, at well-tolerated dosages, provided statistically significant improvements in rheumatoid arthritis signs and symptoms compared with placebo at 24 weeks, the company said in a press release.

The study was designed to evaluate the safety and effectiveness of a single intravenous infusion of TRU-015 compared with placebo for a 24-week period.

Trubion is codeveloping TRU-015 with Wyeth Pharmaceuticals, a division of Wyeth.

The Phase IIb clinical trial included 276 patients with rheumatoid arthritis who were also receiving an autoimmune disease treatment called methotrexate.

The company said it met both its primary and secondary targets at 12 weeks and 24 weeks, respectively.

Specifically, the preliminary analysis shows improvement compared to placebo that was statistically significant in the 800-milligram dose group at 12 weeks.

"We are working with our partner to define the most effective development strategy for TRU-015 in RA that minimizes time-to-market while maximizing our competitive differentiation," said Peter Thompson, president, chief executive and chairman of Trubion. "We currently believe this will include additional clinical study before commencement of a Phase III trial."

The company did not provide added details.

Shares of Trubion rose $1.57, or 9 percent, to close at $19.09. The stock shed $1.40, or 7.3 percent, to $17.67 in after-hours trading.

Shares of Wyeth rose 58 cents to close at $46.30.