Shares of Rigel Therapeutics Inc. lost more than a third of their value Friday on mixed results from a late-stage clinical trial of a rheumatoid arthritis drug the company helped develop.

Rigel's partner AstraZeneca PLC said Friday that fostamatinib met one of its two goals in the study: patients who took the experimental drug and methotrexate, a standard treatment for rheumatoid arthritis, had a bigger improvement in symptoms like joint swelling and tenderness than patients who took methotrexate and a placebo. However, the drug did not meet a second goal related to bone erosion and reductions in joint space.

Rigel doesn't have any approved products, and fostamatinib is its most advanced experimental drug. The company licensed fostamatinib to AstraZeneca in 2010 in a deal that could be worth more than $1 billion if the drug is approved and meets sales targets.

Citi Investment Research analyst Yaron Werber said fostamatinib doesn't compare favorably to existing rheumatoid arthritis drugs or to others in clinical development.

Rigel stock lost $2.56, or 34 percent, to $4.97. Earlier the stock fell to $4.71, its lowest price in almost four years.

The late-stage trial included 923 patients who had not been helped by previous treatment with methotrexate. The results are based on 24 weeks of treatment. AstraZeneca said it expects to report results from two other studies of fostamatinib during the second quarter.

Rigel Pharmaceuticals is based in South San Francisco, Calif. AstraZeneca, which is based in London, is one of the world's largest drug companies. The British company licensed fostamatinib from Rigel in February 2010, paying $100 million upfront.

Werber said fostamatinib doesn't seem to be as effective as biotech treatments like AbbVie Inc.'s Humira and other potential rheumatoid arthritis drugs.

"The outlook for fostamatinib is becoming more challenging given these results and the fact that the arthritis market will be getting very competitive with several oral and biologic drugs in development," he wrote.

U.S.-listed shares of AstraZeneca rose 26 cents to $50.39 in morning trading.