LONDON ( TheStreet) -- An experimental rheumatoid arthritis pill from AstraZeneca ( AZN - Get Report) and Rigel ( RIGL - Get Report) produced mixed results in a late-stage clinical trial, raising additional questions about the drug's future.

In the study known as OSKIRA-1, two doses of fostamatinib significantly improved the signs and symptoms of rheumatoid arthritis compared to a placebo following 24 weeks of treatment. However, fostamatinib failed to show improvements over placebo in bone erosion and joint-space narrowing as measured by X-ray.

The most common adverse events reported in the study included hypertension, diarrhea, nausea, headache and cold symptoms. Detailed safety data was not disclosed, but in previous studies, fostamatinib was shown to cause high blood pressure.

AstraZeneca has been counting on fostamatinib to boost revenue at a time when many of its existing drugs are losing out to cheaper generic alternatives. The mixed results from the OSKIRA-1 study announced Friday will likely be viewed as another setback for the drug.

Last December, a mid-stage study showed fostamatinib to be inferior to Abbvie's ( ABBV - Get Report) Humira. Pfizer ( PFE - Get Report) has also recently launched its own rheumatoid arthritis pill, Xejanz.

AstraZeneca is conducting two additional phase III studies of fostamatinib, with results expected later this quarter. AstraZeneca licensed the drug from Rigel in 2010.

Shares of AstraZeneca were trading slightly lower in London. Rigel closed Thursday at $7.53, ahead of the fostamatinib trial results.

-- Reported by Adam Feuerstein in Boston.

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