LONDON (Reuters) - AstraZeneca's experimental rheumatoid arthritis pill fostamatinib produced mixed results in a late-stage clinical trial, the company said on Friday. It achieved a statistically significant improvement in a test assessing signs and symptoms of the disease, but failed to show an improvement in an X-ray endpoint, according to results from the OSKIRA-1 Phase III study. (Reporting by Ben Hirschler)