AstraZeneca plc's top-line results from a Phase IIb study of fostamatinib in rheumatoid arthritis (RA) showed that the drug was superior to placebo at six weeks, but it fell short of a secondary goal of noninferiority against Humira (adalimumab, Abbott), sending shares of partner Rigel Pharmaceuticals Inc., of South San Francisco, plunging by almost 35 percent.