* Oral fostamatinib worse than Abbott injectable in Phase IIb
* Definitive Phase III results with fostamatinib due in 2013
* Analysts' expectations for AstraZeneca, Rigel product low
* AstraZeneca shares fall 3 percent, Rigel dives 31 percent (Adds latest share price falls, further analyst reaction)
By Ben Hirschler
LONDON, Dec 13 (Reuters) - An experimental rheumatoid arthritis drug from AstraZeneca proved inferior to Abbott Laboratories' Humira in a clinical study, knocking hopes for one of the few late-stage products in the company's pipeline.
The mid-sized study of fostamatinib - given as a pill rather than injected, as is the case with Humira - showed it was not as good as Abbott's market-leading drug in controlling arthritis symptoms, AstraZeneca said on Thursday.
The news is a blow for Britain's second-biggest drugmaker, the shares of which fell 3 percent by 1520 GMT, and a bigger setback for its small partner, Rigel Pharmaceuticals. The U.S. company's shares tumbled 31 percent on Nasdaq.
AstraZeneca is chasing rival Pfizer in the race to develop a convenient oral alternative to anti-TNF injections like the $9 billion-a-year seller Humira, which have led the field for the past decade.
But while Pfizer's new pill Xeljanz won approval from U.S. regulators last month and proved as good or better than Humira in tests, analysts have been sceptical about prospects for fostamatinib after mixed results in earlier studies, including raised blood pressure in some patients.
The Phase IIb monotherapy results will do nothing to inspire confidence that fostamatinib can become the kind of multibillion-dollar seller that AstraZeneca needs to offset expiring patents on its existing best-selling medicines.
Although it proved better than placebo at some doses, the failure to match Humira's efficacy is "likely to limit the product's commercial potential", Panmure Gordon analyst Savvas Neophytou said.
Salveen Richter, of Canaccord Genuity, said the inferiority to Humira was a "significant negative".
With this year's loss of exclusivity on schizophrenia drug Seroquel, and Nexium for stomach acid and cholesterol fighter Crestor set for U.S. patent expiries in 2014 and 2016, AstraZeneca faces one of the industry's steepest patent "cliffs".
The six-month Phase IIb clinical trial, which involved 280 patients, is not yet the definitive test for fostamatinib, however.
A more comprehensive assessment of the drug used in combination with other medicines is being carried out in pivotal Phase III studies that will report results in the first half of 2013 and would form the basis of any regulatory submissions.
Even if approved, however, many analysts believe that fostamatinib, which AstraZeneca licensed from Rigel in 2010, could be sidelined as a marginal product in a highly competitive market.
The setback is the latest in a series of pipeline disappointments for AstraZeneca, which recently appointed a new chief executive - former Roche executive Pascal Soriot - to revive its fortunes.
Rheumatoid arthritis has been one of the most lucrative segments for drugmakers, leading to significant investment in the hunt for new products. The autoimmune disease leads to the body's immune system mistakenly attacking healthy tissue, causing inflammation and pain in the joints. (Editing by Erica Billingham and David Goodman)