* Fostamatinib relieves rheumatoid arthritis symptoms
* New pill fails to impact joint damage measured by X-ray
* Side effects include hypertension and diarrhoea
* Rigel shares slump 33 pct, AstraZeneca little changed (Updates with latest Rigel share price, further analyst comment)
LONDON, April 5 (Reuters) - AstraZeneca's experimental rheumatoid arthritis pill fostamatinib met only one of two goals in a late-stage clinical trial, leaving the future of one of the group's few advanced pipeline products uncertain.
Shares in its smaller U.S. partner Rigel Pharmaceuticals slumped 33 percent in early Nasdaq trade on the news.
Fostamatinib is a potential competitor to injectable drugs like AbbVie's Humira and a new pill from Pfizer called Xeljanz in a $20 billion-plus market. Disappointing results from earlier tests, however, mean investor hopes for the medicine are low.
The latest findings will do little to restore confidence, especially as increased blood pressure and diarrhoea were again noted among patients taking the AstraZeneca product.
"Our misgiving is that fostamatinib appears to be inferior to Humira on efficacy and now the adverse events seem to pitch it below Pfizer's (Xeljanz), so it is hard to see where the product will fit," said Panmure Gordon analyst Savvas Neophytou.
Results of another AstraZeneca-funded study in December found fostamatinib was inferior to Humira in controlling arthritis symptoms.
In the latest trial, AstraZeneca's new drug achieved a statistically significant improvement at 24 weeks, compared to placebo, in a test assessing signs and symptoms of the disease - the so-called ACR20 response rate.
But it failed to show an improvement in an X-ray test measuring the progression of joint damage, according to results from the OSKIRA-1 Phase III study released by the company on Friday.
Side effects seen among patients taking the drug were similar to those experienced in earlier studies, including hypertension, diarrhoea, nausea, headache and common cold.
Hypertension is a particular concern, since raised blood pressure could be a serious obstacle to approval.
ISI Group analyst Mark Schoenebaum said even if fostamatinib was approved in 2014, as AstraZeneca and Rigel hope, its use would be secondary to Xeljanz and other oral arthritis drugs that are expected to be approved over the next few years.
Fostamatinib's ACR20 scores of 44-49 percent were lower than the 52-62 percent seen with Xeljanz in Phase III studies, he noted.
AstraZeneca licensed fostamatinib in 2010 from Rigel and the South San Francisco, California-based company's fortunes are closely tied to the medicine, since it does not yet have any drugs on the market.
Given the doubts surrounding the product, forecasts among AstraZeneca analysts for fostamatinib sales are low, averaging just $171 million in 2016, according to Thomson Reuters data.
Humira, by contrast, has annual worldwide sales of more than $9 billion.
Briggs Morrison, head of global medicines development at Britain's second-biggest drugmaker, said the latest study findings showed the new pill did have an effect in rheumatoid arthritis.
But AstraZeneca is being cautious about talking up fostamatinib's prospects and its researchers want to see more results before deciding their strategy for the drug.
"We will await the results of the remaining Phase III studies, OSKIRA-2 and OSKIRA-3, to further evaluate and characterise the profile of fostamatinib as a potential treatment for rheumatoid arthritis," Morrison said.
The OSKIRA-2 and OSKIRA-3 results are expected later in the second quarter of 2013.
With last 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 "cliffs" of patent expiries.
Worryingly, it has few new drugs in late-stage development to replace them, which is something that new CEO Pascal Soriot aims to address through a radical shake-up of the group's research activities.
Shares in AstraZeneca were little changed in London. (Editing by Helen Massy-Beresford and Clelia Oziel)