UPDATE 2-Astra's Seroquel wins a further U.S. approval

(Adds analyst comment, closing shares)

LONDON, May 14 (Reuters) - AstraZeneca (AZN.L) said on Wednesday that U.S. regulators had approved its Seroquel medicine as a maintenance treatment for patients with bipolar disorder, as an adjunct to lithium or divalproex.

Seroquel, already on sale to treat schizophrenia and as a single agent for bipolar disorder, is the Anglo-Swedish drugmaker's second-biggest selling drug, generating $4 billion of sales in 2007.

However, it is under threat from generic drugmakers seeking to sell a cheap copy, and AstraZeneca is working hard to promote a follow-on, extended release version of the drug called Seroquel XR.

Mike Ward, an analyst with Nomura Code, said the new use was one of a series of indications being pursued by AstraZeneca to develop the Seroquel franchise.

"Eventually the product could receive clearance for use in major depressive disorder and generalised anxiety disorder," he said in a note.

Considered one of the most severe forms of mental illness, bipolar disorder currently affects about 8 million adults in the United States. It is characterised by manic or mixed mood episodes, interspersed with episodes of major depression.

AstraZeneca shares closed 1.9 percent higher at 2,130 pence. (Reporting by Mark Potter, editing by Will Waterman)