AstraZeneca PLC (AZN) has failed to delay generic versions of its top-selling anti-psychotic drug Seroquel entering the U.S. market.

The U.K.'s second biggest drug maker company had wanted to prevent the U.S. Food and Drug Administration giving final marketing approval to generic forms of Seroquel until December. But its suit against the FDA, filed earlier this month, was dismissed on Friday in Washington D.C. by a US judge who refused to grant AstraZeneca an injunction, ruling that the group had failed to make a clear case that it was entitled to one.

The ruling opens the way for doctors in the U.S. to switch to prescribing cheaper generics of the medicine that will quickly become available as Seroquel's protective patent expires Monday.

The medicine maker filed its suit against the FDA earlier this month in an effort to overturn an FDA ruling that generic copies of the antidepressant would not have to carry the same warnings about possible side effects -- including suicidal thoughts and high blood sugar levels -- that were required of AstraZeneca.

Seroquel, AstraZeneca's second best-selling product, generated $5.83 billion last year and is the first of its best-selling drugs to lose U.S. patent protection.

AstraZeneca said Monday that it "continues to believe strongly in the merits of its position and is evaluating its options."

But analysts believe AstraZeneca has little room for maneuver.

"We see AstraZeneca's options as now limited, especially given the speed of the court's decision," said Navid Malik of Cenkos.

Last week, AstraZeneca lost a battle in the U.K. High Court which ruled that its patent on Seroquel is invalid in Britain. The company had hoped to maintain revenues by placing a patent on an extended release version. However, the UK court ruled the patent is invalid following challenges from companies such as Teva Pharmaceutical Industries (TEVA).

AstraZeneca faces one of the highest exposures to drug patent expirations within the pharmaceuticals sector. Its blood-pressure medicine Atacand loses protection this year while an ulcer drug Nexium goes generic in 2014 followed by cholesterol fighter Crestor, its best-selling product, in 2016.

AstraZeneca shares opened down Monday at at 0900 GMT were 0.2% lower at 2,836 pence.

-By Sten Stovall, Dow Jones Newswires; +44 207 842 9292; sten.stovall@dowjones.com