AstraZeneca (AZN) is suing the U.S. Food and Drug Administration to try and extend its exclusive hold on its best-selling anti-depressant drug as a wave of patent expirations and new, more affordable generics continue to weigh on pharmaceutical giants.

The Anglo-Swedish drug maker was denied by the FDA on March 7 for to extend its hold on Seroquel until the end of the year. AstraZeneca has asked the FDA to not grant final marketing approval of generic quetiapine until Dec. 2.

The suit, filed in the U.S. District Court in Washington, D.C., seeks to overturn a recent ruling by the FDA that generic copies of the pill would not have to carry the same warnings about possible side effects, including suicidal thoughts, which has been a major turnoff to these types of drugs, as well as elevated blood sugar.

The drug is the fifth best-selling in the U.S. and has been AstraZeneca’s biggest bread winner.

“AstraZeneca will vigorously defend its legal rights,” the company said in a statement.

The patent on the active ingredient in Seroquel expired in September.

The world’s largest drug companies have been facing a so called “patent cliff” over the past few months that have caused them to lose exclusivity of some of their top-selling drugs and compete with generic companies, which often sell their products at a more affordable price and follow less stringent regulations over labels and testing.

One of the more well-known such cases occurred in November, when Pfizer (PFE) lost exclusivity for its cholesterol pill Lipitor. Bristol-Myers’ (BMY) best-selling blood clot treatment Plavix is set to expire in May.

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