AstraZeneca is suing regulators in the USA in a bid to delay the entry of generic versions of its antipsychotic blockbuster Seroquel.

Last week, the US Food and Drug Administration denied the Anglo-Swedish drugmaker's Citizen Petitions requesting that the agency withhold finally approving any generic with labelling that omits warnings required for branded versions of Seroquel (quetiapine) and Seroquel XR. Those warnings relate to hyperglycaemia and suicidal thoughts.

AstraZeneca has therefore filed a lawsuit against the FDA to overturn the denial and is seeking an injunction barring the granting final marketing approval of copycat forms of the drug until December 2 or "at least until a federal court has a meaningful opportunity to review imminent FDA action regarding the pending generic marketing applications".

In the Citizen Petitions, AstraZeneca says it "raised important issues regarding labelling requirements for generic copies of innovative medicines, as well as data exclusivity rights granted to innovative companies that conduct new clinical trials". The company added that it will "vigorously defend its legal rights".

The patent covering quetiapine ran out in September 2011, and paediatric exclusivity will expire on March 26. Seroquel XR is covered by a formulation patent (including paediatric exclusivity) that expires in November 2017. Regarding the latter drug, AstraZeneca reached a settlement with the USA's Handa Pharmaceuticals in September which prevents the latter from selling a generic form until November 2016.

Revenues from the Seroquel franchise will be boosted if AstraZeneca' can keep the copycat versions off the market for a bit longer. Fourth-quarter sales of the two drugs were up 15% to $1.55 billion; Seroquel XR was up 27% to $398 million.