Now that AstraZeneca's Seroquel has gone off-patent, the floodgates have opened in the USA with a number of Indian drugmakers launching their generic versions of the blockbuster antipsychotic.

This week began with the Anglo-Swedish drugmaker having its lawsuit against the US Food and Drug Administration concerning its stance on copies of Seroquel (quetiapine) thrown out of a US court. Earlier this month, the firm sued the regulator after the latter denied its Citizen Petitions requesting that the agency withhold finally approving any generic with labelling that omits warnings required for branded versions of Seroquel and Seroquel XR. Those warnings relate to hyperglycaemia and suicidal thoughts.

In the USA, the patent covering instant-release quetiapine ran out on March 26 and copies are now going to flood the market. Dr Reddy's Laboratories says the FDA has approved an Abbreviated New Drug Application for its 25 mg, 50 mg, 100 mg, 200 mg, 300 mg and 400 mg versions, noting that the Seroquel brand had US sales of approximately $4.6 billion for the year ending December 2011.

As well as Dr Reddy's, Sun Pharmaceutical Industries and Lupin have also received FDA approval to launch their generics in the US market. The firms noted that they have begun shipping product already.

AstraZeneca's problems in defending Seroquel are not limited to the USA. Earlier this month, the UK High Court stated that the patent on the extended-release version of the antipsychotic, ie Seroquel XR, is not valid. AstraZeneca's patent had been challenged by Accord Healthcare, Intas Pharmaceuticals, Novartis' Hexal and Sandoz generics units and Teva.

The latter has moved quickly and its UK unit launched its versions of Seroquel and Seroquel XL on "day one of patent expiry".