Shire has filed a lawsuit in the USA against Corepharma claiming infringement of two patents, held by its subsidiary Shire Laboratories, covering the epilepsy drug Carbatrol.

The move comes after Corepharma filed for approval of generic versions of the 100mg, 200mg, and 300mg formulations of Carbatrol (carbamazepine). Under US law, the lawsuit immediately triggers a stay of approval of up to 30 months from Corepharma's notice to allow the courts to resolve the suit.

Shire is already defending the product from a patent challenge brought by Nostrum Pharmaceuticals, and the 30-month stay on this expired at the beginning of February. To date the US Food and Drug Administration has not yet approved Nostrum’s copycat version.

The news adds to the pressure on Shire over Carbatrol, which saw first-quarter sales slump 17% to $14.1 million due to limited promotion and supply problems. Carbatrol accounts for around 4% of group sales.

Meanwhile, Shire is also facing challenges to its attention-deficit hyperactivity disorder drug Adderall XR (mixed amphetamine salts), a much more serious problems as this product makes up 50% of its revenues at present. The firm reached a settlement with one challenger, Impax Laboratories, to prevent it launching its generic and is in talks with Barr Laboratories over a similar deal. Teva has also said it plans to launch a version of the drug.