The Anglo-Swedish drugs group said that it would take the case to the Federal Circuit Court of Appeals after a judge in Missouri ripped up the rights protecting Toprol XL.

The unexpected ruling, pre-empting a court case due to start on February 13, paves the way for makers of copycat drugs to launch cheaper generic versions of Toprol within weeks.

Three manufacturers — KV Pharmaceutical, Andrx Corporation and Eon Labs, part of Novartis — are expected to seek immediate approval from American drug regulators to launch their own versions of the drug. Under US law, generic companies are free to launch drugs while patent disputes work through the appeals process, although they are liable to pay damages if the patent-holder wins on appeal.

According to analysts, the ruling could cost AstraZeneca as much as $1.4 billion in lost sales this year and $900 million next year. Patents covering Toprol, AZ’s biggest seller by volume, are due to expire in September 2007 and most analysts had stripped sales estimates from their forecasts for 2008 in anticipation of the launch of cheaper alternatives.

Deutsche Bank, AZ’s broker, gave warning that the ruling would cost the drug group about 30p a share in terms of lost profits, adding that it might also stymie its ability to raise earnings forecasts when it reports full-year figures on February 2.

Shares in AZ closed down 121p at £26.79 after the ruling focused attention on the far more serious challenges that the company faces to patents protecting two of its biggest-selling medicines — Nexium, a stomach ulcer treatment, and Seroquel, an antidepressant. AZ is fighting attempts by Teva Pharmaceuticals, an Israeli generic manfacturer, and Ranbaxy Laboratories, an Indian rival, to launch cheaper versions of Nexium and Seroquel.

Both drugs — with Toprol — are crucial for AZ in the short term. Revenues from the three accounted for a third of AZ revenues in the first nine months of its year. David Brennan, AZ’s new chief executive, is relying on growth from the three to bolster turnover until joint ventures bear fruit.

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