AstraZeneca has won US regulatory approval for a new sustained-release version of its schizophrenia drug Seroquel, which should shore up patent protection for one of its best-selling medicines.

The US Food and Drug Administration has approved Seroquel XR (quetiapine fumarate) extended-release tablets, which has the benefit of offering schizophrenia patients the convenience of once-daily dosing, rather than twice-daily for the currently marketed version. The agency’s approval was based on a placebo-controlled study of over 570 patients experiencing an acute exacerbation of symptoms of schizophrenia of the drug at doses of 400, 600, and 800 mg/day.

Seroquel XR, is going to be launched in the second half of the year and its US patent will expire in 2017 so AstraZeneca can now switch the drug's existing patients to the once-daily formulation. Seroquel is a big earner for the Anglo-Swedish firm and sales in the first quarter rose 13% to $923 million but the US patents on the original formulation of the drug are due to run out in 2011/12. Generic giants Teva Pharmaceutical Industries and Novartis’ Sandoz unit are already waiting in the wings with their copycat versions of Seroquel and legal challenges are set to begin next year.

Analysts are unsure whether Seroquel XR will lead to an increased share of the schizophrenia market, but as a defensive move, this latest approval will prove to be a major bonus, especially given the number of late-stage failures that the company has suffered of late. AstraZeneca is also studying Seroquel's use in adolescent mental illness and plans to seek approval for generalised anxiety and major depressive disorder in 2009.