AstraZeneca, which earlier this week said it is pulling its clotbuster Exanta from the global marketplace on safety issues, is to appeal a US court ruling in January that declared the compound and composition patents for the heart drug Toprol-XL “invalid and unenforceable.”

The decision, by Judge Rodney Sippel, would mean that AstraZeneca could lose out on some valuable income ahead of the drug’s patent expiry on September 17 next year. Toprol-XL reeled in some $1.3 billion in US sales alone in 2005, said a company spokesperson.

It is tough news for a firm that has found itself inadvertently in the quagmire of late. It received a non-approvable letter from the US Food and Drug Administration in 2004 for Exanta - essentially helping to seal the drug’s fate with sales of just $0.5 last year, a far cry from its blockbuster expectations - and has also had to terminate development of two other compounds, one for incontinence and the other for heart arrhythmia.

Meanwhile, two of AstraZeneca's multi-billion dollar products - the gastrointestinal drug Nexium (esomeprazole) and Seroquel (quetiapine fumarate) for schizophrenia - are also facing challenges by generic manufacturers. Nexium sales rose 18% to $4.63 billion in 2005, while Seroquel rocketed 36% to $2.76 billion.

By filing the appeal, AstraZeneca will be hoping that the generics companies concerned – specifically, Novartis' Eon Labs subsidiary, KV Pharmaceuticals and Andrx Corp – may now decide against launching their offerings until after the hearing. The firms’ applications for approval of their generic copies are also awaiting clearance from the US Food and Drug Administration, which could provide some breathing space for the UK giant.

However, investors are breathing heavily down AstraZeneca’s neck: despite a 26% upturn in full year operating profit, the company’s share price has dropped on uncertainty over the future of Toprol-XL.