AstraZeneca faces a significant risk of losing US patent protection on its top selling heart drug Toprol XL (metoprolol succinate), according to analysts from Deutsche Bank.The analysts estimate there is around a 40 percent chance AstraZeneca will lose a ruling on the drug, according to a report from Reuters today. AstraZeneca has maintained its expectation of exclusivity on Toprol XL to hold until Sept. 2007, when US patents expire. The challenges to Toprol XL patents come from KV Pharmaceutical, Andrx Corp. and Eon Labs, a unit of Novartis. Sales of Toprol XL are expected to total around $1.3 billion in 2005.Generic drug makers have also recently challenged AstraZeneca’s two biggest sellers, the anitulcerant Nexium (esomeprazole magnesium delayed-release capsules), and Seroquel (quetiapine fumarate) for schizophrenia.In November AstraZeneca filed a lawsuit against Ranbaxy Laboratories in an attempt to stop the Indian drug maker from launching a copycat version of its top-selling medicine Nexium. Also in November, AstraZeneca filed a lawsuit against Teva Pharmaceuticals for infringement of AstraZeneca’s substance patent protecting Seroquel.