AstraZeneca has lost a US patent protection on its top-selling heart drug Toprol XL, according to a note issued by Deutsche, which acts as broker to the company.
"An announcement was made last night that the Eastern District Court of Missouri has ruled under summary judgement in favour of the generic challengers in the Toprol XL case," Deutsche analysts said in an email.
At 8:24am, AstraZeneca shares were down 3.7 per cent at 2,696 pence. AstraZeneca was not immediately available to comment.
The ruling could open the door to generic competitors as early as this quarter, assuming the Food and Drug Administration approves copycat formulations of the medicine.