A federal court in the USA has cleared Pfizer of infringing a patent, held by Dutch company Synthon, covering a process used in the manufacture of the hypertension drug amlodipine.

Pfizer, which sells amlodipine under the Norvasc brandname, was accused by the US subsidiary of Synthon of infringing its intellectual property last November. The jury also found that Synthon’s patent was invalid on multiple grounds, according to Pfizer.

Norvasc has lost patent protection in a number of markets around the world but is still Pfizer’s second-largest product, with sales of $2.34 billion in the first half of this year mainly coming from the US market.

Pfizer won a key battle to protect Norvasc from generic competition in the USA in January, after a court upheld its patents on the drug and blocked generic company Apotex from launching its rival version until September 2007.