Merck & Co has received a major boost following a court decision that upholds the company's US patent on Singulair and puts a block on Teva’s proposed copycat version of the asthma and allergic rhinitis blockbuster.

In a court in New Jersey, Judge Garrett Brown has ruled in Merck's favour in a patent infringement suit against Teva which was seeking approval from the US Food and Drug Administration to sell a generic version of 4, 5 and 10mg tablets of Singulair (montelukast). The judge also said that Teva committed infringement and issued an injunction blocking the approval of the Israeli firm’s generic versions until the August 2012 expiration of the patent.

Bruce Kuhlik, Merck’s general counsel, said that “we invest heavily in the R&D that is needed to discover innovative medicines like Singulair, and we will vigorously defend our intellectual property rights”. Teva issued a statement saying that the company is “currently reviewing the court's decision to determine its next course of action”.

Teva had argued that the patent for Singulair was invalid because prior research would have taught anyone knowledgeable in drug development how to invent the drug; so-called "prior art" can be grounds for invalidating a patent's claims. The compoany also claimed the patent was unenforceable because Merck misled the US Patent and Trademark Office when it made its initial application.

Merck disputed that view, saying that its researchers took a "tortured path" to develop the drug, a stance backed by Judge Brown. He said that the New Jersey-based drugs giant “put forth sufficient evidence of the failed attempts of others to develop a leukotriene antagonist."

Nevertheless, the patent on Singulair is being re-examined by the USPTO. Sales of the drug in the first half of the year reached $2.3 billion and US revenue made up $1.5 billion of that figure, up 6% on the like, year-earlier period.