AstraZeneca shares are on the rise this morning after a US judge declared that a key patent on cholesterol blockbuster Crestor is valid.

In an eagerly-awaited ruling, Judge Joseph Farnan of the US District Court in Delaware has found that the substance patent protecting Crestor (rosuvastatin), known as the '314 patent, is valid and enforceable. The decision means that the US Food and Drug Administration cannot issue final approvals for Abbreviated New Drug Applications from generic firms prior to the expiration of the aforementioned patent in 2016, virtually guaranteeing a healthy and steady revenue stream from Crestor until then.

In 2007, nine generic drug manufacturers filed ANDAs along with Paragraph IV certifications of non-infringement, invalidity, or unenforceability with respect to the Crestor patents. In return, AstraZeneca and Japan’s Shionogi (the owner of the '314 patent) filed infringement suits against eight manufacturers - Apotex, Aurobindo, Cobalt, Mylan, Par, Sandoz, Sun and Teva.

The generics companies had claimed the Crestor patent was invalid on the grounds it was an obvious invention, also arguing that it was fraudulently obtained and improperly reissued. Specifically they alleged that Shionogi had withheld documents from the US Patent and Trademark Office in order to obtain the patent through deception.

Judge Farnan has dismissed that claim and found that no inequitable conduct was committed by any Shionogi employee. The court also held the '314 patent to be “non-obvious and properly reissued”, noting that Apotex was “liable as a submitter and is therefore bound to the court's decision”.

AstraZeneca chief executive David Brennan said "we are pleased with the court's decision upholding the validity and enforceability” of the substance patent. He added that it “reaffirms the strength of the intellectual property protecting Crestor”.

The ruling was expected to go in AstraZeneca’s favour but the news was still greeted with huge enthusiasm by investors; at 9.20am (UK time), the stock was already up 9.7% to £32.33. The leap highlights the importance of Crestor to the Anglo-Swedish drugmaker’s coffers as a number of key drugs are being hit by generic competition.

First-quarter sales of the lipid-lowerer increased 27% to $1.30 billion and most strikingly in the USA, turnover reached $583 million, a 22% rise. Crestor total prescriptions increased 15%, nearly five times the percentage growth for the US statin market as a whole.