AstraZeneca has filed a lawsuit to prevent the US Food and Drug Administration from approving the sale of generic versions of its multi-billion-dollar cholesterol drug Crestor.

Crestor's initial patent protection has come to an end but the drugs giant recently bagged an extra seven years' exclusivity for a rare paediatric indication, and is now seeking to extend this for all uses of the drug.

The firm has applied for a temporary court order to stop the FDA from allowing more Crestor copycat drugs to flood the market until a final decision on this exclusivity is made.

Senator Bernie Sanders and various US representatives have written to FDA Commissioner Robert Califf urging the regulator to approve the stream of applications to sell generic versions of AZ' drug, noting that this has the potential to "drastically reduce healthcare costs".

It's easy to see why AZ is pulling out all the stops to protect sales of its number-one selling drug, given that it currently pulls in more than $6 billion a year, but revenues could eventually plummet as much as 85 percent after the door to generics is fully opened.

Going to court "is commercially worth it, if generics can be delayed for any significant length of time," Nick Turner, an analyst at Mirabaud Securities LLP in London, told Bloomberg.

Irish drugmaker Allergan is already selling a copycat version of Crestor in the US, following an agreement with AZ allowed its launch 67 days before the end of exclusivity, in what it expects to be the biggest generic launch of the year.