GlaxoSmithKline has temporarily fended off generic competition to its big-selling Flonase allergy treatment, winning a restraining order blocking the sale of copycat versions made by Roxane Laboratories and Par Pharmaceuticals.

Flonase (fluticasone propionate) nasal spray brought in worldwide sales of £656 million ($1.14bn) last year, £506 million of which came from the US market, but saw its last US patent expire in November of last year.

The US Food and Drug Administration approved Roxane’s generic version of the product last week, but was promptly sued by GSK, which argued that the agency had failed to follow its own regulations in approving the rival versions.

Par’s so-called ‘authorised’ generic had already been cleared for marketing as the result of a deal between the generics firm and GSK designed to minimise the impact of competition to the product once copycat versions reached the US market.

The UK-headquartered drugmaker asked the courts for a temporary restraining order to set aside approval of the Flonase generics by the FDA, and this request was granted. The restraining order expires on March 6, when a hearing will take place to examine the cases in more detail.

Both Par and Roxane, a subsidiary of German pharma firm Boehringer Ingelheim, said they had immediately halted all shipments of fluticasone propionate nasal spray.

GSK has long argued that it is not a simple matter to make sure that one nasal spray product is as efficient as another because the active ingredients act locally in the nose, rather than acting systemically via the blood. On that basis it is harder to prove bioequivalence, i.e. that a generic drug can be substituted for the branded version and provide the same efficacy and safety.

And this is complicated further by the fact that, in Flonase, the active ingredient is formulated as a suspension, rather than as a simple solution, claims GSK. So far, the FDA has not prepared any guidance on how to determine if two nasal sprays are equivalent, it says.

The company has made these arguments in two Citizen’s Petitions, which were filed with - but subsequently rejected by - the FDA.