GlaxoSmithKline has lost a legal bid to block generic competition to its $900 million-a-year allergy drug Flonase in the USA.

The world’s second-largest drugmaker won a preliminary injunction against the introduction of a copycat version of Flonase (fluticasone propionate) developed by Roxane Laboratories last month, but this was overturned yesterday US District Judge Andre Davis in a Baltimore court.

Roxane, a subsidiary of Boehringer Ingelheim, said its generic version would be launched straight away. In the meantime, GSK said it would continue its legal efforts to force the US Food and Drug Administration (FDA) to overturn its approval of the generic, but will not appeal injunction ruling.

Flonase contributed £656 million ($1.14bn) to GSK’s coffers last year, £506 million of which came from the US market, but saw its last US patent expire in November 2005.

GSK maintains that, despite the loss of patent protection, 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, ie. that a generic drug can be substituted for the branded version and provide the same efficacy and safety, which is generally done by testing blood levels.

The UK-headquartered firm now hopes to force the FDA to accept this point of view and insist on a full clinical marketing application for other fluticasone nasal spray products, rather than the abbreviated applications used by generic manufacturers.

With Roxane free to launch its product, the way has also been cleared for another generic version developed by Par Pharmaceutical. Par has an agreement to sell an ‘authorised generic’ version of Flonase under an agreement with GSK.

Under the terms of the agreement, Par could launch its product once another fully substitutable generic version of Flonase became available in the US. Par's product is manufactured by a GSK subsidiary and the UK firm stands to receive a share of profits from the generic, although Par had not said it would re-enter the market at the time of writing.