Amgen is facing what could be a lengthy delay in the US approval of its once-monthly dose of anaemia drug Aranesp after the Food and Drug Administration asked for an additional clinical trial.

The US biotech is seeking approval of the once-monthly dose, as well as a once-a-fortnight option – to treat anaemia in patients with chronic kidney disease who do not require dialysis. The FDA also asked for clarification of the data presented on the biweekly dose, as well as additional label language.

At present, Aranesp is approved for dosing weekly or every three weeks in patients with renal disease, as well as in patients with anaemia caused by cancer chemotherapy.

Amgen plans to ask the FDA to consider evaluating additional data from the existing clinical trials programme for once-monthly Aranesp (darbepoetin alfa) – rather than a separate study – in order to progress the application.

The delay is a blow to Amgen, which is facing competition to Aranesp and its short-acting parent Epogen (epoetin alfa), which brought in revenues of $1.31 billion and $1.22 billion, respectively, in the first half of this year. The main threat is Roche’s CERA, a new type of anaemia treatment that is also dosed once-a-month and was submitted for approval in the USA in April.

Amgen has been trying to block CERA in the USA, both by blocking its importation into the country – a move which was unsuccessful - and also via patent infringement proceedings which are still ongoing.