US regulators have extended the review period for GlaxoSmithKline and Danish group Genmab’s marketing application for their leukaemia drug Arzerra, pushing the deadline back by three months until the end of October.

The Food and Drug Administration had accepted the companies’ submission for Arzerra (ofatumumab) for an accelerated review, which is designed to speed up the process to about six months from the normal 10 months, but now the agency says it needs more time to take into account new chemistry and manufacturing data handed over by the companies earlier this month.

GSK bought global rights to Arzerra - a monoclonal antibody designed to treat chronic lymphatic leukaemia, the most common form of adult leukaemia affecting around 90,000 patients in the US alone – from Genmab for a whopping $2.1 billion back in 2006, and the Danish drugmaker stands to receive handsome royalties form the product if it makes to market, particularly as analysts are predicting blockbuster status.

While GSK swallowed news of the delay without much effect, Genmab’s stock sank 7% as shareholder confidence was hit, which is perhaps unsurprising given that Arzerra is the Danish firm’s lead drug and there are high hopes that it will become its first marketed product.

However, despite the setback, analysts seem relatively unconcerned over the Arzerra’s future. Sydbank analyst Rune Majlund Dahl reportedly told clients that he still expects the FDA to issue a green light for the drug, and that the extension to the review period is unsurprising as the newly submitted data will need to be processed, according to Bloomberg.

And it seems staff at the FDA are also in favour with the drug as late last month an FDA Oncology Drugs Advisory Committee voted 10-3 that Arzerra was likely to offer clinical benefit for certain patients with CLL, despite initial concerns that the anti-tumour activity of the drug was difficult to quantify.