Genmab’s shares have slumped after staffers at the US Food and Drug Administration voiced concerns about the benefits of the investigational leukaemia drug Arzerra developed by the Danish drugmaker and GlaxoSmithKline.

In a review posted on the FDA’s website, agency staff questioned whether Arzerra (ofatumumab) provides sufficient benefit to patients with chronic lymphocytic leukaemia to warrant approval. The documents, published ahead of an FDA Oncology Drug Advisory Committee set for May 29, noted that the anti-tumour activity of Arzerra was "difficult to quantify”, adding that "the major issue regarding this application is whether the effect sizes...are reasonably likely to predict clinical benefit."

Genmab and GSK were granted a priority review for Arzerra in April by the FDA but the staffers note that while the agency “acknowledges that the administration of ofatumumab as a single agent yields anti-tumour activity in patients” that alone does not meet the “regulatory standard for having an unmet medical need."

The staffers go on to say that GSK “will need to conduct a comparative study in order to support approval in a patient population...who were only required to be refractory to one drug”. They also questioned GSK’s statement that fatal infections were lower than expected in patients on Arzerra.

In its own briefing documents, GSK says that response rate and safety shown by Arzerra were better than available therapies for CLL patients who have failed other treatment. The company also claims that the drug meets the regulatory requirements for approval.

It would appear that investors in Genmab are not so sure and the firm’s share price sank 23%.