Amgen shares have slipped after preliminary data from an independent study looking at Aranesp for an unapproved use in patients with head and neck cancer undergoing radiotherapy showed the drug fared no better than placebo.

The study, which was conducted by the Danish Head and Neck Cancer Group and involved more than 522 patients, was designed to test whether Aranesp (darbepoetin alfa) enhanced radiation therapy in patients with head and neck cancer. However, the preliminary findings showed that those who were administered the drug were actually 10% more likely to see their tumours grow. The study was temporarily halted in October, with a decision not to resume being made in December, and the Danish group said “we are a bit puzzled by the cause of the specific negative outcome and will on a scientific basis explore that.”

Amgen was made aware of the data in December and although it informed regulators, it did not disclose the findings to investors until the end of last week. The company’s chief executive, Kevin Sharer, said that, in retrospect, the findings should have been revealed even though the company has not seen the raw data from the trial which deals with an off-label use.

The news has led to concerns from industry observers that Amgen needs to take a closer look at its disclosure policies and Credit Suisse issued a note saying that the results might mean that Aranesp can actually cause cancer in some patients.

Aranesp, which is approved to treat anaemia associated with chronic renal failure, and in some patients undergoing chemotherapy, had sales of just over $4 billion in 2006. The company recently noted that the drug failed to show a benefit for patients in a more convenient, once-a-month formulation.