Amgen shares have slipped yet again despite the firm disclosing reasonably promising late-stage clinical data on its investigational osteoporosis drug denosumab.

Results from a two-year 332-patient Phase III study of denosumab published in the Journal of Clinical Endocrinology and Metabolism, show that the drug increased lumbar spine bone mineral density in both early and late-stage postmenopausal women compared with placebo. The drug was also associated with increased BMD in the hip, wrist and total body, compared with placebo.

Javier San Martin, global development lead for the denosumab osteoporosis programme, said that the effect of the drug on wrist BMD, reinforced by the BMD increases at the total body and hip regions, “suggests denosumab has a positive effect on highly cortical sites". The most common adverse events were arthralgia, nasopharyngitis and back pain and the number of these effects was similar in patients taking the Amgen drug and those taking placebo.

However, there was more worrying news when it came to serious adverse events. Patients on denosumab had a higher rate of infections requiring hospitalisation. Those infections included pneumonia, diverticulitis, appendicitis, sepsis, pyelonephritis, urinary tract infection, and cellulites, but none of them “were considered by the site investigators to be related to denosumab treatment”, Amgen said. The infections were common for this subject population and they responded to standard antibiotic therapy.

The company added that it expects the results of a large Phase III study, which will evaluate denosumab's impact on fracture risk reduction, in women with postmenopausal osteoporosis, in the second half of this year.

Questions over Aranesp rebates
At the same time, Amgen’s controversial anaemia drug Aranesp (darbepoetin alfa) is once again back in the headlines. US Senator Charles Grassley has written to the firm asking it to explain why some physician groups received higher rebates for purchasing the drug.

In the letter to Amgen, the leading Republican on the Senate Finance Committee says that data obtained from the company indicated that oncologists in some US states were given much higher rebates than others for Aranesp, which is also under scrutiny by a House committee. He added that there were concerns doctors were profiting by getting reimbursements from the government and private insurers that were higher than the actual cost for the drug.

Marketing of Arsanesp and procrit questioned
Sen Grassley’s letter has come just days after the House of Representatives Energy and Commerce Committee asked Amgen and Johnson & Johnson for records on advertising campaigns and future promotional plans for Aranesp and J&J’s Procrit (epoetin alfa). John Dingell (Democrat), chairman of the Committee, said that “millions of dollars have been poured into aggressive marketing campaigns, despite mounting evidence that these therapies pose serious risks”.

He went on to claim that the drugs are over-prescribed and widely used off-label, “and it's time we evaluate the marketing practices used to push them". Last month, a US regulatory advisory panel has recommended further restrictions on the use of Amgen and J& J’s anaemia drugs in cancer patients.