The US Food and Drug Administration (FDA) has, this week, sent Dendreon a letter requesting more efficacy data for its Biologics License Application (BLA) for Provenge (sipuleucel-T). The so-called Active Cellular Immunotherapy (ACI) is designed to switch a patient's immune system back on, allowing it to destroy cancer tumour cells.

Provenge is being closely watched by both investors and cancer sufferers. If approved, the potential first-in-class drug would provide a real boost to the estimated 670,000 men who are diagnosed with prostate cancer every year globally, according to statistics from the International Agency for Research on Cancer. Accounting for one in nine of all new cancers in males, prostate cancer is the second most common cancer in men, after lung cancer.

Patients currently have "few appealing treatment options", according to Dr Mitchell Gold, CEO of Dendreon.

This latest development may have come as a surprise to many - it is only six weeks after the FDA's Office of Cellular, Tissue and Gene Therapies Advisory Committee advised the regulatory body to approve Provenge. Dendreon has never had a drug approved since it was founded in 1992 and this is its lead candidate. Bad news can wreak havoc on the share price of companies relying on a single late stage drug and Dendreon's share price has plummeted around 60 per cent.

"Given our strong belief in the survival benefit and safety profile of Provenge, coupled with the positive outcome of the Advisory Committee meeting, we are disappointed that this decision will cause a delay in the availability of Provenge for patients who suffer from advanced prostate cancer," said Gold.

According to Dendreon, Provenge is made with activated antigen-loaded Antigen Presenting Cells (APCs), which can potentially stimulate a T cell response against prostate cancer cells. In a Phase III trial Provenge improved survival rates from 21.4 months to 25.9 months versus placebo, although the trail failed to meet its primary goal to slow the spread of the disease.

"This news will come as a disappointment to men with advanced prostate cancer, for whom there is a real need for treatments that will extend life and slow the spread of the disease," said Debbie Clayton, a spokesperson for the UK's Prostate Cancer Charity told DrugResearcher.com.

"Results so far, indicate that this vaccine extends survival by several months but other vaccines currently being researched have also shown similar findings. It is regrettable that this vaccine has not yet been proven to delay the progression of the disease. It is essential to ensure that men are confident that new drugs have been tested thoroughly to ensure that they are safe and effective for treating prostate cancer."

The nearest thing to a cancer vaccine currently on the market is Merck & Co.'s Gardasil, which protects against the human papillomavirus (HPV) that causes 70 per cent of cervical cancers. The company is also collaborating with Biomira on its lung cancer vaccine. Stimuvax is a synthetic 25 amino acid sequence of the cancer-associated marker MUC-1 and is currently in Phase III trials.

GlaxoSmithKline also has a cervical cancer vaccine targeting HPV in development. The pharma giant submitted a BLA for Cervarix to the FDA in March. Also in March, Sanofi-Aventis agreed to pay up to €518m for Oxford Biomedica's lead cancer vaccine Trovax, currently in Phase III trials in renal cancer. A dedicated cancer vaccine, Trovax targets the 5T4 tumour antigen and consists of a poxvirus gene transfer system, which delivers the gene for 5T4 and then stimulates the patient's body to produce an anti-5T4 immune response.

Provenge could also be used to treat various cancers, although Dendreon has concentrated clinical research for that particular drug on prostate cancer. However, the biotech also has a vaccine called Lapuleucel-T designed to treat breast, ovarian and colorectal. Built on the same technology as Provenge, it instead targets the HER2/neu antigen and has completed two Phase I clinical trials.

Dendreon is also conducting preclinical research on the carbonic anhydrase IX (CA9) antigen and the carcinoembryonic antigen (CEA). These targets are present on high percentages of cervical, colon and renal cancers and lung, colon and breast cancers respectively. Dendreon is not restricting its research wholly to anticancer biomolecules however; it is also researching small molecule inhibitors of Trp-p8, an ion channel thought to play an important role in cell growth, division and migration.

Dendreon has announced it will hold a conference call at 4:30pm ET (9:30pm BST) today to discuss the FDA letter and also announce its first quarter results.