How much is 4 months of your life worth? Some men with advanced prostate cancer will be able to find out, as a result of a new drug developed byDendreon Corporation. This drug, Provenge (sipuleucel-T), was recently approved by the FDA to treat advanced prostate cancer.
They are calling Provenge a “vaccine,” but it doesn’t work like vaccines against infections such as measles or mumps. This vaccine is used to treat advanced prostate cancer rather than prevent it. It works by using the patient’s own immune system. The vaccine is made by taking a sample of the patient’s blood, removing the white blood cells, and exposing them to the Dendreon PAP-GM-CSF protein. This specialized fused protein includesprostatic acid phosphatase (PAP), which is an enzyme produced by the prostate and found at increased levels in prostate cancer. Once the vaccine is made, the cells are then infused back into the patient’s vein. The patient receives three of these treatments, 2 weeks apart. Basically the treated cells help stimulate the patient’s immune system to attack his prostate cancer.
The FDA approval of this new vaccine was largely based on the results of Dendreon’s recently completed Provenge Phase III Trial. This was a randomized clinical trial involving 512 men between the ages of 40 and 91 years, of which 341 men received Provenge and 171 men received a placebo. All the men had advanced prostate cancer that was unresponsive to hormone therapy. The common side effects of the Provenge were reported to be minor — such as chills (54%), fevers (29%), and headaches (16%) — and only lasted a couple days after infusion. The men that received the Provenge on average lived 4.1 months longer than the men that took the placebo (25.8 months v.s. 21.7 months). This is a significant break through for men with advanced prostate cancer, who have few treatment options.
There was no evidence that Provenge slowed progression of the disease. How the drug can improve survival rate without slowing disease progression is not clear yet. The FDA approval of Provenge is also limited to only men with prostate cancer (that has progressed despite hormone treatments) with only minimal symptoms at the time of treatment. So this vaccine is not for everyone with prostate cancer. However, it is the first active immunotherapy to demonstrate improvement in overall survival for advanced prostate cancer. As a “proof of concept” drug, it is expected to spark new interest in the development of drugs based on similar principles.
Other prostate cancer vaccines are also in development, including PROSTVAC-VF that uses a genetically modified virus containing prostate-specific antigen. The patient’s immune system is expected to respond to the virus by recognizing and killing the cancer cells with prostate-specific antigen. A randomized phase II clinical study of Therion Biologic’s PROSTVAC-VF was published this January. It involved 122 patients with metastatic prostate cancer who did not respond to hormone therapy, of which 82 received the vaccine and 40 the placebo. This study also demonstrated an increase in survival rate for those taking the vaccine, in this case an average of 8.5 months. This study is small, but the researchers are planning a phase III trial with 600 patients to further evaluate the drug’s effectiveness.
So lets hope that these new drugs represent only the first step in finding effective new ways to treat cancers. In the future, hopefully we’ll be able to “turn on” the body’s own defense mechanisms to treat or prevent many types of cancer.