The trial, sponsored by the National Cancer Institute, will assess the effectiveness of the vaccine, created in the laboratory of Dr. David Peace, assistant professor of medicine, after five years of intensive research.
The vaccine includes a fragment of the protein called prostate-specific antigen, or PSA. PSA is produced by cells lining the tubules of the prostate gland, as well as by prostate cancer cells. It has been used for years as a clinical marker for prostate cancer to screen for and monitor the disease. In the trial, Peace will determine whether PSA can also be used therapeutically — to help the immune system target prostate cancer cells for destruction.
Studies in Peace's laboratory have shown that the vaccine causes subsets of the immune system's white blood cells to morph into highly specific killer white blood cells, called cytotoxic T lymphocytes, which selectively destroy tumor cells that express PSA. The white blood cells Peace used were drawn from healthy individuals as well as patients with advanced prostate tumors.
"Our laboratory findings suggest that the vaccine should be effective in treating advanced prostate cancer, where the patient's immune system is severely challenged," Peace said.
Results of the laboratory study are being presented at the Experimental Biology 2002 meeting in New Orleans on April 21.
Patients with prostate cancer are eligible to participate in the vaccine trial if, after undergoing surgery or radiotherapy to eliminate the tumor, (1) there is a high risk the tumor will recur, (2) the individual's PSA level is rising, or (3) the cancer has spread. Patients are considered to be at high risk for a recurrence of the cancer if their initial PSA level was greater than 10 ng/mL or their tumor grade (Gleason score) is greater than 6. Patients with metastatic prostate cancer must have their tumor under control with hormone blockade or other therapies.
The vaccine is either injected under the skin, along with a compound known to stimulate the immune system, or delivered intravenously, after being loaded onto dendritic cells. A type of white blood cell, dendritic cells activate the immune system by scavenging antigens, like PSA, and presenting them to T-cells, one of the body's most important defense mechanisms.
All patients will continue their regular treatment while participating in the trial.
For purposes of the study, the vaccine was designed to work in patients with a specific immune type, found in about 50 percent of individuals in the United States. However, Peace's laboratory is developing similar PSA vaccines for patients with other immune types.
"The advantage of these kinds of vaccines is that they can be customized to each patient, based on his immune type. The specificity of the vaccine enables the immune system to target the tumor cells with exquisite precision, with minimal risk of damage to the body's normal tissues," Peace said.
According to Peace, the vaccine also holds promise as a treatment for types of breast cancers whose cells express PSA.
Next to skin cancer, prostate cancer is the most common malignant cancer in North American men. It is now the second leading cause of cancer death in men, exceeded only by lung cancer. An estimated 198,100 males developed prostate cancer in the United States last year, and 31,500 died from the disease.
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