Radioimmunotherapy targets sites of metastatic prostate cancer throughout the body using a tumor-specific monoclonal antibodies, lab-made proteins designed to seek and destroy certain types of cancer cells, similar to the way the body’s immune system responds to an infection. The antibodies carry radioactivity with them so the radiation can specifically kill cancer cells on the tumor and minimize damage to surrounding normal tissue.

"Other therapies, such as chemotherapy, have some effect, but entail significant toxicity," says Robert T. O’Donnell, MD, PhD, assistant professor of medicine in the division of hematology and oncology at the University of California, Davis. "Local external beam radiation is useful for individual sites of metastatic prostate cancer, but since the cancer is a systemic disease, other sites of cancer continue to grow and cause problems."

In this Phase I study, O’Donnell and colleagues gave a monoclonal antibody derived from mice to 17 men with advanced prostate cancer. In a Phase I study, a new treatment is given to a small number of patients to find the best way to give a new treatment and how much of it can be given safely.

The patients were divided into three groups based on the percentage of bone affected by cancer. Their pain levels were determined before therapy and afterward by monitoring the use of pain medications on a daily basis, and by assessing pain status on a weekly basis after the medication was administered. The patients’ prostate-specific antigen (PSA) levels were also checked every two months after therapy.

Thirteen of the 17 patients reported they had pain before the therapy; seven of those 17 patients reported partial or complete resolution of their pain for an average of 4.3 weeks after the therapy. Complete resolution of pain occurred in five of 13 patients, and an additional two of 13 patients had at least a 50% reduction in pain.

Phase I studies are open only to patients whose cancer has spread and who would not be helped by other known treatments. While the patients are participating, physicians watch them carefully for any harmful side effects.

In this study, some patients experienced toxicity with the treatment, such as myelosuppression (damage to the blood cell-producing tissues of the bone marrow), but it was "relatively low and manageable," says O’Donnell.

O’Donnell says that more than 31,000 US men die each year from prostate cancer that no longer responds to standard hormonal therapy. Once hormonal therapy is no longer effective, options are limited to treat prostate cancer.

"The antibody is directed against certain antigens on these cancer cells," says Herman Kattlove, MD, a medical editor for the American Cancer Society. "These antibodies have radioactive material attached to them, so it’s like a stealth bomber – when they attach to the [cancer] cells, then the radioactivity that’s attached to the molecule kills the [cancer] cell."

O’Donnell says his study was small, and that more studies are needed to confirm the results. He and his co-researchers are conducting a new study of prostate cancer patients with a newer version of the monoclonal antibody. Later in the study, patients will receive the chemotherapy agent Taxol (paclitaxel) in addition to radioimmunotherapy.

"I think this should provide hope that new forms of therapy, like radioimmunotherapy, are being developed to fight this presently incurable and debilitating cancer," says O’Donnell.

He adds that radioimmunotherapy also has had "encouraging results" in treating other forms of cancer, such as metastatic breast cancer, gastrointestinal tumors, and thyroid cancer.

Kattlove says the new therapy appears very promising.

"As we get better at this, we learn how to design better molecules and find more kinds of antigens in the cancer cells that are more specific for certain types of cancer," he notes. "We might be more successful, especially if we combine it with chemotherapy or with hormonal therapy. It’s a tiny step, but it’s a step in the right direction."