Recent news reports on the study mentioned one patient whose levels of prostate-specific antigen (PSA), a blood marker for prostate cancer, had fallen in nine months from 75 to six. The patient reported disappearance of bone pain attributed to prostate cancer that had spread to the bone, and bone scans showed those metastases shrinking.

The lead author of the study cautions it is too early to know if the therapy will ultimately prove effective against prostate cancer, as the trial is still adding patients.  "What we can say is the two-drug combination appears to be safe and well-tolerated, and we have seen clinical benefit," said Howard I. Scher, MD, chief of the genitourinary oncology service at Memorial Sloan-Kettering.

Cancer experts say Herceptin’s potential new use is just one example of an exciting trend in cancer therapy – targeting drugs at the characteristics that make a patient’s cancer cells different from normal cells. In contrast, radiation and chemotherapy affect any rapidly dividing cells. Although the rapid growth of most cancers means they are sensitive to chemotherapy and radiation, rapidly growing normal cells, such as those of the bone marrow, lining of the mouth, and hair follicles, are sensitive to them as well, which accounts for some of the side effects of those treatments.

Herceptin was originally developed to fight a type of breast cancer in which too many copies of a gene known as HER-2 results in cells with abnormally high numbers of receptors for growth factors, substances that stimulate tumor growth. Herceptin blocks a growth factor from attaching to the cells, thereby slowing tumor growth.

Before Dr. Scher and his colleagues began clinical trials in humans, they tested the drug in animals. In the animal study, Herceptin slowed growth of the kind of human prostate cancer cells usually present when men are first diagnosed. The growth of those cells is stimulated by the male hormone testosterone. By itself, Herceptin didn’t work against cancer cells not dependent on testosterone – the kind that usually are involved when prostate cancer recurs. However, Taxol, a chemotherapy drug often used against breast cancer, worked against both kinds of cells. Researchers got the best results using Herceptin and Taxol together.

The researchers decided to try the two-drug combination in a clinical trial with patients whose prostate cancer had spread beyond the prostate gland and wasn’t stopped by hormone therapy.

Sophisticated tests identified patients with and without the extra copies of the HER-2 gene. Among other things measured by the trial, the responses of those two groups to the drugs will be compared with each other.

Researchers are enthusiastic about the new approach this trial represents.  "It is our feeling that understanding the basic defects that cause a cancer cell to be a cancer cell should lead us to targets for therapy. And this is one of the early trials that has come out of that understanding," said W. Marston Linehan, MD, of the National Cancer Institute (NCI).

Other studies currently in progress are testing Herceptin against lung, colorectal, pancreatic, and salivary gland cancers. A similar approach is also being used to develop drugs that attack other molecular defects in cancer cells.

"A lot of people working in different areas have brought about this advance," said Dr. Linehan.  "We’re starting to see a dividend from our investment over the years. It’s showing up in this kind of work."

Dr. Scher agreed. "We are headed more toward the era of choosing a therapy on the basis of the patient’s particular cancer cell characteristics, rather than on the site where the cancer began, such as lung or prostate," he said. "That’s where cancer therapy is going to go."