Twenty to thirty percent of breast cancers over express a protein referred to as the human epidermal growth factor receptor, better known as HER2 over expression.

Herceptin is an agent that is targeted against the HER2 receptor and helps to slow or stop the spread of cancer cells that over express this protein. Unfortunately, some women that do have the over expression of HER2 on their breast cancer cells do not respond to treatment with Herceptin.

The Journal of the National Cancer Institute has published a report that says among breast cancer patients with HER2 over expressing cancer cells, those whose cells also express a receptor called p95HER2 have a poor anti-cancer response rate with Herceptin.

The study included forty seven women with metastatic breast cancer. All were treated with Herceptin. Nine of these patients also expressed the receptor p95HER2.

The results showed that only 11 percent of the women with the p95HER2 expression showed an anti-cancer response to Herceptin. Of the patients who did not express p95HER2 demonstrated a response of 51 percent. The report also showed that laboratory testing of cancer cells that do express p95HER2 demonstrated anti-cancer activity with a drug called Tykerb.

The researchers concluded that patients with HER2-over expressing breast cancer who also express p95HER2 appear to be more resistant to treatment with Herceptin and "may require alternative or additional anti-HER2–targeting strategies." Patients with HER2-over expressing breast cancer may wish to speak with their physician regarding their individual risks and benefits of participating in a clinical trial further evaluating biologic markers that may help predict responses to certain therapies.