Drugs used to increase the number of red blood cells in patients with anemia could hinder the effectiveness of the breast cancer drug trastuzumab, commonly known as Herceptin, according to a new study.

It's common for breast cancer patients to take anemia medication simultaneously with cancer-treating drugs, because both cancer and the drugs that treat it can cause anemia, said study researcher Dr. Zhen Fan, associate professor at the University of Texas MD Anderson Cancer Center.

But when the red-blood-cell-boosting drug erythropoietin (sold under the brand names Procrit and Epogen) is taken in addition to trastuzumab, the cancer drug's effects are undermined, Fan said.

Trastuzumab is a targeted therapy that blocks the human epidermal growth factor receptor-2 (HER2), which is over-expressed in 25 to 30 percent of breast cancers.

"If confirmed by additional studies, the finding will apply to breast cancer patients whose cancers are HER2-positive and are treated with Herceptin," Fan told MyHealthNewsDaily.

Fan and his colleagues implanted human breast cancer cells in mice to test the effects of the anemia drug with the breast cancer drug.

When the mice received trastuzumab by itself, tumor growth stopped altogether. But when the researchers treated the mice with the anemia drug and trastuzumab, tumor growth was reduced by only half, according to the study.

Erythropoietin hinders the effects of the breast cancer drug because it binds to receptors on cancer tumor cells and activates a molecule that trastuzumab is supposed to block, Fan said. Erythropoietin also shuts down a tumor suppressor gene that scientists say is vital to trastuzumab's effectiveness.

To see if the finding could be confirmed in humans, Fan looked at the medical records of 37 breast cancer patients whose cancer was treated with trastuzumab, chemotherapy and the anemia drug, and compared them with records of 74 breast cancer patients who received only trastuzumab and chemotherapy.

One year after beginning treatment, tumor growth had  halted in 40 percent of the patients who didn't take the anemia drug, compared with 19 percent of the patients who did.

However, it's possible the women who didn't take the anemia drug were healthier to begin with, thereby upping their chances of responding to the treatment, compared with women who needed to take erythropoietin, Fan said. The results need to be confirmed in a larger population.

Now, the researchers are studying how the receptor for erythropoietin is regulated in cancer cells, so patients can take a drug to treat their anemia without compromising their cancer treatment.

This article was provided by MyHealthNewsDaily, a sister site to LiveScience.