HOUSTON - A new study by US researchers shows that a significant percentage of participants with HER2-positive breast cancer had their tumors entirely or almost completely eradicated after taking a combination of two drugs - lapatinib and trastuzumab.

The study was conducted by researchers at the Lester and Sue Smith Breast Center at Baylor College of Medicine, according to the June 1 edition of Texas Medical Center (TMC) news.

Tumors that test positive for HER-2, or human epidermal growth factor receptor 2, are generally aggressive and fast-growing. About a quarter of all breast cancers are positive for HER-2.

Mothaffar Rimawi, medical director of the Smith Breast Center and principal investigator on the study, and Jenny Chang, MD, director of the Methodist Cancer Center, studied 64 women with large tumors that tested positive for HER-2.

For 12 weeks, the women took a daily dose of lapatinib, which blocks HER-1 and HER2 enzymes, and a weekly intravenous dose of trastuzumab, which blocks HER-2 in a different way. Estrogen-receptor positive patients were also given an agent to stop production of estrogen.

The results show that 38 percent of estrogen-receptor negative patients and 21 percent of estrogen receptor-positive patients had their tumors completely removed. Another 34 percent had a substantial reduction in their tumors, said C Kent Osborne, director of the Smith Breast Center and a senior author of the report.

During the past 10 years, the research team conducted two prior clinical studies that showed lapatinib and trastuzumab were effective alone, but the preclinical data suggested the two drugs would likely work better when used together, Chang said.

The team tested the two drugs together on HER-2 positive tumors in mice, whose tumors later completely disappeared, leading to the decision to test the therapy in humans.

The next step in the study will be to test the two-drug combination therapy on smaller tumors and compare 12 weeks of treatment with 24 weeks of treatment to determine optimum duration, according to the TMC news report.