The finding concerning the drug, also known as Iressa, represents the first positive study involving breast cancer for the entire class of drugs known as epidermal growth factor receptor, or EGFR, tyrosine kinase inhibitors, said Dr. Massimo Cristofanilli, the study's principal investigator at the University of Texas M.D. Anderson Cancer Center.

"We initiated this study in 2003 with hopes of reducing the resistance to hormonal therapy," said Cristofanilli. "There was a lot of preclinical work indicating that, in fact, resistance to hormonal therapy is strongly associated with an activated EGFR pathway. Also, EGFR over-expression has been associated with endocrine resistance. If there's a double blockage of the EGFR and the estrogen receptor, you may achieve better control of the disease."

Cristofanilli will present the findings in Chicago June 1 during the annual meeting of the American Society for Clinical Oncology.