Trastuzumab is the front-line drug used to treat women whose tumors overproduce a growth-regulating protein called Her-2. Trastuzumab blocks the Her-2 receptor on cancer cells and inhibits its signaling, thereby shrinking or stabilizing the tumor.

But nearly one-third of tumors with Her-2 over-expression do not respond to trastuzumab, said Blackwell. In the current trial, the majority of patients — 59 percent — had progressed through three, four or five traditional cancer drugs.

Lapatinib represents a new type of therapy because it targets Her-2 plus another growth factor protein called Epidermal Growth Factor (EGFR). “Blocking the action of two growth factors has a more profound effect on inhibiting cell growth than blocking a single growth factor, and we think this dual action on breast cancer cells is responsible for the positive effects we’re seeing,” said Blackwell. The study was funded by GlaxoSmithKline, the maker of lapatinib.