Mechanism of action lapatinib is a 4-anilinoquinazoline kinase inhibitor of the intracellular tyrosine kinase domains of both epidermal growth factor receptor (her1/egfr/erbb1) and human epidermal growth factor receptor type 2 (her2/erbb2)with a dissociation half-life of =300 minutes. lapatinib inhibits erbb-driven tumor cell growth in vitro and in various animal models. an additive effect was demonstrated in an in vitro study when lapatinib and 5-florouracil (the active metabolite of capecitabine) were used in combination in the 4 tumor cell lines tested. the growth inhibitory effects of lapatinib were evaluated in trastuzumab-conditioned cell lines. lapatinib retained significant activity against breast cancer cell lines selected for long-term growth in trastuzumab-containing medium in vitro. these in vitro findings suggest non-cross-resistance between these two agents.