Women with breast cancers that overexpress HER2 are at greater risk for disease progression and death than women whose tumors do not overexpress HER2. Trastuzumab, a recombinant humanized monoclonal antibody against the extracellular domain of the HER2 protein blocks downstream signaling of HER2 and substantially improves the efficacy of chemotherapy in women with metastatic and early-stage HER2-positive breast cancers. Because resistance to trastuzumab eventually results in progressive disease in the metastatic setting and contributes to recurrence following adjuvant trastuzumab-based therapy, it is important to develop agents other than trastuzumab that target HER2 signaling through different mechanisms of action. Lapatinib is an oral, small molecule, dual tyrosine kinase inhibitor of HER2 and EGFR. Lapatinib has shown a lack of cross-resistance with trastuzumab in preclinical studies and activity in women with HER2-positive, metastatic breast cancer that has progressed during trastuzumab treatment. Trastuzumab blocks the downstream signaling of HER2 by binding to the extracellular domain of the receptor. Lapatinib binds to the intracellular domains of HER2 and EGFR and prevents activation of downstream signaling pathways. Because of this different mechanism of action, lapatinib may be effective in trastuzumab-resistant disease. The study will also provide important safety information on trastuzumab and lapatinib combinations immediately following anthracycline exposure, and also provide an initial direct comparison of cardiac effects of trastuzumab and lapatinib when incorporated into a standard sequential AC followed by weekly paclitaxel (neo)adjuvant regimen.

Availability of a second agent that can interrupt HER2-signaling pathways through completely different mechanisms than those of trastuzumab offers the potential for further improvement in the management of patients with HER2-overexpressing breast cancer in both the adjuvant and metastatic setting. Co-administration of both trastuzumab and lapatinib with chemotherapy may be important in improving outcomes in subsets of HER2-positive breast cancers. However, use of two inhibitors of the HER2 pathway will increase costs and may increase toxicity, so it will be important to identify the subsets of patients who would benefit from the dual therapy. Inhibition of HER2 with a single agent clearly is sufficient for many patients as evidenced by the results of the trastuzumab trials. Therefore, co-administration to unselected populations of women with HER2-positive breast cancers would not represent an optimal approach. Given the activity of lapatinib, it is likely that it will also be sufficiently active in inhibiting HER2-pathway activation in some patients to allow for its use as the sole inhibitor of the HER2 pathway. Different populations may also derive greater benefit from one of the HER2-blocking agents relative to the other. Identification of potential predictive factors for pathologic complete response to the combination or to either agent administered alone in neoadjuvant trials would provide important information for adjuvant trials designed to definitively address these important issues.

This study will compare 3 combined chemotherapy regimens: AC followed by paclitaxel plus trastuzumab and lapatinib, AC followed by paclitaxel plus lapatinib, and AC followed by paclitaxel plus trastuzumab given before surgery to patients with HER2-positive breast cancer.