This was a four-arm (parallel group) randomized, open-label, multicenter Phase 3 study to investigate the use of a combination of Lapatinib and Trastuzumab, a sequence of Trastuzumab followed by Lapatinib, and Lapatinib alone, compared to Trastuzumab alone in the adjuvant treatment of Human Epidermal Growth Factor Receptor 2 (HER2) positive early breast cancer.

Treatment allocation was stratified by blocked randomization, with three stratification factors: * Hormone receptor status: Estrogen Receptor (ER) and/or Progesterone Receptor (PgR) positive versus both negative. * Axillary lymph node involvement: not assessed because of neoadjuvant chemotherapy versus node negative versus 1-3 positive nodes versus 4 or more positive nodes. * Timing of adjuvant chemotherapy: concurrently with taxanes and targeted therapy (Design 2) and concurrently with non-anthracycline-based platinum chemotherapy and targeted therapy (Design 2B) versus all other chemotherapy completed before randomization (Design 1). Treatments delivered differed according to the timing and type of adjuvant chemotherapy. The primary objective of this study was to compare disease-free survival (DFS) in subjects with HER2 overexpressing and/or amplified breast cancer randomized to trastuzumab for one year versus lapatinib for one year versus trastuzumab (12 or 18 weeks, according to assigned design) followed by a six week treatment-free interval followed by lapatinib (28 or 34 weeks, according to assigned design) versus trastuzumab in combination with lapatinib for one year (52 weeks). Secondary objectives included treatment comparisons with respect to overall survival, time to recurrence, time to distant recurrence, safety and tolerability, and incidence of brain metastasis. Based on the recommendation of the Independent Data Monitoring Committee (IDMC) at the first interim analysis (18-Aug-2011), the Lapatinib alone arm was discontinued prior to primary analysis due to futility. The IDMC also stated that the other three arms (trastuzumab alone, sequential trastuzumab/lapatinib arm and the combination arm) could continued as planned with no changes.