Lapatinib Resistance in Patients With Breast Cancer

OBJECTIVES: * To identify secondary ErbB2 mutations in tumor tissue samples from patients with ErbB2-positive breast cancer treated with lapatinib ditosylate. * To investigate ErbB2 copy number changes and expression levels. * To determine abnormalities of other pathways (e.g., c-MET and PI3K) as potential mechanisms of resistance. OUTLINE: Previously collected tumor tissue samples\* are obtained for genetic analysis studies. Samples are analyzed for secondary ErbB2 mutations by nested PCR; ErbB2 copy number changes by quantitative PCR and standard histological FISH; and ErbB2 expression levels by quantitative RT-PCR and IHC. Patients also undergo blood sample collection for extraction of DNA (as normal control DNA) and isolation of EpCAM-positive circulating tumor cells using immunomagnetic cell separation technology. Additional research studies may include mutational and amplification analysis of the c-MET and PI3K pathways. NOTE: \*Patients may undergo biopsy if a post-treatment tumor tissue sample is unavailable.