A Study to Examine the Effects of Esomeprazole on the Pharmacokinetics of Orally Administered Lapatinib in Subjects With Metastatic ErbB2 Positive Breast Cancer

Inclusion Criteria: * Metastatic, histologically confirmed breast cancer that overexpresses ErbB2 (3+ by IHC, FISH, or CISH positive). * 18 years to 65 years of age. * Is male or female (of non childbearing potential or willing to use contraception as specified in the protocol). * Is able to swallow and retain oral medication. * ECOG performance status 0 to 2. * Provided written informed consent. * Adequate bone marrow function (as specified in the protocol). * Calculated creatinine clearance (CrCl) greater than or equal to 50 ml/min based on Cockcroft and Gault. * Total bilirubin less than or equal to 1.5 X upper limit of normal of institutional values. * Alanine transaminase (ALT) less than or equal to 3 times the upper limit of the institutional values or less than or equal to 5 times ULN with documented liver metastases * Has a left ventricular ejection fraction (LVEF) within the normal institutional range based on ECHO or MUGA. * Life expectancy of greater than or equal to 12 weeks * Capable of giving written informed consent, which includes compliance with the requirements and restrictions listed in the consent form. Exclusion Criteria: * Pregnant or lactating woman. * Has malabsorption syndrome, a disease affecting gastrointestinal function, or resection of the stomach or small bowel. * Has current active hepatic or biliary disease (with exception of patients with Gilbert's syndrome, asymptomatic gallstones, liver metastases or stable chronic liver disease per investigator assessment) * Has evidence of symptomatic or uncontrolled brain metastases or leptomeningeal disease. Subjects with brain metastases treated by surgery and/or radiotherapy are eligible if neurologically stable and do not require steroids or anticonvulsants. * Is considered medically unfit for the study by the investigator. * Has a known immediate or delayed benzimidazole hypersensitivity reaction or idiosyncrasy to drugs chemically related to the investigational product such as gefitinib \[Iressa\] and erlotinib \[Tarceva\]. * Has received treatment with any investigational drug in the previous four weeks. (with the exception of lapatinib). * Has received chemotherapy, immunotherapy, biologic therapy or hormonal therapy for the treatment of cancer within the past 14 days, with the exception of mitomycin C which is restricted for the past six weeks, and lapatinib. Subjects receiving lapatinib prior to study entry are eligible to participate in the study. * Is receiving any prohibited medication within the timeframe indicated on the prohibited medication list in the protocol. * Has physiological, familial, sociological, or geographical conditions that do not permit compliance with the protocol. * Has inadequate venous access for protocol-related blood draws. * Clinically significant electrocardiogram abnormality. * History of sensitivity to heparin or heparin-induced thrombocytopenia. * Consumption of red wine, seville oranges, grapefruit or grapefruit juice and/or kumquats, pummelos, exotic citrus fruit (i.e. star fruit, bitter melon), grapefruit hybrids or fruit juices from seven days prior to the first dose of study medication.