Inclusion Criteria:

• Provision of written informed consent
• Histologically or cytologically confirmed breast cancer
• Metastatic breast cancer (stage IIIB and IV). EGFR and/or HER-2 expression on the primary tumor is not mandatory.
• Patients should have received at least one course of standard systemic chemotherapy for their metastatic disease. Prior hormonal therapy is allowed.
• Patients should have achieved objective response (CR or PR) or stable disease to previous first or second line treatment.
• There should be at least one month between the end of chemotherapy treatment and trial entry. In case of prior Herceptin administration, 3 months are required to have elapsed before study entry.
• Detection of ≥5 cells/7.5ml of peripheral blood detected by Cell Search System despite the previous administration of chemotherapy and/or hormonal therapy.
• HER-2 expression on CTCs.
• Age 18 years and over
• Adequate Haematological function, Absolute neutrophil count ≥1.5 x 109/L, Platelet count ≥100 x 109/L and Haemoglobin ≥9 g/dL (may be transfused to maintain or exceed this level)
• Adequate Liver Function, total bilirubin <1.5 x upper limit of normal, AST and ALT <2.5 x upper limit of normal in patients without liver metastases and <5 x upper limit of normal in patients with liver metastases
• Adequate Renal function, Serum creatinine ≤1.25 x upper limit of normal or calculated creatinine clearance ≥50 mL/min
• LVEF within institutional normal range
• Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 2
• Life expectancy of at least 12 weeks

Exclusion Criteria:

• Any concurrent systemic treatment for breast cancer (including chemotherapy, radiotherapy, hormonotherapy, monoclonal antibodies)
• Other co-existing malignancies or malignancies diagnosed within the last 5 years with the exception of basal cell carcinoma or cervical cancer in situ
• Any unresolved chronic toxicity greater than Grade 2 (NCI- CTCAE) from previous anticancer therapy (except alopecia)
• Any evidence of severe or uncontrolled systemic disease (e.g., unstable or uncompensated respiratory, cardiac, hepatic, or renal disease)
• Pregnancy or breast feeding (women of child-bearing potential). Women of childbearing potential must practice acceptable methods of birth control to prevent pregnancy
• Treatment with any other investigational agent, or participation in another clinical trial within 28 days prior to enrolment
• Known hypersensitivity to drugs chemically related to lapatinib
• Concomitant requirement for medication classified as CYP3A4 inducers or inhibitors
• Evidence of any other disease, neurological or metabolic dysfunction, physical examination finding or laboratory finding giving reasonable suspicion of a disease or condition that contraindicates the use of an investigational drug or puts the patient at high risk for treatment-related complications