Digoxin Induced Dissolution of CTC Clusters

Inclusion Criteria: * Informed Consent as documented by signature * Diagnosis of adenocarcinoma of the breast with evidence of locoregionally recurrent or metastatic disease not amenable to resection, radiation therapy or systemic therapy with curative intent * Adequate organ and marrow function Exclusion Criteria: * Patients on treatment with digoxin or digitoxin * Patients with atrial fibrillation or atrial flutter * Ventricular Fibrillation or ventricular tachycardia, * Atrioventricular heart block 2nd or 3rd degree, sick sinus syndrome or sinus bradycardia, * Wolff-Parkinson-White Syndrome * Hypokalemia, hypercalcemia, hypomagnesemia, hypoxia, * Hypertrophic cardiomyopathy, aortic aneurysm * Simultaneous intravenous application of calcium salts * Known hypersensitivity to Digoxin, other cardiac glycosides or included compounds * Known drug interactions of ongoing cancer therapy with digoxin * Women who are pregnant or breast feeding, * Intention to become pregnant during the course of the study, * Lack of safe contraception * Other clinically significant concomitant disease states (e.g., renal failure, hepatic dysfunction, cardiovascular disease) * Known or suspected non-compliance, drug or alcohol abuse * Inability to follow the procedures of the study, e.g. due to language problems, psychological disorders, dementia, etc. of the participant * Participation in another study with investigational drug within the 30 days preceding and during the present study, * Previous enrolment into the current study, * Enrolment of the investigator, his/her family members, employees and other dependent persons