Inclusion Criteria:  Written informed consent prior to beginning of trial specific procedures Subject must be female and aged ≥ 18 years on day of signing informed consent ECOG 0-1 Histologically confirmed, HER2 negative breast cancer determined by core biopsy of tumor lesion. Human epidermal growth factor receptor 2 (HER2) negativity is defined as either of the following by local laboratory assessment: In situ hybridization (ISH) non-amplified (ratio ≤ 2.2), or IHC 0 or IHC 1+. Indication for chemotherapy Previous therapy with one chemotherapy line Target lesion (RECIST 1.1) Adequate organ function defined as:  Creatinine Clearance > 50 ml/min ANC ≥ 1.5 x 10 3 /μL Thrombocytes > 100 x 10 3 /μL  Exclusion Criteria:  HER2 positive disease Indication for an anti-hormone treatment Active infection requiring systemic therapy. Active autoimmune disease or other diseases that requires systemic treatment with corticosteroids or immunosuppressive drugs. History of primary or acquired immunodeficiency (including allogenic organ transplant). Active or prior documented inflammatory bowel disease (e.g., Crohn's disease, ulcerative colitis). Severely impaired liver function (Child Pugh C) Hypersensitivity to study medication or any of its components Neuropathy (PNP) > Grade 2 (CTCAE 5.0) Congenital long QT syndrome Preexisting concomitant use of strong CYP3A4 and CYP2C8 inhibiting or inducing drugs Life expectancy of less than three months Pregnancy (contraception is required according tocontraceptive guidance) Lactation Known history of following infections: Human immunodeficiency virus (HIV), History of acute or chronic Hepatitis B or Hepatitis C Has received a live-virus vaccination within 30 days of planned treatment start. Seasonal flu vaccines that do not contain live virus are permitted. Does not agree to blood collection