Phase IIa study, 1:1 randomized, double-blind, placebo-controlled, parallel-design, multi-center study. Each breast cancer patient will be randomly assigned into one of two treatment arms receiving either Myelo001 or placebo as a tablet. Investigational medicinal product is taken as supportive care for 23 consecutive days during chemotherapy treatment. Hematologic and safety parameters as well as actual begin and doses of following chemotherapy cycles will be assessed. A single primary variable will be analyzed with test statistics based on frequent absolute neutrophil measurements. Additionally, in a subgroup of patients biomarkers and pharmacokinetics of Myelo001 will be investigated.