Cancer Stem Cells and Somatic Stem Cells as Potential New Drug Targets, Prognosis Markers, and Therapy Efficacy Predictors in Breast Cancer Treatment

New drug targets, markers of disease prognosis, and more efficient treatment options are an unmet clinical need in breast cancer (BC). We have conducted a pilot study including patients with luminal B stage breast cancer IIA–IIIB. The presence and frequency of various populations of cancer stem cells (CSC) and somatic stem cells were assessed in the blood, breast tumor tissue, and normal breast tissue. Our results suggest that patients with BC can be divided into two distinct groups based on the frequency of aldehyde dehydrogenase positive cells (ALDH1(+) cells) in the blood (ALDH1(hi) and ALDH1(low)). In the ALDH1(hi) cells group, the tumor is dominated by epithelial tumor cells CD44(+)CD24(low), CD326(+)CD44(+)CD24(−), and CD326(−)CD49f(+), while in the ALDH1(low) cells group, CSCs of mesenchymal origin and epithelial tumor cells (CD227(+)CD44(+)CD24(−) and CD44(+)CD24(−)CD49f(+)) are predominant. In vitro CSCs of the ALDH1(low) cells group expressing CD326 showed high resistance to cytostatics, CD227(+) CSCs of the ALDH1(hi) cells group are sensitive to cytostatics. Epithelial precursors of a healthy mammary gland were revealed in normal breast tissue of patients with BC from both groups. The cells were associated with a positive effect of chemotherapy and remission in BC patients. Thus, dynamic control of their presence in blood and assessment of the sensitivity of CSCs to cytostatics in vitro can improve the effectiveness of chemotherapy in BC.