Model for various mechanisms of metronomic chemotherapy. Metronomic chemotherapy may have different mechanisms for eradicating tumors. For example, it may directly affect tumor cells (brown and black ovals), it may cause direct endothelial cell death or growth inhibition (red ovals), or it may decrease the mobilization or viability of bone marrow–derived CEPs, which contribute to the tumor vasculature (pink ovals). Suppression in the levels of viable CEPs detected in PB may serve as a biomarker to define the OBD of chemotherapy drugs given in a metronomic chemotherapy regimen.