Increase in circulating MDSC levels over time in patients with advanced solid tumors is associated with decreased survival times and radiographic disease progression. Gating strategy for BM-MDSCs (A) and whole-blood MDSCs (B) is shown on a representative flow cytometric plot. (C) Random effects regression model and correlation between MDSCs and CTCs. Flow cytometric analysis was performed on peripheral whole blood in a separate cohort of patients with stage IV breast cancer (n = 25) before initiation of therapy and at defined intervals during therapy. Blood for CTC determination by the CellSearch was simultaneously drawn. A significant correlation was found between circulating MDSC levels (%) and CTCs (P = .0001). (D) Survival analysis by circulating MDSC levels (%) at first blood draw in patients with stage IV breast cancer starting a new line of systemic chemotherapy (n = 26). Survival estimates by median percentage of MDSCs (≤ 3.17% and > 3.17%) with the use of the first MDSCs observation (P = .048). (E) Survival estimates by median percentage of MDSCs (≤ 3.04% and > 3.04%) with the use of MDSCs levels drawn at the last visit (P = .018). (F) Survival analysis by circulating MDSC levels at time of study enrollment in patients with stage IV colorectal cancer. Survival estimates by medial percentage of MDSCs (≤ 2.54% and > 2.54%). (G) Analysis of relationship between changes in circulating MDSC levels over time and best radiographic response in patients receiving systemic chemotherapy (n = 25). Plot of MDSCs over time by “best response” defined as patients who achieved complete (CR) or partial radiographic response (PR) while on systemic therapy versus patients who did not. MDSCs were drawn prospectively after every other cycle of therapy. Over time circulating MDSCs were significantly higher in nonresponders than in patients with CR or PR as best response (*P = .015 comparing slopes).