hERG1 blockers can overcome MSC-induced chemoresistance in vivo. NOD-SCID mice (n = 24) were inoculated with 697 cells on day 0 and treated with E4031 daily (20 mg/kg) for 2 weeks beginning 1 week after inoculation. At the end of the treatment, mice were killed, and peripheral blood (PB), bone marrow (BM), and peripheral organs (spleen, liver) were collected. (A) Leukemia engraftment in peripheral blood and bone marrow was quantified by determining the percentage of human CD45-positive (hCD45⁺) cells versus mouse CD45-positive (mCD45⁺) cells by flow cytometry. (B-C) Immunohistochemical staining with anti-hMHCI antibody of the liver and spleen of mice inoculated with the 697 cell line, and either treated (right) or not (left) with E4031. Arrows indicate foci of leukemic blasts. Histograms quantifying the extent of leukemic cell invasion (by anti-hMHCI staining) are shown on the right. The presence and extent of leukemia blast invasion is given as the area positive for anti-hMHCI staining calculated with Leica DC Viewer software at 40× magnification. Images were acquired on a DM 4000B microscope with a DFC 320 camera (Leica Microsystems; PL Fluotar 40×/0.70 objective) Scale bar = 100 μm. (D) Kaplan-Meier plots of overall survival among mice treated with saline only (n = 12) or E4031 (n = 12) after inoculation of 697 leukemic cells.