AMD3100 mobilizes MM cells to the circulation and sensitizes them to bortezomib-induced apoptosis in vivo. (A) Correlation of the number of viable circulating cells with tumor burden in the bone marrow. The number of circulating cells was in direct linear correlation with the tumor burden. Error bars represent SD. (B) Comparison of the slope of the linear correlations, which represents the amount of the mobilized MM cells per 1 unit of tumor burden. AMD3100 showed intensive mobilization of MM cells. Bortezomib did not show a significant decrease compared with the control group. However, the combination of bortezomib and AMD3100 reduces significantly the number of circulating cells compared with AMD3100 alone. (C) The ratio of apoptotic/total MM cells in the circulation detected by in vivo real-time flow cytometry. AMD3100 did not induce apoptosis compared with the control. Bortezomib showed a dose-dependent induction of apoptosis, which was enhanced by daily treatment with AMD3100. Error bars represent SD. (D) Representative in vivo confocal images of BM niches of control and combination of AMD3100 and bortezomib–treated mice, showing viable MM cells (green), apoptotic MM cells (violet), and blood vessels (red). These images show that apoptotic MM cells were present in the circulation of mice treated with the combination of AMD3100 and bortezomib and absent in the control mice. *P = .046.