YM155, alemtuzumab, and the combination of YM155 with alemtuzumab treatment inhibited the growth of MET-1 ATL tumor cells in the MET-1 mouse model of human ATL. (A) The mean concentrations of tumor surrogate marker, human sIL-2Rα levels in MET-1–bearing mice before and after YM155, alemtuzumab, and the combination of YM155 with alemtuzumab treatment (2 and 4 weeks posttherapy). The animals treated with YM155 alone, alemtuzumab alone, and the combination of YM155 with alemtuzumab had significantly decreased values of sIL-2Rα when compared with those of the PBS control group 4 weeks posttherapy (YM155, P < .01; alemtuzumab, P < .01; the combination, P < .001). Furthermore, there were no detectable levels of sIL-2Rα in the mice that received the combination therapy 4 weeks posttherapy. (B) The mean concentration of tumor surrogate marker, human sIL-2Rα levels in MET-1–bearing mice 8 weeks after treatment with YM155, alemtuzumab, and the combination of YM155 with alemtuzumab. The animals receiving the combination of YM155 with alemtuzumab had no detectable levels of sIL-2Rα 8 weeks posttherapy. N.D., not detected.