Activity of B-B4–DM1 in a tumor xenograft model of human CD138⁺ MM. CB-17 SCID mice were inoculated subcutaneously in the interscapular area with 5 × 10⁶ OPM1 (A-B) or OPM2 (C-D) MM cells. Mice were treated intravenously with B-B4–DM1 or control mAbs for 3 consecutive days. Tumor volume was assessed in 2 dimensions using an electronic caliper, and the volume was expressed in mm³ using the formula: V = 0.5a × b², where a and b are the long and short diameters of the tumor, respectively. Tumor volume and survival were calculated as described in “Materials and methods.” ^* indicates that P value is related to the curve with ▪. (E-F) Mice bearing large OPM1 tumors (1309 ± 60 mm³) were treated intravenously with B-B4–DM1 (150 μg DM1/kg) for a total of 3 consecutive days. Figure shows a significant tumor regression 12 days following the beginning of treatment.