Effect of B-B4–DM1 on survival of CD138⁺ and CD138^- MM cells. MM cell lines were exposed to (A) unconjugated B-B4 mAb, (B) immunoconjugate B-B4–DM1, or (C) free DM1 drug at equimolar concentrations. Cell survival was measured using an MTT assay. Data (mean ± SD of triplicate experiments) are expressed as percentage of untreated controls. CD138⁺ MM cell lines evaluated are MM.1S, MM.1R, and Ocy-My5; CD138^- cell lines included the lymphoma cell line SUDHL4 and the Waldenstrom macroglobulinemia cell line WSU-WM.