The effect of cyclolignan PPP on growth in MM cell lines and primary MM cells. A panel of 13 MM cell lines was treated with PPP at indicated concentrations for 48 hours followed by analysis using the resazurin assay. The cell lines were divided into 3 groups showing (A) IL-6–independent EJM, JJN3, Karpas 707, L363, LP-1, OPM-2, and U-1996 cell lines; (B) IL-6–dependent U-1957, U-1958, and U-266-1970 cell lines; and (C) the IL-6–independent RPMI 8226 cell line and its subclones RPMI 8226/Dox40 and RPMI 8226/LR5 with increased resistance to doxorubicin and melphalan, respectively. IL-6 was added to U-1957, U-1958, U-1996, and U-266-1970. Four experiments were performed and 1 representative is shown. (D) Plasma cells were purified from bone marrow samples of 10 patients with MM and treated for 72 hours with indicated concentrations of PPP. At harvest the relative number of viable cells was analyzed using the resazurin assay. (E) Alternatively, primary MM cells from 4 patients were allowed to adhere to BMSCs before treatment with PPP for 72 hours. By using ³H-TdR the amount of DNA synthesis was quantified, whereby ³H-TdR incorporation of the MM cells was calculated as described in “Materials and methods.” The experiments were performed in triplicate, and data are presented as means ± SD. Error bars not visible are included within the symbols. (F) BMSC and SK1069 fibroblasts were treated with PPP at 1 μM for 72 hours. At harvest the relative number of viable cells was analyzed using the resazurin assay. Three experiments were performed and 1 representative is shown whereby data are presented as mean ± SD.