IL-6 secretion in transwell and contact cocultures of BMSCs and MM cells is significantly inhibited by BIBF 1000. (A) Control experiments in BMSC monocultures showed dose-dependent inhibition of IL-6 release by BIBF 1000 (0.5 μM). (B) Basal IL-6 secretion by myeloma cell lines ranged near or below the detection limit of the assay. (C-H) As compared with BMSC monocultures, IL-6 secretion was increased in both transwell (C-E) and contact (F-H) cocultures of BMSCs with U-266, RPMI-8226, or KMS-11 cells. Exposure to BIBF 1000 (0.125-1.0 μM) resulted in almost complete abrogation of IL-6 secretion in both types of cocultures. (I-J) In transwell cocultures of BMSCs and patient-derived CD138⁺-sorted MM cells, a similar increase in IL-6 secretion as compared with the respective monocultures and its dose-dependent inhibition by BIBF 1000 was observed. IL-6 concentrations were determined in serum-free supernatants of 72-hour cultures. Samples were measured in triplicates, and the results are presented as means ± SEs of at least 3 independent experiments. Significance of group differences was analyzed by the Mann-Whitney rank sum test. ^*P < .05, ^**P < .005, ^***P < .001 versus controls without BIBF 1000.