CHIR-258 inhibits FGF-mediated ERK1/2 phosphorylation and induces cytotoxicity in FGFR3-expressing primary MM cells. (A) FGFR3 is expressed at high levels on the surface of t(4;14) primary myeloma cells. Cells were stained with FGFR3 antibody (open curve) or rabbit preimmune serum (shaded curve) and then stained with goat antirabbit FITC. Myeloma cells were identified by CD138 labeling. (B) Primary myeloma cells were incubated in the absence (shaded curve) or presence of aFGF (dashed line) or preincubated with 500 nM CHIR-258 (solid line) for 2 hours and then stimulated with aFGF. ERK1/2 phosphorylation was assessed by flow cytometric analysis. (C) Primary myeloma cells were cultured in growth medium in the presence of DMSO or 500 nM CHIR-258. Cells were harvested after 7 days and stained with annexin V-FITC and analyzed by flow cytometry. Myeloma cells were identified by CD38⁺⁺/CD45^-labeling. The total percentage of CD38⁺⁺/CD45^-/annexin V⁺cells is shown in the upper right quadrant. Shown are representative data from patient 3.