PRO-001 binds to WT and mutant FGFR3, competes with FGF for FGFR3 binding, and inhibits the growth of B9^WT cells. PRO-001 binding to FGFR3 was determined by flow cytometric analysis using a PE-conjugated anti–human secondary antibody. (A) Competition of PRO-001 binding to cell-surface FGFR3 on B9^WT cells. The filled histogram indicates parental B9 cells; dotted gray line, B9^WT without FGF9; solid line, B9^WT in the presence of FGF9. (B) PRO-001 binding to cell-surface WT and mutant FGFR3 on B9 cells. The filled histogram indicates parental cells; thick light gray line, B9^Y373C; thick dark gray line, B9^K650E; dotted gray line, B9^G384D; dotted black line, B9^WT; solid black line, B9^807C. (C) B9 cells were stimulated with FGF and incubated with increasing concentrations of PRO-001 or 100 nM PD173074 for 48 hours, and cell viability was assessed by MTT-based assay. □ indicates unstimulated; ▪, FGF stimulated; ▧, FGF plus 1 μg/mL PRO-001; , FGF plus 3 μg/mL PRO-001; ▨, FGF plus 5 μg/mL PRO-001; ▨, FGF9 plus 100 nM PD173074. Proliferation index (PI) = (OD_d2)/(OD_d0), where OD_d2 and OD_d0 are the optical density (OD) at 48 hours and day 0, respectively. When PI = 1, cells did not proliferate after 48 hours in culture. Values represent mean ± SD of 4 independent experiments.