Preclinical studies of fibroblast growth factor receptor 3 as a therapeutic target in multiple myeloma.

Dysregulation of fibroblast growth factor receptor 3 (FGFR3) by the translocation t(4;14)(p16;q32) occurs in 15% of multiple myeloma (MM) patients and confers a growth and survival advantage to malignant plasma cells. As FGFR3 is a molecular target, we assessed the therapeutic potential of the FGFR-specific tyrosine kinase inhibitors SU5402 and SU10991 in MM. SU5402 inhibited FGFR3 phosphorylation in vitro and in murine MM tumour models. B cells dependent on FGFR3 for survival were specifically sensitive to SU5402. A panel of 11 human myeloma cell lines was studied, five bearing the t(4;14) translocation. The KMS11 human myeloma cell line, which expresses constitutively active mutant FGFR3, displayed an 85% decrease in S-phase cells, a 95% increase in G0/G1 cells, and 4.5-fold increase in apoptotic cells after 72 h treatment with 10 micromol/l SU5402. Activated extracellular signal-regulated kinases 1 and 2 and signal transducer and activator of transcription 3 were rapidly down-regulated after SU5402 treatment. In human myeloma cell lines expressing wild-type FGFR3 the stimulating effect of aFGF ligand was abrogated by SU5402 treatment. Myeloma cells lacking the t(4;14) or with the t(4;14) and a secondary RAS mutation did not respond to therapy. These findings support the development of clinical trials of early intervention with FGFR3 inhibitors in t(4;14) myeloma.