Pre-clinical studies of CHIR258, a small molecule inhibitor that targets FGFR3, for the treatment of t(4;14) multiple myeloma

Abstract

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The t(4;14) translocation that occurs uniquely in a subset (15%) of multiple myeloma (MM) patients results in the ectopic expression of the receptor tyrosine kinase (RTK), FGFR3. Wild-type (WT) FGFR3 induces proliferative signals in MM cells and appears to be weakly transforming in a hematopoeitic mouse model. The subsequent acquisition of FGFR3 activating mutations in some MM is associated with disease progression and is strongly transforming in experimental models. The clinical impact of t(4;14) translocation has been demonstrated in 3 large studies each reporting a marked reduction in overall survival. We have previously shown that inhibition of activated FGFR3 causes morphologic differentiation followed by apoptosis of FGFR3 expressing MM cell lines, validating activated FGFR3 as a therapeutic target in t(4;14) MM and encouraging the clinical development of FGFR3 inhibitors for the treatment of these poor-prognosis patients. CHIR258 is a small molecule kinase inhibitor that targets Class III-V RTKs and inhibits FGFR3 with an IC₅₀ of 5 nM in vitro kinase assays. Potent anti-tumor and anti-angiogenic activity has been demonstrated in vitro and in vivo. We have employed the IL-6 dependent murine myeloma cell line, B9 that has been engineered to stably express either WT FGFR3 (B9-WT) or the K650E mutant FGFR3 (B9-TD) to screen CHIR258 for FGFR3 activity. CHIR258 inhibited FGF-mediated growth of B9-WT and B9-TD with an IC50 of 25nM and 80 nM respectively as determined by MTT assay. Growth of these cells could be rescued by IL-6 demonstrating selectivity of CHIR258 for FGFR3. At concentrations that prevented cell growth, CHIR258 inhibited in vitro FGFR3 autophosphorylation. We then confirmed the activity of CHIR258 against FGFR3 expressing MM cells. CHIR258 inhibited the growth of FGFR3 expressing KMS11 (Y373C), OPM2 (K650E), KMS18 (G384D) and H929 (WT FGFR3) cells with an IC₅₀ of 100 nM (KMS11 and OPM2 cells), 250 nM and 630 nM respectively. U266 cells, which lack FGFR3 expression, displayed minimal growth inhibition demonstrating that at effective concentrations, CHIR258 exhibits minimal nonspecific cytotoxicity on MM cells. Further characterization of this finding demonstrates that inhibition of cell growth is related to G1 cell cycle arrest and dose-dependent inhibition of ERK phosphorylation. CHIR258 also induced delayed, dose-responsive apoptosis of these cells. Apoptotic cells, assessed by annexin V staining, were detectable 2 (KMS11) and 5 (KMS18, OPM2) days after exposure to CHIR258 and increased with dose and days of treatment. Studies assessing in vivo activity of CHIR258 against FGFR3 expressing tumors in xenograft mice are ongoing and will be reported. These data indicate that the small molecule inhibitor, CHIR258 potently inhibits FGFR3 and has activity against human MM cells setting the stage for a Phase I/II clinical trial of this compound in t(4;14) MM.