TKI258 (CHIR-258) Selectively Inhibits FGFR1 Fusion Genes Associated with the 8p11 Myeloproliferative Syndrome.

The 8p11 myeloproliferative syndrome (EMS) is an atypical stem-cell myeloproliferative disorder typically characterised by eosinophilia and associated T- or B-cell lymphoma. The disease is aggressive with transformation to acute leukemia usually occurring within 1–2 years from diagnosis and stem cell transplantation remains the only curative treatment. EMS is caused by constitutive activation of the receptor tyrosine kinase FGFR1 by fusion to unrelated partner genes, the most common being ZNF198. The partner protein promotes dimerisation and ligand-independent activation of the FGFR1 kinase in a manner analogous to BCR-ABL. To investigate the possibility of targeted therapy for EMS, we have studied the effects of TKI258 (formerly CHIR-258) on transformed cell lines and primary cells from patients with FGFR1 translocation fusion genes. TKI258 is an ATP-competitive inhibitor with activity against class III-IV receptor tyrosine kinases (which includes FGFR, VEGFR and PDGFR families) that is currently being assessed for treatment of a variety of malignancies. The growth, as assessed by the MTS assay, of Ba/F3 cells transformed to IL3 independence by the ZNF198-FGFR1 fusion gene was inhibited by TKI258 with an IC50 of 160 nM, compared to an IC50 of 995 nM for Ba/F3 cells transfected with vector only. Western blotting demonstrated dose-dependent inhibition of phosphorylation by TKI258 of ZNF198-FGFR1 and downstream signaling molecules ERK and STAT5. Primary cells from four MPD patients with FGFR1 translocations (ZNF198-FGFR1, FGFR1OP1-FGFR1, MYO18A-FGFR1 and BCR-FGFR1) and three control MPD patients without FGFR1 translocations were exposed to 0, 20 and 100 nM TKI258 in liquid culture for two weeks. All four FGFR1 patients demonstrated substantial reductions in cell numbers in both 20 and 100 nM cultures relative to control whereas no differential cell loss was seen in cells from non-FGFR1 patients. The response of colony growth to inhibition by 20 nM and 100 nM TKI258 was measured in four FGFR1-translocation patients (with and without cytokines) and in ten normal peripheral blood (NPB) controls (with cytokines). To allow sample comparison an inhibition index was calculated as the mean reduction in colony numbers at 20 nM and 100 nM TKI258 at days 7 and 14 relative to control. Four FGFR1-translocation patients showed a significant reduction in colony growth (mean inhibition index = 0.48, range = 0.24–0.60) compared with normal controls (mean index = 0.73, range = 0.48–0.87) (p < 0.01). In summary TKI258 shows significant selective activity against ZNF198-FGFR1-transformed Ba/F3 cells and against primary cells from patients with FGFR1 translocations in both liquid culture and colony assays. TKI258 is therefore a good candidate for targeted therapy for EMS patients, who currently have a poor prognosis and few treatment options.