ATPS-47: ORAL ADMINISTRATION OF THE AXL TYROSINE KINASE INHIBITOR BGB324 PROLONGS SURVIVAL OF GLIOBLASTOMA-BEARING MICE

Overexpression of the Axl receptor tyrosine kinase is associated with a negative outcome for glioblastoma patients. The aim of the study was to evaluate the effect of the specific Axl-inhibitor BGB 324 on glioma cell proliferation and migration in vitro as well as on orthotopic tumor growth in vivo. Axl expression was analyzed in adherent glioblastoma cell lines (N = 32) as well as in spherical stem-like glioblastoma cell lines (N = 9) by microarray and qPCR analyses, Western blot and flow cytometry. Axl was strongly expressed in glioblastoma cells with a proliferative, differentiated phenotype, such as G55 and U87, while its expression was weak in stem-like GBM cells. Full length Axl protein was present at the cell surface, while its soluble form sAxl was detectable in conditioned media and was released from the cells in a metalloproteinase-dependent fashion. The Axl ligand Gas6 was also present in conditioned medium, indicating an autocrine stimulation loop. In addition, unprocessed full length Axl was found in conditioned medium, suggesting an association of Axl with glioblastoma-derived exosomes. Erk1/2 and Akt phosphorylation induced by recombinant Gas6 was abrogated by interfering with Axl tyrosine kinase function using BGB324, resulting in a dose-dependent decrease in proliferation and cell migration. Most importantly, oral administration of 25 mg/kg BGB324 to tumor-bearing mice twice daily significantly increased the median survival of treated animals compared to vehicle controls for both G55 (23 vs 19 days, p < 0.0001) and U87 (26.5 vs 24days, p < 0.001) xenograft models. In summary, targeting Axl by oral administration of BGB324 appears a promising strategy for Axl-overexpressing glioblastoma cells that have undergone proliferative differentiation. The impact of Axl inhibition glioblastoma stem-like cells remains to be elucidated.