Small molecule inhibitors of the transmembrane receptor tyrosine kinase Axl as anticancer therapeutics

4865 Following stimulation by its endogenous ligand Gas6, the transmembrane receptor tyrosine kinase (RTK) Axl mediates oncogenic transformation through its intracellular kinase domain (ICD). The Gas6/Axl RTK signal transduction pathway operates, although not exclusively, through activation of the phosphatidylinositol 3-kinase (PI3K) Akt pathway. Over-expression of Axl has been demonstrated in many cancer cell lines, prominent amongst which are those derived from colon, breast, lung, ovarian and squamous cell carcinoma. Furthermore, increased expression of Axl is an indicator of poor prognosis in several tumor types and Axl kinase has been shown to play a pivotal role in cell survival, proliferation, migration and tumor angiogenesis. Finally, Axl knockdown with retrovirally delivered short hairpin RNA (shRNA) has been shown to reduce the growth of xenografts derived from human breast MDA-MB-231 cells in vivo . Consequently, Axl kinase is a promising potential target for therapeutic intervention. The Cancer Research Technology Development Laboratories has recently screened its library of approximately 56,000 diverse small molecule compounds for inhibitors of Axl. The IMAP-based biochemical screen, using kinase domain human Axl and an ATP concentration of 135 µM revealed two distinct chemical series which progressed into a hit to lead programme. Representatives of both series inhibited Axl kinase with nanomolar potency (lowest IC 50 values of 10 and 31 nM respectively) and had drug-like properties. Subsequently, human breast adenocarcinoma MDA-MB-231 cells stimulated with human Gas6 were incubated with derivatives from these series and levels of phosphoAxl and pSer 473 Akt determined as surrogate biomarkers of Axl kinase activity. Several agents with sub-micromolar activity in these cell-based assays are currently progressing into the lead optimisation stage of the project.