COMBINED DRUG SCREENING AND PHOSPHOPROTEOMICS IDENTIFIES CANDIDATE BRAIN TUMOR THERAPEUTICS AND NOVEL TARGETS IN PRIMARY HUMAN BRAIN TUMOR-INITIATING CELLS

BACKGROUND: “Therapeutic Targeting of Glioblastoma” is a new pan-Canadian research team of the Terry Fox Research Institute and the Canadian Stem Cell Network funded to discover efficacious therapeutics for GBM. The team's goals are also to discover novel signaling pathways regulating GBM cell survival and genetic alterations that mediate drug resistance. As a platform, we use our collection of over 100 primary GBM tumor-initiating lines (BTIC) that are subjected to drug screening by over 1400 compounds, and to genetic and phosphoproteomic analysis. METHODS: We performed drug screening of 21 BTIC lines that represent the molecular heterogeneity of GBM patients, with a library of 110 clinically-relevant kinase inhibitors at three concentrations. We have also completed a phosphotyrosine characterization of 14 BTIC lines using phosphoproteomics. RESULTS: Multiple compounds that exhibit nanomolar cytotoxicity towards all of the lines were prioritized based upon their potency, novelty for GBM, ability to accumulate to relevant concentrations in brain, clinical status and cytotoxicity towards normal cells. Of the several hit compounds that fulfilled all of these criteria, the CDK inhibitor dinaciclib was chosen as our lead compound. We are currently investigating its mechanism of action using phosphoproteomics and RNAi as well as testing its efficacy in vivo in an orthotopic xenograft model as a single agent and in combination with TMZ. Furthermore, we have identified several compounds that exhibit selective cytotoxicity towards only a specific set of BTIC lines, suggesting that genetic differences between the lines account for differential sensitivities. The sensitivity of BTICs to one such compound, EMD-1214063, a selective MET inhibitor, may correlate with Met amplification status. Finally, phosphoproteomics results show activation of RTKs (EGFR, FGFR, PDGFRA, Eph receptor family) and non-receptor kinases (FYN, FAK1, GSK3b, MAPK, CDKs, DYRK) in the majority of BTIC lines, and of adaptor/scaffold proteins and proteins important in adhesion and migration. We are now (1) determining whether drugs that inhibit the activity of identified phosphoproteins suppress BTIC survival in culture and in orthotopic models, (2) validating the importance of those phosphoproteins in GBM survival, and (3) asking whether the prioritized drug hits from our drug screens suppress the phosphorylation of the identified phosphoproteins. CONCLUSIONS: We anticipate that our drug screening approach will generate several clinical candidates that will enable us to proceed to the clinic in the next several years. We also believe that our phosphoproteomics results will lead to identification of novel therapeutic targets for GBM. SECONDARY CATEGORY: Preclinical Experimental Therapeutics.