ATPS-55INFLUENCE OF MAPK AND PI3K PATHWAY MUTATIONS ON RESPONSE TO TARGETED INHIBITORS

RTK/MAPK/PI3K pathway genes are mutated in 90% of glioblastomas. It remains unclear whether targeted inhibitor response is predicted by mutation status. We addressed this issue using isogenic, immortalized mouse and human astrocytes with and without MAPK and/or PI3K mutations. MEK inhibitors showed variable potency (GSK1120212 > PD0325901 > AZD6244) in vitro. Efficacy was independent of MAPK mutations, but increased 2–66 fold with PI3K mutations. In contrast, PI3K inhibitor efficacy (BKM120 > LY294002) was increased 1.5-5 fold when MAPK, PI3K, or both were mutated. We used multiplexed inhibitor bead-mass spectrometry kinome profiling to show that monotherapy efficacy is limited by adaptive kinome responses, including inhibited pathway reactivation and bypass pathway activation. BKM120 induced sustained inhibition of proximal PI3K in mouse astrocytes with mutated MAPK/PI3K (TRP), but distal PI3K reactivation and MAPK activation limited its efficacy. Conversely, AZD6244 induced sustained MAPK inhibition, but also activated PI3K. These findings suggested that MAPK and PI3K were reciprocal, bypass pathways that promote monotherapy resistance. However, dual AZD6244/BKM120 therapy was synergistic in vitro at therapeutic concentrations in cells with mutated MAPK, PI3K, or both. Next, we examined efficacy in an orthotopic, TRP astrocyte allograft model of glioblastoma using maximum tolerated doses of AZD6244 (37 mpk), BKM120 (30 mpk), or both (reduced to 18 and 25 mpk due to toxicity). AZD6244 delayed tumor growth and improved survival compared to control, but BKM120 and dual treatment did not (doubling time 4.1, 2.2, 2.3 vs. 2.3 days; median survival 27, 24, 21 vs. 25 days). Only AZD6244 inhibited targeted pathway signaling in treated tumors, suggesting that BKM120 or dual therapy efficacy was limited by drug bioavailability or potency. Taken together, these results indicate that dual therapy, particularly in tumors with genetically activated MAPK and PI3K signaling, may prove beneficial if sufficient drug bioavailability and potency is achieved.