Inhibition of PI3K/mTOR Pathways with GDC-0980 in Pediatric Leukemia: Impact on Abnormal FLT-3 Activity and Cooperation with Intracellular Signaling Targets

Background: GDC-0980 is a selective small molecule inhibitor of class I PI3K and mTOR pathway with a potent anti-proliferative activity. Objective: We set out to evaluate the efficacy of GDC-0980, in pre-clinical studies, against pediatric leukemia cells. Methods: The anti-neoplastic activity of GDC-0980 was evaluated in vitro using five different pediatric leukemia cells. Results: Our data show that GDC-0980 significantly inhibited the proliferation of leukemia cell lines, KOPN8 (IC50, 532 nM), SEM (IC50 ,720 nM), MOLM-13 (IC50 ,346 nM), MV4;11 (IC50 ,199 nM), and TIB-202 (IC50, 848 nM), compared to normal control cells (1.23 mu M). This anti-proliferative activity was associated with activation of cellular apoptotic mechanism characterized by a decrease in Bcl-2 protein phosphorylation and enhanced PARP cleavage. Western blot analyses of GDC-0980 treated cells also showed decreased phosphorylation levels of mTOR, Akt and S6, not ERK1/2. Notably, FLT3 phosphorylation was decreased in Molm-13 and MV4;11 cells following the application of GDC-0980. We further examined cellular viability of GDC-0980-treated primary leukemia cells isolated from pediatric leukemia patients. This study revealed a potential therapeutic effect of GDC-0980 on two ALL patients (IC50 's, 1.23 and 0.625 mu M, respectively). Drug combination analyses of GDC-0980 demonstrated a synergistic activity with the MEK inhibitor Cobimetinib (MV4-11; 11, CI, 0.25, SEM, CI, 0.32, and TIB-202, CI, 0.55) and the targeted FLT3 inhibitor, Crenolanib (MV4-11; 11, CI, 0.25, SEM, CI, 0.7, and TIB-202, CI, 0.42). Conclusion: These findings provide initial proof-of-concept data and rationale for further investigation of GDC-0980 in selected subgroups of pediatric leukemia patients.