In Vivo Efficacy of PI3K Pathway Signaling Inhibition for Philadelphia Chromosome-Like Acute Lymphoblastic Leukemia

The Philadelphia chromosome (Ph)-like subtype of B-precursor acute lymphoblastic leukemia (ALL) comprises approximately 15% of high-risk ALL, has a kinase-activated gene expression profile similar to that of BCR-ABL1-rearranged ALL, and is associated with a variety of mutations and gene fusions known or predicted to activate oncogenic signal transduction. Children and adults with Ph-like ALL have a very high risk of relapse and poor survival when treated with conventional chemotherapy. Others and we have previously observed constitutive activation of cytokine receptor signaling in Ph-like ALL, particularly of the JAK/STAT and PI3K/Akt/mTOR pathways (Tasian et al., Blood 2012). Preclinical and early clinical studies of JAK inhibition in childhood ALL are in progress. However, the functional role of aberrant PI3K pathway signaling has not been previously investigated in Ph-like ALL. The clinical efficacy of the mTOR inhibitor rapamycin and its analogues has proven suboptimal in various solid and hematologic malignancies, at least in part due to upregulation of Akt signaling, a known sequela of signal transduction inhibitor (STI) monotherapy and a common resistance mechanism. We hypothesized that newer-generation STIs that target multiple PI3K pathway signaling proteins or that selectively inhibit PI3K isoforms may result in superior inhibition of leukemia proliferation and minimize upregulation of alternate signaling pathways. We used patient-derived xenotransplantation models to determine the effects of PI3K pathway STIs upon NOD-SCID-γ-null (NSG) mice well-engrafted with de novo (n = 3) or relapsed (n = 1) childhood Ph-like ALL specimens with JAK2 mutations and/or CRLF2 alterations (Maude et al., Blood 2012). Specifically, we tested the PI3Kα inhibitor BYL719 (30 mg/kg/day), the PI3Kδ inhibitor CAL101 (idelalisib; 30 mg/kg/day), the PI3K/mTOR inhibitor PKI587 (10 mg/kg/day), and the TORC1/TORC2 inhibitor AZD2014 (20 mg/kg/day) to identify the most efficacious PI3K pathway inhibitor(s). Initial pharmacodynamic studies demonstrated that mice treated with each of the four STIs for 72 hours demonstrated potent in vivo inhibition of relevant phosphoproteins in comparison to vehicle-treated mice as measured by phosphoflow cytometric analyses of gated human ALL cells within murine spleens. In particular, both BYL719 and CAL101 treatments resulted in marked inhibition of phosphorylated (p) PI3K, mTOR, S6, and AktS473 via comparison of median fluorescent intensities for STI- vs. vehicle-treated groups with the Mann-Whitney test (p <0.01 for all phosphoproteins). Increased phosphorylation of other measured proteins was not observed, suggesting that proximal inhibition effectively abrogated aberrant PI3K pathway signal transduction with minimal compensatory signaling upregulation. PKI587 treatment robustly inhibited pS6 and p4EBP1 in comparison to vehicle-treated mice (p = 0.001 and 0.003, respectively), but, surprisingly, had minimal effects upon upstream phosphoproteins. AZD2014 inhibited pS6, p4EBP1, and pAktS473, as well as pERK (p < 0.05 for all phosphoproteins). In longer-term therapeutic trials, TORC1/TORC2 inhibition induced stable disease in xenografted mice treated for 4 weeks in comparison to vehicle controls (p < 0.005), while PI3K/mTOR inhibition robustly decreased leukemic burden below pre-treatment levels in blood, bone marrow, and spleen versus vehicle-treated mice (p < 0.001). These studies demonstrate that PI3K pathway inhibition is an effective and biochemically relevant therapeutic strategy for Ph-like ALL. Pharmacodynamic studies and therapeutic trials of the four PI3K pathway STIs are currently ongoing in additional xenograft models of ALLs expressing various defined genetic lesions to delineate the potential therapeutic range of these compounds. Results from these studies will help to improve our understanding of the critical biologic mechanisms involved in Ph-like ALL and to inform our development of clinical trials to test STI-based therapies in patients with these high-risk leukemias. Disclosures: No relevant conflicts of interest to declare.