Therapeutic Utility of PI3Kγ Inhibition in Leukemogenesis and Tumor Cell Survival

Abstract 1492

Aberrant activation of the PI3K/Akt signaling pathway is a frequent event in cancer including various types of leukemia. Consequently, much emphasis has been placed on developing inhibitors that target this pathway. However, this would require an in depth knowledge of the role that specific class I PI3K isoforms (α, β, γ, δ)play in the pathogenesis of a particular hematological malignancy. For instance, PI3Kδ has been shown to be essential for the growth and survival of tumors derived from B cells such as chronic lymphocytic leukemia (CLL). Such knowledge has lead to development of the selective inhibitor GS-1101 (CAL-101) that has shown significant efficacy in clinical trials. Although PI3Kγ plays an important role in modulating the immune function of T cells, its role in leukemogenesis and tumor cell survival is poorly defined. Thus, it is unclear whether an inhibitor that also targets PI3Kγ would be of any benefit in hematological malignancies.

T cell acute lymphoblastic leukemia (T-ALL) is an aggressive cancer resulting from clonal proliferation of T lymphoid precursors. Previous reports suggest that hyperactivation of the PI3K/Akt signaling pathway is a common feature of this disease with the majority of cases due to the loss of function of the tumor suppressor PTEN. However, it remains to be determined whether any particular class I PI3K isoform predominates in T-ALL pathogenesis. We now report that in the absence of PTEN-mediated regulation in T cell progenitors that PI3Kγ can promote leukemogenesis even in the absence of its delta counterpart. However, inactivation of both isoforms was necessary for the suppression of tumor development in animals (< 20% dead at 220 days as compare to >85% for controls), suggesting that PI3Kα and/or PI3Kβ cannot adequately compensate for a deficiency in their γ/δ counterparts. The importance of PI3Kγ in tumor progression was established by the inability of the PI3Kδ selective inhibitor IC87114 to reduce tumor burden in mice (Fig. 1A). In contrast, treatment of PI3Kγ deficient tumors with the same inhibitor dramatically reduced disease in affected tissues (Fig. 1B).

Based on these observations we developed an inhibitor, designated CAL-130, which targets both PI3Kγ and PI3Kδ in an attempt to exploit the addiction of PTEN null T-ALL tumors to both isoforms. IC₅₀ values of this compound were 1.3 nM and 6.1 nM for p110δ and p110γ catalytic domains, respectively, as compared to 115 nM and 56 nM for p110α and p110β. Importantly, this small molecule does not inhibit additional intracellular signaling pathways (>300 kinases tested) that are critical for general cell function and survival. Oral administration of this compound to diseased mice (blast counts > 50 million/ml) for 7 days reduced tumor burden and extended median survival of treated animals to 45 day as compared 7.5 days for the control group (P<0.001). Of note, this inhibitor did not perturb plasma insulin or glucose levels in contrast to the metabolic perturbations associated with tissue-specific deficiencies in PI3Kα and PI3Kβ. The efficacy of this dual inhibitor was not limited to murine tumors as dual inhibition of PI3Kγ and PI3Kδ in primary human T-ALL cells displaying hyperactivation of this signaling pathway also reduced tumor cell survival by promoting activation of pro-apoptotic pathways. This work advances our understanding of the role that distinct PI3K isoforms play in development and survival of T-ALL and suggest that it may be possible to therapeutically exploit the addiction of this hematological malignancy to PI3Kγ and PI3Kδ. Moreover, by selectively targeting a signaling pathway key to tumor survival, it may be possible to limit toxicities associated with conventional chemotherapeutic agents that broadly affect metabolic pathways and DNA replication. Current studies are focused on evaluating the synergistic effect of PI3Kγ/δ blockade in combination with conventional chemotherapeutic agents used in the treatment of T-ALL.

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