PI3Kδ Inhibition Enhances Sensitivity of Primary High-Risk Childhood B-Cell Acute Lymphoblastic Leukemia Cells to Glucocorticoid Chemotherapy

Introduction: Despite excellent 5-year survival for most patients with B-cell acute lymphoblastic leukemia (B-ALL), those with high-risk (HR) B-ALL continue to have inferior outcomes. The prognosis is even worse for patients who respond poorly to chemotherapy, and improvements to current therapy approaches are needed. Glucocorticoids (GCs) are a key component of chemotherapy regimens. Rapid response to GC monotherapy is a well-documented predictor of favorable outcomes, while slow response to GCs correlates with poor outcomes. We hypothesized that enhancing specific GC toxicity to B-ALL cells can improve clinical outcomes for high-risk patients without increasing systemic toxicity. PI3Kδ is a promising target for this purpose. PI3Kδ is a critical component of the B-cell receptor cell-survival pathway; it restrains GC signaling, while GCs repress its expression. PI3Kδ expression is restricted to lymphoid cells, making it an appealing precision target to enhance GC toxicity in lymphoid cells while minimizing risk of increased systemic GC-associated toxicity. We previously demonstrated synergy between dexamethasone (dex) and the FDA-approved isoform-selective PI3Kδ inhibitor idelalisib (idela) in B-ALL cell lines, patient specimens, and patient-derived xenograft models (Kruth et al Blood 2017). In clinical practice, both dex and prednisone (pred) are essential components of childhood ALL chemotherapy. In this study, we sought to elucidate the extent to which idela in combination with dex or pred could induce synergistic killing of HR B-ALL specimens with various genetic backgrounds.

Methods: Peripheral blood or bone marrow from patients ages 1-31 years with newly-diagnosed NCI HR or standard-risk (SR) B-ALL at the University of Iowa Stead Family Children's Hospital was obtained after informed consent on an Institutional Review Board-approved research study. Mononuclear cells were isolated from the specimens via Ficoll gradients and cultured in vitro in 384-well plates. Immediately after isolation, cells were treated with serial dilutions of the GCs dex or pred and idela in RPMI1640+10% FBS medium. Cell viability was measured with PrestoBlue (Invitrogen) after 72 hours of treatment. Survival curves were plotted to determine the LD50 of each drug using GraphPad. Dose effects around the LD50 for each drug were analyzed using Compusyn to determine if the drug combination was synergistic (combination index, CI <1), additive (CI=1), or antagonistic (CI >1).

Results: As of August 1, 2019, 5 HR B-ALL patient specimens of various genetic alterations have been tested. Two patients were high-risk at diagnosis due to steroid pre-treatment (MAP011) or by NCI risk criteria (age >10 years and/or WBC count >50K/mm³; MAP014). Three patients with initially SR B-ALL were upstaged due to minimal residual disease (MRD) >1% at day 8 in peripheral blood (MAP012) or marrow MRD ≥0.01% at end of induction (MAP009, MAP010). In our in vitro studies, only one sample (MAP014) showed reduced viability with GC alone, while 4 specimens (all except MAP012) responded to idela alone. Most specimens showed a synergistic response to dex or pred in combination with idela with greater reduction in cell viability compared to GC monotherapies. Surprisingly, one specimen (MAP010) had an antagonistic response with pred and idela combination. This near-haploid B-ALL specimen had reduced viability with idela monotherapy, but an antagonistic response with single-agent pred, raising the question of whether GC/idela synergy is influenced by genetic background.

Conclusions: In pilot studies, idela sensitized HR B-ALL cells to GC chemotherapy and is a promising strategy for further evaluation in a larger cohort of specimens. Our results highlight the potential impact of genetic heterogeneity within childhood B-ALL upon differential therapeutic responses to standard steroid chemotherapy and demonstrate a potential precision medicine approach to augment GC sensitivity via combination with selective PI3Kδ inhibition.

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