mTOR Kinase Inhibitors Enhance Efficacy of TKIs in Preclinical Models of Ph-like B-ALL

Background and Rationale:

B-lymphoblastic leukemia (B-ALL) is the most common cancer of childhood. While event-free survival (EFS) exceeds 85% for most patients treated with contemporary therapy, outcomes are very poor for children who relapse, highlighting a need for new treatments. In particular, children with Philadelphia chromosome-like (Ph-like) B-ALL (who lack BCR-ABL1 rearrangement) have high rates of relapse and mortality with conventional chemotherapy. Transcriptional profiling and genomic sequencing of Ph-like ALL specimens have identified a variety of alterations that activate oncogenic kinase signaling, including rearrangements (R) of CRLF2, ABL1, and PDGFRB. Addition of the tyrosine kinase inhibitor (TKI) imatinib to chemotherapy has dramatically improved EFS for patients with BCR-ABL1-rearranged (Ph+) B-ALL, and it is hypothesized that TKI addition to therapy will similarly improve outcomes for patients with Ph-like ALL. Our prior preclinical studies in Ph+ B-ALL demonstrated enhanced efficacy of combining TKIs (imatinib or dasatinib) with mTOR kinase inhibitors (TOR-KIs) (Janes et al., Nature Medicine 2010; Janes et al, Leukemia2013). In the current studies, we hypothesized that dual kinase inhibitor therapy would have superior anti-leukemia cytotoxicity in Ph-like ALL and thus investigated combined TKI and TOR-KI treatment using patient-derived xenograft (PDX) models of childhood Ph-like ALL.

Methods:

For in vitro studies, viably cryopreserved leukemia cells from established ABL1-R Ph-like ALL PDX models (2 ETV6-ABL1) were incubated with the TKI dasatinib, TOR-KIs, or both TKI + TOR-KI for 72 hours prior to flow cytometric assessment of cellular viability via Annexin V and propidium iodide staining. Two chemically distinct TOR-KIs (MLN0128 or AZD2014) were used to confirm on-target effects. Additional primary ABL1-R or PDGFRB-R Ph-like ALL specimens were plated in methylcellulose without or with inhibitors in colony-forming assays. Phosphoflow cytometry (PFC) analysis of ALL cells incubated with inhibitors was also performed to measure the ability of TKIs and TOR-KIs to inhibit intracellular ABL1 and PI3K/mTOR signaling pathways. For in vivo studies, Ph-like ALL PDX models were treated with dasatinib, the TOR-KI AZD8055, or both drugs via daily oral gavage for 8 days. Human CD19+ ALL was quantified in murine spleens and bone marrow at end of treatment with quantification of cycling cells by EdU incorporation. PFC analysis of murine bone marrow was also performed 2 hours after drugs were dosed, to measure in vivo inhibition of signaling proteins.

Results:

Combined in vitro treatment with dasatinib and MLN0128 or AZD2014 decreased cellular viability more than inhibitor monotherapy. Similarly, in a set of CRLF2-rearranged samples, mTOR inhibitors augmented killing by the JAK2 inhibitor BBT-594. Incubation of primary ABL1-R or PDGFRB-R ALL cells with both dasatinib and AZD2014 more robustly inhibited colony formation than did inhibitor monotherapy. In in vitro PFC analyses of ABL1-R samples, we observed expected dasatinib-induced inhibition of phosphorylated (p) STAT5. Inhibition of the mTOR substrate pS6 was observed with dasatinib, MLN0128, and AZD2014 with more complete inhibition achieved when dasatinib combined with either MLN0128 or AZD2014. Similarly, in vivo treatment of PDX models with dasatinib and AZD8055 reduced leukemia burden and pS6 signaling more completely than either inhibitor alone. Importantly, dual inhibition decreased the percentage of cycling human ALL cells in murine bone marrow, but preserved cycling in normal mouse bone marrow cells in the same animals. Our data thus provide additional compelling preclinical rationale for combined inhibitor therapy with TKIs and TOR-KIs in Ph-like ALL.