Targeting mTOR and JAK2 in Xenograft Models of CRLF2-Overexpressing Acute Lymphoblastic Leukemia (ALL)
Authors: Maude Shannon L , Tasian Sarah K , Vincent Tiffaney , Hall Junior W , Collins-Underwood Racquel , Mullighan Charles G , Hunger Stephen P , Willman Cheryl L , Loh Mignon L , Grupp Stephan A , Teachey David T 
Published: October 2019

Abstract
Abstract 249
CRLF2 genomic rearrangements that lead to overexpression have been identified in a subset of children with clinically high-risk B-precursor ALL and are highly associated with activating JAK2 mutations (Harvey et al., Blood 2010; Mullighan et al., PNAS 2009 and Nat Genet 2009). These children frequently respond poorly to current intensive cytotoxic chemotherapy regimens and suffer high rates of relapse and mortality. New therapies for these patients are urgently needed. These leukemias exhibit gene expression profiles similar to those of BCR-ABL1 positive ALL, suggesting aberrant kinase activation. We previously demonstrated aberrant JAK/STAT and PI3K/mTOR signaling in CRLF2-overexpressing ALL cell lines and primary human samples in vitro, and thus hypothesize that inhibition of these hyperactive signaling networks has therapeutic relevance.
To further characterize this high-risk subset of ALL, we have established multiple xenograft models of CRLF2-rearranged and JAK2-mutated ALL, providing a robust platform for preclinical testing of signal transduction inhibitors. In this model, primary human ALL samples are intravenously injected into NOD/SCID/γc null (NSG) mice, and engraftment is determined by flow cytometry of peripheral blood for human CD19+/CD45+ blasts. Eighteen of 21 primary cryopreserved specimens provided by the Children's Oncology Group engrafted successfully. In order to biochemically characterize the xenografts, we measured phosphorylation of relevant signal transduction proteins by phosphoflow cytometry. Spleens of mice xenografted with CRLF2-rearranged ALL had uniformly increased surface staining of human CRLF2, a component of the heterodimeric receptor complex for the cytokine, thymic stromal lymphopoeitin (TSLP). In vitro stimulation of the CRLF2-overexpressing ALL xenograft spleens with TSLP induced phosphorylation of STAT5, Akt, S6, and 4EBP1, but not of ERK 1/2. In additon, in vitro JAK inhibition with INCB018424 abrogated TSLP-induced JAK/STAT and PI3K/mTOR signaling. The mTOR inhibitor sirolimus, mTORC1/2 inhibitor PP242, and PI3K/mTOR inhibitor PI-103 potently inhibited phosphorylation of S6 and 4EBP1 in these xenograft specimens. These data suggest that the JAK/STAT and PI3K/mTOR pathways may interact in these CRLF2-overexpressing leukemias.
These data led us to hypothesize that inhibition of the PI3K/mTOR or JAK/STAT pathways may represent potential therapeutic targets; therefore, we utilized these very high-risk ALL xenograft models to study novel, targeted therapies. Once xenografts had engrafted with sufficient disease burden to detect >5% peripheral CD19+/CD45+ blasts, mice were randomized to receive the mTOR inhibitor sirolimus, the JAK inhibitor INCB018424, or vehicle for three to four weeks. Disease burden was assessed weekly by flow cytometric determination of CD19+/CD45+ blast count in peripheral blood, and at sacrifice, by spleen CD19+/CD45+ blast count. To assess potential differential efficacy based on CRLF2 overexpression (CRLF2+) and/or JAK2 activating mutations (JAK2mut), we treated mice with each subtype of ALL. Sirolimus induced a significant decrease in peripheral blast count in 7 of 7 primary ALL xenografts tested (2 JAK2mut/CRLF2+ samples, 1 JAK2mut/CRLF2- sample, 2 JAK2wt/CRLF2+ samples, and 2 JAK2wt/CRLF2- samples) and a significant decrease in spleen blast count in 6 of 7 samples tested. The most profound reduction of disease burden was seen in the JAK2mut/CRLF2+ leukemias. In addition, the JAK inhibitor INCB018424 decreased peripheral blast count and spleen blast count in a JAK2mut/CRLF2+ xenograft. We next determined if sirolimus conferred a survival advantage in xenografts of 2 ALL specimens, a robust responder and an intermediate responder to sirolimus by blast count. Sirolimus treatment significantly prolonged survival of both xenografts (63 days vs. 23 days, p=0.0015; 91 days vs. 58 days, p=0.0027). Additional human ALL xenograft studies of INCB018424 and other kinase inhibitors are ongoing.
The preclinical in vivo efficacy of sirolimus and INCB018424 suggests that novel, targeted therapies have therapeutic potential in CRLF2-overexpressing ALL. Based in part on these data, both INCB018424 and temsirolimus (a parenteral ester of sirolimus) are currently being investigated in multi-center early phase clinical trials for children with relapsed or refractory leukemias.

Disclosures:
No relevant conflicts of interest to declare.