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J.P. Loftus et al.
A
B
Figure 3. In vitro activity of entospletinib in KMT2A-R acute lymphoblastic leukemia (ALL). Viably cryopreserved KMT2A-R ALL PDX cells were exposed in vitro to 0.1% DMSO (vehicle control) or increasing concentrations of entospletinib (200 nM, 500 nM, 1 uM) for 2 hours, then lysed and analyzed by Simple Western. Additional untreated (baseline) cells were lysed immediately following sample thaw. (A) Dose-dependent inhibition of the specified phosphoproteins was observed with ENTO in the ALL135MR PDX model (KMT2A-MLLT1, RAS wild-type), while (B) no treatment effect was seen in the 142MR PDX model (KMT2A-AFF1, NRAS-mutant).
of SFK signaling in CRLF2-rearranged Ph-like ALL with in vitro and in vivo sensitivity to the kinase inhibitor dasa- tinib44,45 and hypothesize that the observed ENTO sensi- tivity in our CRLF2-R infant ALL models could be due to a similar mechanism and signaling dependency.
Superior preclinical activity of combined SYK and MEK inhibition in KMT2A-R acute lymphoblastic leukemia patient-derived xenograft models
Given the surprising observed lack of ENTO activity in our RAS-mutant KMT2A-AFF1 infant ALL PDX models, we hypothesized that dual treatment with ENTO and a MEK inhibitor (MEKi) would have superior therapeutic effects. To test this prediction, we treated RAS-mutant (ALL142MR; infant) and RAS wild-type (ALL3113MR; adult) KMT2A-AFF1 ALL PDX models with ENTO, selumetinib (SEL), or both kinase inhibitors and quantified ALL cell counts in peripheral blood during treatment and in end-study spleens. As expected,40,46 single-agent SEL treatment of the RAS-mutant ALL142MR model apprecia-
bly decreased leukemia burden and augmented anti-ALL effects in combination with ENTO (Figure 6A). Despite its lack of RAS mutation, the ALL3113 model was surprising- ly sensitive to SEL monotherapy41,48 and potent in vivo activity with near-complete leukemia clearance was observed with dual ENTO and SEL treatment (Figure 6B). These in vivo efficacy data in both RAS-mutant and wild- type models, and our additional demonstration of consti- tutive pERK levels and ex vivo signaling inhibition in end- study spleens of both ALL142MR and ALL3113 models (Figure 6C), suggest that MEK inhibition may be a relevant therapeutic strategy for KMT2A-R ALL irrespective of RAS mutation status and may augment SYK inhibitor effectiveness.
Discussion
SYK pathway activation plays a central role in the pro- liferation and survival of malignant B cells, implicating
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haematologica | 2021; 106(4)