LAUSR

Introduction: While children with acute lymphoblastic leukemia (ALL) experience close to a 90% likelihood of cure, the outcome for certain high-risk pediatric ALL subtypes remains poor. Spleen Tyrosine Kinase (SYK), a cytosolic nonreceptor tyrosine kinase, is primarily expressed in the hematopoietic lineage and is essential for B-cell receptor signaling. It is associated with malignant transformation, cancer cell proliferation, and is a prominent tyrosine-phosphorylated protein in B-lineage pediatric ALL (Dolai et al, Cancer Res 76:2766-77, 2016). Fms Related Receptor Tyrosine Kinase 3 (FLT3) is a Class III receptor tyrosine kinase that regulates hematopoiesis. While uncommon in ALL, activating FLT3 mutations and internal tandem duplications are associated with poor outcome. TAK-659 is a dual SYK/FLT3 reversible inhibitor currently undergoing clinical trials against B-cell lymphoma, acute myeloid leukemia and solid tumors. Therefore, it was of interest for the Pediatric Preclinical Testing Consortium to test TAK-659 in vivo against its patient-derived xenograft (PDX) models of pediatric ALL. Methods: SYK and FLT3 mRNA expression in ALL PDXs was quantified by RNAseq (https://pedcbioportal.org) and 8 B-lineage pediatric ALL PDXs (3 B-cell precursor, BCP; 4 mixed-lineage leukemia-rearranged, MLLr; one Philadelphia Chromosome-like, Ph-like) were selected for testing. Engraftment was evaluated by enumerating the proportion of human CD45+ cells (%huCD45+) in the peripheral (PB) blood at weekly intervals. TAK-659 was tested at 60 mg/kg by oral gavage daily for 21 days. Events were defined as %huCD45+ ≥ 25%. At Day 28 post-treatment initiation or event (whichever occurred first), 4 mice/group were humanely killed to assess leukemia infiltration in the spleen and bone marrow (BM). Drug efficacy was assessed by event-free survival (EFS) of treated (T) and control (C) groups by T-C, T/C and stringent objective response measures (Houghton et al, Pediatr Blood Cancer 49:928-40, 2007). Results: The in vivo efficacy of TAK-659 was evaluated against B-lineage PDXs as FLT3 and SYK mRNA expression was higher in the PDXs from this lineage compared with PDXs from T-ALL. TAK-659 was well tolerated and significantly prolonged the time to event in 6/8 PDXs (T-C 0-25.5 days, T/C 1.0-2.2). Only one MLLr-ALL obtained an objective response (partial response) with all other models exhibiting progressive disease. The minimum mean %huCD45+ was significantly reduced in 5/8 PDXs in TAK-659 treated mice compared to control. Despite significant reductions in %huCD45+ in the PB for 5/8 PDXs, TAK-659 did not significantly reduce the spleen or BM infiltration of any PDXs. Conclusion: TAK-659 exhibited low to moderate single-agent in vivo activity against pediatric B-ALL PDXs representative of diverse disease subtypes. (Supported by NCI Grants CA199000 and CA199922) Citation Format: Richard B. Lock, Kathryn Evans, Narimanne El-Zein, Eric J. Earley, Stephen W. Erickson, Beverly A. Teicher, Malcolm A. Smith. Pediatric preclinical testing consortium evaluation of the dual SYK/FLT3 inhibitor TAK-659 in xenograft models of pediatric acute lymphoblastic leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 3039.