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Purpose: To create an in vivo model of PDGFRA D842V-mutant gastrointestinal stromal tumor (GIST) and identify the mechanism of tumor persistence following avapritinib therapy. Experimental Design: We created a patient-derived xenograft (PDX) of PDGFRA D842V-mutant GIST and tested the effects of imatinib, avapritinib, and ML-7, an inhibitor of myosin light-chain kinase (MYLK). Bulk tumor RNA sequencing and oncogenic signaling were evaluated. Apoptosis, survival, and actin cytoskeleton were evaluated in GIST T1 cells and isolated PDX cells in vitro. Human GIST specimens were analyzed for MYLK expression. Results: The PDX was minimally responsive to imatinib but sensitive to avapritinib. Avapritinib therapy increased tumor expression of genes related to the actin cytoskeleton, including MYLK. ML-7 induced apoptosis and disrupted actin filaments in short-term cultures of PDX cells and decreased survival in GIST T1 cells in combination with imatinib or avapritinib. Combined therapy with ML-7 improved the antitumor effects of low-dose avapritinib in vivo. Furthermore, MYLK was expressed in human GIST specimens. Conclusions: MYLK upregulation is a novel mechanism of tumor persistence after tyrosine kinase inhibition. Concomitant MYLK inhibition may enable the use of a lower dose of avapritinib, which is associated with dose-dependent cognitive side effects.

Supplemental Figure 5.Short term culture of cells derived from PDGFRA D842V PDXs are tumor cells.

Supplemental Figure 1.

Supplemental Figure 8.Relative MYLK mRNA expression by qPCR in GIST PDGFRA D842V PDXs treated with vehicle(Veh) or imatinib (IM) for 2 weeks.

Supplemental Figure 3. GIST PDGFRA D842V PDXs are not sensitive to crenolanib.

Supplemental Figure 7.

Supplemental Figure 4.

Purpose:To create an in vivo model of PDGFRA D842V-mutant gastrointestinal stromal tumor (GIST) and identify the mechanism of tumor persistence following avapritinib therapy.Experimental Design:We created a patient-derived xenograft (PDX) of PDGFRA D842V-mutant GIST and tested the effects of imatinib, avapritinib, and ML-7, an inhibitor of myosin light-chain kinase (MYLK). Bulk tumor RNA sequencing and oncogenic signaling were evaluated. Apoptosis, survival, and actin cytoskeleton were evaluated in GIST T1 cells and isolated PDX cells in vitro. Human GIST specimens were analyzed for MYLK expression.Results:The PDX was minimally responsive to imatinib but sensitive to avapritinib. Avapritinib therapy increased tumor expression of genes related to the actin cytoskeleton, including MYLK. ML-7 induced apoptosis and disrupted actin filaments in short-term cultures of PDX cells and decreased survival in GIST T1 cells in combination with imatinib or avapritinib. Combined therapy with ML-7 improved the antitumor effects of low-dose avapritinib in vivo. Furthermore, MYLK was expressed in human GIST specimens.Conclusions:MYLK upregulation is a novel mechanism of tumor persistence after tyrosine kinase inhibition. Concomitant MYLK inhibition may enable the use of a lower dose of avapritinib, which is associated with dose-dependent cognitive side effects.

Supplemental Figure 6.Cell survival of GIST T1 cells treated with imatinib or avapritinib for 72h in complete media.Each data point represents an average of 10 wells.

Introduction: Targeted therapy with tyrosine kinase inhibitors (TKI) including imatinib is effective in treating gastrointestinal stromal tumor (GIST), but resistance often develops within 18 months. Despite a robust immune T cell infiltrate, combining TKIs with immune checkpoint inhibitors has largely been unsuccessful in treating advanced GIST. The superagonist IL15/1L15Rα has shown promise in a number of cancer types. The objective of this study was to determine the effects of TKI therapy on intratumoral T cells in GIST and evaluate IL15/1L15Rα as an adjunctive therapy. Methods: KitV558Δ/+ mice were treated with imatinib (600mg/L in water) and/or IL15/IL15Rα (2ug/12ug/dose, 3x/week), for 1-4 weeks and evaluated by TCRB sequencing (Immunoseq), RNA-seq, and flow cytometry (n=4-5/group). Intratumoral immune cells from KitV558Δ/+ mice were evaluated by single cell RNA sequencing (scRNAseq) (n=3). Results: RNA-seq of sorted CD8 T cells following 1 week of imatinib treatment suggested intratumoral CD8 T cells gain antigen experience but lose their effector profile, as cd44, sell, and tcf7 increased while gzmb and tnfrsf9 decreased (p Citation Format: Andrew Tieniber, Andrew Hanna, Benjamin Medina, Kevin Do, Lillian Levin, Ferdiando Rossi, Ronald DeMatteo. Oncogenic kinase therapy restricts CD8 T cell differentiation and clonal expansion [abstract]. In: Abstracts: AACR Virtual Special Conference: Tumor Immunology and Immunotherapy; 2021 Oct 5-6. Philadelphia (PA): AACR; Cancer Immunol Res 2022;10(1 Suppl):Abstract nr P050.