Myosin Light-Chain Kinase Inhibition Potentiates the Antitumor Effects of Avapritinib in PDGFRA D842V-Mutant Gastrointestinal Stromal Tumor

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.

Abstract P050: Oncogenic kinase therapy restricts CD8 T cell differentiation and clonal expansion

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<0.05). At 1 and 4 weeks of imatinib therapy, flow cytometry of intratumoral CD8 T cells demonstrated increased CCR7 and CD127 and decreased CD69, Tbet, Granzyme B, and Ki67 (p<0.05), confirming an intratumoral phenotypic transition from an activated/effector subtype. Imatinib therapy reduced p-AKT and p-mTOR MFI in intratumoral CD8 T cells by flow cytometry (p<0.05), signifying a deficiency in TCR signaling and/or co-stimulation. TCRB sequencing after 1 week of imatinib treatment showed no difference in T cell clonality, indicating T cells failed to undergo clonal expansion. scRNAseq in tumors from KitV558Δ/+ mice established that IL-15R and IL-15 were primarily expressed on Batf3 dendritic cells and macrophages, while both IL-2R beta and gamma were expressed on NK and T cells. Although IL15/1L15Rα alone had no anti-tumor efficacy, combining IL15/IL15Rα with imatinib improved the anti-tumor response as measured by tumor weight (p<0.05). After combination treatment, CD8 T cells evaluated by flow cytometry exhibited a significant increase in granzyme B, Tbet, CD69, and Ki67, and PI3K signaling was restored (p<0.05). Conclusions: Imatinib therapy in GIST fails to induce T cell clonal expansion and results in a reduced CD8 activation/effector profile. IL15/IL15Rα therapy improves upon the effects of imatinib by rescuing CD8 T cell PI3K signaling and effector response.

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.

The fistula crisis in sub-Saharan Africa: an ongoing struggle in education and awareness

This article examines the social and physical causes of obstetric fistulas, as well as resulting social and psychological consequences. Preventative strategies are addressed using Niger as a microcosm for this devastating condition that affects two million women worldwide.