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2788 Project Stella: Development and Preclinical Assessment of FOLR1-Directed Chimeric Antigen Receptor T Cells in CBF2AT3-GLIS2/RAM AML

Program: Oral and Poster Abstracts
Session: 703. Cellular Immunotherapies: Basic and Translational: Poster II
Hematology Disease Topics & Pathways:
Biological therapies, Chimeric Antigen Receptor (CAR)-T Cell Therapies, Therapies, Immunotherapy
Sunday, December 12, 2021, 6:00 PM-8:00 PM

Quy Le, PhD1*, Sommer Castro, BS1*, Thao T. Tang2*, Cynthia Nourigat-Mckay1*, LaKeisha Perkins1*, Tiffany A. Hylkema, BS1*, Laura Pardo3*, Jenny L. Smith, MSc, MEd1*, Amanda R. Leonti, MS1*, Rhonda E. Ries, MA1*, Lisa Eidenschink Brodersen, PhD1,3, Michael R. Loken, PhD3* and Soheil Meshinchi, M.D., Ph.D.1,4,5

1Clinical Research Division, Fred Hutchinson Cancer Research Center, Seattle, WA
2Clinical Research Division, Fred Hutchinson, Seattle, WA
3Hematologics, Inc, Seattle, WA
4Department of Pediatrics, University of Washington, Seattle, WA
5Children's Oncology Group, Monrovia, CA

Background: A rare but highly aggressive type of AML that is only seen in infants with a unique immunophenotype (RAM phenotype) is caused by cryptic CBFA2T3-GLIS2 (CBF/GLIS) fusion. This infant AML is highly refractory to conventional chemotherapy with near uniform fatality despite highly intensive and myeloablative therapy (PMID 23153540). Transcriptome profiling of CBF/GLIS AML has revealed new insights into the pathogenesis of the fusion and uncovered fusion-specific molecular biomarkers that could be used for risk stratification and to inform treatment (PMID 30592296). Studying the largest cohort of these high-risk infants, we demonstrated several alterations in gene expression and transcriptional networks in these CBF/GLIS-positive patient samples that have potential for therapeutic targeting (PMID 31719049). FOLR1, which encodes for folate receptor alpha, was highly and uniquely expressed in CBF/GLIS AML but was entirely absent in AML with other cytogenetics abnormalities and in normal hematopoietic cells. Furthermore, we recently demonstrated that forced expression of CBF/GLIS enhances the proliferation and alters differentiation in cord blood (CB) CD34+ early precursors towards megakaryocytic lineage that recapitulates acute megakaryocytic leukemia seen in infants (PMID 31719049). Of significance, we showed that FOLR1 surface expression is causally linked to CBF/GLIS-induced malignant transformation, thus making it an attractive antigen for targeted therapies against CBF/GLIS AML cells. Given that chimeric antigen receptor (CAR) T cells are extremely effective at eradicating relapsed/refractory B-ALL malignancies, we developed FOLR1-directed CAR T cells for pre-clinical evaluation in CBF/GLIS AML.

Methods: We generated a FOLR1-directed CAR using anti-FOLR1 binder (Farletuzumab), IgG4 intermediate spacer and 41-BB/CD3zeta signaling domains. The pre-clinical efficacy of FOLR1 CAR T cells was evaluated against CBF/GLIS AML cell lines in vitro and in vivo. CBF/GLIS AML models include CB CD34+ cells transduced with CBF/GLIS expression construct (CBF/GLIS-CB) and WSU-AML cell line. We also engineered Kasumi-1 cell line to express FOLR1 (Kasumi-1 FOLR1+) to evaluate target specificity (Figure 1A).

Results: We tested the target specificity of FOLR1-directed CAR T cells against FOLR1-positive (CBF/GLIS-CB, WSU-AML, Kasumi-1 FOLR1+) and FOLR1-negative (Kasumi-1) cells. CD8 FOLR1 CAR T cells demonstrated cytolytic activity against FOLR1 positive but not FOLR1 negative cells (Figure 1B). Furthermore, both CD8 and CD4 FOLR1 CAR T cells produced higher levels of IL-2, IFN-𝛄, and TNF-𝛂 and proliferated more robustly than did unmodified T cells when co-incubated with FOLR1 positive but not FOLR1 negative cells (Figure 1C). These results indicate highly specific reactivity of FOLR1 CAR T cells against AML cells expressing FOLR1. We next investigated the in vivo efficacy of FOLR1-directed CAR T cells. In CBF/GLIS-CB, WSU-AML, and Kasumi-1 FOLR1+ xenograft models, treatment with FOLR1 CAR T cells induced leukemia clearance, while disease progression occurred in all mice that received unmodified T cells (Figure 1D). Activity of FOLR1 CAR T cells in vivo was target specific, as they did not limit the leukemia progression nor extend the survival of Kasumi-1 xenografts (Figure 1D).

To determine whether FOLR1 is expressed on normal HSPCs, we characterized FOLR1 expression in normal CB CD34+ samples. FOLR1 expression was entirely silent in HSPC subsets (Figure 1E). Consistent with lack of expression, no cytolytic activity was detected against HPSCs Moreover, FOLR1 CAR T cells did not affect the self-renewal and multilineage differentiation capacity of normal HSPCs as compared to unmodified control T cells (Figure 1F), whereas significant eradication of colonies were detected in the CBF/GLIS-CB cells (Figure 1G).

Conclusion: In this study, we demonstrate that FOLR1 CAR T effectively eradicates CBF/GLIS AML cells without compromising normal HSPCs, providing a promising approach for the treatment of high-risk CBF/GLIS AML. Transition of this CAR T to clinical development for infant AML is underway.

Disclosures: Hylkema: Moderna: Current equity holder in publicly-traded company; Quest Diagnostics Inc: Current equity holder in publicly-traded company. Pardo: Hematologics, Inc.: Current Employment. Eidenschink Brodersen: Hematologics, Inc.: Current Employment, Other: Equity Ownership. Loken: Hematologics, Inc.: Current Employment, Other: current equity holder in a privately owned company.

*signifies non-member of ASH