Targeting Myeloid Epithelial Tyrosine Kinase (MERTK) Receptor in Acute Myeloid Leukemia Using a Novel Antibody Drug Conjugate, Rgx-019-MMAE

Abstract Background: Myeloid epithelial reproductive tyrosine kinase (MERTK), a transmembrane protein receptor from the TAM (Tyro3, Axl, Mertk) family, is overexpressed in cancer and associated with reduced apoptosis and chemoresistance, making it an attractive therapeutic target. RGX-019-MMAE, a novel humanized IgG1-MMAE antibody-drug conjugate (ADC) (Inspirna, Inc), selectively binds to MERTK with high affinity on tumor cells, resulting in internalization and degradation of the receptor. It then induces cytotoxicity through the release of the payload, MMAE (monomethyl auristatin E), which disrupts mitosis. We hypothesize that targeting MERTK via RGX-019-MMAE exerts an antileukemic effect. Methods: First, to determine the clinical relevance of MERTK, using the Oregon Health and Science University (OHSU) acute myeloid leukemia (AML) dataset, we divided patients into low and high MERTK mRNA expression based on mean expression and compared their overall survival. To identify the AML subtypes with the highest MERTK expression, we analyzed samples from 810 AML patients at MD Anderson using reverse phase protein arrays (RPPA), as well as 8 AML cell lines and the peripheral blood of 9 AML patients using flow cytometry. Then, the cell lines with the highest MERTK expression were treated with varying doses of RGX-019-MMAE or isotype control for 120 hours and assessed for viability with CellTiter-Glo 2.0. Likewise, we administered RGX-019-MMAE to primary cells from AML patients and determined the anti-leukemic effect. Finally, we investigated the synergistic effect of RGX-019-MMAE with venetoclax, a BCL2 inhibitor, or cytarabine, a chemotherapy agent, in vitro. Results: In the AML OHSU dataset, patients with high MERTK mRNA expression had significantly worse overall survival (p=0.02). RPPA revealed that patients with PTPN11 mutation, t (9;11), and monocytic AML subtypes had significantly higher MERTK protein expression. MERTK protein expression varied in AML cell lines and was highest in Kasumi-1 and OCI-AML3. Treatment of these two cell lines with RGX-019-MMAE resulted in significantly more killing of leukemic cells than the isotype control ADC in a dose-dependent manner (p<0.01). Similarly, in primary AML cells with high MERTK expression, RGX-019-MMAE treatment induced significant cell death compared to the control ADC. In addition, Kasumi-1 and OCl-AML3 cells treated with RGX-019-MMAE combined with venetoclax or cytarabine showed a synergistic cytotoxic effect in a dose-dependent manner. Conclusion: High MERTK expression in monocytic AML suggested that MERTK is a promising therapeutic target in this subtype. RGX-019-MMAE significantly induced AML cell death and produced a synergistic anti-leukemic effect in combination with chemotherapeutic agents and targeted therapy in vitro, suggesting its potential for targeting patients with high-risk AML. Citation Format: Anudishi Tyagi, Maryam Siddiqui, Isabel Kurth, Shugaku Takeda, Amanda Eckstrom, Jenny Borgman, Abhishek Maiti, Venkata Lokesh Battula. Targeting myeloid epithelial tyrosine kinase (MERTK) receptor in acute myeloid leukemia using a novel antibody-drug conjugate, RGX-019-MMAE [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 6341.