Myeloid epithelial reproductive tyrosine kinase (MERTK) receptor is overexpressed in cancers and is associated with poor prognosis. RGX-019-MMAE, a novel humanized IgG1-MMAE antibody-drug conjugate (ADC) (Inspirna, Inc), selectively binds to MERTK with high affinity, 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.

MERTK protein expression was analyzed in 818 AML patients using Reverse Phase Protein Arrays (RPPA). Expression was also assessed by flow cytometry in eight AML cell lines and peripheral blood or bone marrow mononuclear cells from five AML patients. Cell lines with the highest MERTK expression were treated with varying doses of RGX-019-MMAE or naked antibody for 120 h, and viability was measured using CellTiter-Glo 2.0. Similarly, primary cells from five AML patients were treated to assess the anti-leukemic effect of RGX-019-MMAE. Further, the combinatorial effects of RGX-019-MMAE with venetoclax (BCL2 inhibitor) were evaluated in vitro.

Reverse-phase protein array in 818 primary AML samples revealed significantly high MERTK protein expression in monocytic acute myeloid leukemia (AML), especially in those with PTPN11, RAS, CEBPA mutations, t (9;11) translocation, and high WBC count, suggesting its potential as a therapeutic target in AML. We also observed varying degrees of MERTK expression in AML cell lines, with highest expression in Kasumi-1 and OCI-AML3. Treatment of these cell lines with the anti-MERTK antibody-drug conjugate RGX-019-MMAE resulted in significantly more leukemic cell killing than the control antibody in a dose-dependent manner. We validated this finding in MERTK-expressing primary AML samples expressing MERTK. Interestingly, RGX-019-MMAE had no effect on normal hematopoietic stem cells’ clonogenic potential. Further, treatment with RGX-019-MMAE inhibited AML progression in vivo and significantly prolonged survival of AML xenograft-bearing mice in a dose-dependent manner. Moreover, treatment with RGX-019-MMAE sensitized AML cells to venetoclax in a dose-dependent manner.

MERTK is overexpressed in AML and could serve as a therapeutic target. Furthermore, RGX-019 MMAE can be used as a novel therapeutic approach for treating AML, especially in treating monocytic subsets of AML.