Treatment choices for acute myeloid leukemia (AML) patients resistant to conventional chemotherapies are limited and novel therapeutic agents are needed. Interleukin-3 receptor alpha (IL-3Rα, or CD123) is expressed on the majority of AML blasts and there is evidence that its expression is increased on leukemic relative to normal hematopoietic stem cells, which makes it an attractive target for antibody-based therapy. Here we report the generation and preclinical characterization of SGN-CD123A, an antibody-drug conjugate utilizing the pyrrolobenzodiazepine dimer (PBD) linker and a humanized CD123 antibody with engineered cysteines for site-specific conjugation. Mechanistically, SGN-CD123A induces activation of DNA damage response pathways, cell cycle changes, and apoptosis in AML cells. In vitro, SGN-CD123A mediated potent cytotoxicity of 11/12 CD123+ AML cell lines and 20/23 primary samples from AML patients, including those with unfavorable cytogenetic profiles or FLT3 mutations. In vivo, SGN-CD123A treatment led to AML eradication in a disseminated disease model, remission in a subcutaneous xenograft model, and significant growth delay in a multidrug resistance xenograft model. Moreover, SGN-CD123A also resulted in durable complete remission of a patient-derived xenograft AML model. When combined with a FLT3 inhibitor quizartinib, SGN-CD123A enhanced the activity of quizartinib against two FLT3-mutated xenograft models. Overall, these data demonstrate that SGN-CD123A is a potent anti-leukemic agent, supporting an ongoing trial to evaluate its safety and efficacy in AML patients (NCT02848248).
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