Abstract 4421: Targeting myeloid-derived suppressor cells with actinium-225 lintuzumab, a CD33 antibody radioconjugate to enhance antitumor immunity
Authors: Chin Amanda , Chen Mary , Li Jason , McCloskey Megan , Jennings Caroline , Xu Le-Cun , Roy Monideepa , Beckford Denis , Kotanides Helen 
Published: April 2023

Abstract
Introduction: Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of myeloid lineage cells with potent immunosuppressive activity found enriched in cancer patients. Crosstalk between MDSCs and the tumor microenvironment can promote tumor immune evasion and thus there is growing interest in developing therapeutic approaches to intervene in the MDSC suppressive function. Both monocytic-MDSC (M-MDSC) and granulocytic-MDSC (G-MDSC) subpopulations express the cell surface CD33 myeloid marker. For well-defined cell surface markers like CD33, there is considerable interest in the use of radionuclides as therapeutic payloads, particularly α-particle emitters such as actinium-225 (225Ac) since they deliver substantially higher decay energies over a much shorter distance than β-emitters, rendering them more suitable for precise, potent, and efficient target cell killing while minimizing toxicity to surrounding bystander cells. Actimab A, the anti-CD33 antibody lintuzumab armed with the 225Ac radioisotope (CD33 ARC), is currently being evaluated in R/R AML and has demonstrated significant anti-leukemic activity in Phase 1/2 clinical trials. We therefore hypothesized that MDSCs can be directly targeted by the CD33 ARC. Hence, we evaluated the therapeutic potential of the CD33 ARC to deplete MDSCs through preclinical studies in vitro and in vivo with humanized mouse model.
Methods: CD33 ARC was generated by conjugating lintuzumab with p-SCN-Bn-DOTA and subsequently radiolabeled with 225Ac. Primary MDSCs were isolated from cancer patient peripheral blood or healthy donor PBMCs. The specific binding of CD33 ARC to CD33 positive MDSCs and the decreased viability of MDSCs in response to treatment in vitro was characterized by immunophenotyping using flow cytometry. The therapeutic efficacy of CD33 ARC to deplete MDSC in vivo was evaluated in human CD34 reconstituted humanized NOG-EXL mice.
Results: CD33 positive MDSCs (M-MDSCs and G-MDSCs) were identified in human cancer peripheral blood samples, including colorectal and lung. Significantly more MDSCs were found in the peripheral blood of cancer patients in comparison to healthy donors. CD33 ARC treatment of human cancer MDSCs induced a potent dose-dependent reduction in MDSC viability leading to increased depletion of MDSCs. Furthermore, in the humanized NOG-EXL mouse model, CD33 ARC therapy demonstrated in vivo activity of CD33 ARC to deplete human MDSCs.
Conclusions: In this study, we demonstrate CD33 ARC alpha targeted radiotherapy depletes human CD33 positive immune suppressing MDSCs present in multiple cancer types, to enhance antitumor immunity. These findings present a translatable strategy that supports further evaluation of 225Ac lintuzumab as a MDSC targeting agent to improve the efficacy of antitumor therapies.
Citation Format: Amanda Chin, Mary Chen, Jason Li, Megan McCloskey, Caroline Jennings, Le-Cun Xu, Monideepa Roy, Denis Beckford, Helen Kotanides. Targeting myeloid-derived suppressor cells with actinium-225 lintuzumab, a CD33 antibody radioconjugate to enhance antitumor immunity. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4421.