Abstract 629: Activity of VIP943 on AML patient-derived leukemic blasts and healthy donor-derived bone marrow hematopoietic stem cells
Authors: Stelte-Ludwig Beatrix , Schomber Tibor , Frigault Melanie M. , Birkett Joseph , Johnson Amy J. , Rebstock Anne-Sophie , Ludwig Sebastian , Izumi Raquel , Hamdy Ahmed 
Published: March 2024

Abstract
Introduction: In patients with acute myeloid leukemia (AML), CD123 has been shown to have high expression on blast cells and leukemic stem cells (LSC) compared with normal hematopoietic stem cells (HSCs) and other more mature CD34+ subsets (Zhou J, World J Stem Cells 2014). LSCs are inherently resistant to standard of care chemotherapeutics and LSC persistence after chemotherapy is associated with disease relapse. VIP943 is an ADC consisting of an anti-CD123 antibody, a unique linker cleaved intracellularly by legumain which is required for release, and activation of a novel kinesin spindle protein inhibitor (KSPi) payload that accumulates inside the cell. Herein, we evaluated the potential of VIP943 to target LSC and progenitor cells.
Methods: Cytotoxicity assay: Bone marrow aspirates (BMA) and peripheral blood from previously untreated patients with primary or transformed from myelodysplastic syndrome AML were used in this experiment. Cells were treated with VIP943 (66 nM or 330 nM). After the end of the 48 h or 72 h incubation, the CD123+ cells and the CD34+CD38- LSC were detected by flow cytometry. Characterization of progenitor cell population: Fresh BM samples from healthy volunteers (HVBM) were stained with specific monoclonal antibodies to identify progenitor cell populations. Depletion assay: The effect of VIP943 on HVBM was evaluated by measuring the depletion of CD34+ progenitor cell populations in comparison to the anti-CD33-ADC, gemtuzumab ozogamicin (gem-oz). At the end of the incubation time, red blood cells were lysed; the remaining cells were stained with a cocktail of antibodies to discriminate between progenitor cells and analyzed by flow cytometry.
Results: Treatment of patient-derived CD123+ leukemic blasts with VIP943 resulted in a 50% reduction after 48 h and up to 80% at 72 h. A reduction of &gt;70% of CD123+CD34+CD38- LSCs derived from BM aspirates of untreated patients with AML was achieved after incubation with VIP943 up to 72h. In addition, HVBM samples derived from five healthy volunteers were treated with VIP943 or gem-oz in a depletion assay, where gem-oz was toxic to CD34+ cells with an EC50 of 0.16 µM in contrast to VIP943 with an EC50 value of 8.83 µM.
Conclusions: In this in vitro analysis of primary samples from patients with AML, activity of VIP943 is observed in AML patient-derived CD123+ blasts as well as chemoresistant LSCs. At anticipated pharmacologic levels, VIP943 shows no adverse effects on HSCs unlike gem-oz suggesting an improved therapeutic index. In the ongoing first-in-human dose-escalation study in subjects with advanced CD123+ hematologic malignancies, VIP943 demonstrates a promising safety profile (NCT06034275).
Citation Format: Beatrix Stelte-Ludwig, Tibor Schomber, Melanie M. Frigault, Joseph Birkett, Amy J. Johnson, Anne-Sophie Rebstock, Sebastian Ludwig, Raquel Izumi, Ahmed Hamdy. Activity of VIP943 on AML patient-derived leukemic blasts and healthy donor-derived bone marrow hematopoietic stem cells [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 629.