AML-397: Preclinical Efficacy of a PROTAC-based MDM2 Degrader in AML models

Abstract

Mouse double minute 2 homolog (MDM2) is a E3 ubiquitin ligase that promotes p53 tumor suppressor degradation. MDM2 has emerged as a therapeutic target in the treatment of TP53 wild-type expressing acute myeloid leukemia (AML) where MDM2 is overexpressed. Several MDM2 inhibitors are currently in clinical trials. However, p53 stabilization upregulates MDM2, which limits the clinical efficacy of the inhibitors. Proteolysis Targeting Chimeric (PROTAC) molecules recruit targeted proteins for degradation. We developed a MDM2 degrader, MI-265, that binds to MDM2 and targets it for degradation. We tested MI-265 in a panel of 101 primary AML samples in ex vivo cultures. Leukemia stem cells (LSC) were enriched by positive selection with CD34 antibody. After treatment with drugs for 72 hrs, cell death and apoptosis induction were measured by flow-cytometry for DAPI-Annexin V binding. The AML cells were obtained from newly diagnosed AML (N=86) or relapse/refractory AML (N=15). The median IC50 for MI-265 was 120 pM. In contrast, median IC50 for the inhibitor MI-1061 was 20,000-fold higher at 2.3 μM. Sensitivity and resistance to MDM2-degrader and inhibitor were similar in most of the samples suggesting that the mechanism of action of both drugs are also similar. The AML cells were heterogeneous for the expression of mutant FLT-3 (36%), NPM1 (22%), IDH1/2 (16%), CEBPalpha (2%), DNMT3A (2%), and other mutations (10%). Based on classical karyotype, 30% of them had a normal karyotype and 55% had abnormal karyotype. The MDM2 degrader-resistant LSCs (IC50 greater than 1 μM) were analyzed for TP53 mutation status and 8 of 14 were found to harbor mutations in TP53. The degradation of MDM2 and overexpression of p53 was confirmed by immunoblotting after a 4-hr exposure of LSCs to degrader. Moreover, treatment of normal CD34-positive hematopoietic cells in apoptosis assay and colony formation assays suggests lack of toxicity to normal hematopoiesis. Evaluation of efficacy in patient-derived xenografts is ongoing. In conclusion, MI-265 represents a highly potent and efficacious MDM2 degrader, and is currently undergoing extensive preclinical evaluation for the treatment of acute leukemias.