Dual inhibition of HDMX and HDM2 as a Therapeutic Strategy in Leukemia

The p53 protein is the most frequently inactivated tumor suppressor in human cancer. While p53 mutations are found in 50% of all cancers, the p53 pathway can also be suppressed by its interaction with endogenous inhibitors HDMX and HDM2, which are frequently overexpressed in patients with acute myeloid leukemia and other cancers. Thus, pharmacological disruption of both these interactions is an attractive strategy to restore p53-dependent tumor suppressor activity in AML with wild type P53. Strategies targeting HDM2 have recently generated promising results; however, cancer cells are still left vulnerable to p53 inhibition by HDMX, particularly in cancers such as leukemia that overexpress HDMX. In this study, we demonstrate that dual HDMX/HDM2 inhibition using a stapled alpha-helical peptide (ALRN-6924), which has recently entered clinical testing, leads to striking anti-leukemic effects. ALRN-6924 robustly activates p53-dependent transcription at the single cell and single molecule level, and exhibits biochemical and molecular biological on-target activity in leukemia cells in vitro and in a patient who received ALRN-6924 treatment. Dual HDMX/HDM2 inhibition by ALRN-6924 inhibits cellular proliferation by inducing cell cycle arrest and apoptosis in cell lines and primary AML patients’ cells, including in leukemic stem cell-enriched populations, and disrupts functional clonogenic and serial replating capacity. Furthermore, ALRN-6924 leads to significantly improved survival in an AML xenograft model in vivo. At the molecular level, dual HDMX/HDM2 inhibition leads to global transcriptional activation of p53-dependent pathways in leukemia cells. Our study provides insight into the effects of dual HDMX/HDM2 inhibition and proof-of-concept for ALRN-6924 as a novel therapeutic approach in AML and other cancers with high HDMX levels.