Dual Inhibition of Mdmx and Mdm2 Using an Alpha-Helical P53 Stapled Peptide (ALRN-6924) As a Novel Therapeutic Strategy in Acute Myeloid Leukemia

While the TP53 tumor suppressor gene is mutated in more than 50% of human tumors, in Acute Myeloid Leukemia (AML) TP53 mutations are rare, occurring in less than 10% of cases. Yet, functional inactivation of wild-type p53 due to non-mutational abnormalities occurs frequently in AML and other hematological malignancies. A major mechanism of p53 inactivation results from the overexpression of its endogenous inhibitors MDMX (also known as MDM4, HDMX, and HDM4) and MDM2 which are frequently overexpressed in various p53 wild-type human cancers, including AML. Strikingly, MDMX has been reported to be overexpressed in over 92% of AML cases, while MDM2 overexpression is less frequent. Pharmacological disruption of both these interactions has long been sought after as an attractive strategy to fully restore p53-dependent tumor suppressor activity in AML and other cancers with wild-type p53. Nonetheless, selective targeting of this pathway has thus far been limited to MDM2-only small molecule inhibitors which lack affinity for MDMX. ALRN-6924 is an optimized alpha helical p53 stapled peptide and first-in-class dual MDMX/MDM2 inhibitor which has recently entered phase I/II clinical testing (NCT02264613, NCT02909972) in solid tumors and lymphomas with, thus far, excellent tolerability and objective responses as single agent¹. The goal of our study was to evaluate the molecular, cellular, and biochemical mechanisms of action of ALRN-6924 in AML.

We used biochemical affinity studies as well as single molecule FISH and live single cell imaging to assess MDMX/MDM2 binding as well as p21 transactivation by p53 in response to ALRN-6924. Effects on cellular proliferation, apoptosis, DNA repair, cell cycle, clonogenic capacity, and serial replating were determined using AML cell lines and primary human AML patients' cells, including in leukemic stem (CD34+ CD38-) and progenitor (CD34+CD38+) cells. Genome-wide molecular effects were determined by RNA seq. P53 activity in a patient undergoing treatment with ALRN-6924 was measured by intracellular staining of p53 and its target gene p21 in CD34+ cells by flow cytometry. Furthermore, we evaluated ALRN-6924 activity in a xeno-transplantation model of human AML in NSG mice.

We found that MDMX is significantly overexpressed in highly fractionated leukemic stem (Lin-CD34+CD38-CD90-) and progenitor (Lin-CD34+CD38+CD123+CD45+) cells in AML patients compared to identically sorted, age-matched healthy controls (p<0.0001). Dual MDMX/MDM2 inhibition using ALRN-6924 led to striking anti-leukemic effects in suspension culture of AML cell lines and primary cells, with negligible effects on healthy controls. ALRN-6924 robustly activated p53-dependent transcription at the single cell and single molecule level, and exhibited strong biochemical and molecular biological on-target activity in AML cell lines and primary cells in vitro, as well as in a patient who received ALRN-6924 treatment. Co-immunoprecipitation experiments demonstrated that ALRN-6924 disrupts both MDMX/P53 as well as MDM2/P53 interactions at relevant therapeutic dosages in AML cells. RNA seq data showed that dual MDMX/MDM2 inhibition led to global transcriptional activation of p53-dependent pathways in AML cells. Dual MDMX/MDM2 inhibition by ALRN-6924 inhibited cellular proliferation by inducing cell cycle arrest and apoptosis in cell lines and primary AML patients' cells, including in leukemic stem cell-enriched populations. Furthermore, ALRN-6924 disrupted functional clonogenic and serial replating capacity of AML cells, and showed superiority over MDM2-only inhibition. Most strikingly, ALRN-6924 led to highly significant improved survival in an AML xenograft model in vivo (p<0.0001).

Our study provides insight into the molecular, cell biological, and in vivo effects of pharmacological dual MDMX/MDM2 inhibition in AML, which may have important implications for other MDMX/MDM2-related cancers. Our findings provide proof-of-concept for ALRN-6924 as a novel therapeutic in p53-wildtype AML, and provide a rationale for its further preclinical and clinical development in AML and other cancers.

Furthermore, the success of targeting p53 raises the intriguing prospect that the same development path is possible for other helix-in-groove targets, and may thus pave the way for a new class of targeted therapeutics.

¹ ASCO 2017 annual meeting: J Clin Oncol 35, 2017 (suppl; abstr 2505)