Navtemadlin (KRT-232), a Small Molecule MDM2 Inhibitor, Is More Effective Than Decitabine Against Myeloproliferative Neoplasm-Blast Phase in a Patient-Derived Xenograft Model
Authors: Wang Xiaoli , Hu Cing Siang , Gillespie Virginia , Krejsa Cecile Marie , Hoffman Ronald 
Published: November 2021

Abstract
Patients with myeloproliferative neoplasm-blast phase (MPN-BP) have a particularly dismal prognosis with a median survival of less than 6 months with currently available therapies (Mesa Blood 2015). Decitabine is the standard of care for MPN-BP. Recently, using xenotransplantation assays, we have shown that MPN-BP originates at the hematopoietic stem cell (SC) level (Wang Blood 2018) and that MPN-BP CD34 + cells contain higher levels of MDM2 protein compared with their normal counterparts (% of MDM2 + CD34 + cells: MPN-BP: 76.4±3.3; Normal: 17.5±6.4. P &lt;0.05). MDM2 negatively regulates p53 activity and MDM2 inhibitors can activate p53 and induce apoptosis of TP53 WT cancer cells. As mutations or deletions of TP53 occur infrequently in MPN-BP, we examined the effects of a potent MDM2 inhibitor, navtemadlin (KRT-232; Canon Mol. Cancer Ther. 2015) and decitabine as monotherapy or in combination on the depletion or elimination of MPN-BP cells in a patient derived xenograft (PDX) model.
We firstestablished the dynamics of leukemia cell recovery following a single cycle of navtemadlin treatment. Spleen cells with wild type (WT) TP53 and mutations in KRAS, RAD21, KMT2A and ASXL1 were collected from the 4 th generation of MPN-BP PDX mice. Forty days after injection of these spleen cells (4×10 5/mouse) into sublethally irradiated NSG mice, peripheral blood (PB) leukemic burden (hCD34 + cells: 0.39±0.09%) was demonstrated by flow cytometric analysis. Mice were treated with vehicle alone (n=3) or high doses of navtemadlin (100 mg/kg, n=4) by daily oral gavage on day (D) 1-7. Without navtemadlin treatment, leukemia engraftment and leukemic burden continued to increase until the mice died on D15-D23. By contrast, hCD34 + leukemic blasts were almost undetectable on D8 and remained at significantly lower levels in PB of mice treated with navtemadlin on D15, than were detected in mice receiving vehicle alone (D8: Vehicle: 1.8±1.3%; Navtemadlin: 0.1±0.1%. D15: Vehicle: 19.9±6.0%; Navtemadlin: 0.5±0.1%). These findings suggest that navtemadlin monotherapy has the potential to deplete MPN-BP blast cells and prolong survival in MPN-BP PDX mice. To achieve long-term remission and to prevent relapse, we treated MPN-BP PDX mice with multiple cycles of low (50 mg/kg, n=4) and high dose navtemadlin (100 mg/kg, n=5) at three-week intervals based on the dynamics of leukemia cell recovery following a single cycle of navtemadlin treatment. hCD34 + cells, which contain MPN-BP SCs, and hCD45 dimCD33 + leukemic blasts were reduced in the spleen, but not in the marrows of mice during 3 cycles of 100 mg/kg navtemadlin treatment. However, reduced leukemia cell burden in PB persisted during 3 cycles of navtemadlin treatment and was associated with a prolongation in survival, which was dose-dependent (Mean survival after transplantation: Vehicle: 64.0 days; 50mg/kg navtemadlin: 86.0 days; 100mg/kg navtemadlin: 98.3 days). We then examined if a combination of navtemadlin and decitabine is more effective than single agent therapy in depleting MPN-BP SCs. Again, navtemadlin at 100 mg/kg significantly reduced the leukemia cell burden in mouse PB on C1D8 (hCD34 + cells: Vehicle: 4.1±0.8%; Navtemadlin: 0.6±0.2%, an 85% decrease. hCD45 dimCD33 + cells: Vehicle: 1.4±0.3%; Navtemadlin: 0.04±0.03%, a 97% decrease. P &lt;0.001 for both), which persisted during 2 cycles of treatment. By contrast, multiple cycles of decitabine monotherapy (2.5mg/kg, IP, 3 times/week in 21-day cycles) resulted in a modest reduction in hCD34 + cells and hCD45 dimCD33 + cells (11.4% and 30.1% decrease, respectively) in PB of MPN-BP PDX mice, and did not reduce these cells in either the spleen or bone marrow on C1D8. Finally, addition of decitabine to navtemadlin did not further deplete either MPN-BP SCs or leukemia cells and did not improve survival of MPN-BP PDX mice, as compared to treatment with navtemadlin alone. Mean survival after transplantation in the combination study was: Vehicle: 55.5 days; navtemadlin: 70.3 days; Decitabine: 56.3 days; Combination: 64.0 days. Furthermore, toxicity (body weight loss, intestinal pathology) was observed in mice receiving high dose of navtemadlin and decitabine simultaneously, but not when either drug was administered alone. In conclusion, navtemadlin monotherapy, which activates p53, depletes leukemia cell counts and prolongs survival of MPN-BP PDX mice and is a promising agent for patients with WT TP53 MPN-BP.

Disclosures
Krejsa: Kartos Therapeutics, Inc.: Current Employment. Hoffman: AbbVie Inc.: Other: Data Safety Monitoring Board, Research Funding; Kartos Therapeutics, Inc.: Research Funding; Protagonist Therapeutics, Inc.: Consultancy; Novartis: Other: Data Safety Monitoring Board, Research Funding.