Mission: A Phase 3, Randomized, Double-blind, Add-on Study Evaluating the Safety and Efficacy of Navtemadlin Plus Ruxolitinib vs Placebo Plus Ruxolitinib in JAK Inhibitor-Naïve Patients with Myelofibrosis Who Have a Suboptimal Response to Ruxolitinib …

Inclusion Criteria for Ruxolitinib Alone Period:
*  Confirmed diagnosis of PMF, post-PV MF, or post-ET MF, as assessed by the treating physician according to the World Health Organization (WHO) criteria
*  High, Intermediate-1, Intermediate-2 risk category International Prognosis System Score (IPSS)
*  Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 2
*  JAK-inhibitor treatment naive Exclusion Criteria for Ruxolitinib Alone Period:
*  Prior Splenectomy
*  Splenic irradiation within 3 months prior to the first dose
*  Prior BCL-XL, BET, MDM2, PI3K, PIM, or XPO1 inhibitors therapy or p53-directed therapy
*  Eligible for Bone Marrow Transplant
*  Peripheral blood or bone marrow blast count ≥ 10 percent Inclusion Criteria for Randomized Period:
*  PMF, post-PV MF, or post-ET MF that is TP53WT as assessed by central testing
*  ECOG performance status of 0 to 2
*  Treatment with a stable dose of ruxolitinib
*  Suboptimal response to run-in ruxolitinib treatment Exclusion Criteria for Randomized Period:
*  Elevated white blood cell count that doubles (or more) during ruxolitinib treatment and exceeds 50 × 10\^9/L
*  Peripheral blood or bone marrow blast count ≥ 10 percent