A Study of Ruxolitinib in Combination With Ulixertinib in People With Myelofibrosis

Inclusion Criteria: * Patients with a diagnosis of primary myelofibrosis, post-ET myelofibrosis, post-PV myelofibrosis, or post-pre-fibrotic myelofibrosis by WHO 2016 criteria. * Age ≥18 years. * Receiving ruxolitinib monotherapy for at least 3 months with stable dose (10 mg BID to 20mg BID) for at least 4 weeks before first dose of study drug. Note: stable ruxolitinib dosing should be achieved according to strict adherence to dose modification/reduction guidelines detailed in the ruxolitinib package insert, for patients with renal impairment, and/or hepatic impairment. * Must have DIPSS+ intermediate 2 or greater risk disease, or MIPSS70+ intermediate or greater risk disease * Persistent disease despite ruxolitinib monotherapy, as demonstrated by: o Grade 2 or 3 reticulin/collagen fibrosis on bone marrow AND * Splenomegaly (palpable at least 5cm below subcostal margin/or spleen volume \> 450cm\^3) OR * Active symptoms (MPN-SAF TSS score \>10 with at least one MPNSAF TSS score \>5 or two scores \>3) OR * ECOG performance status ≤2 * Participants must have adequate organ and marrow function as defined below unless the elevated laboratory values are attributable to Gilbert's Syndrome with Sponsor review and approval: * Absolute neutrophil count ≥ 0.5 K/mcL * Platelets ≥ 50 K/mcL * Direct bilirubin ≤ 1.5 times institutional upper limit of normal (ULN) * Total bilirubin ≤ 1.5 times institutional upper limit of normal (ULN * AST(SGOT)/ALT(SGPT) ≤ 3 × institution…