• 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 × institutional ULN

⁃ Creatinine clearance ≥ 50 mL/min as calculated by institutional standard

⁃ Bone marrow and peripheral blood blast count \<10%

• Agreeable to the use of adequate contraception to avoid pregnancy (Appendix D). Women of child-bearing potential and men must agree to use adequate contraception (hormonal or barrier method of birth contro) prior to study entry, for the duration of study participation, and 4 months after completion of drug administration (Appendix D). At least two highly effective methods of contraception, one of which must be a barrier method, are required for males and females of childbearing age during dosing and for 4 months after completing treatment. Should a woman become pregnant or suspect she is pregnant while she or her partner is participating in this study, she should inform her treating physician immediately. Men treated or enrolled on this protocol must also agree to use adequate contraception prior to the study, for the duration of study participation, and 4 months after completion of drug administration