• Patients with PMF or post-PV/ET MF requiring therapy and intermediate-1, -2 or high risk disease by the Dynamic International Prognostic Scoring System (DIPSS) , DIPSS-plus MIPSS7021 or MIPSS70-plus v2.0 if PMF and by the Myelofibrosis Secondary to PV and ET - Prognostic Model (MYSEC-PM) if post-PV/ET MF

• Treated with ruxolitinib for ≥12 weeks with a stable dose for the preceding ≥4 weeks. Patients must be on a dose of ruxolitinib of 10mg or 15mg BID at the time of screening.

• Evidence of inadequate response to ruxolitinib: Patients must have palpable splenomegaly ≥5 cm below the left costal margin at study entry AND/OR active MPN symptoms, as defined by the presence of one symptom score ≥5 or two symptom scores ≥3 using the screening symptom form

• Age ≥ 18 years.

• Eastern Cooperative Oncology Group (ECOG) performance status 0, 1 or 2.

• Life expectancy of at least 24 weeks.

• The patient has adequate organ function for all of the following criteria:

• ° Hematologic

• ANC ≥1.5 × 10\^9/L

• Platelets ≥75 × 10\^9/L

⁃ Hepatic

• Total bilirubin ≤1.5 × ULN

• Patients with Gilbert's syndrome with a total bilirubin \>2.0 times ULN and direct bilirubin within normal limits are permitted.

• ALT and AST ≤3 × ULN

• Patients who received chemotherapy must have recovered (Common Terminology Criteria for Adverse Events \[CTCAE\] Grade ≤1) from the acute effects of chemotherapy except for residual alopecia or Grade 2 peripheral neuropathy prior to start of therapy. A washout period of at least 21 days is required between last chemotherapy dose and start of combination therapy (with the exception of hydroxyurea, which may be continued until the day before dosing begins). Patients should not receive hydroxyurea while on treatment.

• Patients who received radiotherapy must have completed and fully recovered from the acute effects of radiotherapy. A washout period of at least 14 days is required between end of radiotherapy and randomization

• The effects of ruxolitinib and abemaciclib on the developing human fetus are unknown. To be eligible for the study, female subjects of childbearing potential (and their male partners) and men (and female partners) enrolled in the study should use two methods of effective contraception (hormonal and barrier method of birth control; abstinence) prior and during the study and also continue to use contraception for 4 months after completion of ruxolitinib and abemaciclib administration. 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 ruxolitinib and Abemaciclib administration.

• Ability to understand and the willingness to sign a written informed consent document.