Inclusion Criteria:

• - Patients must have histologically or cytologically confirmed primary myelofibrosis (PMF), post-polycythemia vera-myelofibrosis (PPV-MF), post-essential thrombocythemia-myelofibrosis (PET-MF), chronic myelomonocytic leukemia, polycythemia vera, or essential thrombocytosis according to the 2008 World Health Organization criteria.

• - Subject has at least 2 symptoms with a score ≥ 3 or a total score of ≥ 12, as measured by the MFSAF(Myelofibrosis Symptom Assessment Form) v4.0.

• - Subject classified as intermediate-2 or high-risk MF, as defined by the Dynamic International Prognostic Scoring System Plus (DIPSS+70).

• - Age > 18 years.

• - ECOG (Eastern Cooperative Oncology Group) performance status 0-2.

• - Subject must have received prior treatment with a single JAK2 inhibitor 4.1.

6 for at least 12 weeks with documented disease progression OR subject must have appearance of new splenomegaly that is palpable to at least 5 cm below the left costal margin (LCM) in subjects with no evidence of splenomegaly prior to the initiation of any first line JAK2 inhibitor.

• - Baseline QTc (corrected QT interval) <0.47 seconds (Bazett formula) - Patients must have normal organ function as defined in protocol.

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

Exclusion Criteria:

• - Patients may not be receiving any other investigational agents.

• - Subjects must not be experiencing toxicity due to prior therapy that has not resolved to ≤Grade 1 by study registration, with the exception of sensory neuropathy related to previous systemic therapy exposure, alopecia and fatigue.

• - Patients that have transformed to Acute Myeloid Leukemia defined by >20% blasts count on peripheral blood smear or bone marrow biopsy evaluation.

• - Uncontrolled inter-current illness including, but not limited to, any other malignancy (with the exception of hormonal therapy for breast cancer/prostate cancer in remission >1 year and for non-hormonal therapies for other cancers in remission for >3 years), other ongoing or active infection, symptomatic congestive heart failure, unstable angina pectoris, uncontrolled cardiac arrhythmia, or psychiatric illness/social situations that would limit compliance with study requirements.

• - Patients with history of hemorrhagic stroke and evidence of uncontrolled bleeding as well as bleeding disorder.

• - Known HIV positive patients on combination antiretroviral therapy are ineligible because these patients are at increased risk of lethal infections when treated with marrow-suppressive therapy.

• - Pregnant or breast-feeding.

This is a prospective phase I dose-escalation study, with the primary objective to access the MTD and find the RP2D of talazoparib, given in combination with standard of care dosing of pacritinib. Subjects must have a diagnosis of a myeloproliferative neoplasm and progressed or become intolerant to any JAK2 directed therapy. To be eligible for the therapy, patients are required to consent to a bone marrow biopsy at the beginning of study treatment. The treatment protocol involves initiating pacritinib on day -7 (lead-in phase, starting day -7 for cycle 1) with a standard of care dose of 200mg twice daily (BID). Subsequently, talazoparib will be given on day 1 of the treatment cycle. Cohort 1 will enroll patients at the starting dose of 0.25mg talazoparib for 14 days (dose level 1) and DLT (dose-limiting toxicity) rate will inform the subsequent dose levels (DLs). Investigators plan to use a Bayesian Optimal Interval Design (BOIN) to determine the maximum tolerated dose (MTD) in patients with Ph-MPNs (Philadelphia chromosome negative myeloproliferative neoplasms) who have either not responded to or cannot tolerate ruxolitinib monotherapy. Subjects who are currently receiving JAK2-directed therapy will undergo a one-day washout period before starting the study treatment, as abrupt discontinuation of JAK2 inhibition can lead to side effects. This brief interval allows for the resolution of any lingering effects before initiating the new treatment. A bone marrow aspirate and or biopsy will be obtained to assess response to therapy at the end of each cycle of treatment (28 days), prior to beginning the next cycle as per physician discretion. Peripheral blood and bone marrow mononuclear cells will be checked for γ-H2AX (histone H2A family X) at set time points as a biomarker assay to assess for DNA damage in the same manner as the pre-clinical models published in our previous studies. Patients may remain on study drug unless they experience an unacceptable toxicity, fail to show a benefit, or they attain remission and it is felt by the investigator that they can proceed to an allogenic stem cell transplant.

Arms

Experimental: Dose Level -1

0.25 mg (PO, QD) Talazoparib (Days 1-7) 200 mg (PO, BID) Pacritinib (Day 1-28, Lead in dosing of Pacritinib day -7 for the first cycle of treatment)

Experimental: Dose Level 1

0.25 mg (PO, QD) Talazoparib (Days 1-14) 200 mg (PO, BID) Pacritinib (Day 1-28, Lead in dosing of Pacritinib day -7 for the first cycle of treatment)

Experimental: Dose Level 2

0.5 mg (PO, QD) Talazoparib (Days 1-14) 200 mg (PO, BID) Pacritinib (Day 1-28, Lead in dosing of Pacritinib day -7 for the first cycle of treatment)

Experimental: Dose Level 3

0.75 mg (PO, QD) Talazoparib (Days 1-14) 200 mg (PO, BID) Pacritinib (Day 1-28, Lead in dosing of Pacritinib day -7 for the first cycle of treatment)

Experimental: Dose Level 4

1 mg (PO, QD) Talazoparib (Days 1-14) 200 mg (PO, BID) Pacritinib (Day 1-28, Lead in dosing of Pacritinib day -7 for the first cycle of treatment)

Interventions

Drug: - Talazoparib

pacritinib in combination with talazoparib

Drug: - pacritinib

pacritinib in combination with talazoparib