This phase I/II trial studies the side effects and best dose of ruxolitinib phosphate when given together with decitabine and to see how well they work in treating patients with acute myeloid leukemia that has come back or is not responding to treatment, or has developed from a type of bone marrow diseases called myeloproliferative neoplasms. Ruxolitinib phosphate may stop the growth of cancer cells by blocking some of the enzymes needed for cell growth. Drugs used in chemotherapy, such as decitabine, work in different ways to stop the growth of cancer cells, either by killing the cells, by stopping them from dividing, or by stopping them from spreading. Giving ruxolitinib phosphate together with decitabine may be an effective treatment for acute myeloid leukemia.

PRIMARY OBJECTIVES:

I. To determine the tolerability of the combination of decitabine and ruxolitinib phosphate (ruxolitinib [DI]) in patients with leukemia. (Phase I) II. To determine the efficacy of ruxolitinib in increasing and prolonging response induced by decitabine alone in patients with post myeloproliferative neoplasm acute myeloid leukemia (AML) (post MPN-AML) alternatively referred to as (myeloproliferative neoplasm - blast phase; MPN-BP). (Compared to historical response rate with decitabine alone) (Phase II)

SECONDARY OBJECTIVES:

I. To compare whether there is a difference in response rate patients with post-MPN AML with janus kinase 2 (JAK2) mutations and patients without JAK2 mutations.

OUTLINE: This is a phase I, dose-escalation study of ruxolitinib phosphate followed by a phase II study.

Patients receive ruxolitinib phosphate orally (PO) twice daily (BID) on days 1-28 and decitabine intravenously (IV) over 1-2 hours on days 1-5. Treatment repeats every 4-6 weeks for up to 24 courses in the absence of disease progression or unacceptable toxicity.

Inclusion Criteria:

• Diagnosis of AML (World Health Organization [WHO] classification definition of >= to 20% blasts)
• In the phase I portion of the study all patients with relapsed or refractory AML are eligible; for the Phase II portion of the study, patients must have AML progressing from prior MPN (MPN-BP) or have myelodysplastic syndrome (MDS)/MPN with more than 20% blasts; temporary prior measures to control blood counts, such as apheresis or Hydrea are allowed; patients with newly diagnosed or previously treated disease are eligible as long as prior therapy does not include hypomethylating agents; prior therapy for ruxolitinib for MPN is allowed
• Serum biochemical values with the following limits unless considered due to leukemia:
• Creatinine =< 1.5 mg/dl
• Total bilirubin =< 1.5 mg/dL, unless increase is due to hemolysis or congenital disorder
• Transaminases (serum glutamate pyruvate transaminase [SGPT]) =< 2.5 x upper limit of normal (ULN)
• Ability to take oral medication
• Ability to understand and provide signed informed consent
• Performance status =< 3, unless directly related to disease process as determined by the principal investigator

Exclusion Criteria:

• Any coexisting medical condition that in the judgment of the treating physician is likely to interfere with study procedures or results including uncontrolled severe infections, as well as uncontrolled cardiac disease, or other organ dysfunction; patients with history of tuberculosis, human immunodeficiency virus (HIV) or hepatitis B and C are excluded
• Nursing women, women of childbearing potential with positive blood pregnancy test within 30 days of study start, or women of childbearing potential who are not willing to maintain adequate contraception (such as birth control pills, intrauterine device [IUD], diaphragm, abstinence, or condoms by their partner) over the entire course of the study
• Incomplete recovery from any prior surgical procedures or had surgery within 4 weeks prior to study entry, excluding the placement of vascular access
• Active clinically serious and uncontrolled infection