• Patients must have morphologically confirmed diagnosis of Philadelphia-chromosome negative MPN in accelerated-phase (10-19% myeloid blasts) or blast-phase (≥ 20% myeloid blasts) arising from polycythemia vera, essential thrombocythemia, primary myelofibrosis, secondary myelofibrosis, or MPN not otherwise specified, as per the World Health Organization (WHO) 2016 classification OR myelodysplastic syndrome (MDS)/MPN overlap syndromes (e.g., chronic myelomonocytic leukemia \[CMML\]) with ≥ 10% blasts

• Patients must not have received prior DNMTi. Previous use of janus kinase (JAK) inhibition, hydroxyurea, and interferon is allowed. There is no required washout period

• Age ≥ 18 years

⁃ Because no dosing or adverse event data are currently available on the use of ASTX727 (35 mg decitabine + 100 mg cedazuridine) in combination with iadademstat in patients \< 18 years of age, children are excluded from this study

• Eastern Cooperative Oncology Group (ECOG) performance status ≤ 3 (Karnofsky ≥ 30)

• Total bilirubin ≤ 1.5 x institutional upper limit of normal (ULN) (unless elevated due to Gilbert's syndrome, thought to be related to MPN-AP/BP, or due to extrasvascular hemolysis. In these cases conjugated bilirubin should be ≤ 2.0 x ULN)

• Aspartate aminotransferase (AST) (serum glutamic oxaloacetic transaminase \[SGOT\])/alanine aminotransferase (ALT) (serum glutamic pyruvic transaminase \[SGPT\]) ≤ 3 x institutional ULN

• Glomerular filtration rate (GFR) ≥60 mL/min/1.73 m\^2 by Modification of Diet in Renal Disease (MDRD)

• Human immunodeficiency virus (HIV)-infected patients on effective anti-retroviral therapy with undetectable viral load within 6 months are eligible for this trial

• For patients with evidence of chronic hepatitis B virus (HBV) infection, the HBV viral load must be undetectable on suppressive therapy, if indicated

• Patients with a history of hepatitis C virus (HCV) infection must have been treated and cured. For patients with HCV infection who are currently on treatment, they are eligible if they have an undetectable HCV viral load

• Patients with a prior or concurrent malignancy whose natural history or treatment does not have the potential to interfere with the safety or efficacy assessment of the investigational regimen are eligible for this trial

• Patients with known history or current symptoms of cardiac disease, or history of treatment with cardiotoxic agents, should have a clinical risk assessment of cardiac function using the New York Heart Association Functional Classification. To be eligible for this trial, patients should be class II or better

• The effects of ASTX727 (35 mg decitabine + 100 mg cedazuridine) and/or iadademstat on the developing human fetus are unknown. For this reason and because DNMT inhibitor and LSD1 inhibitor agents are known to be teratogenic, women of child-bearing potential must agree to use adequate contraception (hormonal or barrier method of birth control; abstinence) for the duration of study participation and 6 months after completion of ASTX727 (35 mg decitabine + 100 mg cedazuridine) and/or iadademstat 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 for the duration of study participation and 6 months after completion of ASTX727 (35 mg decitabine + 100 mg cedazuridine) and/or iadademstat administration

• Women of child-bearing potential must agree not to donate or freeze egg(s) during the course of this study or within 180 days after receiving their last dose of study drug. Male patients must agree not to donate sperm during the course of this study or within 180 days after receiving their last dose of study drug

• Ability to understand and the willingness to sign a written informed consent document. Legally authorized representatives may sign and give informed consent on behalf of study participants

• Patient is able to swallow oral medications

• Patients must have a body weight of at least 50 kg due to the use of flat doses. If a patient is on continued treatment and is receiving benefit, but falls below 50 kg, they may stay on the study per investigator discretion. Otherwise, they will have to come off the study

• Peripheral white blood cell (WBC) count \<25 x 10\^9/L on day 1 prior to treatment initiation. Hydroxyurea is allowed for cytoreduction until 24 hours prior to study treatment