Phase Ib Study of Eltanexor and Venetoclax in Relapsed or Refractory Myelodysplastic Syndrome and Acute Myeloid Leukemia

Inclusion Criteria: \- Age \>/= 18 years at the time of signing the Informed Consent Form (ICF); must voluntarily sign an ICF; and must be able to meet all study requirements. For Myelodysplastic Syndrome (MDS): Morphologically confirmed diagnosis of MDS with increased blasts (\>/= 5%), with a prior DNA methyltransferase inhibitor (DNMTi) treatment and progression after 2 cycles or stable disease after 4 cycles For Acute Myeloid Leukemia (AML): Morphologically confirmed diagnosis of AML in accordance with WHO diagnostic criteria that is relapsed or refractory following \>/= 1 line(s) of therapy. * WBC must be less than 25,000/ul prior to study start (hydroxyurea allowed). * A bone marrow aspirate must be performed, and tissue collected for entrance to the trial unless circulating blasts \>/= 5% in which case, peripheral blood can be used. * Eastern Cooperative Oncology Group Performance Status of 0 - 2. * Must have adequate hepatic and renal function as demonstrated by the following: ALT(SGPT) and/or AST (SGOT) \</= 3x upper limit of normal (ULN); Direct bilirubin \</= 1.5 x ULN; or Total bilirubin \</= 2.5x ULN (known Gilbert's Syndrome as cause of elevated bilirubin is allowed); Calculated creatinine clearance \> 50 ml/min (per the Cockroft-Gault formula). \- Willingness to abide by all study requirements, including contraception, maintenance of a pill diary, and acceptance of recommended supportive care medications. Exclusion Criteria: * Anticancer therapy, including investigational agents \</= 2 weeks or \</= 5 half-lives of the drug, whichever is shorter, prior to C1D1. (Use of hydroxyurea is permitted). * Inadequate recovery from toxicity attributed to prior anti-cancer therapy to \</= Grade 1 (NCI CTCAE v5.0), excluding alopecia or fatigue. * Prior treatment with SINE compounds or other inhibitors of XPO1. * History of allogeneic hematopoietic stem cell transplant (HCT), or other cellular therapy product, within 3 months. * Active acute or chronic GVHD requiring calcineurin inhibitors or steroid dosing \>/= 10mg/day or patients within 4 weeks of stopping calcineurin inhibitors for GVHD. * Radiation therapy or major surgery within 3 weeks. * Active, uncontrolled infection. Patients with infection under active treatment and controlled with antibiotics are eligible. Prophylaxis, even if parenteral, is acceptable. * Inability to swallow oral medications. * Active documented central nervous system leukemia. * Second active malignancy within past 2 years except for basal or squamous cell carcinoma of the skin, ductal carcinoma of breast in situ or cervical carcinoma in situ. * Women of childbearing age or potential must have negative pregnancy test and must not be actively breastfeeding to enroll on the study * Clinically significant cardiovascular disease with major event or cardiac intervention within the past 6 months (e.g. percutaneous intervention, coronary artery bypass graft, documented cardiac heart failure) as determined by the investigator. * Any condition not listed but deemed by the investigator to make the patient a poor candidate for clinical trial and/or treatment with investigational agents.