Venetoclax, SL-401, and Chemotherapy for the Treatment of Blastic Plasmacytoid Dendritic Cell Neoplasm

Inclusion Criteria: 1. Treatment naïve or relapsed refractory patients with histologically confirmed diagnosis of blastic plasmacytoid dendritic cell neoplasm (BPDCN) per 2016 WHO criteria 2. Front line participants may have received emergent chemotherapy prior to study enrollment: 1. One prior cycle of SL-401, or other BPDCN-directed therapy, will be allowed prior to entering the study. 2. Prior or concomitant doses of ARA-C (cytarabine) or Hydroxyurea are allowed on before or during the study for proliferative disease due to BPDCN. 3. Relapsed/refractory participants may have received at least one prior cycle of therapy. 4. Age ≥ 18 years 5. ECOG performance status 0, 1, or 2 (see APPENDIX B) 6. Adequate organ function as defined by: * Albumin ≥ 3.2 g/dL (in the absence of receipt of intravenous albumin in the previous 72 hours) * Serum creatinine \< 1.5x ULN * Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) \< 2.5x ULN * Total bilirubin \< 1.5x ULN (if total bilirubin is \> 1.5x but \< 3x ULN, and thought to be elevated due to Gilbert's disease or the patient's BPDCN, the subject may be eligible but must discuss with the PI) 7. Ability to understand and the willingness to sign a written informed consent document. 8. Able to adhere to study visit schedule and other protocol requirements including follow-up for survival assessment 9. Women of child-bearing potential and men enrolled on this protocol must agree to use adequate contraception for the duration of study participation and for 2 months after completion VEN administration. Acceptable birth control methods allowed to be used while on study include: * Birth control pills or injections * Intrauterine devices (IUDs) * Double-barrier methods for example condom in combination with spermicide. Males should not donate sperm while on study and for at least 8 weeks after the last dose of SL-401. 10. Left ventricular ejection fraction ≥ institutional lower limit of normal by MUGA scan or echocardiogram within 30 days of first protocol treatment. Exclusion Criteria: 1. Participants is pregnant or breastfeeding 2. Known active hepatitis B or C infection, or known seropositivity for human immunodeficiency virus (HIV) 3. Major surgery or radiation therapy within 14 days prior to the first study dose 4. Symptomatic or untreated leptomeningeal disease or spinal cord compression 5. Participants with active heart disease (New York Heart Association (NYHA) class 3-4 as assessed by history and physical examination, unstable angina/stroke/myocardial infarction within the last 6 months) 6. Malabsorption syndrome or other conditions that preclude enteral route of administration 7. Other severe acute or chronic medical or psychiatric condition or laboratory abnormality that in the opinion of the investigator may increase the risk associated with study participation or investigational product administration or may interfere with the interpretation of study results and/or would make the participants inappropriate for enrollment into this study