Inclusion Criteria:

          1. Disease Characteristics:

             a. Confirmation of CD123 positivity by flow cytometry or IHC. Patients who received
             prior CD123-targeting agents will be allowed as long as the blasts still have
             detectable CD123 expression.

          2. Expansion inclusion:

               -  Cohort 1 - Patients with relapsed or refractory BPDCN with 1-3 prior lines of
                  therapy

               -  Cohort 2 - Patients will have relapsed AML.

               -  Cohort 3 - Patients will have relapsed or refractory ALL (including any subtypes:
                  B-cell, T-cell, Ph+, and Ph-).

               -  Cohort 4 - Patients will have relapsed or refractory "other" hematologic
                  malignancies not included in the cohorts above (eg, high-risk/very high-risk MDS,
                  MPN, CMML, BP- CML). Other CD123+ malignancies may be considered upon discussion
                  with the Sponsor.

               -  Cohort 5 - Patients will have relapsed or refractory (to non-intense therapies)
                  AML.

               -  Cohort 6 - Patients with frontline BPDCN who have not received prior systemic
                  therapy.

        Note: Patients in Cohort 6 may have received local therapy (radiotherapy, surgical
        excision, photodynamic therapy). Eligible patients must have a recurrence or progression in
        the field of local therapy OR disease outside the field of local therapy.

        Exclusion Criteria:

          1. Patients who, in the judgment of their treating physician, have appropriate standard
             of care therapies will be excluded from Cohorts 1 through 5.

          2. Frontline BPDCN patients with central nervous system (CNS) disease will be excluded. A
             lumbar puncture must be performed during the 28-day screening period, prior to drug
             administration. Relapsed or refractory BPDCN patients with a known history of CNS
             disease must have been treated locally, have at least 1 lumbar puncture with no
             evidence of CNS disease, and must be clinically stable prior to first dose. Concurrent
             therapy for CNS prophylaxis or continuation of therapy for controlled CNS disease is
             permitted with the approval of the Sponsor.

          3. Patients with a history of veno-occlusive disease of the liver.

          4. Patients with a history of Grade 4 capillary leak syndrome, or non-cardiac Grade 4
             edema are ineligible, eg, related to tagraxofusp-erzs or other etiology.

          5. Interval from prior cancer therapy: 1. For frontline BPDCN patients with prior local
             therapy (eg, radiotherapy), patients must not have received treatment within 14 days
             prior to drug administration on this study. 2. Relapsed or refractory BPDCN patients
             must not have received any anti-cancer therapy including chemotherapy, immunotherapy,
             radiotherapy, hormonal, biologic, or any investigational agents within 14 days prior
             to drug administration on this study. Patients must have recovered to baseline from
             all acute toxicity from this prior therapy.

        Note: the exception that patients who have received a checkpoint inhibitor must not have
        received that therapy within 28 days prior to drug administration on this study.