Phase 1a is an open-label, multi-dose, single-arm, dose-escalation study to define the toxicity profile, pharmacokinetics, and antitumor activity of SGN-33 in patients with myelodysplastic syndrome (MDS), acute myelogenous leukemia(AML), and CD33+ myeloproliferative diseases. Phase 1b includes patients with AML or MDS treated at the highest tolerated dose from phase 1a.

Inclusion Criteria:

1. Patients must have a diagnosis of MDS or AML.
2. Patients must have an ECOG performance status ≤ 2 and a life expectancy > 3 months.

Exclusion Criteria:

1. Patients who have received prior therapy with gemtuzumab ozogamicin (Mylotarg®) or other anti-CD33 monoclonal antibody treatment.
2. Patients with a prior allogeneic transplant.
3. Patients with known leptomeningeal or CNS involvement of leukemia. Patients with onset of CNS symptoms within the past 12 months will also be excluded.
4. Patients receiving chemotherapy within the last four weeks.