CD33 and acute myeloid leukemia: CD33 has been found to be highly expressed on the leukemia cells of most AML patients and has developed as a targetable antigen.60 Gemtuzumab ozogamicin is an ADC of a CD33-directed humanized monoclonal antibody (mAb) that is covalently linked to N-acetyl gamma calicheamicin (cytotoxic drug).