CD33 and acute myeloid leukemia: Gemtuzumab ozogamicin, a CD33-targeted drug, was approved by the US FDA in 2000 as the first ADC (31) for the stand-alone treatment of relapsed and/or refractory (R/R) acute myeloid leukemia, but it was withdrawn from the market by Pfizer voluntarily in 2010 because its effectiveness was seriously challenged by the adverse event (AE) profile.