CD33 and acute myeloid leukemia: Gemtuzumab ozogamicin was originally approved for monotherapy in CD33+ AML patients aged ≥60 years in May 2000 but unfortunate features such as hepatotoxicity and BM toxicity caused the FDA to retract approval (Fig. 1).363 Further clinical trials were conducted to determine safety and efficacy of gemtuzumab ozogamicin at lower doses in combination with currently approved therapies, it was shown to prolong OS in AML patients, and it was well tolerated.