A recombinant, humanized anti-CD33 monoclonal antibody attached to the cytotoxic antitumor antibiotic calicheamicin. In this conjugate, the antibody binds to and is internalized by tumor cells expressing CD33 antigen (a sialic acid-dependent glycoprotein commonly found on the surface of leukemic blasts), thereby delivering the attached calicheamicin to CD33-expressing tumor cells. Calicheamicin binds to the minor groove of DNA, causing double strand DNA breaks and resulting in inhibition of DNA synthesis. or A drug that was used to treat acute myeloid leukemia (AML) that recurred (came back). It was used in older patients who were not able to take other anticancer drugs. Gemtuzumab ozogamicin was taken off the market but continues to be studied in clinical trials for the treatment of certain types of leukemia. It contains a monoclonal antibody that binds to a protein called CD33, which is found on some leukemia cells. It also contains a toxic substance, which may help kill cancer cells. Gemtuzumab ozogamicin is a type of antibody-drug conjugate. Also called Mylotarg.