Mylotarg is a novel immunoconjugate directed against the CD33 antigen found on most leukemia cells. This humanized murine IgG4 monoclonal antibody is tagged with the toxin, calicheamicin. In equal molar concentrations, calicheamicin is about 3200 times more potent than adriamycin. In a Phase I study involving adult patients with relapse AML, Mylotarg has been shown to have significant anti-leukemia activity with little toxicity. The most concerning side effects of Mylotarg were prolonged neutropenia and thrombocytopenia. Phase II studies have also demonstrated good efficacy with little toxicity.