Schematic structure of GO. The humanized IgG₄ anti–CD33 antibody (hP67.6) contains amino acid sequences that are approximately 98.3% of human origin. Lysine residues on hP67.6 are linked to N-acetyl γ-calicheamicin dimethyl hydrazide via a hybrid 4-(4′-acetylphenoxy)butanoic acid linker. The labile hydrazone bond leading to drug release under acidic conditions, presumably within lysosomes, is shaded in green. GO has approximately 50% of the antibody loaded with 4 to 6 mol of the toxic moiety per mole of antibody; the remaining 50% of antibody is unbound. Reproduced with permission from Macmillan Publishers Ltd.¹³⁰