Conjugation of protected β-lapachone to Gem-IgG1 antibodies, which contain the variable region of gemtuzumab, results in homogeneous, systemically non-toxic and conditionally stable CD33+-specific antibody–drug conjugates with in vivo efficacy against a xenograft murine model of acute myeloid leukaemia. Here, CD33 is linked to acute myeloid leukemia.