In vitro experimental At-211-anti-CD33 antibody therapy of leukaemia cells overcomes cellular resistance seen in vivo against gemtuzumab ozogamicin

Monoclonal anti-CD33 antibodies conjugated with toxic calicheamicin derivative (gemtuzumab ozogamicin, GO) are a novel therapy option for acute myeloid leukaemia (AML). Key prognostic factors for patients with AML are high CD33 expression on the leukaemic cells and the ability to overcome mechanisms of resistance to cytotoxic chemotherapies, including drug efflux or other mechanisms decreasing apoptosis. Alpha particle-emitting radionuclides overwhelm such anti-apoptotic mechanisms by producing numerous DNA double-stranded breaks (DSBs) accompanied by decreased DNA repair. We labelled anti-CD33 antibodies with the alpha-emitter At-211 and compared survival of leukaemic HL-60 and K-562 cells treated with the At-211-labelled antibodies, GO or unlabelled antibodies as controls. We also measured caspase-3/7 activity, DNA fragmentation and necrosis in HL-60 cells after treatment with the different antibodies or with free At-211. The mean labelling ratio of At-211-labelled antibodies was 1:1,090 +/- 364 (range: 1:738-1:1,722) in comparison to 2-3:1 for GO. Tumour cell binding of At-211-anti-CD33 was high in the presence of abundant CD33 expression and could be specifically blocked by unlabelled anti-CD33. At-211-anti-CD33 decreased survival significantly more than did GO at comparable dilution (1:1,000). No significant differences in induction of apoptosis or necrosis or DNA DSB or in decreased survival were observed after At-211-anti-CD33 (1:1,090) versus GO (1:1) treatment. Our results suggest that At-211 is a promising, highly cytotoxic radioimmunotherapy in CD33-positive leukaemia and kills tumour cells more efficiently than does calicheamicin-conjugated antibody. Labelling techniques leading to higher chemical yield and specific activities must be developed to increase At-211-anti-CD33 therapeutic effects.