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Gemtuzumab ozogamicin (GO) consists of the cluster of differentiation 33 (CD33) antibody linked to calicheamicin. The binding of GO to the CD33 antigen on leukemic cells results in internalization and subsequent release of calicheamicin, thereby inducing DNA strand breaks. We hypothesized that the poly (ADP-ribose) polymerase inhibitor olaparib might inhibit DNA repair initiated by GO-induced DNA strand breaks, thereby increasing cytotoxicity.The human myeloid leukemia cell line HL-60 and a GO-resistant variant (HL/GO20) were used.The 50% growth-inhibitory concentrations (IC50) were 24 ng/ml for HL-60 cells and 550 ng/ml for GO-resistant variant HL/GO20 cells. HL/GO20 cells were also refractory to GO-induced apoptosis. CD33 positivity was reduced in HL/GO20 cells. Olaparib-alone did not inhibit the cell growth and did not induce apoptosis in either HL-60 cells or HL/GO20 cells at concentrations of up to 10 μM. When cells were treated with different concentrations of GO in the presence of 10 μM olaparib, the IC50 of GO for HL-60 cells was 13 ng/ml. The combination index was 0.86, indicating synergistic cytotoxicity of GO and olaparib in combination. Such a combination was ineffective for HL/GO20 cells.GO and olaparib exerted synergistic cytotoxicity in CD33-positive myeloid leukemia cells in vitro.

Clofarabine is transported into leukemic cells via the equilibrative nucleoside transporters (hENT) 1 and 2 and the concentrative nucleoside transporter (hCNT) 3, then phosphorylated by deoxycytidine kinase (dCK) and deoxyguanosine kinase (dGK) to an active triphosphate metabolite. Cytarabine uses hENT1 and dCK for its activation. We hypothesized that cytarabine-resistant leukemia cells retain sensitivity to clofarabine.Human myeloid leukemia HL-60 cells and cytarabine-resistant variant HL/ara-C20 cells were used in the present study.Despite 20-fold cytarabine resistance, the HL/ara-C20 cells exhibited only a 6-fold resistance to clofarabine compared to HL-60 cells. The intracellular concentration of the triphosphate metabolite of cytarabine was reduced to 1/10, and that of clofarabine was halved in the HL/ara-C20 cells. hENT1 and dCK were reduced, but hCNT3 and dGK were not altered in the HL/ara-C20 cells, which might contribute to their retained capability to produce intracellular triphosphate metabolite of clofarabine.Clofarabine was cytotoxic to leukemia cells that were resistant to cytarabine.