Ubenimex Enhances the Gemtuzumab Ozogamicin-Induced Cytotoxicity Against Myeloid Leukemia Cells.

Gemtuzumab ozogamicin (GO) is a humanized anti-CD33 antibody conjugated to a derivative of a tumoricidal antibiotic calicheamicin. GO alone induces complete remission in 26% of AML patients in first relapse. One way to improve the response rate is to combine GO with other agents. A quantitative relationship has been shown between rates and degrees of CD33 expression and GO-induced cytotoxicity in vitro by using gene transfer to manipulate CD33 expression in myeloid cell lines. We first studied CD33 expression on myeloid leukemia cells by stimulating with several agents in vitro, and found that ubenimex, but not G-CSF, M-CSF, or ATRA, increased CD33 expression on both HL-60 and K562 cells. Ubenimex is an aminopeptidase inhibitor isolated from Streptomyces olivoreticuli and commercially available as an oral agent for clinical use for AML treatment. To investigate whether ubenimex enhances the cytotoxicity of GO, HL-60 and K562 cells were cultured with GO and/or ubenimex for 3 days. When GO was administered, cell viabilities of HL-60 and K562 were reduced to 31.5% and 90.3% as compared with control, respectively. Treatment with ubenimex alone did not influence viabilities of either HL-60 or K562. However, when cells were preincubated with ubenimex and then cultured with GO, cell viabilities decreased to 18.5% and 81.4% for HL-60 and K562, respectively, indicating that pretreatment with ubenimex enhanced GO-induced myeloid leukemia cell death in vitro. We next assessed the mechanism of cell death. The treatment with GO alone and ubenimex alone induced apoptosis in 39.2% and 2.9% of HL-60 cells, respectively. When HL-60 cells were preincubated with ubenimex and then cultured with GO, the number of apoptotic cells increased to 62.9%, demonstrating that ubenimex augments GO-induced apoptosis. Our data suggest that the priming of AML cells with ubenimex should improve the clinical efficacy of GO.