Targeting Bcl-2 Family Proteins in Adult T-Cell Leukemia/Lymphoma: In Vitro and In Vivo Effects of a Novel Bcl-2 Family Inhibitor ABT-737.

Abstract 1685

Poster Board I-711

Adult T-cell lymphoma/leukemia (ATLL) is a T-cell malignancy caused by human T-lymphotrophic virus type I (HTLV-I), and its therapeutic outcome is still remains very poor. Therefore, novel therapeutic strategies are needed to improve patient outcome. In this study, we elucidated the therapeutic potential to target anti-apoptotic Bcl-2 family proteins for the treatment of ATLL by using ABT-737 (Abbott Laboratories, Abbott Park, IL, USA), a small molecule inhibitor of Bcl-2, Bcl-X_L and Bcl-w. We first validated the rationale of this study by assessing the expression of Bcl-2 family proteins among 25 lymph-node specimens derived from ATLL patients by using immunohistochemistory. Both or either of Bcl-2 and Bcl-X_L proteins was highly expressed in 80% of specimens. We next examined the cytotoxicity of ABT-737 against ATLL cell lines. ABT-737 significantly inhibited growth of MT-1, MT-2 and HuT 102 cells with a concentration of 50 percent inhibition (IC₅₀) at 72 h of 2.4, 0.23 and 0.008μM, respectively. We then elucidated the mechanism of growth inhibition induced by ABT-737 using MT-1 and MT-2 cells. ABT-737 induced apoptosis in MT-1, MT-2 cells with cleavage of caspase 9, 3 and PARP. ABT-737 also induced apoptosis in fresh tumor cells derived from patients with ATLL. We next elucidated the potential of ABT-737 to enhance the cytotoxicity induced by conventional chemotherapeutic agents. The interaction between them was evaluated using the Chou-Talalay method by determining the combination index. ABT-737 synergistically enhanced the cytotoxicity and apoptosis induced by either of doxorubicin, vincristine or etoposide, which is a current key drug to treat ATLL. Most importantly, ABT-737 significantly inhibited tumor growth of in vivo ATLL model using SCID mice inoculated by HuT 102 cells subcutaneously. The mean tumor volume, weight and serum level of soluble interleukin-2 receptor á of ABT-737 (100mg/kg/day)-treated mice were significantly lower than those of vehicle-treated mice after treatment for 21 days. Moreover, massive induction of apoptosis in tumors treated by ABT-737 was observed by immunofluorescent TUNEL assay. These results suggest that ABT-737 used either alone or in combination with conventional cytotoxic drugs, represents a promising novel targeted approach to overcome drug resistance and improve patient outcome in ATLL.