Substantial growth inhibition and apoptosis without evidence of differentiation were induced in most malignant lymphocytic cells treated with 1-2 microM As2O3. As2O3 may prove useful in the treatment of malignant lymphoproliferative disorders.

All-trans retinoic acid (tRA) and arsenic trioxide (As 2 O 3 ) induce non-cross-resistant complete clinical remission in patients with acute promyelocytic leukemia with t(15;17) translocation and target PML-RAR␣, the leukemogenic protein, by different pathways suggesting a possible therapeutic synergism. To evaluate this possibility, this study examined the effect of As 2 O 3 on tRA-induced differentiation and, conversely, the effect of tRA on As 2 O 3 -induced apoptosis. As 2 O 3 at subapoptotic concentrations (0.5 M) decreased tRA-induced differentiation in NB4 cells but synergized with atRA to induce differentiation in tRA-resistant NB4 subclones MR-2 and R4 cells as measured by nitroblue tetrazolium reduction and tRA-inducible genes (TTGII, RAR␤, RIG-E). tRA cleaved PML-RAR␣ into distinct fragments in NB4 but not in tRAresistant MR-2 or R4 cells, whereas As 2 O 3 completely degraded PML-RAR␣ in all 3 cell lines. As 2 O 3 -induced apoptosis was decreased by tRA pretreatment of NB4 cells but not of R4 cells and was associated with a strong induction of Bfl-1/A1 expression, a Bcl-2 protein family member. Severe combined immunodeficient mice bearing NB4 cells showed an additive survival effect after sequential treatment, but a toxic effect was observed after simultaneous treatment with tRA and As 2 O 3 . These data suggest that combined As 2 O 3 and tRA treatment may be more effective than single agents in tRAresistant patients. Although in vitro data do not always translate to in vivo response, toxicity and potential drug antagonism may be diminished by decreasing the concentration of As 2 O 3 when given at the same time with therapeutic levels of tRA. IntroductionAcute promyelocytic leukemia (APL) is a specific type of acute myeloid leukemia characterized by the t(15;17) translocation that fuses the PML gene on chromosome 15 to the retinoic acid receptor ␣ (RAR␣) gene on chromosome 17 to form the fusion gene and leukemogenic protein PML-RAR␣. 1,2 The specific sensitivity of APL cells to all-trans retinoic acid (tRA)-induced differentiation has been exploited to achieve a 90% remission rate with a projected 60% to 70% cure of patients with APL when combined with chemotherapy. [3][4][5][6][7] However, the disease may relapse with resistance to further tRA and chemotherapy treatment. 8 The recent discovery that treatment with As 2 O 3 induces durable remission in APL patients in relapse after tRA or chemotherapy has provided a novel therapy for APL patients. 9-11 However, the reported chronic toxicities and carcinogenicity of As 2 O 3 has hampered its acceptance as a first-choice drug, 12 even though limited side effects were found in relapsed APL patients successfully treated with As 2 O 3 . 10,11 Consideration of the facts that the toxicity of As 2 O 3 is dose dependent and reversible and that small amounts of As 2 O 3 have been used in traditional Chinese medicine suggests that it may be possible to use As 2 O 3 as a first-choice drug if low concentrations prove effective.Arsenic trioxide induces both apoptos...

Low concentrations of As2O3 (≤1 μmol/L) induce long-lasting remission in patients with acute promyelocytic leukemia (APL) without significant myelosuppressive side effects. Several groups, including ours, have shown that 0.5 to 1 μmol/L As2O3 induces apoptosis in APL-derived NB4 cells, whereas other leukemic cells are resistant to As2O3 or undergo apoptosis only in response to greater than 2 μmol/L As2O3. In this report, we show that the ability of As2O3 to induce apoptosis in leukemic cells is dependent on the activity of the enzymes that regulate cellular H2O2 content. Thus, NB4 cells have relatively low levels of glutathione peroxidase (GPx) and catalase and have a constitutively higher H2O2content than U937 monocytic leukemia cells. Glutathione-S-transferase π (GSTπ), which is important for cellular efflux of As2O3, is also low in NB4 cells. Moreover, As2O3 further inhibits GPX activity and increases cellular H2O2 content in NB4 but not in U937 cells. Selenite pretreatment of NB4 cells increases the activity of GPX, lowers cellular H2O2 levels, and renders NB4 cells resistant to 1 μmol/L As2O3. In contrast, concentrations of As2O3 that alone are not capable of inducing apoptosis in NB4 cells induce apoptosis in the presence of the GPx inhibitor mercaptosuccinic acid. Similar effects are observed by modulating the activity of catalase with its inhibitor, aminotriazol. More important from a therapeutic point of view, U937 and HL-60 cells, which require high concentrations of As2O3 to undergo apoptosis, become sensitive to low, clinically acceptable concentrations of As2O3 when cotreated with these GPx and catalase inhibitors. The induction of apoptosis by As2O3 involves an early decrease in cellular mitochondrial membrane potential and increase in H2O2 content, followed by cytochrome c release, caspase 3 activation, DNA fragmentation, and the classic morphologic changes of apoptosis.