Combined effect of all-trans retinoic acid and arsenic trioxide in acute promyelocytic leukemia cells in vitro and in vivo

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Abstract
All-trans retinoic acid (tRA) and arsenic trioxide (As2O3) induce non–cross-resistant complete clinical remission in patients with acute promyelocytic leukemia with t(15;17) translocation and target PML-RARα, the leukemogenic protein, by different pathways suggesting a possible therapeutic synergism. To evaluate this possibility, this study examined the effect of As2O3 on tRA-induced differentiation and, conversely, the effect of tRA on As2O3-induced apoptosis. As2O3 at subapoptotic concentrations (0.5 μM) 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, RARβ, RIG-E). tRA cleaved PML-RARα into distinct fragments in NB4 but not in tRA-resistant MR-2 or R4 cells, whereas As2O3 completely degraded PML-RARα in all 3 cell lines. As2O3-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 As2O3. These data suggest that combined As2O3 and tRA treatment may be more effective than single agents in tRA-resistant 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 As2O3 when given at the same time with therapeutic levels of tRA.

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