['Chronic Lymphocytic Leukemia (CLL) is not curable in patients that are not eligible for allogeneic stem cell transplantation. Therefore, new treatment options are highly desirable. Chemically modified nonsteroidal anti-inflammatory drugs (NSAIDs), such as nitric-oxide-donating acetylsalicylic acid (NO-ASA), have been described to possess antineoplastic capacity. Recently, we could demonstrate a potent apoptosis induction in primary CLL cells in vitro and tumor growth inhibition by para-NO-ASA in a xenograft mouse model. However, little is known about the impact of positional isomerism of NO-ASA on its antineoplastic capacity in CLL.', 'Primary CLL cells were treated with the meta-or para-isomer of NO-ASA at varying concentrations and durations. Viability was assessed flow cytometrically by annexin V-FITC/PI staining and by CellTiter-Glo luminescence cell viability assay. Caspase and PARP cleavage as well as involvement of β-catenin/Lef-1 signaling was determined by immunoblotting. For caspase inhibition, BD™ ApoBlock was used. Nude mice were xenografted with JVM3 cells and treated with meta-NO-ASA, para-NO-ASA or vehicle control.', 'The meta-isomer was entirely ineffective in inducing CLL cell apoptosis in concentrations up to 100 μM, while para-NO-ASA acted in the low micromolar range. meta-NO-ASA, in contrast to para-NO-ASA, did not alter caspase activity. While para-NO-ASA action involved inhibition of β-catenin/Lef-1 signaling, meta-NO-ASA did not show any impact on this signaling pathway. Further, meta-NO-ASA did not significantly reduce tumor growth in a CLL xenograft mouse model, while para-NO-ASA was highly potent.'] Is the Antineoplastic Effect of Nitric Oxide-Donating Acetylsalicylic Acid ( NO-ASA ) in Chronic Lymphocytic Leukemia ( CLL ) Cells Highly Dependent on its Positional Isomerism?