Pixantrone is a novel aza-
anthracenedione active against aggressive
lymphoma and is being evaluated for use against various hematologic and solid
tumors. The
drug is an analog of
mitoxantrone, but displays less
cardiotoxicity than
mitoxantrone or the more commonly used
doxorubicin. Although
pixantrone is purported to inhibit
topoisomerase II activity and intercalate with
DNA, exact mechanisms of how it induces cell death remain obscure. Here we evaluated the effect of
pixantrone on a panel of solid tumor cell lines to understand its mechanism of cell killing. Initial experiments with
pixantrone showed an apparent discrepancy between its anti-proliferative effects in MTS assays (short-term) compared with clonogenic assays (long-term). Using live cell videomicroscopy to track the fates of cells, we found that cells treated with
pixantrone underwent multiple rounds of aberrant cell division before eventually dying after approximately 5 d post-treatment. Cells underwent abnormal mitosis in which chromosome segregation was impaired, generating
chromatin bridges between cells or within cells containing micronuclei. While
pixantrone-treated cells did not display γH2AX foci, a marker of DNA damage, in the main nuclei, such foci were often detected in the micronuclei. Using
DNA content analysis, we found that
pixantrone concentrations that induced cell death in a clonogenic assay did not impede cell cycle progression, further supporting the lack of canonical DNA damage signaling. These findings suggest
pixantrone induces a latent type of DNA damage that impairs the fidelity of mitosis, without triggering DNA damage response or mitotic checkpoint activation, but is lethal after successive rounds of aberrant division.