Mechanisms of Action and Reduced Cardiotoxicity of Pixantrone; a Topoisomerase II Targeting Agent with Cellular Selectivity for the Topoisomerase II  Isoform

The switch from myocyte hyperplasia to hypertrophy occurs during the early postnatal period. The exact temporal sequence when cardiac myocytes cease dividing and become terminally differentiated is not certain, although it is currently believed that the transition takes place gradually over a 1-2-week period. The present investigation has characterized the growth pattern of cardiac myocytes during the early postnatal period. Cardiac myocytes were enzymatically isolated from the hearts of 1, 2, 3, 4, 6, 8, 10, and 12-day-old rats for the measurements of binucleation, cell volume and myocyte number. Almost all myocytes were mononucleated and cell volume remained relatively constant during the first 3 days of age. Increases in cell volume and binucleation of myocytes were first detected at day 4. Myocyte volume increased 2.5-fold from day 3 to day 12 (1416 +/- 320 compared to 3533 +/- 339 microns 3). The percentage of binucleated myocytes began to increase at day 4 and proceeded at a high rate, reaching the adult level of approximately 90% at day 12. Myocyte number increased 68% during the first 3 days (from 13.6 +/- 3.5 x 10(6) at day 1 to 22.9 +/- 5.6 x 10(10) at day 3) and remained constant thereafter. To confirm that no further myocyte division exists after 4 days, bromodeoxyuridine (Brdu) was administered to 4-day-old rats and the fate of DNA-synthesizing myocytes was examined 2 h and 2, 4, 6 and 8 days after Brdu injection. About 12% of myocytes were labeled with Brdu at 2 h and all were mononucleated at that time. Gradually, these Brdu-labeled myocytes became binucleated. However, the percentage of labeled myocytes in all groups was identical, indicating that DNA-synthesizing myocytes were becoming binucleated without further cell division after 4 days of age. Within 8 days after injection, approximately 82% of total labeled myocytes were binucleated, while the others remained mononucleated. Sarcomeric alpha-actinin was fully disassembled in dividing myocytes of 2-day-old rats, while typical alpha-actinin striations were present in dividing myocytes of 4-day-old rats. The results from this study suggest that a rapid switch from myocyte hyperplasia to hypertrophy occurs between postnatal day 3 and 4 in rat hearts.

Phosphorylated H2AX (gamma-H2AX) is essential to the efficient recognition and (or) repair of DNA double strand breaks (DSBs), and many molecules, often thousands, of H2AX become rapidly phosphorylated at the site of each nascent DSB. An antibody to gamma-H2AX reveals that this highly amplified process generates nuclear foci. The phosphorylation site is a serine four residues from the C-terminus which has been evolutionarily conserved in organisms from giardia intestinalis to humans. Mice and yeast lacking the conserved serine residue demonstrate a variety of defects in DNA DSB processing. H2AX Delta/Delta mice are smaller, sensitive to ionizing radiation, defective in class switch recombination and spermatogenesis while cells from the mice demonstrate substantially increased numbers of genomic defects. gamma-H2AX foci formation is a sensitive biological dosimeter and presents new and exciting opportunities to understand important biological processes, human diseases, and individual variations in radiation sensitivity. These potentialities demonstrate the importance of understanding the parameters and functions of gamma-H2AX formation.

Doxorubicin (Adriamycin) is a potent and broad-spectrum antineoplastic agent prescribed for the treatment of a variety of cancers, including both solid tumours and leukaemias. Unfortunately, despite its broad effectiveness, long-term therapy with doxorubicin is associated with a high incidence of a cumulative and irreversible dilated cardiomyopathy. Numerous mechanisms have been proposed to account for this toxicity. Although there is general consensus that doxorubicin undergoes redox cycling to generate free radicals that are responsible for mediating the various cytopathologies associated with drug exposure, the source and subcellular targets continue to be debated. This short review provides a synopsis of the evidence implicating cardiac mitochondria as key intracellular targets, both as sites of generation of highly reactive free radical intermediates as well as targets for the interference with cell calcium regulation and bioenergetic failure that are hallmarks of doxorubicin-induced cardiac failure.