Microcalorimetric evaluation of the effect of combined chemotherapeutic drugs

The DNA methylation inhibitor 5-azacytidine modulates 6-thioguanine toxicity in mammalian cells

In order to assess the effects of combining two antimetabolites used separately to treat human leukemias, we carried out an experimental study by exposing V79 Chinese hamster cells, a 6-thioguanine (6-tG)-sensitive cell line, to sequential and concurrent treatments with 5-azacytidine (5-azaC) and 6-tG. In this paper, we demonstrate that there is a clear dependency for the way in which this combination was tested. Pre-treatment with 5-azaC made V79 cells more resistant to 6-tG by a substantial reduction in 6-tG incorporation into DNA; this effect could still be detected for several cell divisions after the removal of 5-azaC, and was achieved neither by reduced cell growth nor by the induction of hypoxanthine-guanine-phosphoribosyltransferase (HGPRT-) mutants. The reverse order of treatment produced a higher toxic effect than exposure to each prodrug alone.

Direct microcalorimetry as a technique in cell cultures

In vitro studies of energy metabolism in isolated cells contribute to improved knowledge of human energy metabolism under normal and pathological conditions. In every cellular system energy is taken up, metabolized and finally transformed into heat, which is dissipated into the environment. Thus, energy turnover of isolated cells can be assessed by microcalorimetric determination of their heat production. Microcalorimeters of the thermopile heat conduction type facilitate direct physical determination of thermogenesis with a sensitivity of 0.2 microW; 10(4)-10(5) cells being sufficient for one measurement. Peltier elements are sandwiched between the sample and a precisely thermostated heat sink, creating a detectable voltage proportional to the heat production. For adequate interpretation of the results, simultaneous biochemical investigations of relevant metabolic pathways are required. Up to now, numerous studies with blood cells, skeletal and heart muscle cells, hepatocytes, endothelial cells, fibroblasts and adipocytes have been performed in relation to various diseases and under the influence of certain hormones and pharmacological agents.

Serum-liposome interaction is an oxygen-dependent process

Measurements of heat dissipation, oxygen concentration and average vesicle size were correlated to study the effect of serum components on different types of liposome. The results indicate that the interaction between serum components and liposomes is exothermic and oxygen dependent, and leads to disruption of vesicles. The dependence of this effect on serum concentration, vesicle surface charge and type of liposome was also evaluated. Serum components did not produce any effect on conventional liposomes in the absence of oxygen. Moreover, in hypoxic conditions the serum-liposome interaction was delayed. Both results suggest that this interaction is an oxygen-dependent event. Finally, we confirmed that sterically stabilised liposomes remain unalterated in the presence of serum.