Microcalorimetric evaluation of the effects of methotrexate and 6-thioguanine on sensitive T-lymphoma cells and on a methotrexate-resistant subline

Isothermal microcalorimetry, a new tool to monitor drug action against Trypanosoma brucei and Plasmodium falciparum

Isothermal microcalorimetry is an established tool to measure heat flow of physical, chemical or biological processes. The metabolism of viable cells produces heat, and if sufficient cells are present, their heat production can be assessed by this method. In this study, we investigated the heat flow of two medically important protozoans, Trypanosoma brucei rhodesiense and Plasmodium falciparum. Heat flow signals obtained for these pathogens allowed us to monitor parasite growth on a real-time basis as the signals correlated with the number of viable cells. To showcase the potential of microcalorimetry for measuring drug action on pathogenic organisms, we tested the method with three antitrypanosomal drugs, melarsoprol, suramin and pentamidine and three antiplasmodial drugs, chloroquine, artemether and dihydroartemisinin, each at two concentrations on the respective parasite. With the real time measurement, inhibition was observed immediately by a reduced heat flow compared to that in untreated control samples. The onset of drug action, the degree of inhibition and the time to death of the parasite culture could conveniently be monitored over several days. Microcalorimetry is a valuable element to be added to the toolbox for drug discovery for protozoal diseases such as human African trypanosomiasis and malaria. The method could probably be adapted to other protozoan parasites, especially those growing extracellularly.

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.

Microcalorimetric evaluation of the effect of combined chemotherapeutic drugs

A study of the effect of combined antineoplastic drugs in vitro was carried out by microcalorimetric monitoring of the metabolic activity of treated cells. Power-time curves of growing T-lymphoma cell suspensions, treated with single or combined drugs, were recorded. The extent of the effect was evaluated by changes in the slopes of the microcalorimetric curves and the kinetics of the drug action were interpreted from the time at which these changes reached their maximum value. The method was validated using two well-established drug combinations, the potentiatory effect of dipyridamole on methotrexate cytotoxicity, and the synergism between methotrexate and 6-thioguanine. In the first case, where one drug is not toxic, the modulation may be evaluated by comparing the inhibition produced by the toxic drug alone and in combination with its modulator. Otherwise, when both drugs are toxic, the combined effect must be evaluated by means of their combination index. The measurement procedure is simple, the electric signal is well suited to automation of data acquisition and the response may be evaluated within 5 to 6 h of drug administration. Moreover, we demonstrate that microcalorimetry is a reliable method for the detection of modulatory effects in combination chemotherapy.