The application of heat conduction microcalorimetry to study the metabolism and pharmaceutical modulation of cultured mammalian cells

The heat produced by animal cells in culture can be used as the primary ind icator of the kinetics of their metabolism because the scalar flux of it is a function of the metabolic flux. The validity of the relationship between heat and metabolism was demonstrated theoretically through the concept of thermal advancement and in experiments by the use of continuous cultures. T his validation permitted the application of heat flux as a probe of the met abolic state of cells in culture. It consisted of an on-line heat conductio n microcalorimeter that measures the instantaneous heat flow and dividing t he smoothed signal with one obtained simultaneously using a dielectric spec trometer that records the change in capacitance as an estimate of the amoun t of viable biomass. In this mini-review, it is shown with Chinese hamster ovary cells (CHO320) genetically engineered to produce interferon-gamma IFN -gamma) that heat flux is an early signal of deteriorating metabolism in cu ltures that produce considerable amounts of toxic lactate under fully aerob ic conditions. The early detection favours the use of heat flux as the cont rol variable in fed-batch cultures. This is a particularly useful finding i n the context of the pharmaceutical industry because it will help to ensure the high fidelity of the cytokines, antibodies and vaccines produced in la rge-scale cultures. The monotonic relationship between the fluxes for heat and metabolism means that the enthalpy balance method can be employed to te st the validity of the growth reaction for cells in culture. This showed th at the crucial ratio between the substrates, glucose and glutamine, in the culture medium was incorrect at 5.5:1 instead of about 3:1, depending on th e phase of the culture. Together with other changes to the medium compositi on, an improved formulation was made that ensured faster cell growth and gr eater specific rate (flux) of IFN-gamma constitutive secretion while decrea sing glucose utilisation and, most importantly, halving the excretion of la ctate, that is toxic to the cells and harmful to the fidelity of their seco ndary products. Indirect calorimetry (oxygen uptake rate, OUR) is often fav oured over the direct technique, but the former only measures aerobic metab olism. The environmental conditions in cultures favours lactate production even under fully aerobic conditions. Developments in measuring OUR mean tha t the stationary liquid phase balance can be used successfully to make the calorimetric:respirometric (CR) ratio a valuable tool in optimising cell cu lture to grow cells that synthesise the maximum amounts of the high fidelit y secondary products. Besides the value of heat flux in improving the cultures of animal cells pr oducing heterologous products, three different techniques are examined that should be valuable in the testing the many compounds that are produced on a speculative basis as potential drugs. They are: (i) a thin-film thermopil e transducer as an immunosensor; (ii) infra-red imaging of cells cultured i n multi-well microtitre plates and (iii) integrated circuit (IC) calorimetr y for small samples and low detection limit. One or more of these methods c ould well find favour with industry in the near future. (C) 2001 Elsevier S cience B.V. All rights reserved.