Glucose diffusion coefficients determined from concentration profiles in EMT6 tumor spheroids incubated in radioactively labeled L-glucose

Abstract

A method for performing and evaluating autoradiography of diffusible 14C labeled substances in multicellular tumor spheroids is presented that allows one to obtain a diffusion coefficient of the substance investigated from each individual spheroid. Application of the method with 14C labeled L-glucose resulted in a glucose diffusion coefficient of 5.10-6 cm2/s. It also revealed problems of the method at very short incubation times of about 10 s or less. These problems are most likely caused by the large penetration depth of β particles irradiated by 14 labels (as compared to 3H labels) which tends to transform steep 14C concentration gradients into much more shallow optical density gradients during exposure. This transformation can be corrected for by deconvolution of the recorded optical density distributions. Basic data and mathematical tools necessary for the process of deconvolution are presently being developed. It is planned to use this method for determining diffusion coefficients of other substances of interest. One such group of substances are the metabolic waste products, most importantly lactate. Another group consists of larger molecules, e.g. peptides and comprises the various growth factors important in tumor biology. Since for members of this latter group little is known about their velocity of penetration into tissue, model calculations may be applied to predict a range of incubation times suitable for determining diffusion coefficients. Moreover, the algorithms for data analysis will have to be modified to allow for receptor binding of the substance under study. Finally, penetration of anticancer drugs and hypoxic markers into tumor spheroids is to be studied. In particular, the method presented allows for directly assessing the effects of chemical modifications of these substances which are intended to improve penetration.