Assessment of in Vitro Drug Sensitivity of Human Tumor Cells Using [3H]thymidine Incorporation in a Modified Human Tumor Stem Cell Assay

We have developed a method for performing in vitro drug testing on primary human tumor explants which is a variant of the human tumor stem cell assay (HTSCA) described previously by Salmon et al. (N. Engl. J. Med. 298: 1321, 1978). The method utilizes a cell-containing liquid top layer and a soft-agar bottom layer. Tumor growth is measured by [3H]thymidine incorporation into material precipitable by 5% trichloroacetate. Results show linear correlations with number of cells plated and with a number of colonies per plate measured using the HTSCA, when cell aliquots from one sample are used. In vitro drug sensitivity, as determined by inhibition of [3H]thymidine incorporation, correlates with HTSCA results in 54 of 61 determinations (89%). Of 22 experiments in which drug sensitivity curves were compared, 21 (95%) were similar in both systems. The [3H]thymidine method yields results more quickly (5 days after samples are plated) and with smaller variances than those measurements obtained using the HTSCA. Normal human skin muscle, lung, and colon tissue and a human fibroblast cell line do not incorporate significant amounts of [3H]thymidine into trichloroacetic acid-precipitable material. Thus, normal cell components plated in tumor samples do not interfere with assay results. Standard scintillation counting is used; optical counting, either visual or automated, is not required. Therefore, the measurement of in vitro drug sensitivity by inhibition of incorporation of radiolabeled precursors deserves further evaluation as a predictor of in vitro response.