Perplexing peroxisome proliferators.

Soft agarose culture human tumour colony forming assay for drug sensitivity testing: [3H]-Thymidine incorporation vs colony counting Summary In vitro drug sensitivity testing, both by optical colony counting and by a [3H]-TdR incorporation assay, was performed on human tumour cells proliferating in soft agar cultures. Cells from two different human tumour cell lines, 5 different human tumour xenografts, and 94 different primary human tumour specimens of various histologic types were studied. Regression analysis comparing the results of the colony counting assay and the [3H]-TdR assay revealed good to excellent correlations between the two assay endpoints for quantitating the effect of in vitro anticancer drug exposure for a large number of different agents. The presence of pre-existing tumour cell aggregates complicates the performance of the optical colony counting assay. The [3H]-TdR incorporation assay is more sensitive and reproducible than the colony counting assay when performed on samples containing a large number of initially seeded tumour cell aggregates. Use of short-term in vitro tumour colony forming *assays (in agar, agarose, or methyl cellulose media) for assessing anticancer drug effects and for screening for new anticancer agents has been a popular area of oncologic research since the original publications by Salmon's group in 1977-1978 (Hamburger & Salmon, 1977; Salmon et al., 1978). In the last seven years, more than 700 publications have described the general use of soft agar tumour colony forming assays for quantitation of cancer cell proliferation and anticancer drug effects in vitro (Human Tumor Cell Cloning Bibliography, 1984). Promising reports of good correlation between results of in vitro soft agar colony forming assays and patients' clinical response or resistance to chemotherapeutic agents have been published (Salmon et al. Despite this extensive international experience, major questions remain about the biologic significance, technical performance and clinical utility of such assays