In vitro assays for new drug screening: comparison of a thymidine incorporation assay with the human tumor colony-forming assay.

Because of technical limitations with the human tumor colony-forming assay (HTCFA), we determined the feasibility of using a thymidine incorporation assay (TIA) for new drug screening. Concordance between the TIA and the HTCFA was obtained (r = 0.840) with twenty coded compounds. Toxic agents without proven clinical efficacy were active in both assays, while non-toxic substances were inactive. Growth rates were higher with the TIA (75%) than with the HTCFA (45%), and the TIA could be completed in 6 days compared to 14-21 days with the HTCFA. We concluded that the TIA is a useful adjunct to in vivo tumor models for screening new anticancer agents.