Suitability of different cytotoxicity assays for screening combination effects of environmental chemicals in human fibroblasts

Cytotoxicity screening assays measuring survival, growth, colony forming ability, DNA and protein synthesis in human fibroblasts were tested for their suitability to determine combination effects. Thereby, the dose-response curves of a hydrophilic substance A alone and after pretreatment with a membrane damaging substance B were compared. Substances B were applied at concentrations which did not induce toxic effects in the assays (noec). Synergistic combination effects were demonstrated by reduction of the EC(20) value of substances A in the combination in comparison to substances A alone. The following substance pairs (substance B/substance A=membrane damaging/hydrophilic) were tested: n-dodecylbenzenesulfonic acid/2,4-dichlorophenoxyacetic acid, 2,4,6-trichlorophenol/2,4-dichlorophenoxyacetic acid, 1,1,2,2-tetrachloroethane/4-chloroaniline pentachlorophenol/CrCl3. While survival, growth, and DNA synthesis assays were suitable methods for detecting synergistic combination effects, the growth assay was the most sensitive. Here, all four substance pairs showed synergistic combination effects.