Evaluation of consistency for multiple experiments of a single combination in the time-dependence mixture toxicity assay

摘要

Test-to-test consistency was evaluated for a single binary combination of organic chemicals using an assay that examined toxicity over multiple exposure times. Six experiments were conducted. The toxicity of 3-chloro-2-butanone (3C2B), methyl crotonate (MC) and the mixture (MX) was evaluated in each experiment at 15, 30 and 45-min of exposure using the Microtox^® system. Concentration-response (x/y) curves were generated via the five-parameter logistic minus one-parameter (5PL-1P) curve-fitting function and used to develop predicted x/y curves for the dose-addition (DA) and independence (I) models of combined effect. A variety of toxicity (e.g., EC50) and time-dependent toxicity (TDT) endpoints, 5PL-1P parameters, and various combined effects metrics (e.g., MX/DA) were calculated. Test-to-test consistency was evaluated via the coefficient of variation (CV) or, for TDT, the standard deviation of mean values. In the study, CVs obtained for single-chemical and mixture toxicity endpoints (EC25, EC50, and EC75) at each exposure time were <20, as were those for the predicted DA and I curves. For the MX/DA metric, mixture toxicity was consistent with that predicted for DA at each exposure time in each experiment with CVs <6, despite some substantial variation in TDT for MC-alone at the EC25 and for the 30 to 45-min time-interval. There was lower variation in TDT for 3C2B and for MX. Mean and CV values for 5PL-1P-derived slope and asymmetry parameters were also assessed to provide bases for comparisons in future reports.