Time-dependence in mixture toxicity prediction

Abstract

The value of time-dependent toxicity (TDT) data in predicting mixture toxicity was examined. Single chemical (A and B) and mixture (A + B) toxicity tests using Microtox® were conducted with inhibition of bioluminescence (Vibrio fischeri) being quantified after 15, 30 and 45-min of exposure. Single chemical and mixture tests for 25 sham (A1:A2) and 125 true (A:B) combinations had a minimum of seven duplicated concentrations with a duplicated control treatment for each test. Concentration/response (x/y) data were fitted to sigmoid curves using the five-parameter logistic minus one parameter (5PL-1P) function, from which slope, EC25, EC50, EC75, asymmetry, maximum effect, and r2 values were obtained for each chemical and mixture at each exposure duration. Toxicity data were used to calculate percentage-based TDT values for each individual chemical and mixture of each combination. Predicted TDT values for each mixture were calculated by averaging the TDT values of the individual components and regressed against the observed TDT values obtained in testing, resulting in strong correlations for both sham (r2 = 0.989, n = 25) and true mixtures (r2 = 0.944, n = 125). Additionally, regression analyses confirmed that observed mixture TDT values calculated for the 50% effect level were somewhat better correlated with predicted mixture TDT values than at the 25 and 75% effect levels. Single chemical and mixture TDT values were classified into five levels in order to discern trends. The results suggested that the ability to predict mixture TDT by averaging the TDT of the single agents was modestly reduced when one agent of the combination had a positive TDT value and the other had a minimal or negative TDT value.

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

“…A previous study demonstrated that, for binary combinations, averaging the 25, 50 and 75% effect-level TDT values for the individual chemicals over the 15 to 45-min interval could be used to predict MX TDT over the same interval (Dawson et al 2014a). This was done in this study to determine test-to-test consistency of the prediction (Supplementary Table S10).…”

“…Utilizing the quorum-sensing activity of the marine bacterium Aliivibrio fischeri (Urbanczyk et al 2007), inhibition of bioluminescence was assessed using a calibrated Microtox ® 500 analyzer (Dawson et al 2014a, b). Six experiments were conducted.…”

“…Mean TDT values for a given time-interval were calculated as follows: $${mTDT}_{t1:t2}=\frac{\left({TDT}_{25}+{TDT}_{50}+{TDT}_{75}\right)}{3}$$ in which t 1 : t 2 represents the selected time-interval (i.e., 15 to 30, 30 to 45 or 15 to 45-min). It was previously shown that mTDT 15:45 values for single-chemicals were of value in predicting mTDT 15:45 for binary mixtures (Dawson et al 2014a). …”

“…By calculating TDT for each single-chemical and mixture at each effect-level and time-interval it was possible to compare observed mixture TDT (MX TDT obs ) with predicted mixture TDT (MX TDT pred ) from the single-chemical data (Dawson et al 2014a). The MX TDT obs value for a given experiment was its mTDT 15:45 value.…”

“…These studies have examined toxicity of selected mixtures of industrial organic chemicals (e.g., Gagan et al 2007, Dawson et al 2008), developed and evaluated the 5PL-1P function for concentration-response curve-fitting (Dawson et al 2012, 2016) and examined the viability of using time-dependent toxicity (TDT) of the individual chemicals for predicting TDT of the mixtures (Dawson et al 2014a). The design of these studies has included multiple tests of individual chemicals (Dawson et al 2010, 2011, 2014b, 2016) thereby allowing for assessment of test-to-test consistency of those data, but none of these studies had evaluated such consistency over several tests of the same mixture.…”