Toxicity assessments for organic chemical mixtures are often described as being approximately additive. Recent mixture studies with soft electrophiles have suggested that agents with less-than fully time-dependent toxicity (TDT) may actually induce toxicity by more than one mode of toxic action within the same series of concentrations. To evaluate this concept further, four Michael acceptor electrophiles, each with a different rate of in chemico reactivity and different level of TDT, were tested with each other and in sham combinations (a single chemical tested as if it were a binary mixture) using the Microtox system. For each binary combination, each agent was tested alone and in a mixture, with toxicity assessed as inhibition of bioluminescence at 15-, 30- and 45-min of exposure. Each single agent and mixture test included seven duplicated concentrations and a duplicated control treatment. To evaluate relative reactivity, each agent was also tested with the model nucleophile glutathione (GSH). Agents with greater in chemico reactivity (mean RC(50) mM) showed greater toxicity (mean 45-min EC(50) - mM) but these were inversely related to the TDT levels of the agents. Combined effects for the sham combinations, as quantified by additivity quotient values for the EC(50) of the mixture, tended to be close to 1.00 (i.e., the dose-addition EC(50)-AQ). For true binary combinations (i.e., two chemicals tested together), the EC(50)-AQ tended to be increasingly above 1.00 when TDT levels of the agents in the mixture were more disparate. The results of this study with Michael acceptors suggested that: (i) when reactivity was fast, there was most likely a single prominent mode of toxic action, i.e., electro(nucleo)philic reactivity, leading to time-dependent toxicity at the full or high levels, (ii) when the reaction rate for a chemical was slower, two modes of action, electro(nucleo)philic reactivity and narcosis, were apparent such that the time-dependent toxicity level was lower as well, (iii) mixtures of the former agents show a combined effect that was strictly dose-additive, whereas (iv) mixtures which included one (or more) agent with a lower reaction rate had a combined effect that was approximately additive rather than strictly dose-additive.
"a full TDT – agent typically has time-dependent toxicity (TDT) > 100% partial TDT – agent typically has TDT between about 10 and 90% no TDT – agent typically has TDT <10% (see Dawson et al., 2008; 2010; 2011 "
[Show abstract][Hide abstract] ABSTRACT: In mixture toxicity, concentration-effect data are often used to generate conclusions on combined effect. While models of combined effect are available for such assessments, proper fitting of the data is critical to obtaining accurate conclusions. In this study an asymmetry parameter (s) was evaluated for data-fitting and compared with our previous approach. Inhibition of bioluminescence was assessed with Vibrio fischeri at 15, 30 and 45-min of exposure with seven or eight concentrations and a control (each duplicated) for each single-chemical (A or B) and mixture (A:B). Concentration-effect data were fitted to sigmoid curves using the four-parameter logistic function (4PL) and the five-parameter logistic minus one-parameter (5PL-1P) function. For the 4PL, parameters included minimum effect, maximum effect, EC(50) and slope, while for the 5PL-1P the minimum effect parameter was removed and an asymmetry parameter was added. A total of 72 mixture toxicity data sets were evaluated, representing 432 single-chemical and 216 mixture curves. Mean coefficients of determination (r(2)) for all 648 curves showed that the 5PL-1P gave better fitting (0.9982 ± 0.0018) than the 4PL (0.9973 ± 0.0030). For both functions, the sum-of-squares of the residuals (SS-Res) was determined for each curve. The 5-parameter rational regression best described the relationship between the decrease in sum-of-squares of the residuals (i.e., 4PL: SS-Res - 5PL-1P: SS-Res) and log s, with fitting improved the most at low values of s (s<0.8). This held even when curves with r(2) values ≤ 0.9970 were removed from the analyses. Subsequent review of the combined effects obtained via the 4PL and the 5PL-1P functions resulted in a change in the interpretation of combined effect in 39/216 (18%) cases.
"Previous work has shown that many soft electrophiles possess partial TDT, and this had been taken as potential evidence for multiple modes (or mechanisms) of toxic action for such agents (Dawson et al. 2008, 2010; Gagan et al. 2007). The results of this study support this idea, with further support coming from the variability in asymmetry parameter values with respect to differences in TDT. "
[Show abstract][Hide abstract] ABSTRACT: Four ethyl α-halogenated acetates were tested in (1) sham and (2) nonsham combinations and (3) with a nonreactive nonpolar narcotic. Ethyl iodoacetate (EIAC), ethyl bromoacetate (EBAC), ethyl chloroacetate (ECAC), and ethyl fluoroacetate (EFAC), each considered to be an SN2-H-polar soft electrophile, were selected for testing based on their differences in electro(nucleo)philic reactivity and time-dependent toxicity (TDT). Agent reactivity was assessed using the model nucleophile glutathione, with EIAC and EBAC showing rapid reactivity, ECAC being less reactive, and EFAC lacking reactivity at ≤250 mM. The model nonpolar narcotic, 3-methyl-2-butanone (3M2B), was not reactive. Toxicity of the agents alone and in mixture was assessed using the Microtox acute toxicity test at three exposure durations: 15, 30 and 45 min. Two of the agents alone (EIAC and EBAC) had TDT values >100%. In contrast, ECAC (74 to 99%) and EFAC (9 to 12%) had partial TDT, whereas 3M2B completely lacked TDT (<0%). In mixture testing, sham combinations of each agent showed a combined effect consistent with predicted effects for dose-addition at each time point, as judged by EC(50) dose-addition quotient values. Mixture toxicity results for nonsham ethyl acetate combinations were variable, with some mixtures being inconsistent with the predicted effects for dose-addition and/or independence. The ethyl acetate-3M2B combinations were somewhat more toxic than predicted for dose-addition, a finding differing from that observed previously for α-halogenated acetonitriles with 3M2B.
Archives of Environmental Contamination and Toxicology 03/2011; 61(4):547-57. DOI:10.1007/s00244-011-9663-1 · 1.90 Impact Factor
"). Experimental mixture EC 25 , EC 50 , EC 75 , and slope values were calculated and converted to additivity quotient (AQ) values, which are similar to toxic unit values often calculated in mixture toxicity studies (Dawson et al. 2008). Herein, AQ values were calculated as AQ = experimental value/predicted value for dose-addition. "
[Show abstract][Hide abstract] ABSTRACT: The concept of multiple modes of toxic action denotes that an individual chemical can induce two or more toxic effects within the same series of concentrations, for example, reactive toxicity and narcosis. It appears that such toxicity confounds the ability to develop precise predictions of mixture toxicity and makes it more difficult to clearly link a dose-additive combined effect to agents in the mixture having a single common mechanism of toxic action. This initial study of a three-part series begins to examine this issue in greater detail by testing three α-halogenated acetonitriles: (1) in sham combinations, (2) in true combinations, and (3) with a nonreactive nonpolar narcotic. Iodo-, bromo-, and chloro-derivatives of acetonitrile were selected for testing based on their electro(nucleo)philic reactivity, via the S(N)2 mechanism, and their time-dependent toxicity individually. Reactivity of each agent was assessed in tests with the model nucleophile glutathione (GSH). Each acetonitrile was reactive with GSH, but the nonpolar narcotic 3-methyl-2-butanone was not. In addition, toxicity of the agents alone and in mixtures was assessed using the Microtox(®) acute toxicity test at three time points: 15, 30, and 45 min of exposure. Each of the three agents alone had time-dependent toxicity values of about 100%, making it likely that most of the toxicity of these agents, at these times, was due to reactivity. In contrast, the nonpolar narcotic agent lacked time-dependent toxicity. In mixture testing, sham combinations of each acetonitrile showed a combined effect consistent with predicted effects for dose-addition at each time point, as did the sham combination of the nonpolar narcotic. Mixture toxicity results for true acetonitrile combinations were also consistent with dose-addition, but the acetonitrile-nonpolar narcotic combinations were generally not consistent with either the dose-addition or independence models of combined effect. Based on current understanding of mixture toxicity, these results were expected and provide a foundation for the second and third studies in the series.
Archives of Environmental Contamination and Toxicology 11/2010; 59(4):532-41. DOI:10.1007/s00244-010-9518-1 · 1.90 Impact Factor
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