Synergistic toxicity of some sulfonamide mixtures on Daphnia magna

In livestock farming, sulfonamides (SAs) are used prophylactically and simultaneously in large numbers of animals. Therefore, traces of these compounds, alone or in combination, have been repeatedly detected in the environment. Synergistic interactions among chemicals in such mixtures represent an area of concern for the regulatory authorities. In this study, the acute toxic effects of binary and ternary mixtures of SAs were evaluated in Daphnia magna, in order to verify whether, based on their individual toxicity, they jointly exert a larger effect than would be predicted by individual actions alone. First, following the Concentration Addition (CA) principle, some preliminary observations were made by testing a number of drug combinations with an expected 50% effect. Then, mixtures more recognised for their synergistic effect (four binary and two ternary) were assayed in a range of reducing concentrations. The data acquired were processed using CompuSyn software, which integrates the different shape of the curves obtained in calculating the Combination Index (CI) for the evaluation of synergistic effects. For binary mixtures, synergy was also evaluated using the curvilinear isobologram method for heterodynamic drugs. Results indicate that most of the selected mixtures exhibit a synergistic effect using the CI methodology. For binary mixtures, these findings were also confirmed by isobologram analysis. Detected synergies indicate that the CA is not always precautionary as a reference model for the evaluation of the aquatic toxicity of SAs mixtures.