Comparison of binary mixtures of dibutyl phthalate and diisobutyl phthalate cytotoxicity towards skin and lung origin cells in vitro

In the light of current literature on the subject and legal acts that classify phthalates as a group of endocrine-disrupting chemicals, it is crucial to accurately assess their combined effect, because the disruption of endocrine homeostasis can lead to multi-directional implications causing disorders of the efficiency and functions of the body, and combined exposure to phthalates is common. The possibility of interactions between them during exposition to their mixtures is also extremely important. The aim of the study was to evaluate the cytotoxic effect of two phthalates: dibutyl and diisobutyl (which are structural isomers) on cells derived from the human respiratory system (A549) and human skin (A431) in vitro, and then to evaluate the interactions between the tested phthalates with the combined exposure of both types of cells to their equimolar (1:1) and non-molar mixtures, in which 1:3 and 3:1 ratios were used. Comparing the IC50 values determined for equimolar mixtures of DBP and DIBP (1:1) with the theoretical values, calculated on the basis of the IC50 values of each tested compound applied individually to cells, an antagonism of the cytotoxic effect in terms of cell metabolic activity and integrity of cell membranes were observed on A549 cells, while on A431 cells an equimolar mixture of the tested compounds showed a synergistic effect in terms of cytotoxicity assessed by the MTT test, while in the NRU test the differences between the obtained IC50 values and the calculated theoretical values were not statistically significant. In the combined exposure to equitoxic (1:1) mixtures of the tested compounds, antagonism of the cytotoxic effect regarding the ability of cells to proliferate was observed when both A549 and A431 cells were exposed to the lower of the concentrations tested, which was reflected in the calculated Synergy Index (SI) values. Taking into consideration that the results of cells co-exposure to chemicals can differ significantly from single exposure pattern of toxicity, it is essential to the take effects of co-exposure into account when performing the risk assessment.