Toxicokinetics and structure-activity relationships of nine para-substituted phenols in rat embryos in vitro

Integrating in vitro data and physiologically based kinetic (PBK) modelling to assess the in vivo potential developmental toxicity of a series of phenols

Toxicity outcomes derived in vitro do not always reflect in vivo toxicity values, which was previously observed for a series of phenols tested in the embryonic stem cell test (EST). Translation of in vitro data to the in vivo situation is therefore an important, but still limiting step for the use of in vitro toxicity outcomes in the safety assessment of chemicals. The aim of the present study was to translate in vitro embryotoxicity data for a series of phenols to in vivo developmental toxic potency values for the rat by physiologically based kinetic (PBK) modelling-based reverse dosimetry. To this purpose, PBK models were developed for each of the phenols. The models were parameterised with in vitro-derived values defining metabolism and transport of the compounds across the intestinal and placental barrier and with in silico predictions and data from the literature. Using PBK-based reverse dosimetry, in vitro concentration–response curves from the EST were translated into in vivo dose–response curves from which points of departure (PoDs) were derived. The predicted PoDs differed less than 3.6-fold from PoDs derived from in vivo toxicity data for the phenols available in the literature. Moreover, the in vitro PBK-based reverse dosimetry approach could overcome the large disparity that was observed previously between the in vitro and the in vivo relative potency of the series of phenols. In conclusion, this study shows another proof-of-principle that the in vitro PBK approach is a promising strategy for non-animal-based safety assessment of chemicals.

Comparative toxic potency ranking of chlorophenols

Chlorophenols are prevalent in all media of the environment. The most common environmental source of pentachlorophenol (PCP) and other chlorinated phenols are via the lumber industry as a wood preservative and as a pesticide in plant production. The US Environmental Protection Agency’s (EPA) contaminant candidate list (CCL) includes a majority of these compounds as unregulated contaminants. Except for pentachlorophenol, there is a lack of human or animal data base which can be used for human health risk assessment. The specific aim of this study is to develop a rationale to use in vivo nonmammalian, in vitro mammalian and nonmammalian, micro-organism toxicity data base, structural activity, mechanistic and toxicokinetic data bases for developing a relative toxic potency ranking scheme of chlorophenols. Although the toxic potency of chlorophenols was found to increase with the number of chlorines, the potency decreases if the chlorines are attached in the ortho position of the molecules. Based on the LOAELs and mammalian in vitro data, the relative potency of chlorophenols determined to be best estimated by the ratios of log Kow to the 0.55 power. The relationship of the toxic potency derived from such an approach is largely presumptive.

Relative embryotoxic potency of p-substituted phenols in the embryonic stem cell test (EST) and comparison to their toxic potency in vivo and in the whole embryo culture (WEC) assay

The applicability of the embryonic stem cell test (EST) as an alternative for in vivo embryotoxicity testing was evaluated for a series of five p-substituted phenols. To this purpose, the potency ranking for this class of compounds derived from the inhibition of cardiomyocyte differentiation in the EST was compared to in vivo embryotoxic potency data obtained from literature and to the potency ranking defined in the in vitro whole embryo culture (WEC) assay. From the results obtained it appears that the EST was able to identify the embryotoxic potential for p-substituted phenols, providing an identical potency ranking compared to the WEC assay. However, the EST was not able to predict an accurate ranking for the phenols compared to their potency observed in vivo. Only phenol, the least potent compound within this series, was correctly ranked. Furthermore, p-mercaptophenol was correctly identified as a relative potent congener of the phenols tested, but its ranking was distorted by p-heptyloxyphenol, of which the toxicity was overestimated in the EST. It is concluded that when attempting to explain the observed disparity in potency rankings between in vitro and in vivo embryotoxicity, the in vitro models should be combined with a kinetic model describing in vivo absorption, distribution, metabolism and excretion processes of the compounds.

Comparative QSAR: on the toxicology of the phenolic OH moiety

In this report we consider the effect of substituents on phenol toxicity and show how the parameters used in Quantitative Structure-Activity Relationships (QSAR) can be used to draw mechanistic inferences of value in understanding the reasons behind the various types of toxicity. In particular, we are interested in gaining clearer insight into mechanisms via the Hammett-type parameters sigma, sigma(-), sigma(+) and octanol/water parti tion coefficients. Particular attention is given to the role of radical reactions and their role in attacking DNA to cause cancer or estrogenic toxicity.

Structure-hepatic disposition relationships for phenolic compounds

Phenolic compounds are widely used in therapeutic, environmental, and industrial applications. The present work seeks to define the hepatic disposition of 11 phenolic compounds with varying lipophilicities and molecular weights. The hepatic disposition kinetics were studied in a once-through in situ rat liver perfusion preparation in order to avoid extra-hepatic metabolism and recirculation effects. The phenols were administered using the impulse-response technique and the time course of hepatic venous effluent concentration was examined by moments and a two-compartment dispersion model. While the extraction of the phenolic compounds was relatively independent of lipophilicity, the estimated permeability-surface area (PS) product for influx of solutes into the hepatocytes could be related to the compounds' octanol-buffer partition coefficients (log Papp). This log PS-logPapp relationship was consistent with that reported earlier for another series of solutes with a wide range of lipophilicity. The metabolites produced from each of the phenolic compounds used in this study had mean transit times similar to those of their corresponding parent phenols, suggesting that the metabolites were not trapped in the liver as a consequence of their higher polarity. It is concluded that the strong solute lipophilicity-toxicity and lipophilicity-skin penetration relationships often seen for aqueous solutions of phenols are not evident for the hepatic extraction of these compounds. Such a conclusion is consistent with the hepatic extraction of phenolic compounds being mainly determined by a blood flow limitation in delivery of the phenol to the liver, rather than the intrinsic liver metabolic enzyme activities at the doses injected.

Comparative QSAR in toxicology: examples from teratology and cancer chemotherapy of aniline mustards

During the past 30 years, thousands of quantitative structure-activity relationships (QSAR) have been published for all sorts of chemicals acting on many forms of life or parts thereof (DNA, enzymes, organelles, etc.). Very little effort has been made to show the relationship among these equations. In this report, we discuss two examples, the toxicity of phenols to rats and the effect of aniline mustards on a variety of living systems, where the electronic effects in the QSAR can be correlated to QSAR from physical organic chemistry. This enables one to make better mechanistic deductions about the biological structure-activity relationships. From this, it is concluded that radicals formed from the phenols cause birth defects.

Disposition kinetics of chemicals with carboxylic, hydroxyl, and amino groups in the near-term fetus of the rat: effect of van der Waals volume on disposition kinetic characteristics

Disposition kinetics from the near-term fetus of the rat were studied by subcutaneous injection of chemicals into the fetus. These studies were conducted with a series of chemicals having carboxylic, hydroxyl, and amino groups. In homologous groups, the mean residence time (MRT) increased with molecular weight, and clearance (Cl) decreased. These pharmacokinetic parameters in each series were correlated significantly with van der Waals volume (Vw). MRT of the hydroxyl group series was higher and Cl lower when compared with chemicals of other series with similar Vw. The steady-state volume of distribution (Vss) of each series showed no remarkable change with molecular weight. Vss of the hydroxyl group series was the highest, followed in order by those of carboxylic and amino groups. The tissue/plasma concentration ratio of the amino group series was higher than those of other series.