Acute and short-term toxicity of 2,3,4,6-tetrachlorophenol in rats

Assessment of the inhibition risk of chlorophenol substances on cytochrome P450 via cocktail inhibition assays

Chlorophenols (CPs) are widespread pollutants in nature. CPs have raised significant concern due to their potential hepatotoxic effects on humans. This research aimed to ascertain the inhibitory potential of eleven CPs (2-CP, 3-CP, 4-CP, 2,4-DCP, 2,3,4-TCP, 2,4,5-TCP, 2,4,6-TCP, 2,3,4,5-TeCP, 2,3,4,6-TeCP, 2,3,5,6-TeCP, and PCP) on nine human CYP isoforms (CYP1A2, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1, and 3A4). The CPs that inhibit the activity of CYP isoforms were detected with human liver microsomes (HLM) using a cocktail approach in vitro. The results demonstrated that trichlorophenols, tetrachlorophenols, and PCP strongly inhibited CYP2C8 and CYP2C9. The half inhibition concentration (IC₅₀) value of 2,3,4,6-TeCP and PCP for CYP2C8 inhibition was 27.3 μM and 12.3 μM, respectively. The IC₅₀ for the inhibition of 2,4,6-TCP, 2,3,4,6-TeCP and PCP towards CYP2C9 were calculated to be 30.3 μM, 5.8 μM and 2.2 μM, respectively. 2,3,4,6-TeCP, and PCP exhibited non-competitive inhibition towards CYP2C8. 2,4,6-TCP, 2,3,4,6-TeCP, and PCP exhibited competitive inhibition towards CYP2C9. The inhibition kinetics parameters (Ki) were 51.51 μM, 22.28 μM, 37.86 μM, 7.27 μM, 0.68 μM for 2,3,4,6-TeCP-CYP2C8, PCP-CYP2C8, 2,4,6-TCP-CYP2C9, 2,3,4,6-TeCP-CYP2C9, PCP-CYP2C9, respectively. This study also defined clear structure-activity relationships (SAR) of CPs on CYP2C8, supported by molecular docking studies. Overall, CPs were found to cause inhibitory effects on CYP isoforms in vitro, and this finding may provide a basis for CPs focused on CYP isoforms inhibition endpoints.

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.