Characterization and Biological Potency of Mono- to Tetra-Halogenated Carbazoles

Abstract

This paper deals with the characterization and aryl hydrocarbon receptor (AhR) agonist activities of a series of chlorinated, brominated, and mixed bromo/chlorocarbazoles, some of which have been identified in various environmental samples. Attention is directed here to the possibility that halogenated carbazoles may currently be emitted into the environment as a result of the production of carbazole-containing polymers present in a wide variety of electronic devices. We have found that any carbazole that is not substituted in the 1,3,6,8 positions may be lost during cleanup of environmental extracts if a multilayer column is utilized, as is common practice for polychlorinated dibenzo-p-dioxin (dioxin) and related compounds. In the present study, (1)H NMR spectral shift data for 11 relevant halogenated carbazoles are reported, along with their gas chromatographic separation and analysis by mass spectrometry. These characterization data allow for confident structural assignments and the derivation of possible correlations between structure and toxicity based on the halogenation patterns of the isomers investigated. Some halogenated carbazoles exhibit characteristics of persistent organic pollutants and their potential dioxin-like activity was further investigated. The structure-dependent induction of CYP1A1 and CYP1B1 gene expression in Ah-responsive MDA-MB-468 breast cancer cells by these carbazoles was similar to that observed for other dioxin-like compounds, and the magnitude of the fold induction responses for the most active halogenated carbazoles was similar to that observed for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). 2,3,6,7-Tetrachlorocarbazole was one of the most active halogenated carbazoles and, like TCDD, contains 4 lateral substituents; however, the estimated relative effect potency for this compound (compared to TCDD) was 0.0001 and 0.0032, based on induction of CYP1A1 and CYP1B1 mRNA, respectively.

Halogenated carbazoles induce cardiotoxicity in developing zebrafish (Danio rerio) embryos

“…Therefore, the present study's results are consistent with the in vitro study in that all halogenated carbazoles were several orders of magnitude less potent than the dioxin analogue. It was also reported that only 2 lateral substituents (3 and 6) are necessary for high CYP1A induction for the chlorinated carbazoles . However, in our in vivo study, the 2 and 7 positions were necessary to become stronger CYP1A inducers.…”

“…In 1 study, 3‐chlorocarbazole, 3,6‐dichlorocarbazole, and 36‐DBCZ were found to have dioxin‐like activity using ethoxyresorufin O ‐deethylase induction in rat hepatoma cells . In the other study, halogenated carbazoles showed structure‐dependent induction of cytochrome P450 (i.e., CYP1A and CYP1B) gene expression in aryl hydrocarbon–responsive MDA‐MB‐468 breast cancer cells, and the highest induction was similar to that observed for other dioxin‐like compounds . Therefore, it would be of great significance to investigate if those in vitro AhR‐mediated toxicities of polyhalogenated carbazoles could also be observed in an in vivo animal model.…”

“…Because 2367-TCCZ is identical to 2378-TCDD in halogen substitution positions, this suggests that halogenated carbazoles are less potent than their dioxin analogues in inducing CYP1A expression. In an in vitro study using a cell line assay, the median effect concentration values for CYP1A induction were 2500 nM, 3000 nM, 2700 nM, 150 nM, 450 nM, and 0.046 nM for 3-BCZ, 27-DBCZ, 36-DBCZ, 1368-TBCZ, 2367-TCCZ, and 2378-TCDD, respectively [11]. Therefore, the present study's results are consistent with the in vitro study in that all halogenated carbazoles were several orders of magnitude less potent than the dioxin analogue.…”

A review of persistent organic pollutants: dioxins, furans, and their associated nitrogenated analogues

“…However, halogenation of carbazole is a key subject in medicinal chemistry and materials science and for this reason they are starting materials for the synthesis of numerous bioactive molecules of pharmaceutical importance despite their well-known toxicological characteristics [91]. These nitrogen containing heterocyclic compounds such as the mono-and polyhalogenated carbazoles (PHCZs) which are commonly known as nitrogenated dioxins are suggested to originate from either anthropogenic or natural sources and have been reported to possess toxicities comparable to those of dioxin and furan compounds basically because of their structural similarities [92]. From a chemical standpoint, the toxicity of polyhalogenated carbazoles (PHCZs) has been attributed to their resemblance with polychlorinated dibenzo-p-dioxin and dibenzofuran, with very close correlation between their planar molecular conformation and dioxin-like toxicological characteristics [23].…”

“…It is important to note that halogenated carbazoles are structurally similar to polychlorinated dibenzo-p-dioxin and halogenated dibenzofurans; therefore they are believed to be extremely toxic because of their dioxin-like characteristic behaviour attributed to similarity in their molecular conformations [32]. Polyhalogenated carbazoles (PHCs) which has been established to be non-degradable in the soil [73] display toxicities similar to those of dioxins and furans and are precursors for carcinogenesis, developmental defects and lowered immunity in animals [92].…”