Pharmacokinetics and tissue distribution of the aryl hydrocarbon receptor (AHR) ligand, 6-formylindolo[3,2-b]carbazole (FICZ) in rat: implications for AHR activation and Cytochrome P450 enzyme activity

The aryl hydrocarbon receptor (AHR) ligand, 6-formylindolo[3,2-b]carbazole (FICZ), plays a pivotal role in modulating various biological processes, including circadian rhythms and maintenance of cellular homeostasis. This study aimed to investigate the pharmacokinetics, tissue distribution, and metabolic clearance of FICZ in vivo using high-performance liquid chromatography (HPLC) with fluorescence detection and an ethoxyresorufin-O-deethylase (EROD) assay. FICZ was administered intraperitoneally to rats in varying doses (42.6 µg/kg, 85.2 µg/kg, 127.9 µg/kg, and 426.4 µg/kg) at multiple time points (1, 3, and 6 h). The liver was identified as the primary organ for FICZ distribution, with peak concentrations observed at 1 h, whereas metabolism by cytochrome P450 enzymes, particularly CYP1A1, led to a significant reduction in FICZ levels over time. Additionally, heart, testis, and brain tissues exhibited varying FICZ accumulation patterns, with dose-dependent distribution observed in testes. The findings highlight the tissue-specific pharmacokinetics of FICZ, with its distribution influenced by both dose and exposure time. EROD activity, a marker for CYP1A1 induction, peaked at 3-h post-administration and was inversely correlated with FICZ degradation. The rapid metabolism of FICZ, exacerbated by its susceptibility to photodegradation, complicates the detection and quantification of endogenous FICZ in vivo. This study presents the first comprehensive assessment of FICZ pharmacokinetics and tissue distribution in various rat tissues, including the liver, heart, brain, prostate, and testis, while also exploring its link to CYP1A1 modulation. This study underscores the complexity of FICZ’s pharmacokinetic, suggesting that both dose- and tissue-specific responses play crucial roles in AHR activation and subsequent enzyme activity. These results contribute to a deeper understanding of FICZ’s biological effects, offering insights into its therapeutic potential and toxicological implication.