Xenobiotic metabolism response to 3R4F or THS2.2 exposure: from pre-clinical to clinical
Cytochrome P450s (CYP) are prominent xenobiotic metabolism enzymes that detoxify or activate xenobiotic compounds. CYP1A2 is a key factor in the activation metabolism of various constituents found in cigarette smoke (CS), in particular of polycyclic aromatic hydrocarbons, and is indicative of CS exposure. The Tobacco Heating System (THS) 2.2 is designed to heat tobacco instead of burning it, thereby significantly reducing the levels of harmful chemicals in the aerosol compared with those found in cigarette smoke. As part of the pre-clinical THS 2.2 assessment, mRNA and protein abundance of CYP1A2 was measured in the livers of ApoE-/- mice exposed to CS from a 3R4F reference cigarette, aerosol from THS 2.2, or fresh air (sham). Elevated levels of CYP1A2 were observed in the livers of mice in the 3R4F group, but not in the THS 2.2 group, compared with the sham group. The combined enzymatic activity of CYP1A1 and CYP1B1 was further shown to be lower in human organotypic bronchial and nasal cultures exposed to THS 2.2 aerosol than in those exposed to 3R4F CS. Overall, gene expression levels of xenobiotic metabolizing enzymes and transporters were less impacted by exposure to THS 2.2 aerosol than exposure to 3R4F CS in all pre-clinical models tested. In addition, a causal biological network model that captures the xenobiotic metabolism response in the respiratory tract demonstrated weaker perturbation upon THS 2.2 aerosol compared with CS exposure at nicotine-matched exposure concentrations. Finally, in four clinical studies conducted with THS 2.2, CYP1A2 activity was found to be lower in the plasma of subjects who abstained from smoking or switched to THS 2.2 for five days compared with subjects who continued smoking. Taken together, these results demonstrate an overall reduced impact on xenobiotic metabolism for exposure to THS 2.2 compared with cigarette in non-clinical in vitro and in vivo models as well as clinical studies.