Uptake, disposition and acute effects of inhaled organic solvents : Sex differences and influence of cytochrome P450 2E1 in human volunteers

Abstract: The main aim of the present thesis was to study the influence of sex, cytochrome P450 2E1 and chlorzoxazone on the uptake and metabolism of solvents in humans. Sex differences in acute effects of organic solvents were also assessed. Study I: Many solvents, including acetone and toluene, are to a large extent metabolised by cytochrome P450 2E1 (CYP2E1). Also some pharmaceuticals such as chlorzoxazone are mainly metabolised by CYP2E1. Volunteers were exposed in an exposure chamber to acetone or toluene vapour at a dose equal to the Swedish occupational exposure limit, with and without intake of chlorzoxazone. The solvents were measured in inhaled air, exhaled air, capillary blood and urine. The levels of chlorzoxazone, its metabolite 6‑hydroxychlorzoxazone and hippuric acid, a metabolite of toluene, were also measured. Chlorzoxazone intake did not result in significant effects on acetone or toluene disposition; however, we found indications of slightly decreased biotransformation of acetone and possibly also of toluene after intake of chlorzoxazone. In addition, both acetone and toluene seem to delay the metabolism of chlorzoxazone. The most likely explanation for the effect is competitive inhibition of CYP2E1. Study II: To study the reliability of chlorzoxazone as an in vivo probe for CYP2E1 activity in humans, a series of experiments were performed. The influence of genotype, sampling time, dose, and ethanol intake were studied. In addition estimates of the intra- and inter-individual and short-term and long-term intra-individual variability in metabolic ratio were made. A positive correlation with body weight suggested dose-dependent metabolism of chlorzoxazone. The intra-individual metabolic ratio in long-term (yearly intervals) and short-term (weekly intervals) variability were similar. The metabolic ratio decreased with increasing chlorzoxazone dose, which supported that metabolism was dose-dependent. CYP2E1 genotypes or ethanol intake the preceding evening did not influence the metabolic ratio. In summary, the metabolism of chlorzoxazone is relatively stable over time, but appears to be dose-dependent at commonly used doses. It is therefore advisable to adjust the dose for body weight. Study III-V: To study possible sex differences in acute effects and the influence of sex on uptake and metabolism of solvents, women and men were exposed to vapours of m‑xylene, of 2‑propanol and to clean air in an exposure chamber. The volunteers were exposed for two hours at the Swedish occupational exposure limit. Effect measurements included blinking frequency, pulmonary function, nasal swelling, colour vision and inflammatory markers in nasal lavage. In addition, the subjects rated symptoms in a questionnaire. Body fat, weight and height were measured to assess the body build. The solvents and their metabolites were analysed in inhaled air, exhaled air, capillary blood, saliva and urine. The metabolic biotransformation capacity was assessed by phenotyping for CYP2E1 with chlorzoxazone and by genotyping for several metabolic enzymes. The women tended to rate symptoms higher than men. However, there were no significant sex related differences regarding symptom ratings in response to the solvent exposure. Women showed a small but significant decrease in some pulmonary function parameters after exposure to m‑xylene. The respiratory uptake was higher and the volume of distribution larger in males. The women had a slightly shorter half time of 2-propanol in blood but approximately four times higher 2-propanol levels in exhaled air at 10 min post-exposure and onwards. The most marked sex difference was that of salivary acetone, where levels increased more than 100-fold in women, but only about 10-fold in men after exposure to 2-propanol compared to clean air.