Occupational health topics series on the effects of chemicals: epidemiological and toxicological risk assessments of ortho-toluidine for bladder cancer

Objectives: Our aim was to report integrative evidence for the health risk assessment of ortho-toluidine (OT) in bladder cancer in a mini-review of the recent studies of humans, experimental animals, and OT skin permeability. Methods: Bladder cancer cases were identified in workers in Japan who were occupationally exposed in 2014-2017 to aromatic amines, primarily to the human carcinogen OT. Results: A key epidemiological study of 98 aromatic amine–exposed workers in Japan showed a clear OT exposure–response relationship with a standardized incidence ratio. A rat model experimental study also indicated that OT and acetoaceto-o-toluidine are potent bladder carcinogens. Multiple mechanisms of OT-related bladder cancer have been proposed: metabolic activation to reactive metabolites that bind DNA and proteins, mutagenicity, oxidative DNA damage, chromosomal damage, and cytotoxicity by OT. Recent comprehensive analyses of DNA adducts in rats identified a number of common oxidative DNA adducts, including 8-OHdG, in the rat urothelium and indicated that oxidative stress may play a crucial role in the development of urinary cancer caused by OT. The skin permeability of 6 aromatic amines (o-toluidine, aniline, p-toluidine, o-anisidine, 2,4-dimethylaniline, and o-chloroaniline) was examined with the use of a 3-dimensional (3D) reconstructed human skin model; ∼70%-80% of the 6 aromatic amines had permeated through the 3D skin within 8 hours. Genotoxic potency testing in a human urothelial cell line using γ -H2AX, a marker of DNA damage, suggested that OT exhibited strong γ -H2AX generation. Conclusions: Prolonged dermal exposure to OT along with other genotoxic aromatic amines over many years may contribute to the development of bladder cancer.