Rat Metabolism Study Suggests 3‑(3,5-Di-tert-butyl-4-hydroxyphenyl)propionic Acid as a Potential Urinary Biomarker of Human Exposure to Representative 3‑(3,5-Di-tert-butyl-4-hydroxyphenyl)propionate Antioxidants

3-(3,5-Di-tert-butyl-4-hydroxyphenyl)­propionate antioxidants, a family of synthetic phenolic antioxidants (SPAs) widely used in polymers, have recently been identified in indoor and outdoor environments. However, limited information is available concerning human exposure to these novel contaminants. In the present study, seven 3-(3,5-di-tert-butyl-4-hydroxyphenyl)­propionate antioxidants were analyzed in human urine samples of donors from the United States. None of the target SPAs were initially detected in the urine samples either before or after hydrolysis by β-glucuronidase, prompting us to probe the major metabolites of these SPAs. We conducted rat metabolism studies with two representative congeners, tetrakis­(3-(3,5-di-tert-butyl-4-hydroxyphenyl)­propionate) (AO1010) and N,N′-bis­[3-(3,5-di-tert-butyl-4-hydroxyphenyl)­propionyl]­hydrazine (AO1024). Neither AO1010 nor AO1024 was detected in rat urine, while 3-(3,5-di-tert-butyl-4-hydroxyphenyl)­propionic acid (fenozan acid) was identified as a urinary biomarker for these 3-(3,5-di-tert-butyl-4-hydroxyphenyl)­propionate antioxidants. Surprisingly, fenozan acid was detected in 88% of the human urine samples before hydrolysis (geometric mean: 0.69 ng/mL) and 98% of the samples after hydrolysis (geometric mean: 10.2 ng/mL), indicating prevalent human exposure to 3-(3,5-di-tert-butyl-4-hydroxyphenyl)­propionate antioxidants. To our knowledge, this is the first study reporting the occurrence of fenozan acid in urine, where it can act as a potential biomarker of human exposure to 3-(3,5-di-tert-butyl-4-hydroxyphenyl)­propionate antioxidants.