Metabolism and urinary excretion kinetics of 2,4-di-tert-butylphenol after a single oral dose: Characterisation of suitable exposure biomarkers for human biomonitoring.

2,4-Di-tert-butylphenol (DTBP) is a degradation product of the antioxidant and plastic additive tris(2,4-di-tert-butylphenyl)phosphite. DTBP can migrate into food or drinking water from high production volume polyolefins containing this additive, leading to potential human exposures, confirmed by findings of DTBP in blood and urine. Natural sources of DTBP have also been described. We have recently developed a sensitive analytical method for the quantification of urinary DTBP and its postulated oxidised metabolite 2-tert-butyl-4-(2-hydroxy-1,1-dimethylethyl)phenol (DTBP-4-OH) after enzymatic cleavage of glucuronides. This method has now been employed in a human metabolism study. Five adult volunteers were orally dosed with 9.715mg of DTBP and their urine voids were quantitatively and separately collected for 48h. DTBP and DTBP-4-OH were quantifiable in all post-dose samples and biphasic excretion kinetics were observed. Urinary peak concentrations of the oxidised metabolite DTBP-4-OH were around six times higher than those of DTBP and occurred around the same time (1.5-3.8h). Overall, we recovered 62.2% of the oral dose as DTBP-4-OH and 9.3% as parent DTBP within 48hours. Over 95% of the excretion occurred within the first 24hours. External contamination with DTBP e.g. from commonly used polypropylene tubes was observed, potentially interfering with the determination of parent DTBP. Hence, we strongly recommend using DTBP-4-OH as the most robust and most sensitive urinary exposure biomarker in human biomonitoring studies. With the toxicokinetic information provided here, reverse dosimetry of DTBP-4-OH enables an informed exposure and risk assessment for DTBP both in environmentally and occupationally exposed populations. (247 words).