Human Metabolism and Urinary Excretion Kinetics of Nonylphenol in Three Volunteers after a Single Oral Dose

Abstract

Nonylphenol (NP) is an endocrine-disrupting anthropogenic chemical that is ubiquitous in the environment. Human biomonitoring data and knowledge on internal NP exposure are still sparse, and its human metabolism is largely unknown. Therefore, in this study, we investigated human metabolism and urinary excretion of NP. Three male volunteers received a single oral dose of 1 mg ¹³C₆-labeled NP (10.6-11.7 μg/kg body weight). Consecutive full urine voids were collected for 48 h. A metabolite screening identified nine ring- and/or side chain-oxidized metabolites. We chose the most promising hits, the alkyl chain-oxidized metabolites hydroxy-NP (OH-NP) and oxo-NP, for quantitative investigation next to the parent NP. For this purpose, we newly synthesized specific n - 1-oxidized monoisomeric analytical standards. Quantification of the polyisomeric metabolites was performed via online-solid phase extraction-LC-MS/MS with stable isotope dilution using a previously published consensus method. Alkyl chain hydroxylation (OH-NP) constituted the major metabolism pathway representing 43.7 or 62.2% (depending on the mass transition used for quantification) of the NP dose excreted in urine. The urinary excretion fraction (F_UE) for oxo-NP was 6.0 or 9.3%. The parent NP, quantified via an analogous isomeric ¹³C₆-NP standard, represented 6.6%. All target analytes were excreted predominately as glucuronic acid conjugates. Excretion was rather quick, with concentration maxima in urine 2.3-3.4 h after dosing and biphasic elimination kinetics (elimination half-times first phase: 1.0-1.5 h and second phase: 5.2-6.8 h). Due to its high F_UE and insusceptibility to external contamination (contrary to parent NP), OH-NP represents a robust and sensitive novel exposure biomarker for NP. The novel F_UEs enable to robustly back-calculate the overall NP intakes from urinary metabolite levels in population samples for a well-informed cumulative exposure and risk assessment.