Separating the Role of Gut Enzymatic Transformation in Modulating Internal Exposure to Three Major Phthalates

The gastrointestinal tract (GIT) is crucial in the absorption and metabolism of xenobiotics, including phthalates─widespread environmental contaminants associated with various health risks. Estimating human exposure to phthalates via biomonitoring is challenging due to their complex metabolic pathways, resulting in a mass-balance gap between internal and external exposure. The relative contributions of the GIT and liver to phthalate metabolism remain underexplored. This study investigated the metabolism of three representative phthalate diesters─dibutyl phthalate (DBP), di(2-ethylhexyl)phthalate (DEHP), and diethyl phthalate (DEP) in GIT. We first incubated these diesters in simulated stomach and small intestine fluids to identify the primary enzyme responsible for their hydrolysis. The kinetics were further investigated under varying pH conditions (4.0, 5.0, 6.0, 7.0 or 7.5) to mimic the small intestine environment. Next, using a refined human physiologically based toxicokinetic model, we quantified the relative contributions of preabsorption intestinal versus postabsorption hepatic biotransformation to the body burden of phthalates. Our results suggested that DBP and DEHP were extensively metabolized (>90%) in the GIT by lipase, with comparatively lower hepatic involvement, while DEP underwent minimal preadsorption metabolism (13%) in the GIT, highlighting the influence of structure-dependent differences on metabolic rates. This study emphasized the importance of incorporating both intestinal and hepatic metabolism into toxicokinetic analyses. The findings demonstrate the GIT's critical role in limiting phthalate bioavailability, underscoring the need to account for the intestinal first-pass effect in toxicokinetic models to enhance predictions of phthalate pharmacokinetics and health impacts.