IMPROVING IN VITRO PREDICTIONS OF IN VIVO PAH BIOAVAILABILITY
Date
2018-10-24
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Type
Thesis
Degree Level
Doctoral
Abstract
Exposure assessment for incident ingestion of polycyclic aromatic hydrocarbon (PAH) contaminated soil typically assumes an absorption factor of 100%. However, gastro-intestinal (GI) absorption of PAHs from soil is known to be less than 100% and will vary based on the soil. The research herein investigates factors affecting desorption of soil PAHs and absorption into mammalian systemic circulation in order to develop an in vitro bioaccessibility model that is predictive of in vivo bioavailability, aka the absorption factor. In vivo bioavailability is determined using the juvenile swine model, a mammalian system, to determine PAH soil bioavailability. The Fed organic estimation of the human simulation test (FOREhST) is the in vitro model compared against in vivo bioavailability. The hypotheses of this thesis are (1) PAH bioavailability can be partially explained by chemical partitioning, as measured by fugacity capacity, (2) PAH bioaccessibility measurements are dependent upon energetic input of the model, (3) PAHs interact with each other influencing partitioning, bioaccessibility and bioavailability, and (4) PAH-PAH interactions at the cellular level, using an intestinal porcine enterocyte cell line (IPEC-J2), alter partitioning into cellular components and rates of metabolism affecting bioavailability measurements.
Within a soil, fugacity predicts PAH exposure (Exposure = 0.21 log Fugacity + 0.68, r2 = 0.96, p < 0.005, n=14), however between soils, fugacity does not predict plasma content of PAH compounds, with the exception of benzo(a)pyrene. Soil fugacity capacity predicts the PAH soil concentration for all five PAHs with an average slope of 0.30 (μg PAH g-1soil) Pa-1 and r2’s of 0.64-0.73. As a result of soil fugacity capacity predicting soil concentration, soil fugacity capacity was correlated to PAH bioavailability for these historically contaminated soils, with r2's of 0.45-0.66, however benzo(k)fluoranthene and benzo(a)pyrene had much weaker correlations with r2 values of 0.13 and 0.14, respectively. These findings suggest that soil and chemical dependent properties of fugacity and fugacity capacity can partially explain the variability associated with PAH exposure, soil concentration and bioavailability.
Shaking method significantly affected PAH bioaccessibility in the FOREhST model, with PAH desorption from the high energy FOREhST an order of magnitude greater compared to the low energy FOREhST. PAH-PAH interactions significantly influenced PAH bioavailability and when these interactions were used in a linear model, the model predicted benzo(a)anthracene bioavailability with a slope of 1 and r2 of 0.66 and for benzo(a)pyrene bioavailability has a slope of 1 and r2 of 0.65. When spiking low levels of benzo(a)anthracene into the FOREhST model with soil, a significant increase (p < 0.05) in bioaccessibile benzo(a)pyrene was observed. When spiking low levels of fluoranthene into the FOREhST model with soil, no significant differences in benzo(a)anthracene was observed.
Co-exposure of IPEC-J2 cells to fluoranthene/benzo(a)anthracene mixture significant increases the partitioning to media, opposed to partitioning to cellular components. Furthermore, a fluoranthene/benzo(a)anthracene mixture significantly increases the metabolism of benzo(a)anthracene from media when compared to solo exposure of benzo(a)anthracene. Notably, a chrysene/fluoranthene/benzo(a)anthracene mixture results in a significant increase of benzo(a)anthracene partitioning to media while no significant difference in the disappearance of benzo(a)anthracene from media was observed compared to solo benzo(a)anthracene exposure. Co-exposure of IPE-J2 cells to benzo(a)anthracene/benzo(a)pyrene mixture significantly increases the partitioning to media compared to solo benzo(a)pyrene exposure but no significant difference in benzo(a)pyrene metabolism.
PAH in vivo bioavailability is a function of a multitude of factors, including but not limited to PAH mixtures influencing PAH partitioning to soil, simulated intestinal fluid, and cellular components, and PAH mixtures influencing their own relative metabolism. By accounting for the partitioning effects of PAH mixtures through the use of statistical modelling tools, co-inertia (COIA) and structural equation modelling (SEM), better in vitro predictions are made. PAHs mixtures influence PAH cellular partitioning and metabolism, influencing PAH bioavailability, however the simple 3 PAH mixtures used here may not wholly explain the complicated cellular interactions influencing partitioning and metabolism.
Description
Keywords
Polycyclic Aromatic Hydrocarbons, Bioaccessibility, Bioavailability, Soil
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Toxicology Centre
Program
Toxicology