Can in vitro assays account for interactions between inorganic co-contaminants observed during in vivo relative bioavailability assessment?

In vitro assays act as surrogate measurements of relative bioavailability (RBA) for inorganic contaminants. The values derived from these assays are routinely used to refine human health risk assessments (HHRA). Extensive in vitro research has been performed on three major inorganic contaminants; As, Cd and Pb. However, the majority of these studies have evaluated the contaminants individually, even in cases when they are found as co-contaminants. Recently, in vivo studies (animal model) have determined that when the three aforementioned contaminants are present in the same soil matrix, they have the ability to influence each other's individual bioavailability. Since in vitro assays are used to inform HHRA, this study investigated whether bioaccessibility methods including the Solubility/Bioavailability Research Consortium (SBRC) assay, and physiologically based extraction test (PBET), have the ability to detect interactions between As, Cd and Pb. Using a similar dosing methodology to recently published in vivo studies, spiked aged (12 years) soil was assessed by evaluating contaminant bioaccessibility individually, in addition to tertiary combinations. In two spiked aged soils (grey and brown chromosols), there was no influence on contaminant bioaccessibility when As, Cd and Pb we present as co-contaminants. However, in a red ferrosol, the presence of As and Pb significantly decreased (p < 0.05) the bioaccessibility of Cd when assessed using gastric and intestinal phases of the SBRC assay and the PBET. Conceivable, differences in key physico-chemical properties (TOC, Fe, Al, P) between the study soils influenced contaminant interactions and bioaccessibility outcomes. Although bioaccessibility methods may not account for interactions between elements as demonstrated in in vivo models, in vitro assessment provides a conservative prediction of contaminant RBA under co-contaminant scenarios.

Influence of sample matrix on the bioavailability of arsenic, cadmium and lead during co-contaminant exposure.

In this study, the influence of sample matrix on the relative bioavailability of arsenic (As), cadmium (Cd) and lead (Pb) was assessed following exposure of C57BL/6 mice to spiked aged (12years) soils. AIN93G mouse chow was amended with individual and tertiary As, Cd and Pb soil combinations which were administered to mice over a 9day exposure period. Contaminant relative bioavailability was calculated by comparing As urinary excretion and Cd-kidney/Pb-liver accumulation to corresponding values for compounds used to derive the respective toxicity reference value. Strong linear dose-responses were observed for mice exposed to AIN93G mouse chow augmented with individually spiked soil with As, Cd and Pb. When mice were exposed to co-contaminants, As relative bioavailability (RBA) decreased similar to results from previous co-contaminant salt experiments presumably due to the influence of Cd on phosphate transport proteins, which are utilized for As absorption. However, a decrease in Cd-kidney and Pb-liver accumulation was also observed following co-co-exposure. It was postulated that this resulted from interactions with other (essential) metals (e.g. iron, aluminium, manganese, magnesium) within the soil matrix and their influence on absorption via divalent metal transporters.

Influence of co-contaminant exposure on the absorption of arsenic, cadmium and lead.

Incidental ingestion of contaminated soil and dust is a major pathway for human exposure to many inorganic contaminants. To date, exposure research has focused on arsenic (As), cadmium (Cd) and lead (Pb), however, these studies have typically assessed metal(loid) bioavailability individually, even when multiple elements are present in the same matrix. As a consequence, it is unclear whether interactions between these elements occur within the gastro-intestinal tract, which may impact absorption and accumulation. In this study, the influence of contaminant co-exposure was assessed using a mouse bioassay and soluble forms of As, Cd and Pb supplied in mouse chow as individual, binary and tertiary elemental combinations. Arsenic urinary excretion and Pb-liver accumulation were unaffected by As-Pb co-exposure (1-10 mg As kg-1 and 3-30 mg Pb kg-1) while Cd-kidney accumulation was unaffected by the presence of As and/or Pb. However, Cd co-exposure decreased As urinary excretion and increased Pb-liver accumulation. It was hypothesized that Cd influenced arsenate absorption as a consequence of the impairment of phosphate transporters. Although the reason for increasing Pb-liver accumulation following Cd co-exposure is unclear, enhanced Pb accumulation may occur as a result of transport protein overexpression or changes in divalent metal compartmentalization.

Assessment of arsenic speciation and bioaccessibility in mine-impacted materials.

Mine-impacted materials were collected from Victoria, Australia and categorized into three source materials; tailings (n=35), calcinated (n=10) and grey slimes (n=5). Arsenic (As) concentrations in these materials varied over several orders of magnitude (30-47,000mgkg(-1)), with median concentrations of 500, 10,800 and 1500mgkg(-1), respectively. When As bioaccessibility was assessed using the Solubility Bioaccessibility Research Consortium (SBRC) assay, As bioaccessibility ranged between 4 and 90%, with mean gastric phase values of 30%, 49% and 82% for tailings, calcinated and grey slimes, respectively. An analysis of variance (ANOVA) determined that As bioaccessibility was significantly different (P<0.05) between source materials. This was due to differences in As mineralogy, soil particle size as well as the concentration and nature of Fe present. X-ray Absorption Near Edge Structure (XANES) analysis identified arseniosiderite, yukonite, realgar, loellingite and mineral sorbed arsenate species in mine-impacted materials. Despite differences in physicochemical properties, 'mine wastes' are often reported under a generic descriptor. Outcomes from this research highlight that variability in As bioaccessibility can be prescribed to As mineralogy and matrix physicochemical properties, while categorizing samples into sub-groups can provide some notional indication of potential exposure.