October 1, 2010

Oral Bioaccessibility Data: Missing the Point in Exposure Assessment of Soil-borne Chemicals

Dr Sohel Saikat

Centre for Radiation, Chemicals and Environmental Hazards London

Outline

Concepts

Why Does Bioaccessibility Matter?

Some Observation on the In-vitro Bioaccessibility Data

Conclusion and Way forward

Concepts - Relationships

Total ≥≥≥≥ acid extrac. ≥≥≥≥ bioaccess≥≥≥≥ bioavail.

Bioavailable. Bioaccessible.

The Concept

Definitions

Bioavailability, absolute : Is a measure of uptake or fraction of an administered dose absorbed by the body

Relative bioavailability : Comparative bioavailabilities of different forms of a chemical or for different exposure media cont aining the chemical

Bioaccessibility, oral : Is a measure of dissolution , or fraction of contaminant released into solution from the soil duri ng digestion

[Bioavailability requires in-vivo test with live animal; bioaccessibility can be measured in-vitro (with artificial non-animal bench-top procedure)]

In-vivo Bioavailability

Routinely used in US

Animals as surrogate of human

But not feasible

Costs and time

Expertise

Reproducibility

Ethical reservation

In-vitro Bioaccessibility

Absorption depends on solubility

Incorporates certain human gut parameters depending on the specific test

• Stomach pH and small intestine pH

• Stomach mixing

• Stomach emptying rates

• Cost and time reductions

• Conforms to the three ‘ R’principle (Reduce, Refine and Replace)

Maddaloni and Beringer 2005

In-vitro Method Development

Contaminated soils

In-vivo study In-vitro studyCompare

BioaccessibilityBioavailability

Why Does Bioaccessibility / Bioavailability Matter?

Toxicity related to absorbed dose not to applied do se

Some chemicals are readily absorbed from the gut than others

Default assumption that a chemical is equally bioavailable in all media

Tox study is based on high doses with most soluble form of chemical

May not be appropriate where contaminant is bound

Bioavailability in Risk Assessment

Contaminants Phases

In pore water

Weakly absorbed

Associated with carbonate

Associated with Fe, Mn oxides

Complexed by organics

Associated with sulphide

In the mineral lattice

Free aquoion

Inorganiccomplexes

Org. complexes & chelates

T

O

T

A

L

Total = available + sequestered

HIGH

LOW

Factor: pH, redox condition, grain size, cations and anions etc.

RBA ESTIMATES: Soil-Lead at 20 Sites

Relative Bioavailability (RBA) per Tissue(Rank Ordered)

0.0%

20.0%

40.0%

60.0%

80.0%

100.0%

120.0%

0 2 4 6 8 10 12 14 16 18 20

Test Materials

RB

A (

test

vs

PbA

c)

Pt. Est.

Blood

Liver

Kidney

Bone

EPA defaultRBA = 60%

Impact of Food: Human Study withPb contaminated Bunker Hill Soil

Group(n=6)

Age(yrs)

Weight(kg)

Pb Dose(ug)

Soil Dose(mg)

Bioavailability(%)

Fasted 28 59.7 213 72.9 26.2(18.0-35.6)

Fed 28 67.9 242 82.9 2.52(0.2-5.2)

Maddaloni et al. 1998

Soil Treatment: Impact on PbBioavailability

Group

Age (yrs)

Weight

(kg)

Pb Dose (µg)

Soil Dose (mg)

Bioavailability (%, Absolute)

Untreated 28.9 62.6 231.5 44.5 34.7

(16.5-54.2)

Amended 31.4 69.6 251.2 59.2 19.9 (10.8-38.5)

Maddaloni et al. 2005

Comparative Results for Pb Bioavailability: animal, in vitro & human

Animal

In vitro (pH 2.3)

Human

% Reduction in Bioavailability

38 38 43

Relative Costs:Remedies for Metal-Contaminated Soils

0 0.5 1 1.5 2

$ (Millions)

Phosphate Inactivation

Soil Cap

Asphalt Cap

Phytoextraction

Soil Wash

Excavate & Landfill

Net Present Cost for 1 Hectare Site

1.62

0.79

0.25

0.16

0.14

0.06

• Use the Integrated Exposure Uptake Biokinetic Model to predict blood lead levels in young children

• Absolute bioavailability of soluble lead in food/water = 50%

• Relative bioavailability (RBA) of lead in soil = 60%

• 30% absolute bioavailability (model input)

• A sensitive parameter in predicting blood lead levels and potential risks to children

• RBA of 80% = soil cleanup goal of 265 mg/kg• RBA of 60% = soil cleanup goal of 400 mg/kg (defaul t)• RBA of 40% = soil cleanup goal of 530 mg/kg • RBA of 20% = soil cleanup goal of 1075 mg/kg

Bioavailability Assumptions inEvaluating Risks to Children from Lead

Some Observation on the In-vitro Bioaccessibility Data

Criteria for Credible In-vitro Method

Ability to predict bioavailability

Data are reproducible

Robustness and sensitivity

Model validation, QA and QC (Reference soils)

(based on ECVAM, ICCVAM and US EPA validation and r egulatory acceptance criteria)

Lead – Correlation Between In Vivo RBA and In Vitro Bioaccessibility (IVBA)

Arsenic – Correlation Between In Vivo RBA and In Vitro Bioaccessibility (IVBA)

Dutch Study with RIVM and TNO TIM Models

Number of soils 90 16 16

average 0.72 0.63 0.11Lowest value 0.11 0.40 0.04Higest value 1.77 1.03 0.21

Percentile 50 0.67 0.64 0.12Percentile 60 0.71 0.68 0.12Percentile 70 0.78 0.69 0.12Percentile 80 0.91 0.71 0.15Percentile 90 1.02 0.81 0.17

relative oral bioavailability factoraverage physiological state

IVD (I) IVD (II) Tiny-TIM

Round Robin Study - Data Comparability

Methods: PBET, SBRC, RIVM, EDTA+CH 3COOH

Lab

mg/kg

lkjhgfedcba

100

90

80

70

60

50

40

30

20

Individual Value Plot of Arsenic Bioaccessibility (mg/kg) in Soil 1

Median: 47.5

Range: 20-89

Total As: 112

Lab

mg/kg

lkjhgfedcba

40

35

30

25

20

15

10

Individual Value Plot of Arsenic Bioaccessibility (mg/kg) in Soil 2

Median: 20

Range: 13-38

Total As: 120

Methods: PBET, SBRC, RIVM, EDTA+CH 3COOH

Round Robin Study - Data Comparability

Lab

mg/kg

lkjhgfedcba

8000

7000

6000

5000

4000

3000

2000

1000

0

Individual Value Plot of Arsenic Bioaccessibility (mg/kg) in Soil 4

Median: 218.5

Range: 121-7011

Total As: 10307

Methods: PBET, SBRC, RIVM, EDTA+CH 3COOH

Round Robin Study - Data Comparability

Data Comparability (summary) (excludes overseas and one UK lab)

Test soils Arsenic (mg/kg)

Lead (mg/kg)

Nickel (mg/kg)

Soil 1 (n = 8) R: 20-77 Med: 43

R: 1-39 Med: 10

R: 4-23 Med: 8

Soil 2 (n = 8) R: 13-88 Med: 18

R: 1462-8219 Med: 1911

R: 1-5 Med: 2

Soil 3 (n = 8) R: 121-7011 Med: 194

R: 2920-84979 Med: 10480

R: 5-25 Med: 9

US Soil (n = 3)

R: 5-9 Med: 5

R: 348-542 Med: 477

R: 1.35-1.75 Med: 2

Labs using the same method and same operating procedure

produce comparable results.

Labs using different methods produced different results

Arsenic – Correlation Between In Vivo RBA and Bioaccessibility (UBM)

Arsenic – Correlation Between In Vivo RBA and Bioaccessibility (RIVM method)

Some Conclusions to Date

In-vitro bioaccessibility data can make the exposure a ssessment more realistic

Evidence on in-vitro methods in predicting bioavailab ility is inconclusive for most of the chemicals

Variability of in-vitro data between the methods using the same samples

Laboratories use same method irrespective of chemical, chemical form, matrix

Way Forward

Coordinated leadership and approach in undertaking fur ther research required

Continue effort in in-vitro method development and validation with appropriate data

What can be done to increase confidence in in-vitro data?

Using multiple lines of evidence to support in-vitro bioaccessibility method/data:

• consolidation of available knowledge

• integration of geochemistry with exposure science

Increasing confidence: integrated approach

Geochemistry

Exposure Toxicology

In-vitro approach

-Method screening

-Geochemical matching

-Geochemical classification

-Biomarker

Qualitative use of in-vitro bioaccessibilityas a line of evidence

Highly contaminated

×

Moderately contaminated

√ + ??

Background √

Fixed √ Moderately

fixed √

Mobile √ +?

Thank-you

Sohel Saikat (sohel.saikat@hpa.org.uk)

www.hpa.org.uk