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Reference
Endpoint:
dermal absorption in vitro / ex vivo
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
1 (reliable without restriction)
Qualifier:
according to
Guideline:
OECD Guideline 428 (Skin Absorption: In Vitro Method)
GLP compliance:
yes
Specific details on test material used for the study:
- Name of test material (as cited in study report): Lead Oxide
- Supplier: EBRC Consulting GmbH, Zeppelinstrasse 8, D-30175 Hannover, Germany
- Substance type: Solid
- Physical state: Yellowish powder
- Analytical purity: 99.8%
- Batch No.: 210213
- Stability under test conditions: 2 years
- Storage condition of test material: Ambient temperature in the dark
Radiolabelling:
no
Species:
other: human
Type of coverage:
open
Vehicle:
other: 1% hydroxypropylmethyl cellulose in water
Duration of exposure:
6 hours, with an additional 18 hour monitoring period
Doses:
- Nominal doses: 0, 100, and 1000 ug/cm2
- Actual doses: 0, 111, and 1033 ug/cm2
Details on study design:
Test preparations (7.6 uL) were applied over the surface of the stratum corneum of exposed skin. At 6 hours post-exposure, skin was washed with a 2% soap solution and then dried with tissue swabs. At 24 hours post-exposure, each diffusion cell was disconnected and the underside of the skin was rinsed with receptor fluid. The skin was removed and the donor and receptor chambers were transferred into separate pots of receptor fluid and test items were extracted for 30 minutes. The skin was dried and the stratum corneum was removed with 25 successive tape strips, which were pooled into vials. Throughout the study, receptor fluid was collected in four fractions (0-6 hours, 6-12 hours, 12-18 hours, and 18-24 hours). All samples were analyzed for lead concentration by Harwell Scientifics using ICP-MS.
Details on in vitro test system (if applicable):
SKIN PREPARATION
- Source of skin: 40-year-old patient undergoing routine surgery
- Ethical approval if human skin: Informed consent received
- Type of skin: A single, full-thickness human breast skin sample
- Preparative technique: Split-thickness membranes prepared by pinning the skin, with the stratum corneum up, onto a raised corkboard and cutting at a setting of 200-400 um depth using a Zimmer electric dermatome.
- Thickness of skin: Split-thickness skin samples were 390-400 um thick
- Membrane integrity check: Assessed by penetration of tritiated water (250 uL) applied to skin surface with the donor chamber occluded. All skin samples with a tritiated water permeability coefficient (kp) less than 0.0025 cm/hour were accepted into the study.
- Storage conditions: Full-thickness skins were dried and stored at -20 degrees C. Split-thickness skins were used immediately after being cut.

PRINCIPLES OF ASSAY
- Diffusion cell: Automated flow-through diffusion cell apparatus (Scott/Dick, University of Newcastle-upon-Tyne, United Kingdom)
- Receptor fluid: Phosphate buffered saline containing 5% bovine serum albumin, Streptomycin, and Penicillin G, pH 7.4
- Flow-through system: Flow-through cells were connected to multi-channel peristaltic pumps from their afferent ports. The receptor chamber volume was 0.25 mL and the peristaltic pumps maintained a flow-rate of 1.5 mL/hour.
- Test temperature: 32 degrees C
- Occlusion: None; the chambers were left open to the atmosphere.
- Other: Surface area of exposed skin within diffusion cells was 0.64 cm2.
Signs and symptoms of toxicity:
not specified
Dermal irritation:
not specified
Absorption in different matrices:
For the 111 ug/cm2 dose: 95.63% of the applied lead was removed during the wash at 6 hours post-exposure. At 24 hours post-exposure, 99.51% of the applied dose was not absorbed. The absorbed dose into the receptor fluid was <0.01% (5 ng/cm2) of the applied dose and the dermal delivery (absorbed dose plus the amount in the skin itself) was 0.13% (148 ng/cm2) of the applied dose.

For the 1033 ug/cm2 dose: 96.10% of the applied lead was removed during the wash at 6 hours post-exposure. At 24 hours post-exposure, 98.28% of the applied dose was not absorbed. The absorbed dose was < 0.01% (10 ng/cm2) of the applied dose and the dermal delivery was 0.05% (479 ng/cm2) of the applied dose.
Time point:
24 h
Dose:
111 ug/cm2
Parameter:
percentage
Absorption:
ca. 0.13 %
Time point:
24 h
Dose:
1033 ug/cm2
Parameter:
percentage
Absorption:
ca. 0.05 %
Conclusions:
The authors concluded that the apparent decrease in absorption as a function of dose may be attributable to the considerable background from endogenous lead in the donor skin restricting the reliability of the absorption factor at the low dose.
Executive summary:

This study assessed the in vitro absorption of lead oxide in human skin samples at 0, 111, and 1033 ug/cm2 concentrations over 24 hours. For the 111 ug/cm2 dose, 95.63% of the applied lead was removed during the wash at 6 hours post-exposure. At 24 hours post-exposure, 99.51% of the applied dose was not absorbed. The absorbed dose into the receptor fluid was <0.01% (5 ng/cm2) and the dermal delivery was 0.13% (148 mg/cm2) of the applied dose. For the 1033 ug/cm2 dose, 96.10% of the applied lead was removed during the wash at 6 hours post-exposure. At 24 hours post-exposure, 98.28% of the applied dose was not absorbed. The absorbed dose was <0.01% (10 ng/cm2) and the dermal delivery was 0.05% (479 ng/cm2) of the applied dose. The authors concluded that the apparent decrease in absorption as a function of dose may be attributable to the considerable background from endogenous lead in the donor skin restricting the reliability of the absorption factor at the low dose.

Description of key information

Short description of key information on absorption rate:

Dermal absorption of lead through unabraded human skin is considered to be minimal (<0 .1%) and thus absorption of inorganic lead compounds through the skin has previously been considered to be of less significance than absorption through the respiratory and gastrointestinal routes.

Key value for chemical safety assessment

Additional information

Animal studies serve to validate mechanistic inferences derived from observational human studies. The majority of information pertaining to lead toxicokinetics has been accurately defined in humans of different ages and degrees of susceptibility to lead toxicity. A number of toxicokinetic models have been developed to predict the effects of external lead exposure upon internal or systemic levels of lead. The Integrated Exposure Uptake Biokinetic (IEUBK) is now widely applied to assess relationships between environmental lead exposure and blood lead in children. Due to limitations in the ability of the IEUBK model to assess the deposition and subsequent remobilisation of lead from bone, use of the IEUBK model is generally restrict to predict exposures in chidren six years of age or younger.

Physiologically-based pharmacokineitc models (e.g. the O'Flaherty Model) have been developed to predict lead uptake in humans of all ages but is most commonly applied in the assessment of adult exposures. Both the O'Flaherty and IEUBK models are available as computer simulation models and are discussed in greater detail in section 7.10.5.

Lead is most easily taken up into the body through inhalation or ingestion – dermal uptake makes a negligible contribution to systemic lead levels. Once taken up into the body, lead is not metabolized. However, lead will distribute to a variety of tissue compartments such as blood, bone and soft tissues. The half-life of lead in the body varies as a function of body compartment. Lead in blood has a half life of 30 – 45 days – measurement of lead in blood thus provides an integrated assessment of average lead exposure (via all routes) over the preceding month. Lead is retained far longer in bones. Depending upon bone type, the retention time of lead can vary between 8 and 30 years. Such lead can both serve as a source of endogenous lead exposure and as a cumulative index of exposure over a time frame of years. Lead excretion is primary via urinary and biliary excretion routes.

Discussion on absorption rate:

Human data are available and superced the animals studies that have been conducted - one of which is described here. Detailed studies on dermal uptake in humans are described in section 7.10.5. Dermal absorption of lead through unabraded human skin is considered to be minimal and thus absorption of inorganic lead compounds through the skin has previously been considered to be of less significance than absorption through the respiratory and gastrointestinal routes. The most recent guideline-conformed in-vitro dermal absorption study (Toner and Roper, 2005) has established absorption of lead to be less than 0.1%. Other quantitative estimates of dermal absorption are limited in reliability with the most rigorous study (Moore et. al. 1980) suggesting uptake on the order of 0.01 – 0.18%. However, the data from many published studies on this aspect largely lack compliance with current guideline requirements, and their reliability and relevance for human health risk assessment is questionable.