1. Percutaneous Absorption
Leena A. Nylander-French, Ph.D., CIH
11/3/00
Percutaneous Absorption
Transdermal absorption/percutaneous absorption
Toxicants pass through the cell layers before entering the small blood and lymph capillaries in the dermis
A complex event with many key factors relating to the physical, chemical, and biochemical constitution of the skin overlaid with the vast range of physicochemical behavior of the penetrant
4/23/2017
Dermal Toxicology

2. Pharmaceutical Research
Local or systemic pharmacological response using dermally applied drugs
Current research is divided
restraint (slow release technology) and
enhancement (occlusion, permeation enhancers)
4/23/2017

3. Occupational and Environmental Exposures
Accidental or deliberate (chemical warfare)
commercial and home and garden pesticides
polymer and paint chemicals
detergents and cleaning chemicals
a broad range of heavy industrial chemicals
unscheduled exposures to environmental accidents and
mishandling of toxic waste disposal
4/23/2017

4. Percutaneous Absorption
Dermal absorption prevalent for any compound, with the exemption of highly volatile chemicals
Research is directed towards understanding transdermal flux rates and the toxicological consequences of penetration
At the practical end, such data contribute to risk assessment
4/23/2017

5. Factors Affecting Percutaneous Absorption
Biological
skin age
skin condition
anatomical site
skin metabolism
circulatory effects
Physical
drug concentration
surface area
exposure time
occlusion
vehicle
Physicochemical
hydration
drug-skin binding
temperature
4/23/2017

6. Mechanisms of Percutaneous Absorption
Mechanisms by which chemicals cause visible effects on the skin differ from chemical to chemical
disruption of lipids and membranes
protein denaturation
keratolysis
cytotoxicity
4/23/2017

7. Mechanisms of Percutaneous Absorption
Leena A. Nylander-French, Ph.D., CIH
11/3/00
Mechanisms of Percutaneous Absorption
The rate-determining barrier is Stratum Corneum (nonviable epidermis), which is densely packed keratinized cells (nuclei lost, biologically inactive)
SC contains 75-80% lipophilic materials
very little triglycerids (0%)
cholesterol (27%)
cholesterol esters (10%)
various ceramides (41%; amides and/or esters of saturated and unsaturated fatty acids)
4/23/2017
Dermal Toxicology

8. The Steps Involved in Percutaneous Absorption
Leena A. Nylander-French, Ph.D., CIH
11/3/00
The Steps Involved in Percutaneous Absorption
1. Partitioning
2. Diffusion
3. Partitioning
4. Diffusion
5. Capillary uptake
Mukhtar, H., Pharmacology of the Skin. CRC Press, Inc., Boca Raton, FL.
4/23/2017
Dermal Toxicology

9. The Putative Pathways of Penetration Across the Stratum Corneum
Leena A. Nylander-French, Ph.D., CIH
11/3/00
The Putative Pathways of Penetration Across the Stratum Corneum
Mukhtar, H., Pharmacology of the Skin. CRC Press, Inc., Boca Raton, FL.
4/23/2017
Dermal Toxicology

10. Mechanisms of Percutaneous Absorption
Appendageal transport
a negligible contribution to the overall percutaneous flux across human skin
however, transport through the appendageal route has been shown to be significant during the initial (non-steady-state) period of percutaneous absorption
remains controversial; question of the participation of the hair follicles in percutaneous absorption
4/23/2017

11. Mechanisms of Percutaneous Absorption
Permeation pathways
Polar (hydrophilic)
Path through corneocytes with their desmosomal connections
Nonpolar (lipophilic)
Agents dissolve in and diffuse through the lipid matrix between the protein filaments
Regional variations in skin permeability are correlated with quantitative differences in lipid content rather than SC thickness or cell number
4/23/2017

12. Percutaneous Transport
Molecules traverse membranes either by
passive diffusion
solute flux is linearly dependent on the solute concentration gradient
active transport
typically involves a saturable mechanism
Percutaneous flux is directly proportional to the concentration gradient and, therefore, transport across the skin occurs primarily by passive diffusion
4/23/2017

13. Percutaneous Transport
At steady state, the flux due to passive diffusion may be described by Fick’s 1st law
J = kp  a
J = flux of the permeant (moles/cm2s)
kp = permeability coefficient of the permeant through the
membrane (cm/s)
∆a = activity gradient across the membrane (moles/cm3)
4/23/2017

14. Percutaneous Transport
kp is the inverse of the “resistance”, which the membrane offers to solute transport, and is defined by
kp = KD / h
K = membrane-aqueous phase partition coefficient of
the solute
D = diffusion coefficient of the solute in the membrane
(cm2/s)
h = diffusion path length through the membrane
4/23/2017

15. Percutaneous Transport
Leena A. Nylander-French, Ph.D., CIH
11/3/00
Percutaneous Transport
The flux rate is a rate process Rate = Driving Force / Resistance
Driving force for diffusion is the activity gradient (concentration gradient across the permeability barrier)
Molecular flux across the membrane can be determined by the solute’s size and lipophilicity if the driving force remains the same
Octanol/Water partition coefficient (Ko/w) has been chosen to be used as the index of lipophilicity
4/23/2017
Dermal Toxicology

16. Example of Human Dermal Exposure Assessment
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17. Laboratory Human Volunteer Study
Leena A. Nylander-French, Ph.D., CIH
11/3/00
Laboratory Human Volunteer Study
1.0 ml of jet fuel is applied at two sites
Exposure study was done inside a fume-hood to prevent inhalation exposure
Surface area of exposure is 20 cm2
30 minute exposure
Up to 4 subjects were exposed each day
Nurse was around to draw blood and help with data collection
Tenax® tubes were used to measure evaporation from arm
4/23/2017
Dermal Toxicology

18. Leena A. Nylander-French, Ph.D., CIH
11/3/00
Study Population
5 male and 5 female adult volunteers
Breathing-zone, dermal tape-strip, breath, urine, and blood samples
Exclusion criteria
occupational exposure to PAH (e.g., auto mechanics)
cardiovascular disease
atopic dermatitis
smoking
use of prescription medication for illness
alcohol consumption during the study
Sample size issue
-not done for deterministic models
-when looking at population kinetic models, of course sample size must be addressed
4/23/2017
Dermal Toxicology

19. How to estimate permeation of JP-8 components across the skin
Leena A. Nylander-French, Ph.D., CIH
11/3/00
How to estimate permeation of JP-8 components across the skin
Fick’s Law of Diffusion L1
x0, C(x0) L2
J = -D
The data was used to measure the permeation of chemical across the skin. Permeation is a concept derived from Fick’s Law of diffusion for homogeneous membranes, which the skin is assumed to be.
Diffusion across a homogeneous membrane.
Flux is dependent on the concentration gradient and a proportionality constant called the diffusion coefficient
Flux = mass/(area*time)
Diffusion coefficient = area/time
Kp = distance/time
x1, C(x1)
Permeability Coefficient Kp (cm/h)
4/23/2017
Dermal Toxicology

20. Leena A. Nylander-French, Ph.D., CIH
11/3/00
Calculation of Kp
Using Fick’s law of diffusion, flux was estimated by calculating the slope of the cumulative mass/area vs. time plot. The permeability coefficient was then calculated using this expression.
Straight portion of the curve is used
No lag-time is observed
Flux is greater for the aliphatic components because there’s more aliphatic components in JP-8 than aromatics
4/23/2017
Dermal Toxicology

21. Human Skin Permeability Coefficients (x 10-5)
Leena A. Nylander-French, Ph.D., CIH
11/3/00
Human Skin Permeability Coefficients (x 10-5)
Subject
naphthalene
1-methyl naphthalene
2-methyl naphthalene
decane
undecane
dodecane 1
16.0
1.3
5.3
0.85
0.021
0.13 2
3.4
2.8
3.1
0.86
0.036
0.15 3
3.2
3.0
0.76
0.023
0.30 4
3.7
2.7
1.2
0.025
0.20 5
5.4
2.9
0.33
0.067 6
5.7
3.3
0.80
0.048 7
4.1
0.71
0.033 8
0.56
0.041 9
0.22
0.076
0.11 10
4.2
3.5
0.083
0.12
mean
0.65
0.045
0.16 SD
3.8
0.59
0.74
minimum
maximum
rat Kp
51.0
5.5
2.5
1.4
Here are the permeability coefficients for each individual. I’d like to focus on some patterns that arose from comparison of Kp between this study and Kp estimated using rat skin, as well as across compounds. [change all to same exponent and remove exponent from table]
Compare the Kp for human from rat in vitro suggests that human Kp is about an order of magnitude smaller.
Aromatic Kp is larger than aliphatic Kp
Naphthalene has the highest Kp value
Rat Kp from McDougal et al. (2000)
4/23/2017
Dermal Toxicology

22. Estimation of the Internal Dose
Leena A. Nylander-French, Ph.D., CIH
11/3/00
Estimation of the Internal Dose
M = Kp  CJP-8  A  t
Mrat = 1.29 mg
Mpig = 0.53 mg
Mhuman = 0.13 mg
10 
4 
1 hr
hands ≈ 840 cm2
3 mg/ml
rat, pig, human
The permeability coefficient, combined with Fick’s law of diffusion, can be used to estimate the internal dose using this equation.
Method of estimating the total amount of JP-8 that is absorbed into the systemic circulation.
Rat and pig skin models overestimate the amount absorbed; therefore, the internal dose estimated using the rat and pig Kp will lead to conservative estimates in a risk assessment.
Another problem with this approach is that the estimated internal dose does not take into account the distribution and clearance of chemicals in vivo.
Rat Kp from McDougal et al. (2000)
Pig Kp from Muhammad et al. (2004)
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Dermal Toxicology

23. Dermatotoxicokinetic ModelsB C D
surface
surface
surface
surface k0 k0 k0 k0
stratum corneum
stratum corneum
skin
skin k1 k1 k2 k1 k2 k1
viable epidermis
viable epidermis
blood k3
blood k3
Four DTK models that were evaluated in this study. Each of these data-based compartment models was used to quantify the absorption, distribution, and elimination of chemicals following dermal exposure to JP-8. Mass-balance differential equations were used to describe the kinetic behavior of chemicals in each of these compartments.
Start with a a simple model that treats the skin as 1 compartment for uptake, distribution, and elimination of chemicals.
In the 2-compartment models, the skin is divided into the stratum corneum and the viable epidermis.
Additional compartment for storage of chemical. k3 k2 k3 k2 k5 k4
blood k4
blood k4
storage k6 k5
storage

24. Which is the Optimal Model?
surface k0
stratum corneum k1
viable epidermis
Here are the predictions made by each of the DTK models for naphthalene, 1-methyl naphthalene, 2-methyl naphthalene, decane, undecane, and dodecane. Data are shown for one individual because it gets too messy if all data and model predictions are shown.
Model were compared: (1) using visual inspection, and (2) using a likelihood ratio test based on the values of the objective function for each model.
Optimal model is D.
Model D consists of 5 compartments representing the surface, stratum corneum, viable epidermis, blood, and storage compartment. k3 k2
blood k4 k6 k5
storage
data ( ■ ), model A (-----), model B (------), model C ( ), model D ( )

25. U.S. Air Force Study
85 fuel-cell maintenance workers from six Air Force bases
Breathing-zone, dermal tape-strip, breath, and urine samples
Work diaries
Questionnaires

26. Whole-body dermal exposure to naphthalene [ln(ng/m2)]

27. Dermal exposure and urinary 1-naphthol level [ln(ng/l)]
Predictor
Parameter
Estimate
Standard Error
P-Value
Relative
Contribution (%)
0.27
(n = 83)
Intercept
ln(breathing-zone naphthalene)
Smoking (0 = no, 1 = yes)
3.48
0.50
0.28
1.32
0.10
0.16
0.0101
<0.0001
0.0808
88.2
11.8
0.318
(n = 72)
ln(end-exhaled breath naphthalene)
6.14
0.43
0.36
0.75
0.08
0.17
0.0399
87.2
12.8
Chao et al. Environ Health Perspect 114: , 2006

28. Dermal exposure and urinary 2-naphthol level [ln(ng/l)]
Predictor
Parameter
Estimate
Standard
Error
P-Value
Relative
Contribution (%)
0.26
(n = 83)
Intercept
ln(breathing-zone naphthalene)
ln(dermal naphthalene)
Smoking (0 = no, 1 = yes)
5.11
0.33
0.11
0.34
1.53
0.13
0.04
0.18
0.0013
0.0114
0.0119
0.0603
51.1
35.8
13.1
0.31
(n = 72)
ln(end-exhaled breath naphthalene)
6.80
0.30
0.10
0.45
0.87
0.12
0.05
0.19
<0.0001
0.0128
0.0709
0.0238
52.9
32.3
14.8
Chao et al. Environ Health Perspect 114: , 2006.

29. Occupational Exposure to JP-8 in the US Air Force
personal breathing zone
end-exhaled breath
Average concentration of naphthalene in air, skin, and breath
Exposure Category
Media
Low
Medium
High
Air (µg/m3)
1.9
29.8
867
Skin (ng/m2)
344
483
4188
Breath (µg/m3)
0.7
0.9
1.8
tape-strip
One of the purposes of modeling in this study was to be able to predict the internal dose for dermal exposures that take place in the real-world. An example comes from exposures of USAF personnel. In a typical workplace scenario, where personnel handle jet fuel, workers are exposed vial dermal contact and inhalation.
Dermal and inhalation exposures were measured in USAF personnel in a previous exposure assessment study. Dermal exposures were measured using tape strips, and inhalation exposures were measured using a passive sampler located in the personal breathing zone of the workers. Exhaled-breath measurements were also collected to estimate the contribution of both exposure routes to internal dose. These data were collected from low, medium, and high exposure groups, with the exception of the low-exposure group which did not have dermal exposure measurements because they did not handle the liquid form of JP-8.
Egeghy, P. P., Hauf-Cabalo, L., Gibson, R., and Rappaport, S. M. (2003). Benzene and naphthalene in air and breath as indicators of exposure to jet fuel. Occup. Environ. Med. 60,
Chao, Y. C., Kupper, L. L., Serdar, B., Egeghy, P. P., Rappaport, S. M., and Nylander-French, L. A. (2006). Dermal Exposure to Jet Fuel JP-8 Significantly Contributes to the Production of Urinary Naphthols in Fuel-Cell Maintenance Workers. Environ Health Perspect. 114,

30. Physiologically Based Toxicokinetic (PBTK ) Model for Naphthalene
DTK PBTK
CALIBRATION DATA
Volunteer study
USAF study
PARAMETER VALUES
PB = 54.7
PF = 40.4
PD = 12.7
Kps = 5.210-5 cm/h
Kpv = 2.0 cm/h
Data-based compartmental models are informative for describing and quantifying the kinetic behavior of chemicals, it is difficult to
generalize to other populations, exposure scenarios, compounds, and exposure routes.
Physiologically-based compartmental models allow such extrapolations to be made.
In this study, the DTK model provided the backbone to PBTK modeling human exposures to jet fuel.
Each of the rate constants were defined in the PBTK model based on the anatomy and physiology of humans, whereas in the DTK model, they were determined empirically.
Some of the parameters had to be adjusted because values do not exist for naphthalene exposures in humans in vivo.
Calibration dataset came from the human dermal exposure study, and from the air force study.
These are the optimized parameter values.
Kps was determined using this equation, which is based on Fick’s law of diffusion.
Kim et al. Environ Health Perspect 115: , 2007

31. Contribution of Dermal Exposure to Internal Dose
Estimated contribution of dermal exposure to the end-exhaled breath concentrations of naphthalene relative to inhalation exposure.
This analysis was based on three US Air Force personnel whose end-exhaled breath concentrations represented the 10th, 50th, and 90th
percentiles. The ratio of INHAL1pred to INHAL1adj is a measure of the relative percent contribution of dermal exposure to the
internal dose.
Kim et al. Environ Health Perspect 115: , 2007

32. Summary of Findings
In vivo studies may be used to make reasonable predictions of dermal absorption and penetration of JP-8 components in humans
Human permeability coefficients are 10-fold lower than estimates made in vivo; however, there is a wide range of Kp values among study volunteers
A two-compartment model of the skin best describes the toxicokinetic behavior of dermal exposure to aromatic and aliphatic components of JP-8
Dermal exposure to JP-8 contributes significantly to urinary 2-naphthol but not to 1-naphthol levels among the fuel-cell maintenance workers
Dermal exposures may contribute up to 35% of the internal dose of naphthalene
The 4 major findings from this study are:

33. IMPROVED EXPOSURE and RISK ASSESSMENT
Summary
viable epidermis
blood
QL x EL
Kpv x Aexp / PD
Epidermal exposure
kuptake
stratum corneum
QE / PE QE
QF / PF
storage QF
other
Inhalation exposure QP
QP / PB QO
QO / PO
Integration of exposure assessment, biological monitoring, and toxicokinetic modeling tools have been used to better characterize and quantify dermal exposures to JP-8 components.
The combination of these tools can be used to improve human environmental health risk assessment by reducing the uncertainty associated with extrapolation from animal studies to human exposure scenarios.
Quantification of keratin adducts obtained from the stratum corneum of exposed individuals will allow us to investigate the importance of dermal penetration, metabolism, and adduction of keratin as well as to make accurate prediction of the contribution of dermal exposure to the systemic dose for inclusion in exposure- and risk-assessment models.
IMPROVED EXPOSURE and RISK ASSESSMENT