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
9/17/2018
Dermal Toxicology

2. Transdermal control
Local or systemic pharmacological response using dermally applied drugs
Current research is divided
restraint (slow release technology) and
enhancement (occlusion, permeation enhancers)
9/17/2018

3. Toxicological hazard 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
9/17/2018

4. Percutaneous Absorption
With the exemption of highly volatile chemicals, the principal organ exposed to these hazards is skin
Research in this area is directed towards understanding transdermal flux rates and the toxicological consequences of penetration
At the practical end, such data contribute to risk assessment
9/17/2018

5. 1. Factors Affecting Percutaneous Absorption
Biological factors
skin age
skin condition
anatomical site
skin metabolism
circulatory effects
9/17/2018

6. 1. Factors Affecting Percutaneous Absorption
Physicochemical factors
hydration
drug-skin binding
temperature
9/17/2018

7. 1. Factors Affecting Percutaneous Absorption
Physical factors
drug concentration
surface area
exposure time
occlusion
vehicle
9/17/2018

8. 2. 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
9/17/2018

9. 2. Mechanisms of Percutaneous Absorption
Leena A. Nylander-French, Ph.D., CIH
11/3/00
2. 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)
9/17/2018
Dermal Toxicology

10. Figure 5. The sequential steps involved in percutaneous absorption
Leena A. Nylander-French, Ph.D., CIH
11/3/00
Figure 5. The sequential 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.
9/17/2018
Dermal Toxicology

11. Leena A. Nylander-French, Ph.D., CIH
11/3/00
Figure 6. The putative pathways of penetration across the Stratum Corneum
Mukhtar, H., Pharmacology of the Skin. CRC Press, Inc., Boca Raton, FL.
9/17/2018
Dermal Toxicology

12. 2. Mechanisms of Percutaneous Absorption
Appendageal transport makes 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
Appendageal transport remains controversial
resent research has again raised the question of the participation of the hair follicles in percutaneous absorption
9/17/2018

13. 2. 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
9/17/2018

14. 3. 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
9/17/2018

15. 3. 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)
9/17/2018

16. 3. 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
9/17/2018

17. 3. Percutaneous Transport
Leena A. Nylander-French, Ph.D., CIH
11/3/00
3. Percutaneous Transport
The flux rate is a rate process rate = (driving force) / (resistance)
The 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
9/17/2018
Dermal Toxicology

18. 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
9/17/2018
Dermal Toxicology

19. 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 chemicals in JP-8 (e.g., auto mechanics)
cardiovascular disease
atopic dermatitis
smoking
use of prescription medication for illness
alcohol consumption during the study
9/17/2018
Dermal Toxicology

20. 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
x1, C(x1)
Permeability Coefficient Kp (cm/h)
9/17/2018
Dermal Toxicology

21. Leena A. Nylander-French, Ph.D., CIH
11/3/00
Calculation of Kp
9/17/2018
Dermal Toxicology

22. 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
Rat Kp from McDougal et al. (2000)
9/17/2018
Dermal Toxicology

23. Estimation of the Internal Dose
Leena A. Nylander-French, Ph.D., CIH
11/3/00
Estimation of the Internal Dose
hands ≈ 840 cm2
3 mg/ml
1 hr
M = Kp  CJP-8  A  t
rat, pig, human
4 
Mrat = 1.29 mg
Mpig = 0.53 mg
Mhuman = 0.13 mg
10 
Rat Kp from McDougal et al. (2000)
Pig Kp from Muhammad et al. (2004)
9/17/2018
Dermal Toxicology

24. Metabolism of Xenobiotics
Most foreign compounds are lipophilic and able to penetrate lipid membranes and to be transported by lipoproteins in the blood
These lipophilic compounds are substrates for biotransforming enzymes
Epidermis is the major site in the skin for metabolism of xenobiotics, steroids, and vitamins
9/17/2018

25. Metabolism of Xenobiotics
After invasion, the xenobiotic substance is first chemically activated (usually by oxidation)
phase I metabolic reaction, where a polar reactive group is introduced into the molecule, rendering it a suitable substrate for phase II metabolism
cytochrome P-450 isoenzymes
localized mainly in the endoplasmic reticulum (microsomal fraction)
activities about 1-5% of those in the liver
Pre-carcinogenic chemicals can be converted to carcinogenic metabolites
9/17/2018

26. Marzulli, F. N. and Maibach, H. I. , 1996. Dermatotoxicology, 5th ed
Marzulli, F.N. and Maibach, H.I., Dermatotoxicology, 5th ed. Taylor & Francis, Washington, DC.
9/17/2018

27. Figure 7. Schematic of metabolism of xenobiotics
Drug Or
Xenobiotic
Active Xenobiotic
(e.g., epoxides)
Elimination
Binding to Macromolecules
(e.g., membranes, proteins,
DNA, RNA)
Chemocarcinogenesis, Mutagenesis,
Teratogenesis, Sensitization
Transferases,
Epoxyhydrase,
NQR
P-450
Marzulli, F.N. and Maibach, H.I., Dermatotoxicology, 5th ed. Taylor & Francis, Washington, DC.
9/17/2018

28. Metabolism of Xenobiotics
Activated metabolite is transformed by phase II enzymes (transferases, reductases)
all major transferases are found in the skin (about 10% of hepatic activities)
NAD(P)H-quinone reductase (NQR)
epoxide hydrolase
Formation of highly hydrophilic metabolites, which are more readily excreted (e.g., mercapturic acids)
9/17/2018

29. Metabolism of Xenobiotics
Some foreign compounds (e.g., electrophiles that undergo nuclear substitution) are not transformed by phase I enzymes but react directly at the site of contact; ultimately eliminated by phase II enzymes
e.g., mono- and multifunctional acrylates
Skin metabolizing enzymes differ both quantitatively and qualitatively from those in the liver, particularly by their relative proportions, composition, and interactions
9/17/2018