Physiological Modeling of the Dermal Absorption
of Octamethylcyclotetrasiloxane (D4)
MB Reddy,1 RJ Looney,2 MJ Utell,2 ML Jovanovic,3 JM
McMahon,3 DA McNett,3 KP Plotzke3 and ME Andersen4
CH3 CH3
1
Quantitative and Computational Toxicology
Group, The Center for Environmental
Toxicology and Technology, Colorado State
University, Fort Collins, Colorado 80523
2
University of Rochester School of Medicine,
Rochester, New York 14642
3
Toxicology, Health and Environmental
Sciences, Dow Corning Corporation, Midland,
Michigan 48686
4
CIIT Centers for Health Research, Research
Triangle Park, North Carolina
Si
O
CH3
CH3
Si
O
Si
D4
O
CH3
O
Si
CH3 CH3
CH3
Abstract

Studies of human dermal absorption of octamethylcyclotetrasiloxane, D4, through axilla skin in vivo and
through abdominal skin in vitro have recently been completed. A mathematical model describing the
dermal absorption of D4 was developed and combined with an inhalation PBPK model for this material.
The model includes volatilization of D4 from the skin surface, evaporation of chemical out of the skin
after the skin surface had been cleared of the chemical, and a deep skin compartment. The in vivo
dermal absorption study of D4 in the rat provided evidence that a model structure including elimination
from the skin by evaporation was appropriate. Concentrations of D4 in exhaled air and blood plasma
from human, in vivo exposures were used to estimate the model parameters. Following either inhalation
or dermal exposures, D4 blood plasma concentrations increased with time relative to exhaled air
concentrations. The PBPK model for both dermal and inhalation exposures required the inclusion of a
pool of unavailable D4 created in the liver, transported in the blood, and cleared in the liver to describe
this behavior. Model calculations indicated that during the human, in vivo, dermal exposure, more than
90% of the applied dose evaporated from the skin surface before it could be absorbed into the skin. Of
the D4 absorbed into the skin, the majority was eliminated by evaporation before systemic absorption
could occur. For men and women, respectively, about 0.1 and 0.5% of the applied dose of D4 entered
the cutaneous blood within 24 hours of the exposure.
Introduction




Octamethylcyclotetrasiloxane (D4) is a lipophilic (logKo/w  5), semi-volatile (vapor pressure  0.68
mmHg at 20C) compound primarily used as an intermediate in the manufacturing of high molecular
weight silicone polymers and as an ingredient in some consumer products.
Recently, several studies evaluating the dermal absorption of D4 have been completed:
 in vivo dermal absorption through human axilla skin
 in vitro dermal absorption through human abdomen skin
 in vivo dermal absorption through rat skin
Here, we analyze and interpret these data.
A compartment model describing the dermal absorption of volatile chemicals was combined with a
human D4 PBPK model for the analysis of the human, in vivo, dermal absorption data.
Background


During dermal absorption, the absorbing
chemical must diffuse through the
stratum corneum and viable epidermis.
Once the chemical reaches the highly
vascularized dermis, it enters the blood
stream and systemic circulation.
For clarity, we define the following:
The amount absorbed is the amount of
chemical that has passed through the skin
into the blood combined with the amount of
chemical remaining in the viable skin layers.
The amount penetrated is the amount of
chemical that has passed through the skin into the blood.
Often, the amount absorbed (particularly in vitro) is used as an estimate of the amount penetrated for
lipophilic materials
Model






For dermal exposures to neat, volatile or semi-volatile chemicals, the skin surface is exposed to the
chemical until all of the chemical has left the skin surface due to evaporation or dermal absorption.
While D4 remains on the skin, the mass transfer of D4 from the skin surface by evaporation was
modeled as a zero-order process and the dermal absorption of D4 was modeled as a first-order
process (Figure 1).
After the skin cleared of chemical (i.e., after all the D4 left the skin surface by evaporation or dermal
absorption), the model included D4 mass transfer from the skin back into the air.
The model also included a deep skin compartment.
The dermal absorption model was combined with a human inhalation PBPK model for D4 (Figure 1,
Table 2), which required several features for describing D4 kinetics:
 mass transfer limitations in the slowly perfused compartment and fat tissue
 a pool of unavailable D4 that was produced in the liver, traveled through the bloodstream, and
cleared in the fat
PBPK model equations were solved using Berkeley Madonna and the multiple curve-fitting algorithm
was used to fit the model output to human, in vivo data (i.e., Experiment 1 data) to calculate
parameters for the dermal absorption model (Table 1).
Table 1. Parameters for the dermal absorption model.
kv, g/cm2/min
k1, cm3/min
k-1, min-1
k-2 , min-1
kd , min-1
k-d , min-1
men
women
0.0098
0.0068
0.00050
0.00015
3.8
0.18
0.014
0.036
0.060
0.038
0.010
0.000010
Cin
alveolar space
Cout
lung blood
(a)
exposed skin
rapidly perf. tissue
fat
deep fat
venous blood
arterial blood
slowly perf. tissue
blood lipid
liver
metabolism
(continued)
(continued from last page)
(b)
venous
blood
(c)
clearance
in fat
evaporation of D4 on
the skin surface
neat D4
k-1
k1
kd k-d
deep
comp.
production
in liver
evaporation of D4 that has
absorbed into the skin
kv
skin
venous
blood
k-2
venous
blood
skin
k-2
venous
blood
kd k-d
deep
comp.
Figure 1. Schematic diagram of (a) the PBPK model [1, 4], (b) the sub-model for the pool of
unavailable D4, and (c) the compartment model of dermal absorption before and after the D4
dose has evaporated or absorbed into the skin.
Table 2. Parameters used in the D4 human PBPK model [4].
Parameter
Value
alveolar ventilation
QP
7.6 L/min
cardiac output
QC
5.9 L/min
fraction
liver
0.227
of blood
fat
0.052
flow to
rapidly perf. tissue
0.472
tissues
slowly perf. tissue
0.249
fraction
liver
0.0314
of body
fat
0.23
weight in
rapidly perf. tissue
0.05
tissues
slowly perf. tissue
0.5396
(continued)
(continued from last page)
allometric constant for
metabolic clearance
0.097 L/min/kg0.7
allometric constant for
volume of distribution
1.2 L/kg
clearance for metabolites
0.038 L/min
partition
blood:air
0.94
coefficients
liver:blood
8.9
fat:blood
490
slowly perf. tissue:blood
3
rapidly perf. tissue:blood
8.4
parameters for
slowly perfused comp.
0.36 L/min
mass transfer limitations
mass transfer into deep fat
0.0038 min-1
in tissues
mass transfer from deep fat
0.0021 min-1
parameters for
metabolism
production and clearance first order production rate
0.053 min-1
of unavailable D4 in blood clearance into fat
0.014 L/min
Experiment 1
Dermal Absorption through Human Skin In Vivo

Exhaled D4 conc., ug/L

Three male and three female subjects had 0.7 and 0.5 g of [13C]-D4 applied to each axilla, respectively.
For all subjects, there was a 5-min pause before the test chemical was applied to the second axilla. This
work was conducted at the University of Rochester.
After the exposure, samples of expired air were collected in a 5-liter Tedlar bag using a Hans Rudolf nonrebreathing valve .
The amount of [13C]-D4 in plasma and exhaled air (Figure 2) was measured using GC/MS.
100
A
10
1
0.1
0.01
0.001
0.0001
0
4
8
12 16 20 24
time, h
D4 conc. in plasma, ug/L

8
B
6
4
2
0
0
4
8
12 16 20 24
time, h
Figure 2. Measured (symbols) and calculated (solid lines) D4 concentrations in (A) exhaled breath
and (B) blood plasma for men () and women () after a dermal exposure. The error bars
represent one standard deviation for n = 3.
Experiment 2
Dermal Absorption through Rat Skin In Vivo





A 2.5 cm2 aluminum skin depot was glued to the back of F344 female rats housed in Roth-style glass
metabolism cages for the collection of expired air and excreta.
For all three dose levels, 1.9, 4.8 and 9.7 mg/cm2, groups of 4 rats were sacrificed at 1, 6 and 24 h
following the exposure. Another group of rats was washed at 24 h but not sacrificed until 168 h.
At the time of sacrifice, the exposed site was wiped, washed with a soap solution, dried, washed with
70% ethanol, dried, and then tape stripped to remove the stratum corneum.
The amount of 14C in the urine, feces, skin depot, charcoal basket, skin washes, tapes, excised skin at
the exposure site, carcasses, the CO2 and volatiles absorbents, and cage washes was determined by
liquid scintillation counting.
The amount absorbed was calculated as the amount expired either as parent compound or CO2 and the
amount in the urine, feces, excised skin, carcass. The amount penetrated included the same sans D4 in
excised skin (Figure 3).
0.8
absorbed
0.6
0.4
penetrated
0.2
Dose = 1.9 mg/cm2
0.0
cum. % of dose
absorbed and penetrated
0
50
100
time, h
1.0
0.8
absorbed
0.6
0.4
penetrated
0.2
Dose = 4.8 mg/cm2
0.0
0
50
100
time, h
150
200
150
200
cum. % of dose
absorbed and penetrated
cum. % of dose
absorbed and penetrated
1.0
1.0
0.8
absorbed
0.6
0.4
penetrated
0.2
Dose = 9.7 mg/cm2
0.0
0
50
100
150
200
time, h
Figure 3. For Experiment 2, the cumulative amount of D4 absorbed and penetrated as a function
of time for all doses. The skin of rats sacrificed at 168 h was cleaned 24 hours following the
exposure. Points are connected; the lines do not represent model simulation.
Experiment 3
Dermal Absorption through Human Skin In Vitro




Skin disks obtained from abdominal skin of 6 human cadavers were dermatomed to a thickness of 356457 m and mounted on Bronaugh flow-through diffusion cells in a 32C water bath.
Skin integrity was verified using [3H]-H2O.
About 10.7 mg/cm2 [14C]-D4 was applied to an exposure area of 0.64 cm2 on the skin. The application
site was covered with a charcoal basket to trap any D4 that evaporated.
The receptor medium (Hank’s Balanced Salt Solution with 0.6% HEPES, 0.005% Genetecin and 4%
BSA adjusted to a pH of 7.4) flowed continuously through the receptor chamber and was collected
directly into liquid scintillation vials for the determination of the cumulative amount of D4 penetrated
(Figure 4).
Cumulative amount of
D4 penetrated, ug/cm2
1.6
applied dose of D4:
 - 11 mg/cm2
 - 16 mg/cm2
 - 7.3 mg/cm2
 - 8.4 mg/cm2
 - 13 mg/cm2
 - 7.9 mg/cm2
1.2
0.8
0.4
0.0
0
4
8
12 16 20 24
time, h
Figure 4. For Experiment 3, the cumulative amount of D4 that penetrated through human skin in
vitro (i.e., into receptor medium) as a function of time for six experiments. Points are
connected; the lines do not represent model simulation.
Results: Model Structure





The model for describing the dermal absorption of D4 included volatilization of neat D4 from the skin
surface, evaporation of D4 from the skin after the skin had cleared of chemical, and a deep skin
compartment (Figure 1c).
Dermal absorption models do not usually include evaporation of chemical back out of the skin following
an exposure, but the in vivo dermal absorption study of D4 in the rat (Figure 3) provided evidence that
this model structure was appropriate. During Experiment 2:
 The amount of D4 that absorbed decreased significantly with time, but there were no corresponding
increases in the amount penetrated. This provides evidence that some D4 was eliminated from the
skin by evaporation before penetration occurred.
 For volatile or semi-volatile chemicals that can be eliminated from the skin by evaporation, the
amount absorbed may be significantly higher than the amount penetrated.
Calculated D4 plasma concentrations more closely matched the experimental data than calculated
concentrations in exhaled air because the human inhalation PBPK model also matched blood plasma
concentration data more closely.
Peak D4 plasma and exhaled air concentrations probably occurred before the earliest samples were
collected at one hour following the exposure. Because data were unavailable at early times when peak
blood concentrations and D4 evaporation occurred, model predictions at early times require confirmation.
Model parameters were calculated for men and women separately (Table 1) because D4 concentrations
in plasma and exhaled air were higher for women than for men.
Results: Calculations for Experiment 1




By including the dermal exposure route in a human D4 inhalation PBPK model, it was possible to calculate that
during human, in vivo, dermal exposure (Experiment 1):
 all the applied D4 would be cleared from the skin within 5 minutes due to evaporation of neat D4 and
dermal absorption
 more than 90% of the applied dose evaporated from the skin surface before it could be absorbed into the
skin
 the majority of the D4 that had absorbed into the skin was eliminated from the skin by evaporation before
penetration into systemic blood could occur
 the maximum D4 plasma concentration was more than 100 mg/L
 for men and women, respectively, about 0.1 and 0.5% of the applied dose of D4 penetrated the axilla skin
in 24 h following a dermal exposure
The calculation that more than 90% of the applied neat D4 evaporated is consistent with the other experiments:
 Experiment 2: more than 92% of the applied dose was recovered from the charcoal filter covering the
exposed site at 24 hours for all doses
 Experiment 3: an average of 88.2% of the applied dose was recovered from the charcoal filter covering
the exposed site at 24 hours
For Experiment 1, with an average applied dose of 30.5 mg/cm2, about 0.3% of the applied dose penetrated.
For Experiment 3, with an average applied dose of 10.7 mg/cm2, about 0.01% of the applied dose penetrated.
This discrepancy could be because of different doses or regional differences in skin properties (e.g., axilla skin
has been shown to absorb more parathion, malathion and hydrocortisone than other regions [2,3]).
For Experiment 1, the amount penetrated could be calculated because the data contained information pertaining
to the amount of D4 that reached systemic circulation (e.g., plasma concentrations), but the amount absorbed
could not be calculated.
Model Structure


After an inhalation exposure has ended and during dermal exposures, the ratio of the concentration of
chemical in the venous return to the concentration in exhaled breath (i.e., Cv/Cex) is expected to remain
constant over time [4].
Surprisingly, after human inhalation exposures to 10 ppm [14C]-D4, the ratio Cv/Cex increased with time
(Figure 5). To describe this behavior, the PBPK model was modified to include a pool of unavailable D4
that was produced in the liver, moved through the blood, and was cleared in the fat (Figure 1) [1].
Because the ratio Cv/Cex also increased with time following the human, in vivo, dermal exposures, the
same model structure is appropriate for inhalation and dermal exposures and blood sequestration of D4
is equally important for describing D4 kinetics following dermal and inhalation exposures.
10000
1000
Cv / Cex

Figure 5. The ratio Cv/Cex as a function of
time for inhalation () and dermal ()
exposures.
100
10
1
0
5
10 15 20 25 30
time, min
Summary



The dermal absorption model included volatilization of D4 from the skin surface, evaporation of D4 out of
the skin after the skin surface had been cleared of the chemical, and a deep skin compartment.
By including the dermal exposure route in a human D4 inhalation PBPK model, it was possible to
calculate that during human, in vivo, dermal exposure (Experiment 1):
 more than 90% of the applied dose evaporated from the skin surface before it was absorbed
 of the D4 that was absorbed into the skin, most was eliminated by evaporation before penetration
into systemic blood occurred
 about 0.3% of the applied dose of D4 penetrated in 24 hours
For highly lipophilic and semi-volatile chemicals that can eliminate from the skin by evaporation, the
amount penetrated may be significantly less than the amount absorbed.
References
[1]
[2]
[3]
[4]
Andersen, ME, Sarangapani, R, Reitz, RH, Gallavan, RH, Dobrev, ID and Plotzke, KP. 2001.
Physiological modeling reveals novel pharmacokinetic behavior for inhaled
octamethylcyclotetrasiloxane in rats. Toxicol Sci 60:214-231.
Feldmann, RJ and Maibach, HI. 1967. Regional variation in percutaneous penetration of 14C cortisol in
man. J Invest Dermatol 48:181-183.
Maibach, HI, Feldmann, RJ, Milby, TH, and Serat, WF. 1971. Regional variation in percutaneous
penetration in man - pesticides. Arch Environ Health 23:208-211.
Reddy, MB, Andersen, ME, Morrow, PE, Dobrev, ID, Varaprath, S, Plotzke, KP and Utell, MJ. In press,
2003. Physiological modeling of inhalation kinetics of octamethylcyclotetrasiloxane (D4) in humans
during rest and exercise. Toxicol Sci.
Acknowledgements

M. Reddy received support from grant number F32 ES11425-02 from the National Institute of
Environmental Health Sciences (NIEHS), NIH. This work is solely the responsibility of the authors and
does not necessarily represent the official views of NIEHS, NIH. The support of many of our colleagues
at CETT, especially that of R. Yang, is gratefully acknowledged.