Physiological Modeling of the Dermal Absorption of Octamethylcyclotetrasiloxane D4

1
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

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
2
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.

3
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.

4
Background

• During dermal absorption, the absorbingchemical
  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 ofchemical
  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

5
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).

6
Table 1. Parameters for the dermal absorption
model.
men 0.0098 0.00050 3.8 0.014 0.060 0.010
women 0.0068 0.00015 0.18 0.036 0.038 0.00001
0
kv, g/cm2/min k1, cm3/min k-1, min-1 k-2 ,
min-1 kd , min-1 k-d , min-1
7
alveolar space
Cin
Cout
lung blood
(a)
exposed skin
arterial blood
venous blood
slowly perf. tissue
rapidly perf. tissue
fat
deep fat
blood lipid
liver
metabolism
(continued)
8
(continued from last page)
(b)
venous blood
venous blood
productionin liver
clearance in fat
(c)
evaporation of D4 onthe skin surface
evaporation of D4 that has absorbed into the skin
kv
neat D4
k-1
k-2
venous blood
skin
k1
venous blood
k-2
skin
k-d
kd
deep comp.
k-d
kd
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.
9
Table 2. Parameters used in the D4 human PBPK
model 4.
Parameter QP QC liver fat rapidly perf.
tissue slowly perf. tissue liver fat rapidly
perf. tissue slowly perf. tissue
Value 7.6 L/min 5.9 L/min 0.227 0.052 0.472 0.249
0.0314 0.23 0.05 0.5396
alveolar ventilation cardiac output fraction of
blood flow to tissues fraction of body weight
in tissues
(continued)
10
(continued from last page)
allometric constant formetabolic
clearance allometric constant forvolume of
distribution clearance for metabolites bloodair l
iverblood fatblood slowly perf.
tissueblood rapidly perf. tissueblood slowly
perfused comp. mass transfer into deep fat mass
transfer from deep fat first order production
rate clearance into fat
parameters for metabolism partition coefficien
ts parameters for mass transfer limitations
in tissues production and clearance of
unavailable D4 in blood
0.097 L/min/kg0.7 1.2 L/kg 0.038
L/min 0.94 8.9 490 3 8.4 0.36 L/min 0.0038
min-1 0.0021 min-1 0.053 min-1 0.014 L/min
11
Experiment 1Dermal Absorption through Human Skin
In Vivo

• 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 non-rebreathing valve .
• The amount of 13C-D4 in plasma and exhaled air
  (Figure 2) was measured using GC/MS.

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.
12
Experiment 2Dermal 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).

13
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.
14
Experiment 3Dermal Absorption through Human Skin
In Vitro

• Skin disks obtained from abdominal skin of 6
  human cadavers were dermatomed to a thickness of
  356-457 ?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 (Hanks 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).

15
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
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.
16
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.

17
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.

18
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.

Figure 5. The ratio Cv/Cex as a function of time
for inhalation (?) and dermal (?) exposures.
19
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.

20
References

• 1 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
  60214-231.
• 2 Feldmann, RJ and Maibach, HI. 1967. Regional
  variation in percutaneous penetration of 14C
  cortisol in man. J Invest Dermatol 48181-183.
• 3 Maibach, HI, Feldmann, RJ, Milby, TH, and
  Serat, WF. 1971. Regional variation in
  percutaneous penetration in man - pesticides.
  Arch Environ Health 23208-211.
• 4 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.