Octamethylcyclotetrasiloxane or D4 (CAS #556-67-2) is a low molecular weight siloxane (LMWS) fluid with a low surface tension, low aqueous solubility, high lipid solubility, and low vapor pressure (Figure 1, Table 1). It is present at 40-60% by weight in personal care products such as antiperspirants, cosmetics, and hair care products (2-4). Because of its widespread use in these products, a typical woman is exposed to an average daily intake (ADI) of 0.158 mg/kg/day (11.1 mg/day) of D4 from daily use of such products (4).
 |
Figure 1. Three-dimensional structure of D4. Unique chemical properties were caused by methyl groups (white), which shield the oxygen atom (red) and reduced the polarizability of the molecule. |
D4 is also used to manufacture polydimethylsiloxane (PDMS) polymer. PDMS is commonly used in orthopedic and breast implants (5). Although breast implants are composed mostly of stable high molecular weight siloxanes (HMWS), low molecular weight siloxanes (LMWS) still exist in the polymer as impurities (5-9). LMWS consist of both cyclic and linear molecules with repeating units of dimethylsiloxane, of which D4 is a major component (47%) (7-9). Residual LMWS ranged from 0.2 to 2% by weight for the silicone gel and 0.01% to 0.1% for the silicone envelope (1.04-10.4 mg) (5-9). Numerous studies have documented the migration of significant amounts of LMWS out of breast implants into surrounding breast tissues and to the liver (8-11). This would add to the dermal or inhalation exposures from personal care products in a typical woman.
The disposition of [14C]D4 in mice and rats revealed wide tissue distributions after intravenous (IV), subcutaneous, and inhalation exposures, respectively (1,12-14). Female rats exposed to D4 via inhalation induced more liver metabolizing enzymes such as cytochrome P450 (CYP) and retained higher amounts of D4 than did male rats (1,12,15). CYP is a family of enzymes that play a major role in the oxidative and reductive metabolism of many drugs, many xenobiotics, and steroids. Two major and three minor metabolites of D4 have recently been identified in rat urine (16). At high doses, mice and rats had enlarged lungs and liver as well as developmental effects such as decreased live litter size, number of pups, and number of uterine implantation sites (1,4,12,13,15). There is little information concerning the effects of the highly lipid soluble D4 in humans, but it is not expected to be inert if it accumulates in the body (9-12).
Previously, Andersen et al. (16) used physiologically based pharmacokinetic (PBPK) modeling to determine D4 disposition in Fisher 344 rats after inhalation doses (16). Although the model provided a reasonable simulation of the disposition of D4 during exposure, it underestimated the postexposure levels of D4 experimentally found in blood and tissues. It failed to account for the gradual rate of decline of D4 exhalation in rats after exposure, and the authors reported no extrapolation to human exposure (16).
A goal of this study was to develop a validated PBPK model to relate external exposure of D4 to internal target dose (bioavailability) in women with silicone breast implants. The model was first calibrated using previously reported data on tissue distribution in female rats after single and repeated IV administrations of 14C-D4 (1). We validated the predicted results in rats and in humans after inhalation exposure using published independent data (12,17,18). We then used the validated model to predict the pharmacokinetics of D4 as leached from saline-filled breast implants in women.
PBPK model. The PBPK model (Figure 2) uses the actual physicochemical properties of D
4 and physiologic measurements of animals or humans as a basis for calculating the disposition of D
4. Material balances were written for D
4 for each of the model's compartments: lungs, blood, two fat compartments, richly perfused tissues, gastrointestinal tract, kidneys, and liver (see Appendix for details). This model was based on a previously published PBPK model for styrene (
19) and 4,4´-methylenedianiline (MDA) (
20).
 |
Figure 2. Flow diagram for the PBPK model. |
In this model, D4 was metabolized in the liver, and all the metabolites were excreted in the urine or feces (1,12). Generally lipids are transported in blood by various lipoproteins such as low-density lipoproteins (LDL), high-density lipoproteins (HDL), and chylomicrons (22). The highly lipid-soluble D4 is likely to be transported in blood by one or more of these carriers. We found it necessary for D4 to be bound to two lipoproteins of different binding affinities (weakly bound and strongly bound) to account for D4 kinetics in blood (1,12). Physiologic values (e.g., blood flows; tissue and organ volumes) in the model were taken from the literature (Table 2). We adjusted other parameters needed to calibrate the model--such as tissue
distribution coefficients, metabolism and excretion parameters, and the blood:air distribution coefficient (Hair, reciprocal of Henry's Law constant)--to fit the experimentally determined tissue distribution plasma as reported by Kirkpatrick et al. (1). We solved the equations using an adaptive grid Runge-Kutta method (Mathcad 8.0; MathSoft, Inc., Cambridge, MA, USA). The solutions to these multiple stiff ordinary differential equations describe the time course of D4 in the rat. The model predicts the time course of D4 at target tissues, as well as excretion rates, and the variation of these responses as a function of the dose, duration, and route of exposure. Once we obtained the best fit for the rat IV experiments, the model parameters were scaled up to the known human physiology based on power law (Y = a M3/4) (23). The model was validated in both the rat and human by independent prediction and comparison to inhalation results in both rat and human (12,17).
Model calibration. We calibrated the PBPK model using blood/tissues distribution data by Kirkpatrick et al. (1). Briefly, three groups of 20 Sprague-Dawley (SD) rats (10 males and 10 females) were injected intravenously with single 7 mg/kg and 70 mg/kg and repeated 7 mg/kg daily doses of 14C-D4 for 14 days. Blood samples were taken from tail vein at predose, 10, 20, 40 min and 1, 2, 4, 6, 12, 24, 36, and 48 hr postdose from two groups of rats comprising five animals of each sex. The animals were subsequently sacrificed and the liver, kidneys, lungs, and samples of fat were taken from each animal for radioactive determination of tissue distribution of D4. In another group of 10 rats (5 males and 5 females), D4 was administered intravenously via tail vein at 7 mg/kg 14C-D4 as described above. These animals were placed in metabolism cages and urine, feces, and expired air were collected from each animal 0- to 6-hr, 6- to 12-hr, and subsequent 24-hr intervals for up to 5 days. The animals were sacrificed at completion and the liver, kidneys, lungs, samples of fat, gastrointestinal (GI) tract, and remainder of carcass taken for measurements of radioactivity.
Model validation. Rat inhalation model. We extrapolated the rat IV model to simulate rat inhalation exposure as reported by Plotzke et al. (12). In this study, F344 rats were exposed to 7, 70, or 700 ppm 14C-D4 by inhalation for 6 hr. Up to 168 hr after exposure, total body burden, excretion in urine, feces, exhalation, and accumulation in target tissues (liver, lungs, perirenal fat, ovaries, vagina, and testes) were measured (12).
Human inhalation model. We scaled the rat calibrated PBPK model to humans as described previously, and then compared the human model prediction for inhalation exposure with experimental data obtained from an independent inhalation study (17). In the human inhalation exposure study, Utell et al. (17) exposed 12 volunteers using a mouthpiece-exposure system for 1 hr to either air or 10 ppm (122 µg/L) D4 vapor. Utell et al. (17) divided each exposure into three rest periods of 10, 20, and 10 min, respectively, and two exercise periods, each of 10-min duration. The order of D4 exposure was randomized. Exhaled air samples were collected before, immediately after exposure, and at 1 and 6 hr after exposure. D4 was extracted from plasma samples with tetrahydrofuran and the D4 analysis was performed using gas chromatography-mass spectrometry analysis as described elsewhere (21).
Breast implant exposure. In our simulation, the maximum dose of residual D4 that could migrate from silicone breast implants was estimated to be 0.1% wt of the silicone implant envelope (0.15 mg/kg or 10.4 mg) (5). The residual D4 was left by incomplete devolatization of medical polymer (5,7-9).
Lykissa et al. (9) measured in vitro the diffusion rate of LMWS (D4-D7) from explanted silicone gel-filled breast implants into various surrounding media. The highest reported diffusion rate (40 µg/g implant/day) was for a lipid-rich medium such as that found in breast tissue, and the lowest reported rate was for an aqueous extraction media (< 1 µg/g implant/day). Given the in vitro results for D4 into lipid rich medium, the estimated diffusivity of D4 in the breast implant shell was 5.4 
10-8 cm2/sec. Using published values of the diffusivity for different compounds in PDMS and free volume theory, we also estimated the diffusivity of D4 to be 2 10-8 cm2/sec in the breast implant shell (24,25). Thus, the leaching rate of D4 from the shell to the surrounding fatty tissues of the breasts was estimated to be 95% removal in 30 days. We obtained this rate by solving one-dimensional diffusion equation inside a breast implant shell (5-7 g) exposed to an infinite sink (the body) using estimated values of the diffusivity (5.4 10-8 cm2/sec, 0.3 mm shell thickness) of D4 (26). This conservative estimate for diffusion in a rubbery polymer allows for all of the D4 to be removed from the breast implant shell rapidly within a month (9). In the actual human body, external mass transfer resistance will slow the rate down and allow the D4 to persist in the body even longer (27). The dose of D4 from silicone gel implant is expected to be higher (9), because the initial mass of D4 in the silicone gel leaching out of the silicone polymer envelope (which is permeable to its own components) is higher, even years after implantation (9-11).
The predicted and experimental plasma and fat results in the SD rats are shown in Figures 3 and 4, respectively. The model parameters in the rat are shown in Table 3. The experimental data showed higher accumulation of radioactivity in female rats than in male rats at all doses (
1,12). We used only the data from female rats to develop our model because our goal was to assess the exposure of D
4 in women.
The plasma radioactivity profiles of 14C-D4 showed nonlinear kinetics for different dosage regimens (Figure 3). Single IV dose of 7 mg/kg 14C-D4 had two half-lives (t1/2 = 2.1 hr and 12.7 hr) and 14-day repeated daily dose of 7 mg/kg 14C-D4 had three half-lives (t1/2 = 3.2 hr, 15.9 hr, and 32.7 hr). Table 4 shows the tissue distribution results obtained in the IV rat studies. The highest radioactivity accumulated in fat followed by richly perfused tissues (i.e., lungs, brain), blood, liver, and kidneys. Approximately 60-80% of the absorbed D4 dose was exhaled and excreted in the urine and feces. Figure 4 shows the predicted and experimental time and dose response of 14C-D4 in fatty tissues of the treated rats.
 |
Figure 3. Predicted and experimental D4 plasma concentrations in rats exposed by IV. |
 |
Figure 4. Predicted and experimental D4 fat concentrations in rats exposed by IV. |
Table 5 shows the pharmacokinetic results obtained in the IV rat studies. For the repeated doses, the exposure as defined by area under the curve (AUC = 43.4 vs. 663.3 µg hr/mL) substantially increased, but the clearance remained the same.
To validate the rat model, we compared the inhalation results obtained from the model with independent inhalation exposure data in F344 rats reported by Plotzke at al. (12). The F344 rat results were obtained using higher metabolism rate than in SD rats (12). Also, the higher ventilation rate in the rat produced lower capture efficiency than in the human ventilation model. As in the IV rat model, the experimental inhalation results in F344 rats also indicated a biphasic plasma profile as well as slow clearance in fat, in fairly good agreement with the predictions of the model (Figure 5). These results showed less accumulation in fat compared to rat IV exposure results. After 60 hr, the simulated plasma results were below the reported experimental data, which were reported as the limit of detection (0.01 µg/g). Hence, we cannot really compare, but the model does not conflict with the experimental results. Previous rat experiments indicated rapid clearance of D4 in plasma (1).
 |
Figure 5. Predicted and experimental D4 plasma and fat concentrations in rats exposed by inhalation. Limit of detection = 0.01 µg/g. |
To validate the human model, we predicted the absorption, distribution, metabolism, and excretion of D4 after an inhalation dose and compared it to previously published experimental results by Utell et al. (17). Figure 6 displayed the predicted and experimental profiles of D4 after a single dose of 11.1 mg by inhalation for 1 hr in human plasma and fat, respectively. As shown, the model was an excellent predictor of the experimentally determined plasma concentrations.
 |
Figure 6. Predicted and experimental D4 plasma and fat concentrations in humans exposed by inhalation. |
Table 6 shows the simulated results for tissue distribution, exhalation, and excretion in the urine and feces for human inhalation and implantation. Table 7 shows the pharmacokinetics of D4 in plasma. Like fat in rats, the human fat showed highest accumulation of D4 regardless of dose, regimen, or route of exposure.
A single low-dose D4 exposure by inhalation showed fast plasma absorption (Cmax = 69.4 ng/mL at tmax = 1 hr) and fast excretion [43.8% exhaled, 6.1% in urine, 2% in feces; Clearance = 407 mL/hr (Table 7)]. Repeated daily exposure to the same low dose of D4 for 14 days showed higher Cmax value (76 ng/mL) at 145 hr after exposure, increased area under the curve (AUC = 3,628 ng hr/mL), increased volume of distribution (Vd = 1,022 L) and reduced systemic clearance (Clearance = 357 mL/hr). The implantation of silicone breast implants had a lower Cmax of 2 ng/mL compared to a Cmax of 69.4 ng/mL and 76 ng/mL after single or repeated inhalation of 11.1 mg/day for 7 days, respectively. Comparison of different routes of exposure in human showed largest volume of distribution (Vd = 4,100 L) after implantation, indicating longer retention in the body. We predicted that D4 systemic exposure would be longer after implantation (t1/2 = 18 days for D4 in fat) compared to inhalation (t1/2 = 11 days in fat). The maximum amount retained in fat for the breast implant case was 174 ng/mL at tmax =11.7 days after implantation compared to a corresponding Cmax = 330 ng/mL at tmax = 34 hr after inhalation.
Women are prone to bioaccumulate D
4 when exposed daily to such multiple personal care products as antiperspirants, skin care, or hair care products. A mean dose of 0.158 mg/kg D
4 per day by inhalation was reported in a recent abstract by Shipp et al. (
4). Added to this would be the estimated dose (10.4 mg/30 days/60 kg = 0.0057 mg/kg/day) of D
4 leached from the saline-filled silicone breast implants (
5-9). For the first time, the results of the PBPK model suggest that women accumulate D
4 in their fatty tissues (e.g., breasts), richly perfused tissues, liver, and kidneys. The D
4 accumulation increases with the dose, the regimen of dosing (single vs. repeated), and the routes of exposure (inhalation vs. implantation).
The resulting tissue distribution is attributed to the physical properties of D4, which is highly lipid soluble and very insoluble in water (Figure 1, Table 1). Thus, once lipid-containing tissue (e.g., breast tissue) is exposed to D4--as occurs when D4 leaches from breast implants--D4 is rapidly absorbed and only slowly desorbed with a very long half-life (fat t1/2 = 18.2 days). D4 is retained in the body if during exposure it contacts the lipophilic tissues. Thus neither inhalation exposure (about a 10% capture of the intake dose) nor dermal contact (0.5% absorption) is an efficient way to deliver D4 into internal target organs in the body (17,28). By contrast, leaching from an implant directly into breast tissue (mostly fat) would have great potential for allowing accumulation of D4 in the body. Repeated exposures increase accumulation in target tissues since the frequency of exposure is shorter than the elimination half-life, especially in certain target tissues.
Fat is the primary tissue depot following all routes of exposure, at low and high doses of D4, with significant longer half-life than that of plasma (Table 4 and 5). In the rat, the highest D4 accumulation was in the fat, followed by richly perfused tissues (e.g., lungs, brain), blood, liver, and kidneys (Table 4). Single-dose inhalation results in rats were similar, with rapid clearance from plasma and a longer half-life in fat (Figure 5). The single inhalation route in rats also had poor absorption, which was consistent with both our inhalation model and published results (12). Predictions of the PBPK model regarding plasma, tissue distributions, and excretions were consistent with the above reported experimental results (1,12).
In the human inhalation case, the model predicted that the biphasic plasma half-life would be 1.7 and 7.4 hr, and the fat half-life would be 11.1 days. Similarly, the model predicted that after desorption of D4 from the breast implant in the human, the plasma half-life would be 7.8 hr and the fat half-life would be 18.2 days (Table 7). If D4 exposures were repeated in either case at a frequency shorter than the fat half-life, the net result would be accumulation in the fat tissues, because the input would exceed the elimination output.
These results were not evident from previous human exposure studies, because the target tissue disposition was not accurately determined (17,18). Utell et al. (17) exposed human volunteers to 129-137 mg D4; the lungs only captured 10% of D4 exposure by inhalation (13 mg out of 129-137 mg of inhaled D4). Thus most of the inhaled dose (124 mg out of 137 mg) was not absorbed, which is expected from a combination of poor gas mixing in the lung, and alveolar tissue mass transfer resistance to hydrophobic materials such as D4 because of the aqueous nature of the alveolar membrane. Utell et al. (17) reported that D4 was rapidly cleared from plasma, but they did not identify D4 tissues distribution or excretion as predicted by our model. Our PBPK model did confirm the plasma measurements reported by Utell et al. (17) as well as a nonlinear clearance with two half-lives of 30 min and 330 min and a mean peak value of 79 ng/g (Table 7 and Figure 6).
The pharmacokinetic results shown in Tables 5 and 7 for rats and humans, respectively, showed a disproportionate increase in area under the curve and volume of distribution at high dose and repeated exposures of D4. The predicted D4 plasma and fat behavior are similar to that observed with other volatile lipophilic chemicals such as styrene (12,20). Because the exposure was repeated daily, the D4 concentrations in fat increased, as shown in Table 6. However, systemic clearance remained about the same regardless of the route of exposure (Table 7). The shape of the plasma and fat concentration-time curves shown in Figures 3-5 also suggested probable saturation of the elimination processes. On repeated dosing, saturation of the elimination processes may increase the delivered dose of D4 to target organs such as fat, richly perfused tissues (e.g., lungs, brain), liver, and kidneys in both rats (Table 4) and humans (Table 6). In the liver and kidneys, such accumulation could produce liver enlargement confirming experimental results found in mice and rats (1,13,15). McKim et al. (15) reported some preliminary results suggesting that repeated inhalation exposure to high concentrations of D4 produced liver enlargement with significant induction of cytochrome P450 CYP2B1/2 in rats. Analysis of the rat excretion results (Table 4) showed no compelling evidence of D4 CYP enzyme induction. Furthermore, there is a more reliable way (e.g., using yeast that contains human genes) to determine whether D4 chemically induced P450 in humans.
The model predicted 43% and 53% of the dose of D4 in exhaled air after single and repeated inhalation exposures, respectively. Similarly, 59% of D4 was exhaled in air after breast implantation (Table 6). The model results were comparable with the exhalation data reported by others (1,3,12,15,17). The PBPK model used the blood:air partition coefficient to estimate the exhalation data of D4. The blood:air partition coefficient that best fit the experimental data [shown in Table 3 as Hair (20.0)], was not consistent with the water:air partition coefficient values for D4. The published Henry's law constants for D4 (air:water) have been reported in the range of 3-32, which is equivalent to water:air partition coefficient of 0.33 to 0.031 (27,29-32). The discrepancy was attributed to both the low solubility of D4 in water (56 ppb) and its binding to blood lipoproteins, which increased the value of the blood:air partition coefficient relative to its corresponding water:air partition coefficient (33). Beliveau and Krishnan (33) indicated that the blood:air partition coefficients of lipophilic volatile organic compounds (e.g., D4, styrene) could not be determined using water:air partition coefficients. They published a methodology to account for this solubility enhancement of protein binding for estimating the blood:air partition coefficients using oil:water and n-octanol:hemoglobin:water mixtures instead of a simple water:air measurements (33). The high lipid affinity of D4 for blood proteins makes this effect even more significant for this case. We suspected that a smaller blood:air partition coefficient (< 20) was used in a reported model by Anderson et al. (16). This could have led the authors to underestimate the postexposure blood and tissues levels of D4 in rat (16).
We developed a PBPK model to determine the time course of D4 in the human body after exposure at different doses and routes in rats and humans. The simulation results suggest that D4's unique physicochemical properties play an important role in the bioavailability of this chemical. It is absorbed rapidly and retained in the fatty tissues for a longer time when it contacts lipophilic tissues directly (e.g., breast implants). The repeated daily inhalation exposures to D4 would accumulate this compound in fat, liver, and kidneys significantly to saturate its elimination processes. This accumulation is likely to occur in women because physiologically women have a large volume of adipose tissues and a slower metabolism than do men. Future studies are expected to extend our toxicokinetic evaluation of D4 such that we could accurately assess the safety of D4 based on its bioavailability and target doses, not by consideration of the dose level alone.
Appendix
The material balances for both D4 and its metabolites for the model shown in Figure 2 were solved using MathCad 8.0 (MathSoft, Inc., Cambridge, MA, USA). We used an adaptive grid Runge-Kutta method for the system of stiff ordinary differential equations. We calibrated the rat model using an IV dose; this was introduced as a forcing function partitioned to all of the compartments in the model. In that way, the D4 was distributed to all compartments in the body, allowing the numerical procedure to converge to a solution. In the human inhalation or breast implant case, the D4 was introduced directly to the lung or into the systemic circulation at the mix point, similar to previous models (12,13). A nomenclature list is included at the end of this appendix. The following equations are the material balances for D4:
For the lung,
For the richly perfused tissues,
For the deep fat compartment,
For the weakly bound fat compartment,
For the liver, WIDTH="251" HEIGHT="36" ALIGN="top" ALT="Equation 5">
For the GI tract,
For the kidneys,
We determined the total metabolites generated by integrating the accumulation equation below,
In the blood, D4 exists dissolved in the plasma (aqueous), weakly bound to protein, and strongly bound to protein. The venous blood is well mixed before it is returned to the arterial circulation. At this mix point, the D4 protein binding is calculated. For D4 in the plasma,
For the weakly bound D4 in the blood,
For the strongly bound D4 in the blood,
For the IV case in the rat, we solved the above equations with the initial conditions that all the compartments except the weakly bound D4 in the blood have zero concentration at t = 0. The weakly bound D4 in the blood was assigned an initial concentration of D4 equal to 5% of the IV dose at t = 0; this was a better fit of the experimental data.
For the rat, the IV dose was input using the following forcing function.
For the human, the IV dose was replaced by either an inhalation dose or implant dose. Using the data of Utell et al. (11), we exposed subjects to 10 ppm D4 by inhalation for 1 hr. This exposure period consisted of 10 min rest, 10 min exercise, 20 min rest, 10 min exercise, and finally 10 min rest before the D4 inhalation was terminated. To model this, we defined five forcing functions covering each period of exposure. The inhalation rate was set at 10 L/min during rest, and 30 L/min during exercise. Using Utell et al.'s (11) published capture efficiencies of the human lung of 0.12 during rest and 0.07 during exercise, we delivered a total dose of 11.1 mg of D4 to the human body during the exposure. The following equations were used,
During the exposure, Qair in equation 1 is also stepped between Qrest and Qexercise during the 1-hr exposure. After exposure, the lung ventilation rate was set to the resting value. In the human case, unlike the rat, all of the compartments contained no D4 at t = 0. Also, unlike the rat where 5% of the D4 was initially bound to the blood plasma, in the human this value was set to 0.8% of the dose delivered by the functions
.
For the human breast implant exposure, the inhalation forcing functions were replaced by a first-order desorption.
