Rebuttal and critical review of Andersen et al.'s [D.sub.4] PBPK model. (Correspondence).

The letters of Meeks and Andersen et al. regarding our paper in EHP (1) included inaccurate statements and misconceptions about our pharmacokinetic model of [D.sub.4].

After reviewing Andersen et al.'s recent paper (2), we found several shortcomings. First, Anderson et al. (2) used an unconventional experimental method to underestimate the affinity of [D.sub.4] for blood and fat; these partition coefficients were not comparable to those obtained for other lipid soluble organic chemicals. They further reduced these experimental measurements in order to "fit" a 10-compartment model, which included 3 deep compartments in the lungs, fat, and liver. Andersen et al. used these low values to underestimate potential [D.sub.4] accumulation in fat and increase its clearance.

Second, Andersen et al. (2) did not validate and verify their PBPK model using independent data from intravenously (iv) treated rats. When we used Andersen et al.'s parameters for [D.sub.4] [blood:air partition coefficient ([P.sub.b:a]), fat:blood partition coefficient ([P.sub.fat]), and metabolism rate ([V.sub.max])] in our own model, our results did not fit the iv experimental rat data, especially regarding [D.sub.4] tissue distribution in fatty tissues. Andersen et al.'s conclusions about the disposition and fate of [D.sub.4] also were not substantiated by the experimental rat inhalation data because high lipid solubility and slow desorption would favor accumulation in fatty tissues, as in the case with styrene.

Third, in their letter, Andersen et al.'s criticism about the dose rate of [D.sub.4] from a breast implant was incorrect; the dose rate reported was for [D.sub.4] leaching from the saline-filled breast implants and not from the silicone gel-filled breast implants.

We question the validity of Andersen et al.'s model (2) and believe that their predictions about the safety assessment of [D.sub.4], a component in silicone personal products and breast implants, may be misleading.

Andersen et al. (2) used a low [P.sub.b:a] (0.88) in their model, despite reporting a measured experimental value of 4.3. They also used an unconventional method to measure the [P.sub.b:a] and blood:tissue partition coefficient ([P.sub.b:t]).

To measure the [P.sub.b:a] and [P.sub.b:t] of [D.sub.4], Andersen et al. (2) placed liquid [D.sub.4] and matrices such as blood, fat, lung, and liver in separate glass scintillation vials. All of the vials were subsequently placed in an enclosed 500-mL beaker. The [D.sub.4] was not in physical contact with the blood or any other matrices throughout the experiment. Using this method, a low volatility compound like [D.sub.4] would have to vaporize, diffuse through a gas space, and diffuse into a stagnant blood or tissue phase with liquid mass transfer resistance. This process would take time to reach equilibrium, but did Andersen et al. allow enough time for equilibrium to occur? Shields et al. (3), who measured [D.sub.4] concentrations in indoor air using a state-of-the-art analytic method, indicated that the sampling intervals for [D.sub.4] should be in weeks, not hours, in order to reach equilibrium. Andersen et al. (2) reported that they agitated for 24 or 48 hr and measured [P.sub.b:a] at two unknown time points. In fact, if the samples were allowed to reach equilibrium, their measurement of the [P.sub.b:a] of [D.sub.4] (4.3) might reach our estimated value of 20. The measured concentrations of [D.sub.4] in blood based on molecular diffusion between the vapor phase of [D.sub.4] and blood are not reliable unless they used long sampling intervals (3). Because Andersen et al. (2) did not describe internal standards for the experiment, it is likely that the percentage recovery was low after 24-48 hr. The same method was also used to underestimate other partition coefficients for fat, lungs, and kidneys.

A more accurate and direct measurement of [P.sub.b:a] (or [P.sub.tissues]) would be to place several milliliters of the viscous [D.sub.4] liquid in direct contact with the tested matrix (e.g., whole blood, fat, liver, etc.) in a closed scintillation vial (4). The headspace (air) concentration and matrix concentration of [D.sub.4] should then be quantified during several time intervals following agitation. This minimizes the equilibrium problems not addressed by Andersen et al. (2).

The physical properties of [D.sub.4] (Table 1) play an important role in its tissue distribution and excretion; thus it is important that the use of arbitrary "fitted" parameters be avoided. This arbitrary low value of [P.sub.b:a] used by Andersen et al. (2) differed by a factor of 5 from the in vitro evaluation. Similarly, the partition coefficients used for fat and other tissues also varied widely from their experimental data (2). For example, Andersen et al. used a [P.sub.fat] of 550.6 for instead of their experimentally determined value of 2,089 so their model would fit the data.

The low [P.sub.b:a] value is not comparable to those of other organic chemicals with properties similar to those of [D.sub.4]. As shown in Table 1, the higher the volatility, the smaller the value of [P.sub.b:a] of an organic compound. For example, because benzene is more volatile than styrene, it has a smaller [P.sub.b:a] (75% smaller) than styrene (Table 1). According to Andersen et al.'s results (2), [D.sub.4] would be more volatile than benzene in blood. This is inconsistent with the observed volatility because benzene has a boiling point of 80.1[degrees]C, whereas [D.sub.4] has a boiling point of 175[degrees]C (Table 1). Because [D.sub.4] has a lower volatility than both styrene and benzene, its [P.sub.b:a] would be expected to be at least as large as the values reported for these two chemicals, and not smaller (Table 2). The [D.sub.4] [P.sub.b:a] would not be expected to have a value as low as 0.88, which is outside the range of all of the chemicals listed in Table 2. Ramsey and Andersen (16) reported a [P.sub.b:a] for styrene of 40.2 (Table 1).

Under the scenario of Andersen et al. (2), if both 1 [micro]g [D.sub.4] and 1 [micro]g of a much more volatile component such as benzene or another chemicals in Table 2 were added to blood, the [D.sub.4] would vaporize more readily. This is due to its partition coefficient, which favors transfer to the gas phase. Thus, [D.sub.4], which boils at 175[degrees]C, would be more volatile than benzene, which boils at 80.1[degrees]C, a situation which makes no sense.

As we discussed in our paper (1), the physical properties of [D.sub.4] favored its absorption into fat. High absorption of [D.sub.4] (100%) by the iv route and slow desorption, as well as a long half-life in fat ([t.sub.1/2] = 18 days), were attributable to the high [P.sub.b:a], [P.sub.fat], and high lipid solubility of [D.sub.4] [log octanol/water partition coefficient ([K.sub.ow]) = 5.1]. For similar reasons, other highly lipid soluble organic compounds such as styrene, with high [P.sub.b:a] and [P.sub.fat], tend to accumulate in the fat tissue of rats and humans (5-7).

To compensate for this estimate of a thermodynamic property in blood and to "fit" the rat data for inhalation exposure, Andersen et al. (2) modified their basic model with 6 compartments to a refined model with 10 compartments, including deep compartments (deep lung, deep liver, deep fat). But any scenario can be fitted by simply adding more compartments. However, adding more mass balance equations requires more biochemical parameters, which may not be available or accurately measured.

In our study, we derived the [P.sub.b:a] as the reciprocal of the [D.sub.4] Henry's Law Constant, which is its published water:air partition coefficient (a value ranging from 3 to 32) (8-11). The value we used in our model was within the range reported by these independent investigators (8-11). Still, we included in our paper (1) a discussion of the discrepancy caused by blood to the aqueous Henry's Law Constant of [D.sub.4], and we also cited the paper that supported these observations (4). We believe that Andersen et al. (2) did not take into account the [P.sub.b:a] of lipophilic organic compounds described by Beliveau and Krishnan (4).

Even though our [P.sub.b:a] is significantly larger than that reported by Andersen et al. (2) we predicted that the absorbed [D.sub.4] would be mostly exhaled [range, 42-59% in humans; see Table 6 of our paper (1)]. We do not understand Andersen et al.'s comment in their letter that we did not predict significant exhaled [D.sub.4]. Recalculating the exhaled air amount using the following material balance on the exhaled air may clarify our concerns to Andersen et al.

(1) Exhaled = [[integral].sup.t.sub.0][Q.sub.air][C.sub.air]dt

In preparing this response, we ran our PBPK model again using Andersen et al.'s fitted values (2) for [P.sub.b;a] [P.sub.fat], and [V.sub.max] for metabolism rate (8 times higher than our [V.sub.max] value). The results in Figure 1 show that the model using parameters employed by Andersen et al. (2) predicted poorly the [D.sub.4] levels in fat while predicting reasonable plasma [D.sub.4] levels following a single, low-dose iv injection. Using Andersen et al.'s parameters (2), we found that > 80% of [D.sub.4] is exhaled after iv exposure. Therefore, this would cause underprediction of [D.sub.4] accumulation in fat, experimentally found to be 16% of the iv dose. As shown in Figure 5 in our paper (1), our model gave excellent simulations of the rat iv (12) and rat inhalation data (13).

[FIGURE 1, 5 OMITTED]

Two structurally different PBPK models could not both be correct, and both models fit the rat inhalation reasonably well. This leads us to believe that there are other shortcomings in Andersen et al.'s study (2):

* Andersen et al. (2) based their model on a rat inhalation study in which the absorption and elimination rates are compromised. Only 10% of the exposed [D.sub.4] in the air is absorbed, compared to 100% absorption of [D.sub.4] with iv exposure. The dose absorption is limited from the mass transfer resistance in the lungs. Using a low [P.sub.b:a], Andersen et al. reported that > 50% of the [D.sub.4] absorbed is eliminated in the expired air, whereas they assumed the metabolism rate of [D.sub.4] to be 8 times higher in their model than in ours. The unusual kinetics could not be confirmed by other published studies.

* Andersen et al.'s model lacks validation and verification using independent data such as included in the rat iv study (12), so their conclusions about the disposition of [D.sub.4] are best described as preliminary.

* Andersen et al.'s model is not accurate because they failed to measure partition coefficients for both parent compounds and metabolites for the 10 compartments including 3 deep compartments (lungs, liver, and fat). Instead, they have to curve fit, leading to errors and uncertainty regarding [D.sub.4] distribution in fatty tissues especially.

* Andersen et al.'s conclusions on [D.sub.4] kinetics even contradicted what others (13,14) reported regarding [D.sub.4] kinetics. In fact, they reported that [D.sub.4] plasma and tissue distributions resemble other volatile organic compounds such as styrene, which were found to accumulate in fat tissues of both experimental animals and exposed workers (5-7).

* Andersen et al. (2) failed to determine accurate [D.sub.4] pharmacokinetic data which show that [D.sub.4] is retained in fat. Because 8-10% of [D.sub.4] dose was found in fat 7 days postexposure and because rats were to be exposed daily for 14 days, it is hard to believe that [D.sub.4] would not retain and accumulate in the body.

In our paper (1), we estimated the maximum dose rate of residual [D.sub.4] that could migrate from the silicone envelope of a breast implant to be 5.7 [micro]g/kg/day based on Fick's Law of Diffusion. We estimated a leaching rate of 95% in 30 days for the thin shell of a saline-filled breast implant surrounded by fatty tissues. The diffusivity of 5.4 x [10.sup.-8] [cm.sup.2]/sec was consistent with published values for other chemicals (15). Our reported dose rate was the dose rate of [D.sub.4] leaching from saline-filled breast implants. The dose rate of [D.sub.4] leaching out of implanted silicone gel-filled breast implants could be easily determined, if needed.

In their letter, Andersen et al. correctly identified a typographical error in the Appendix regarding the material balance on the lung. The correct equation is as follows:

[V.sub.lung] = d[C.sub.lung] / dt = [Q.sub.t][C.sub.ai] - [Q.sub.t][C.sub.lung][H.sub.air] - [Q.sub.air][C.sub.lung]

In this equation, [C.sub.ai] is a venous blood concentration as defined by the equation at the mix point [Appendix of our paper (1)]. It is not an arterial concentration, as suggested by Andersen et al. In this nomenclature, a = average. Thus, unlike the claims of Andersen et al. in their letter, this model does not artificially limit the exhalation of [D.sub.4]. Any introduced [D.sub.4] will flow through the lung in a physiologically realistic manner, despite claims to the contrary. The above equation is equivalent to the tubular equilibrium lung used in the styrene model (16). The capture efficiency we used in both the rat and human models was similar and was only used to determine the delivered dose to the rat or human body as described in the Appendix of our paper (1). In the reference (14) cited in our paper, the delivered dose was experimentally determined by measuring the gas flow and inlet and outlet concentrations of [D.sub.4] at the rebreathing tube connections. We used the same model structure for both the rat and the human. Andersen et al. agreed that our rat inhalation model was correct because the human model had an identical structure.

In their letter, Andersen et al. also claimed that the "accumulation in the strongly bound fat compartment would always be zero." Figure 2 shows the accumulation in this compartment for F344 rats after low-dose inhalation (13).

[FIGURE 2 OMITTED]

It is informative to use animal data in a PBPK model to predict [D.sub.4] dose metrics in an animal body. This approach also allows the determination of the internal dose in target tissues, which can then be extrapolated to humans and correlated with the toxicity. However, models should be physiologically realistic and should not be used to predict phenomena beyond the reasonable bounds of the data by "fitting" highly restrictive cases. In an accurate model, the following problems should be avoided:

* Artificially high pulmonary clearance of [D.sub.4] resulting from use of a [P.sub.b:a] that is not comparable to one obtained experimentally.

* Use of unconventional methods to reduce the potential of accumulation in target organs.

* Overestimation of the rate of metabolism, which is caused by a reduced absorbed dose resulting from inhalation exposure.

* Inappropriate use of the inhalation model for [D.sub.4] to examine the disposition and fate of [D.sub.4] leached from silicone breast implants.

Because of these problems with Andersen et al.'s model (2), the authors underestimated the potential bioavailability of [D.sub.4] and were unable to predict its bioaccumulation after repeated exposures or long-term exposure that occurs when [D.sub.4] leaches from silicone breast implants.

Table 1. Comparison of physical properties of [D.sub.4], styrene, and
benzene.

Property                      [D.sub.4]       Styrene        Benzene

Melting point ([degrees]C)      17.5           -30.6           5.5
Boiling point ([degrees]C)     175.4           145-146        80.1
Vapor pressure (mmHg)           1 (25          4.5 (20         2.3 (3
                              [degrees]C)    [degrees]C)    [degrees]C)
[P.sub.b:a]                      0.88 (a)       40-52         17.8
Solubility in water             56 ppb         300 ppm         --
Log [K.sub.ow]                   5.1            2.95           2.14

(a) We used a value of 20 for [P.sub.b:a].

Table 2. Blood:air ([P.sub.b:a]) and blood:fat partition coefficients
([P.sub.fat]) of some known VOCs.

Compound                 Log [K.sub.ow]    [P.sub.fat]    [P.sub.b:a]

Hexane                       3.87             69.43          2.29
Isoprene                     2.42             38.5           1.87
1,1,1-Trichloroethane        2.48             45.66          5.76
Tetrachloroethene            3.40             86.67          18.9
Benzene                      2.14             28.03          17.8
Toluene                      2.64             56.72          18
p-Xylene                     3.15             42.32          41.3
Styrene                      2.95             86.47          40.2
Chlorobenzene                2.86             21.5           59.4

REFERENCES AND NOTES

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(12.) Kirkpatrick D. [sup.14]C-[D.sub.4] Pharmacokinetics in the Rat Following Intravenous Administration. Huntingdon, UK: Huntingdon Research Center Ltd. Sponsored by the Silicone Environment Health and Safety Council. Midland, MI:Dow Corning Corporation, 1995.

(13.) Plotzke KP, Crofoot SD, Ferdinandi ES, Beattie JG, Reitz RH, McNett DA, Meeks RG. Disposition of radioactivity in Fischer 344 rats after single and multiple inhalation exposure to [sup.14]C]octamethylcyclotetrasiloxane ([[sup.14]C][D.sub.4]). Drug Metab Dispos 28:192-204 (2000).

(14.) Utell MJ, Gelein R, Yu CP, Kenaga C, Geigel E, Torres A, Chalupa D, Gibb FR, Speers DM, Mast RE, et al. Quantitative exposure of humans to an octamethylcyclotetrasiloxane ([D.sub.4]) vapor. Toxicol Sci 44:206-213 (1998).

(15.) Favre E, Schaetzel P, Nguyen QT, Clement R, Neel J. Sorption diffusion and vapor permeation of various penetrants through dense poly(dimethylsiloxane) membranes: a transport analysis. J Membr Sci 92:169-184 (1994).

(16.) Ramsey JR, Andersen ME. A physiologically based description of the inhalation pharmacokinetics of styrene in rats and humans. Toxicol Appl Pharmacol 73:159-175 (1984).

Hoan-My Do Luu
Joseph C. Hutter
Office of Science and Technology
Center for Devices & Radiological Health
U.S. Food and Drug Administration
Rockville, Maryland
E-mail: [e-mail]