Proposed PBPK model to predict infant exposure: Clewell and Gearhart's response. (Correspondence).

In her letter, Soldin makes a number of points regarding the potential human toxicity of perchlorate (Cl[O.sub.4.sup.-]), one of the chemicals mentioned in our paper (Clewell and Gearhart 2002). The purpose of our article (Clewell and Gearhart 2002) was to describe the potential use of physiologically based pharmacokinetic (PBPK) modeling to address the challenges encountered in calculating chemical transfer in breast milk. We presented Cl[O.sub.4.sup.-] as one of several examples of how PBPK modeling can be used to predict neonatal dose where little or no data are available in humans. The model includes descriptions for Cl[O.sub.4.sup.-] and iodide in the lactating rat and neonate and the extrapolation of radioiodide to human lactation. We suggested that the successful extrapolation of the rat radioiodide model to predict human milk increases confidence in using a similar PBPK-based approach to estimate infant Cl[O.sub.4.sup.-] dose (Clewell and Gearhart 2002).

With respect to the PBPK model for Cl[O.sub.4.sup.-], Soldin expresses concern regarding the model structure, stating that Cl[O.sub.4.sup.-] is not transported into the thyrocyte. This contention is based solely on in vitro electrogenicity studies in frog oocytes (Eskandari et al. 1997) and Chinese hamster ovary cells (Yoshida et al. 2002), the results of which are susceptible to two explanations: a) Cl[O.sub.4] blocks [I.sup.-] transport but is not transferred into the thyroid, and b) Cl[O.sub.4.sup.-] competes with [I.sup.-] and is transported by sodium iodide symporter (NIS) at a 1:1 ratio with [Na.sup.+] (Dohan et al. 2000; Nilsson 1999; Riedel et al. 2001a, 2001b). These three studies did not measure thyroid Cl[O.sub.4.sup.-] levels, nor did they offer concrete evidence disproving the translocation of Cl[O.sub.4.sup.-] into the follicle.

Perchlorate transfer into the thyrocyte is evidenced by the effect of Cl[O.sub.4.sup.-] on internalized thyroid iodide (Hildebrandt and Halmi 1981). In the presence of propylthiouracil (which blocks iodide organification) and thyroid stimulating hormone (which increases inorganic iodide uptake), Cl[O.sub.4.sup.-] exposure caused a significant discharge of the internal iodide. This suggests that Cl[O.sub.4.sup.-] enters the thyroid cell and displaces iodide from binding sites in the thyrocyte. The model description is also supported by measured thyroid:serum Cl[O.sub.4.sup.-] ratios > 1 and up to 30 (Chow and Woodbury 1970; Yu et al. 2002a). These high anion concentrations cannot be explained without active sequestration, and the possibility of analytical interference by metabolites is ruled out by a double-labeled [sup.36]Cl[sup.18][O.sub.4.sup.-] study that showed no appreciable metabolism in the rat (Anbar et al. 1959). Furthermore, time-course data show two distinct phases in Cl[O.sub.4.sup.-] uptake in the thyroid (half-life = 2 and 33 min), which Chow and Woodbury (1970) suggested represent the rapid transport into the interstitial and cellular compartments and the slow equilibration in the luminal fluid, respectively. This same behavior was also observed in the clearance curve from an intravenous dose (Yu et al. 2002a). Thus, when taken together, the weight of evidence suggests that Cl[O.sub.4.sup.-] is a truly competitive inhibitor .(Chow and Woodbury 1970; Hildebrandt and Halmi 1981; Wolff 1998; Yu et al. 2002a). Our mode of action hypothesis, which served as the foundation for the kinetic models, was based upon the entire body of available information.

A second concern mentioned by Soldin was the inclusion of active uptake of Cl[O.sub.4.sup.-] in the skin. In the model, the skin compartment actively sequesters Cl[O.sub.4.sup.-] and iodide from the blood but is not a route of entry. This description of ion transport in the skin is supported by the presence of NIS (Kotani et al. 1998) and the accumulation of iodide and Cl[O.sub.4.sup.-] in rat skin (Brown-Grant 1959; Yu et al. 2002a), as well as Cl[O.sub.4.sup.-] induced inhibition of iodide uptake in the skin of fetal and neonatal rats (Yu et al. 2002b; Zeghal et al. 1995). Because the skin is not the target tissue, it is not critical to prove the mode of ion transport. However, it is important to accurately describe skin concentrations, since blood levels and, therefore, milk transfer and thyroid kinetics, could be affected by the transfer into such a large organ.

Finally, Soldin suggests that the dose-response relationship for Cl[O.sub.4.sup.-] toxicity is known from a variety of human exposure scenarios. We contend, however, that quantitative information on dose response in humans is incomplete at best and essentially nonexistent in the newborn. Although thyroid hormones have been measured in Cl[O.sub.4.sup.-]-exposed infants and children in epidemiologic studies (Crump et al. 2000; Lamm and Doemland 1999; Li et al. 2000), neurologic end points have not been correlated to a neonatal Cl[O.sub.4.sup.-] dose. Because Cl[O.sub.4.sup.-] inhibits iodide uptake in the thyroid and in milk (Brown-Grant 1957; Potter et al. 1959; Howard et al. 1996) and is present in the milk of lactating rats (Clewell and Gearhart 2002), it would seem prudent to quantitatively determine the extent of neonatal exposure to Cl[O.sub.4.sup.-] and the resulting inhibition of neonatal thyroid iodide. The PBPK models presented in our paper (Clewell and Gearhart 2002) enable us to develop a reasonable estimate of dose in the population of interest, the human neonate, thereby providing a vital piece of information needed for a more informed human health risk assessment.

Rebecca A. Clewell
Geo-Centers, Inc.
Wright-Patterson AFB, Ohio

Jeffery M. Gearhart
Mantech Environmental Technology, Inc.
Dayton, Ohio
E-mail: [e-mail]

REFERENCES

Anbar M, Guttmann S, Lewitus Z. 1959. The mode of action of perchlorate ions on the iodine uptake of the thyroid gland. Int J Appl Fladiat Isot 7:87-96.

Brown-Grant K. 1957. The iodide concentrating mechanism of the mammary gland. J Physiol 135:644-654.

Brown-Grant K, Pethes G. 1959. Concentration of radioiodine in the skin of the rat. J Physiol 148:68,1-693.

Chow SY, Woodbury DM. 1970. Kinetics of distribution of radioactive perchlorate in rat and guinea-pig thyroid glands. J Endocrinol 47:207-218.

Clewell RA, Gearhart JM. 2002. Pharmacokinetics of toxic chemicals in breast milk: use of PBPK models to predict infant exposure. Environ Health Perspect 110:A333-A337.

Crump C, Michaud P, Tellez R, Reyes C, Gonzalez G, Montgomery EL, et al. 2000. Does perchlorate in drinking water affect thyroid function in newborns or school age children. J 0ccup Environ Med 42(6):603-612.

Dohan O, De la Vieja A, Carrasco N. 2000. Molecular study of the sodium-iodide symporter (NIS): a new field in thyroidology. Trends Endocrinol Metab 11:99-105.

Eskandari S, Loo DD, Dai G, Levy 0, Wright EM, Carrasco N. 1997. Thyroid [Na.sup.+]/[I.sup.-] symporter. Mechanism, stoichiometry, and specificity. J Biol Chem 272:27230-27238.

Hildebrandt JD, Halmi NS. 1981. Intrathyroidally generated iodide: the role of transport in its utilization. Endocrinology 108(3):842-849.

Howard BJ, Voigt G, Segal MG, Ward GM. 1996. A review of countermeasures to reduce radioiodine in milk of dairy animals. Health Phys 71(5):661-673.

Kotani T, Ogata Y, Yamamoto I, Aratake Y, Kawano JI, Suganuma T, et al. 1998. Characterization of gastric [Na.sup.+]/[I.sup.-] symporter of the rat. Clin Immunol Immunopathol 89.271-278.

Li FX, Byrd DM, Deyhle GM, Sesser DE, Skeels MR, Katkowsky SR, et al. 2000. Neonatal thyroid stimulating-hormone level and perchlorate in drinking water. Teratology 62(6):429-431.

Li Z, Li IX, Byrd D, Deyhle GM, Sesser DE, Skeels MR, et al. 2000. Neonatal thyroxine level and perchlorate in drinking water. J Occup Environ Med 42(2):200-205.

Nilsson M. 1999. Molecular and cellular mechanisms of transepithelial iodide transport in the thyroid. Biofactors 10:277-285.

Potter GD, Tong W, Chaikoff IL. 1959. The metabolism of [I.sup.131]-labeled iodine, thyroxine and triiodothyronine in the mammary gland of the lactating rat J Biol Chem 234:350-354.

Riedel C, Dohan O, De la Vieja A, Ginter CS, Carrasco N. 2001a. Journey of the iodide transporter NIS: from its molecular identification to its clinical role in cancer. Trends Biochem Sci 26:490-496.

Riedel C, Levy O, Carrasco N. 2001b. Post-transcriptional regulation of the sodium/iodide symporter by thyrotropin. J Biol Chem 276:21458-21463.

Wolff J. 1998. Perchlorate and the thyroid gland. Pharmacol Rev 50:89-105.

Yoshida A, Taniguchi S, Hisatome I, Royaux IE, Green ED, Kohn LD, et al. 2002. Pendrin is an iodide-specific apical porter responsible for iodide efflux from thyroid cells. J Clin Endocrinol Metab 87(7):3356-3361.

Yu KO, Narayanan L, Mattie DR, Godfrey R J, Todd PN, Sterner TR, et al. 2002a. The pharmacokinetics of perchlorate and its effect on the hypothalamus/pituitary-thyroid axis in the male rat Toxicol Appl Pharmacol 182(2):148-159.

Yu KO, Mahle DA, Narayanan L, Godfrey RJ, Buttier GW, Todd PN, Parish PN, et al. 2002b. Kinetics of perchlorate induced inhibition of iodide uptake in tissues of the pregnant rat and fetus [Abstract]. Toxicol Sci 66(suppl 1):139.

Zeghal N, Redjem M, Gondran F, Vigouroux E. 1995. Analysis of iodine compounds in young rat skin in the period of suckling and in the adult. Effect of perchlorate [in French]. Arch Physiol Biochem 103:502-511.