FUNDAMENTAL AND APPLED TOXCOLOGY 29, 63-70 (1996) Artile No. 0006 Evaluating Lead Bioavailaility Data y Means of a Physiologially Based Lead Kineti Model 1 J. POLAK,* E. J. O'FLAHERTY,* G. B. FREEMAN,! J- D. JOHNSON,! S. C. LAO.J AND P. D. BERGSTROMt *'Department of Environmental Health, University of Cininnati College of Mediine, Cininnati, Ohio 4267-006; tbattelle Laoratories, Columus, Ohio 43201; and tatlanti Rihfield Company, Denver, Colorado 80202 Reeived Otoer 24, 1994; aepted June 8, 199 Evaluating Lead Bioavailaility Data y Means of a Physiologially Based Lead Kineti Model. POLAK, J., O'FLAHERTY, E. J., FREEMAN, G. B., JOHNSON, J. D., LAO, S. C, AND BERGSTROM, P. D. (1996). Fundam. Appl. Toxiol. 29, 63-70. A method of ioavailaility estimation is presented in whih a physiologially ased kineti model of lead kinetis isfitsimultaneously to lood and one lead onentrations after a period of exposure to dietary lead. Optimization of the simultaneous fit, varying only frational asorption, gives the est estimate of frational ioavailaility for eah treatment group. The analysis was applied to data from three separate studies in whih rats were fed for 30 onseutive days purified diets ontaining lead added as lead aetate, mine waste-ontaminated test soils, or mine waste itself. Frational asorption dereased as lead intake inreased, regardless of the soure of the lead; ut the magnitude of this dose dependene was lead soure-dependent. There were no differenes in lead asorption y male and female rats when lead intake was expressed per unit ody weight Frational asorption varied from 4 to %, at low exposure rates (1-2 mg lead/kg/day) when lead aetate was added to the diet, to 0.24% at a high exposure rate (24 mg/kg/day) when a mine waste-ontaminated test soil was added to the diet. Comparison of the results of this analysis with the results of a more onventional analysis, in whih the one and lood lead onentrations were separately ompared with one and lood lead onentrations in rats given daily injetions of lead aetate intravenously for 29 onseutive days, demonstrated that the standard analysis failed to reveal the dose dependene of frational asorption. O 1996 Sodety of ToikxJofj The ioavailaility of lead from different environmental matries is known to vary widely (Steele et al., 1990). A numer of ioavailaility. studies are in progress or have reently een pulished (Freeman et ai, 1992, 1994; Weis etal., 1994; Ghantous et al., 1994). n a typial study design, a solule lead salt or lead-ontaining soil or mine waste is fed to experimental animals for a period of time judged suffiient to ahieve reasonale lead onentrations in lood and soft tissues. Conentrations of lead are measured in 1 This work was supported y the Atlanti Rihfield Company. lood, seleted soft tissues, and often in one at the end of the exposure period. The ioavailaility of lead from an oral soure is estimated y omparison of tissue onentrations at the end of the oral study with tissue onentrations measured at the end of a series of daily intravenous injetions of a solule lead salt given for the same time period. This method of estimating ioavailaility requires multiple experimental studies, and generates a separate estimate for eah set of tissue onentrations. We propose an approah to ioavailaility estimation that does not require intravenous solule salt study groups, and that yields a single estimate ased on the omined tissue onentration data for eah dose and lead soure. The method relies on a physiologially ased model of lead kineti ehavior in rats (PBK model) (O'Flaherty, 1991). The est simultaneous model fit to tissue onentrations from a single study group is ahieved y optimizing the value of frational asorption from the gastrointestinal trat. The optimized value is the est single estimate of ioavailaility in that study group. The method is applied to the results of three studies in whih rats were fed lead at various levels in mine waste, mine waste-ontaminated soils, or as solule lead aetate, or were administered lead aetate intravenously, for 30 days. Eah data set is analyzed independently to estimate ioavailaility, in the ase of the oral studies, or as a hek on the operation of the model, in the ase of the intravenous studies. The results of the first and seond studies have een pulished (Freeman et al., 1992, 1994). The experimental data are reanalyzed in this paper. METHODS Study (Freeman et al, 1992). n the first study, young, rapidly growing male and female Sprague-Dawley rats (7-8 weeks old at the start of the study; rats per sex per treatment group) were given one of two mine waste-ontaining soils, mixed in the diet to provide a total of 8 different dose levels, for 30 onseutive days. The soils were olleted from residential areas in Butte, Montana. The test soils had geometri mean volumeased partile diameters of 48 y.m (Test Soil D and 42 pm (Test Soil FD, with ranges up to 194 and 182 /im, respetively. Test Soil H had 4.8 times as muh lead as Test Soil. Both test soils were added to the feed at 0.2, 63 0272-090/96 $12.00 Copyright O 1996 y the Soiety of Toxiology All rights of reprodution in any form reserved. Downloaded from https://aademi.oup.om/toxsi/artile-astrat/29/1/63/167771 on 01 Marh 2018
64 POLAK ET AL. TABLE 1 Asolute Perentage Bioavailaility of Lead from Test Soils Added to Feed (Study ) % Soil in diet ppm lead in diet Lead dose rate, mg/kg/day for 30 days Perentage ioavailaility; lood onentrations Using standard method Perentage ioavailaility; one onentrations Using PBK Model Perentage ioavailaility 0.2 0. 0.2 2 0. 2 1.62 (soil ) 4.0 (soil ) 7.82 (soil ) 16.2 (soil ) 19. (soil in) 40. (soil ) 78.2 (soil ) 19 (soil ) 0.124 0.314 0.680 1.20 1.67 3.36.42 12.7 0. 0.01 0.916 1.88 2.31 4.40 7.63 23.8 1.31 (9.04) 2.29 (3.12) 1.27 (1.3) 2.18 (0.82) 1.63 (0.7) 2.07 (7.7).72 (3.69) 2.23 (2.20) 3.41 (1.23) 0.36 (1.9) 0.48 (0.64) 0.087 (0.38) 0.48 (0.2) 0.3 (0.16) 0.39(1.8) 0.8 (0.6) 0.36 (0.36) 0.43 (0.18) 1.60 1.20 0.9 0 7 0.2 0.30 0.90 0.8 0.41 0.37 0.24 " Standard error in parentheses. * Not distinguishale from akground exposure. These alulations would have required extrapolation of V dose onentration data. 0., 2, and %, produing target lead onentrations of 1.62, 4.0, 7.82, 16.2, 19., 40., 78.2, and 19 ppm for the eight treatment groups, four for eah test soil (Tale 1). Additional treatment groups ( rats per sex per group) were given lead aetate mixed in the feed for 30 days to provide five different onentrations seleted to raket the test soil diet lead levels. Dietary lead onentrations were 1,, 2, 0, and 20 ppm for the five lead aetate treatment groups. A ontrol group of five male and five female rats served to estalish akground tissue lead levels. All rats were fed ad liitum AN-76 meal (Ziegler Brothers, Gardners, PA) into whih the appropriate amounts of test soil or lead aetate had een mixed. Test soils were haraterized physially and mineralogially, and the homogeneity and staility of the test soils and solutions during the ourse of the study were ensured. Clinial status, ody weight, and food onsumption were monitored throughout the in-life phase of the study. The animals were killed y intraperitoneal injetion of sodium pentoarital on the 30th day of the study. At termination, lood, one (right femur), and liver were taken from all rats for lead analysis. Study (Freeman et al, 1994). n the seond study, young male and female Sprague-Dawley rats (7-8 weeks old at the start of the study; rats per sex per treatment group) were given either one of three different onentrations of lead aetate mixed in the feed for 30 days, or daily intravenous injetions of lead aetate for 29 onseutive days. nlusion of the intravenous treatment group allowed asolute lead ioavailaility to e alulated for oth oral lead aetate and the previously tested mine waste soils, sine intravenous lead was presumed to e 0% ioavailale. All rats were fed ad liitum AN-76 meal into whih the appropriate amounts of lead aetate had een mixed. Dietary lead onentrations were 1, 2, and 20 ppm for the three treatment groups. Dose rates were 0.02, 0.20, and 2.0 mg lead/kg ody weight/day for the three intravenous treatment groups. The daily intravenous dose was given y injetion into the lateral tail vein. The design and ondut of the study were the same as those of Study. Study (Freeman et al, 1993). The test sustanes were four different mine wastes. Three (B1-B3) were omposites taken either from the surfae of a single tailings pile or from a neary reek hannel (the Bingham Creek area in Utah). The fourth (B4) was a waste omposite taken from the interior of a tailings pile loated in Butte, Montana. The ulk samples were dried, sieved to exlude partiles >20 (im, homogenized, and separated into susamples for soil and mineralogi haraterization and for the rat feeding studies. The geometri mean volume-ased partile diameters of the mine wastes were 38 /im (Bl), 23 ^m (B2), 21 urn (B3), and 23 ixm (B4), with ranges up to 23, 23, 23, and 300 fim, respetively. Young male Sprague-Dawley rats (7-8 weeks old at the start of the study; 6 rats per dose group) were given one of the four mine wastes, mixed in the diet to provide three dose levels of eah waste, for 30 days. n addition, male rats (7-8 weeks old at the start of the study; 8 rats per dose group) were given daily intravenous injetions of lead aetate for 29 days. All rats were fed ad liitum AN-76 meal into whih the appropriate amounts of the mine wastes had een mixed. Doses were,, and 20 ppm lead in the diet for the mine wastes, and 0.2, 1.0, and 2.0 mg lead/kg ody weight/day for intravenous administration. The daily intravenous dose was given y means of an indwelling venous atheter. The design and ondut of the study were the same as those of Study. Kineti modeling. The physiologially ased model of lead kinetis has een desried in detail (O' Flaherty, 1991). t inorporates rat ody 30 40 0 90 70 80 90 FG. 1. Model growth urve, fit to ody weight data from male rats in Study. Downloaded from https://aademi.oup.om/toxsi/artile-astrat/29/1/63/167771 on 01 Marh 2018
and one growth from irth on, and takes into aount growth-related inreases in organ and tissue volumes as well as the age-dependent mehanisms y whih lead is assimilated into and released from one. nput data for the simulations were the akground exposure, measured ody weights, feed onsumption, onentration of lead in feed, age of the rats when the experiment was started, and duration of ontrolled exposure. Model growth urves, whih determine organ and tissue weights and the values of physiologi funtions, were ased on the reorded ody weights of male and female rats in eah study. Feed onsumption urves, together with the onentration of lead in feed, gave the daily intake of lead for males and females in eah dose group. The value of frational asorption during the period of ontrolled oral lead administration was determined y visually optimizing the model simulation for growing rats of the experimental sex, age, and ody weight range, given lead for 30 days at the experimental daily rate in addition to their akground exposure, to the experimental lead onentrations in lood and one. Frational asorption was the only parameter whose value was varied in the optimization proess. All other model parameter values were held onstant, or were assigned their default age and ody-weight dependenies, during the optimization proess. As an independent hek on model performane, the intravenous experiments were modeled y simulating 29 onseutive daily injetions of lead aetate and assuming 0% ioavailaility. LEAD BOAVALABLTY USNG A PBK MODEL 6 r 300 o 20 200 - - - O 0-0 - 0.01 A h, n 0. 1.00 DOSE LEVEL [mg P/kg].00 FG. 3. Experimental and simulated one lead onentrations in the intravenous lead aetate groups, Studies and in. Simulated onentrations shown y ars; ranges of measured onentrations shown y lines onneting symols. The 0.2 and 2.0 ppm dose levels were inluded in oth Studies and. RESULTS Body weights during the ontrolled exposure period were independent of treatment group in all three studies. Figure illustrates the fit of the model growth urve to the mean ody weights for male rats in Study. This growth urve, whih estalishes organ and tissue weights, lood flows, and exretion rate as funtions of rat age, is a key determinant of the output of the model. Use of an experimentally determined growth urve allows the simulations to e tailored to the speifi group of rats in eah study. 120 o- 0 S 80 uj 60 o Q 40 20-0.01 0. 1.00 00 DOSE LEVEL [mg P/Vg ) FG. 2. Experimental and simulated lood lead onentrations in the intravenous lead aetate groups, Studies and. Simulated onentrations shown y ars; ranges of measured onentrations shown y lines onneting symols. The 0.2 and 2.0 ppm dose levels were inluded in oth Studies and. The range of measured lood lead onentrations at 2.0 ppm in Study was 13 216 /ig/dl, eyond the sale of this graph. The range at 2.0 ppm in Study was 96-236 /ig/dl. From the model with frational asorption set at its default value of 1 % for rats onsuming a typial mixed diet, akground exposure to lead in food, drinking water, and air from irth until day of termination of the ontrolled exposure was determined to have een equivalent to dietary levels etween 0.6 and 1.0 ppm in the three studies, had the diet een the only soure of lead. This akground was inluded in eah simulation as a ontinuous dietary exposure from irth until the end of the study. Experimental and predited lood lead onentrations in the intravenous lead aetate groups from Studies and H agreed well, validating this appliation of the model and onfirming that the injeted lead was lose to 0% ioavailale (Fig. 2). Simulated one lead onentrations for intravenous lead aetate groups, however, were onsistently lower than measured femur onentrations. Figure 3, again from Studies and T, illustrates the differenes. nterpretation of this disparity is ompliated y the fat that the model simulates a mean one lead for the entire skeleton, while measurements were made on a single one, the femur. n addition, fators related to the nature of the injetion itself ould have influened the distriution of lead in the intravenous lead aetate groups (see Disussion). Figures 4 and, for the Study male and female rats given mine waste-ontaining soils mixed in the diet, illustrate the general quality of the optimized model fits to lood and one lead onentrations in the various dietary lead groups. From the eight dietary levels, the two lowest are exluded for males and the three lowest for females eause lood lead onentrations in these groups were similar to or elow lood lead onentrations in the ontrol groups. The simulated onentrations of lead in lood and one were suess- Downloaded from https://aademi.oup.om/toxsi/artile-astrat/29/1/63/167771 on 01 Marh 2018
66 POLAK ET AL. 2 0 7 0 12 17 200 DOSE LEVEL [ppoij 2 0 7 0 12 17 200 DOSE LEVEL [ppm] FG. 4. (a) Experimental and simulated lood lead onentrations and () experimental and simulated one lead onentrations in male rats in the lead-ontaining test soil treatment groups, Study. Simulated onentrations shown y ars; ranges of measured onentrations shown y lines onneting symols. fully fit simultaneously to the experimentally measured lood and one onentrations at eah dose level. Appliation of the standard method for estimation of ioavailaility, y omparison of individual measured lood and one lead onentrations in Study test soil treatment groups to onentrations estimated y interpolation etween measured lood and one lead onentrations in the Study intravenous lead aetate groups, yields the perentage ioavailaility values shown in Tale 1. The three highest test soil dose levels for the male rats, and the four highest for the female rats, were exluded from this alulation eause they would have required extrapolation of the referene (intravenous lead aetate) tissue onentrations eyond the range of atual measurement. Frational asorption alulated in this way is independent of test soil, sex, and dose level, and is dependent on the tissue used for the alulation. Tale 1 also gives the values of perentage ioavailaility estimated y visual optimization of the physiologially ased model fit to omined tissue onentrations at eah test soil dose level. n oth males and females, lead ioavailaility was not different etween the two test soils. However, frational asorption determined in this manner dereases smoothly as lead onentration inreases. n all treatment groups (Tale 1), the female rats appear i 26 0 7 0 12 17 200 DOSE LEVEL [ppm] 2 0 7 0 12 17 200 DOSE LEVEL [ppm] FG.. (a) Experimental and simulated lood lead onentrations and () experimental and simulated one lead onentrations in female rats in lead-ontaining mine waste test soil treatment groups, Study. Simulated onentrations shown y ars; ranges of measured onentrations shown y lines onneting symols. Downloaded from https://aademi.oup.om/toxsi/artile-astrat/29/1/63/167771 on 01 Marh 2018
LEAD BOAVALABLTY USNG A PBK MODEL 67 2.0 1. P 1 1 1 1 ehave very muh like the mine waste test soils used in Study. The differene etween B3 and the other three mine wastes is also apparent when the ioavailaility alulation is made using the standard method (Tale 3), although again with this method no onsistent dose dependene is detetale. DSCUSSON 0. 0.0.. 1 1 20 DOSE LEVEL [mg PtAg BWAtay] FG. 6. Perentage asorption as a funtion of lead intake per unit ody weight in male and female rats in lead-ontaining mine waste test soil treatment groups, Study. to have asored a smaller fration of dietary lead than the males. However, within eah treatment group the females were smaller, and onsumed more food per unit ody weight, than the males. When the perentage of asorption is related to atual lead intake per unit ody weight instead of to nominal exposure, as shown in Fig. 6, it is lear that frational asorption of lead from the mining waste soils is not different for males and females. The overall dose dependene of frational asorption is striking. At the lowest intake rate usale for this alulation, 0.680 mg/kg/day in male rats, 1.6% of the mine waste-ontaining soil lead was asored. At the highest intake rate, 23.8 mg/kg/day in female rats, only 0.24% was asored. Tale 2 presents the same omparison of ioavailaility, as alulated y the standard and PBK methods, for the oral lead aetate-treated rats in Studies and. As for the mine waste soils, estimates of ioavailaility made y the standard method are tissue-dependent ut neither dose- nor sex-dependent. n ontrast, estimates of ioavailaility made y the PBK method are dose-dependent, although not as markedly nor as onsistently as in the mine waste-ontaminated test soil studies (Fig. 7). As expeted, lead is more ioavailale from lead aetate than from the mine waste-ontaining soils. At the lowest intake rate, 0.732 mg/kg/day in male rats, 3.9% of the lead was asored, although estimated asorption was atually 4.8% at a slightly higher dose rate. At the highest intake rate, 26.2 mg/kg/day in female rats, 1.7% of the lead was asored. A summary of the results of Study is given in Tale 3. Perentage asorption alulated y the PBK method onsistently dereases with inreasing dose even within the relatively narrow dietary lead onentration range in this study. Lead appears to e somewhat more ioavailale from mine waste B3 than from the other three mine wastes, whih 2 A purified diet was seleted for these studies in order to minimize akground lead exposure and maximize frational asorption from the ontrolled lead soure. The nutritionally omplete ATN-76 diet, whih more losely simulates the low-fier diets of most U.S. adults than does rat how, maximizes lead asorption eause it is relatively low in alium. Sine alium and lead are asored through similar mehanisms, homeostati regulation of alium asorption results in greater frational asorption of oth alium and lead from low-alium diets (Mahaffey et al., 1973). High-alium rat how diets, in ontrast, attenuate the ioavailaility of lead. Partile size is also a key fator mediating lead ioavailaility. Barltrop and Meek (1979) showed that frational lead asorption from metalli partiles in the rat gastrointestinal trat was dereased as partile size inreased within a size range of -200 /xm mean partile diameter. Partile size ranges were restrited in the urrent study in order to approximate the range of soil partile sizes typially ingested y hildren (Duggan et al., 198). The geometri mean volume-ased partile diameters of all the test sustanes are within the metalli partile size range reported y Barltrop and Meek (1979) to favor high frational asorption. Bioavailaility of lead from a limited size range of small leadontaining partiles inorporated into a purified diet matrix would e expeted to e maximized. Dose-dependent ioavailaility of lead from the rat gastrointestinal trat has een reported efore. t has een most strikingly demonstrated in fasted adult rats, in whih frational asorption of a single oral 1 mg/kg dose of solule lead was found to e 42% (Aungst et al., 1981). Only at a dose of 0 mg/kg was frational asorption around 1-2%, of the order of those estimated for the -30 mg/kg lead aetate treatment groups in the present study. These figures are onsistent with the finding that lead is also muh etter asored y humans in the fasted than in the fed state (Rainowitz et al., 1980). The present study represents the first doumentation of a onsistent dose dependene of frational ioavailaility of oth solule lead and lead emedded in a mineral matrix, in fed rats. The oserved dose dependene is less pronouned for lead aetate than it is for lead in mine waste or in mine waste-ontaminated soils. This differene is onsistent with the suggestion of Chaney (1991) that soils present in the Downloaded from https://aademi.oup.om/toxsi/artile-astrat/29/1/63/167771 on 01 Marh 2018
68 POLAK ET AL. TABLE 2 Asolute Perentage Bioavailaillty of Lead from Lead Aetate Added to Feed (Studies and ) ppm lead in diet in Lead dose rate, mg/kg/day for 30 days Perentage ioavailaility; lood onentrations Using standard method Perentage ioavailaility; one onentrations Using PBK Model Perentage ioavailaility Study 1 2 0 20 0.079 0.732 1.86 7.67 16.8 0.096 0.834 2.01.1 26.2 63. (21.ir 14.2 (2.3). (1.7) 28.6 (14.3) 14.6 (3.0) 13.2 (2.4) 4.0 (2.93).82 (0.67) 6.17 (0.6) 1.4 (2.9) 3.88 (0.4) 4.07 (0.46) * 3.90 4 78 3.30 2.70 2.90 2.00 1.47 1.70 Study 1 2 20 0.02 1.38 1.3 0.04 1.0 17.9 2.84 (16.34) 7.3 (1.61).72 (1 4) 30.78 (.7) 6.86 (19.28).13 (1.26) 8.47 (19.3) 14.37 (1 39) * 2.40 1.7 4.16 1.0 Standard error in parentheses. * Not distinguishale from akground exposure. These alulations would have required extrapolation of V dose onentration data. gastrointestinal trat may adsor lead, rendering it unavailale for asorption. However, in Study, inreases in dietary lead onentration were not always aompanied y orresponding inreases in dietary soil onentration (Tale 1). While the data in Tale 1 are not inonsistent with the onept that the amount of soil in the gastrointestinal trat may e an important determinant of lead ioavailaility, the oserved smooth derease in frational ioavailaility with inreasing dietary lead onentration (Fig. 6) must have een - m 111 O a. UJ 4-3 2 1 20 2 DOSE LEVEL [ms/kg/da] FG. 7. Perentage asorption as a funtion of lead intake per unit ody weight in male and female rats in lead aetate treatment groups. Studies and. 30 determined at least in part y fators ating in addition to any effet of adsorption onto soil. The mine wastes and mine waste-ontaining soils tested in these studies represent a spetrum of lead-ontaining soil ontaminants. The test soils in Study were omposite residential soils that had een sujeted to natural weathering proesses and may have ontained lead from other soures in addition to mine waste. The mineral omposition and metal onentrations of the two soils were distintly different (Freeman et ai, 1992), ut the geometri means and standard deviations of the soil partile diameters were similar. Bioavailaility of lead from the two soil matries was also not different. Bioavailaility of lead from residential soils is of the greatest environmental onern from the point of view of exposure of hildren playing in ontaminated areas. However, there also is interest in the ioavailaility of lead from matries more losely representative of the mine waste itself. n Study, mine wastes were mixed diretly with the diet. Samples Bl - B3 had een sujeted to weathering and leahing at the surfae of the tailings pile and stream ed. Sample B4, from the interior of the tailings pile, was representative of "pure" mine waste, unontaminated y other soures and not sujeted to weathering. The ioavailaility of lead from Sample B3 was greater than that from the other mine waste samples (Tale 3). Sample B3, from the reek ed adjaent to the tailings pile from whih Samples Bl and B2 were olleted, had a higher ph (~) than Samples Bl and B2 (~2.6), and a different mineralogi omposition. Sample B4 was similar in its omposition to the residential test soils in Study. Downloaded from https://aademi.oup.om/toxsi/artile-astrat/29/1/63/167771 on 01 Marh 2018
LEAD BOAVALABimY USNG A PBK MODEL 69 TABLE 3 Asolute Perentage Bioavailaility of Lead From Mine Wastes Added Diretly to Feed: Male Rats Only (Study H) ppm lead in diet Lead dose rate. mg/kg/day for 30 days Perentage ioavailaility; lood onentrations Using standard method Perentage ioavailaility; one onentrations Using PBK model Perentage ioavailaility Bl 20 B2 20 B3 20 B4 20 0.38 0.73 1.39 0.36 0.66 1.37 0.43 0.71 1.44 0.38 0.7 1.46 0.36 (1.04) 1.68 (0.93) 3.86 (0.86) 0.37 (1.07) 0.63 (0.86) 1.74 (0.2) 3.44 (1.3) 7.32 (1.7).69 (1.01) 0.36 (1.09) 3.04 (1.00) 1.19 (0.43) 0.6 (0.11) 0.83 (0.08) 1.23 (0.09) 0.1 (0.1) 0.81 (0.11) 0.79 (0.07) 1.87 (0.34) 1.64 (0.23) 2.2 (0.23) 1.29 (0.29) 1.13 (0.19) 0.93 (0.14) 1.6 1.3 1.3 2.2 1.3 1.0 3.7 3.1 2.2 1.9 1.7 0.9 ' Standard error in parentheses. Appliation of a kineti model in order to estimate ioavailaility from a single set of data olleted at the end of a study period requires the assumption that frational asorption has een essentially onstant during the study. Frational asorption is known to e dependent on age in very young rats. t is high prior to weaning and delines thereafter. Fores and Reina (1972), however, showed that this deline ours very rapidly and that frational asorption has stailized at adult levels y age 30 days (Fig. 8). Therefore, the assumption that frational ioavailaility is essentially onstant in rats 7-8 weeks old and older is reasonale. 20 30 40 0 60 70 80 90 0 AG ld«ys] FG. 8. The deline of perentage asorption from the gastrointestinal trat in rats immediately after weaning. Data from Fores and Reina (1972). Thirty days is suffiient time to allow ahievement of an approximate steady state etween lead in lood and lead in soft tissues. However, lead is not near steady state in one after only 30 days. The one to lood lead onentration ratios are lower in these animals than they would have een if exposure had een allowed to ontinue to ahievement of steady state. The tales show that ioavailaility as estimated y the standard method is onsistently lower when one onentrations are used as the asis for the alulation than when lood onentrations are used. Possily, the non-steady-state onditions in one ontriute to some extent to this differene. Of greater proale importane in this regard, however, are the high one lead onentrations in the intravenous dose groups, whih were invarialy underpredited y the PBK model. Model ehavior was onsistent with oth lood and one lead onentrations in the oral dose groups in these studies. A possile explanation for the disrepany etween predited and atual one lead onentrations in the intravenous lead aetate groups is that on intravenous injetion, equiliration of lead etween plasma and the red ell annot keep pae with the rapid influx of lead into the lood plasma. This equiliration proess has een shown to require several minutes in vitro (Simons, 1993). Furthermore, the equilirium ratio is onentration-dependent. At low total lood lead onentrations, the red ell ontains at least 99% of the lead in the lood; ut inding in the red ell is apaity-limited, so that the fration of lood lead that is in the red ell at equilirium dereases with Downloaded from https://aademi.oup.om/toxsi/artile-astrat/29/1/63/167771 on 01 Marh 2018
70 POLAK ET AL. inreasing lood lead onentration (Klaassen and Shoeman, 1974). Both these fators onentration dependene and time dependene would tend to shift lood lead away from the red ell and toward the plasma and, onsequently, to shift the uptake of lead toward peripheral tissues, inluding one, during periods of very rapid entry of lead into the plasma. Estimation of frational ioavailaility of lead y the PBK model method has revealed an internal oherene of these data sets that was not apparent when the standard method was used for estimating ioavailaility. This outome, suggesting that an integrated approah to interpretation of tissueand time-dependent onentration measurements is worthwhile, supports use of a PBK model for this purpose. The new approah offers several additional advantages over the traditional method now in use: 1. Sine diret tissue onentration omparisons etween oral and intravenous routes of administration are not made, there is no requirement that the oral dose levels of interest must e raketed y intravenous doses that generate the same range of ending tissue onentrations. 2. Without the requirement for intravenous dose groups, fewer animals need e used, and invasive tehniques are not required. 3. Frational ioavailaility is alulated diretly for eah oral dose. Rather than multiple estimates of ioavailaility for eah treatment group, the aggregate information from a single treatment group generates the single est estimate of frational ioavailaility at that dose level. 4. Blood and tissue lead levels need not e at steady state, nor even at approximate steady state. Physiologially ased models are apale of simulating dynami as well as steadystate kinetis. The potential for extension of this methodology to other agents and to other experimental or oservational situations in whih exposure is known or ontrolled and tissue onentrations have een measured is apparent. One validated, physiologially ased models an e applied in other studies in whih it is desired to estimate the magnitude of a partiular physiologi proess. By integrating all the availale experimental information into a single model struture that reflets the funtion of the total system, the physiologially ased model an assist in illuminating and larifying the relationships among key determinants of kineti ehavior. REFERENCES Aungst, B. J., Dole, J. A., and Fung, H.-L. (1981). The effet of dose on die disposition of lead in rats after intravenous and oral administration. Toxiol. Appl. Pharmaol. 61, 48-7. Barltrop, D., and Meek, F. (1979). Effet of partile size on lead asorption from the gut. Arh. Environ. Health 34, 280-28. Chaney, R. L. (1991). 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