Biokinetic model of americium in the beagle dog
A. Luciani1, E. Polig2, R. D. Lloyd3, S. C. Miller3
1
ENEA – Radiation Protection Institute – via dei Colli, 16 – 40136 Bologna, (Italy)
2
Forschungszentrum Karlsruhe – HS/ÜM – Postfach 3640 – 76021 Karlsruhe (Germany)
3
University of Utah – Radiobiology Division, Dept. Radiology – 2334 CAMT, 729 Arapeen Drive,
Salt Lake City, UT 84108-1218 (U.S.A.)
Abstract. A biokinetic model of the systemic distribution of americium in the beagle dog is presented. The
model is based on a previous biokinetic model of plutonium. The data sets used for the development of the
model are the measurements of excreted activity (urine and feces) and organ burdens (skeleton, liver and other
soft tissues). In developing the model, the part of the plutonium model representing the skeleton, which was
based on histomorphometric and autoradiographic investigations, has not been modified with regard to both its
structure (compartments for the trabecular/cortical volume, surface and marrow) and the remodelling rates. Other
skeletal parameters such as the transfer rate from marrow to blood and the partitioning of transferred activity
from the blood to trabecular/cortical surface and volume were optimized to describe the element-specific
biokinetics in the skeleton. The model well describes the fractions of americium in the skeleton, liver and soft
tissues and the total fraction excreted in urine and feces. Particularly it demonstrates the possibility of describing
the behavior of americium in the skeleton with a model substantially analogous to the model of plutonium in
humans. However it differs from the ICRP model of the skeleton with regard to the fundamental structure and
the predictive power. This study will be the starting point for a future improvement of the currently used
americium model for humans, particularly for the skeleton.
1. Introduction
In the past decades a number of experiments were carried out to investigate the metabolism of
plutonium and americium in beagle dogs. These experiments were designed to study the long term
effects to be expected from internal burdens of these nuclides and to evaluate the possible risks to
humans.
The experimental data on plutonium metabolism were analysed previously to develop a biokinetic
model of the systemic distribution and dosimetry of 239Pu in the beagle dog [1]. The compartmental
structure of the biokinetic model was based on the simple structure previously used to demonstrate the
possibility of modelling the microdistribution of 239Pu in the beagle skeleton [2]. The model
parameters were determined from retention and excretion measurements by optimization procedures,
whereas the portion representing the skeleton was defined on the basis of the histomorphometric and
autoradiographic investigations.
Up to now the results of the corresponding americium experiments have not yet been analysed for
developing a biokinetic model, but only empirical retention functions in the main deposition organs
were calculated for different injection levels [3]. By keeping the structure of the model for plutonium,
the main purpose of the present work is the development of a compartmental model able to describe
the available data for americium retention and excretion.
2. Materials and methods
2.1. Experimental data
Since 1950’s several experimental investigations on the metabolism of americium in beagle dogs were
performed. Young adult beagles were given a single intravenous injection of 241Am (III) citrate
ranging from 0.067 to 167 kΒq·kg-1 body mass [3, 4, 5]. At the time of introduction into the
experiment, the dogs were on the average about 18 months old, an age at which skeletal maturity is
reached. For the purpose of the present work, only the data from dogs injected with the lowest levels
(below 3.7 kBq·kg-1, i.e. 0.1 µCi·kg-1) were considered, in order to exclude any acute toxic effect
(damage of tissue cells and interference with bone remodelling) that could significantly disturb the
normal physiology of an organ.
1
In vivo measurements, based on total and partial body counting, allowed the estimation of the retention
in liver and non-liver tissues (mainly skeleton). For several dogs that were subject to autopsy, the
distribution of 241Am in individual bones, liver and other soft tissues are also available. It was found
that the ratio of the skeletal 241Am to total non-liver 241Am averaged 0.885 with no significant
dependence upon either injection level or time after injection [3]. Therefore in vivo measurements of
non-liver tissues, multiplied by 0.885, were used to evaluate retention values in skeleton. This
provided additional data for the skeletal retention other than the values obtained by autopsy analyses.
Analogously, in the present work, a similar approach was used to extend the retention data for soft
tissue other than liver, for which only a limited number of measurements is available. Thus, in vivo
measurements of non-liver tissues were multiplied by 0.115 in order to obtain retention values for the
other soft tissues. This additional source of data will enhance the accuracy in model fitting.
Measurements of the cumulative urine and fecal excretion of americium are available only for four
beagle dogs and during a short time after injection (up to 21 days). These dogs were given injections
significantly above the quoted limit for low levels of 3.7 kBq·kg-1 (about 104 kBq·kg-1 for two dogs
that died because of radiation effects, and 33.3 kBq·kg-1 for the other two dogs).
2.2. Biokinetic model
As a starting point, the structure of the biokinetic model for plutonium metabolism in the beagle dog
was adopted (Fig. 1). It closely resembles the human model for actinides, as recommended in
Publication 67 [6] of the International Commission on Radiological Protection (ICRP). The ICRP
skeletal model was modified to achieve a better agreement with the physiology of bone remodelling
and autoradiographic analyses [7]. As in ICRP model the liver and the remaining soft tissues are
considered separately. The three compartments of the ICRP model for the remaining soft tissues were
reduced to a single compartment. Owing to the lack of excretion data for low injection levels and the
limited number of data even for the high level cases, no attempt was made to model accurately the
excretion pathways for americium. The ICRP compartment “kidney” was lumped into the soft tissue
compartment. The remaining excretion compartments of the ICRP model have no feedback to the
main system and, therefore, can be dropped without affecting the computations for the remaining
compartments.
Fst
Trab.
marrow
rm
Fvt
rm
Fsc
Trab.
surface
Trab.
volume
frt
rl
fft
Liver
fl
Blood
Cort.
marrow
Fvc
Cort.
surface
Cort.
volume
rs
frc
fs
ffc
Other
soft
tissues
fe
FIG. 1. Compartmental model of the systemic distribution of
are partitioning factors, r(F)-parameters are transfer rates.
2
241
Am in the beagle dog; f-parameters
All transfer rates from the blood to other compartments were determined as the product of the general
blood clearance (r) times the fraction (f) of the clearance to the respective compartment. The fraction
for the whole skeleton, fskel, is partitioned into the four pathways to the respective surface and volume
compartments of trabecular and cortical bone (fskel = frt + fft + frc + ffc). Once fskel is given, the four
partitioning factors are calculated using the expressions available in the literature [7], depending on
the affinity ratio of resting to forming bone surfaces (qrf) and cortical to trabecular bone surfaces (qct),
the remodelling rates of trabecular and cortical bones (Fvt, Fvc), the mean wall thickness (MWT) of the
trabecular bone units, the osteonal radius (Ro) and the Haversian canal radius (Rh), the surface to
volume ratios (Svt, Svc) and the trabecular and cortical surface (St, Sc). With exception of the affinity
ratios, all these parameters depend on the morphometry and physiology of the beagle skeleton and,
therefore, are not related to specific radionuclide characteristics. On the other hand the affinity ratios
are typical for the radionuclide considered. For americium qrf = 1.0 was chosen because no
experimental values are available. Parameter qct will be determined by model optimization. Parameters
values and derived quantities are listed in Table I and have been presented previously [1].
Table I. Histomorphometric parameters of the skeleton.
Parameter
Volume turnover trabecular bone (%·y-1)
Volume turnover cortical bone (%·y-1)
Mean wall thickness (µm)
Osteonal radius (µm)
Haversian canal radius (µm)
Trabecular surface to volume ratio (cm-1)
Cortical surface to volume ratio (cm-1)
Trabecular surface (cm2)
Cortical surface (cm2)
Affinity ratio resting/forming
Symbol
Value
Fvt
Fvc
MWT
Ro
Rh
Svt
Svc
St ,
Sc
qrf
100
5
40
75
15
288
54
15,560
13,440
1.0
2.3. Optimization procedure
The model calculations and optimization procedures were performed using a commercially available
software1 package. The partitioning factors fskel, fl, fs for the skeleton, liver and soft tissue
compartments, respectively, and the clearance rates r, rm, rl and rs from blood, bone marrow, liver and
soft tissue are determined by minimizing a suitable objective function, which is a measure of the
deviation between observations and model predictions. The total excreted fraction of americium in
urine and feces is derived from the other partitioning factors (fe = 1 – fskel – fl – fs). For the
optimization all the measurements are equally weighted.
3. Results
The optimum parameter values were determined by fitting the model predictions to the 241Am
retention in the skeleton, liver, non-liver soft tissues (other soft tissues) and the total cumulative
excretion. A few blood concentration values were available up to about 25 hrs after injection. But
because of the short interval of observation they were disregarded for the optimization. The results of
the optimization procedure are given in Table II. The fitted values (second column) are listed together
with the uncertainty as percentage standard deviation (third column). For derived parameters (frt, fft, frc,
ffc and fe), calculated as functions of other fitted parameters, the uncertainties are estimated by using
the standard formula for error propagation.
1
SAAM User Guide. SAAM Institute, Inc. 4530 Union Bay Place, NE, Seattle, WA 98105, USA.
3
Generally the uncertainties amount to a few percent and even in the worst cases, as for the blood
clearance (r), the affinity ratio of cortical to trabecular bone surfaces (qct), the clearance rate of bone
marrow (rm), and the excretion fraction (fe), are below 20 %.
The blood clearance rate for americium is greater by a factor of 1.7 than for plutonium (1.57 d-1 for
americium and 0.90 d-1 for plutonium). The calculated blood retention decreases more slowly than the
few measured blood concentrations [8]. This would suggest an even a higher blood clearance rate.
As for plutonium, the liver and the skeleton are the main deposition organs. The total fraction of the
clearance to the skeleton and the liver (fskel + fl) is 0.85, very close to the value of 0.81 for plutonium
[1], but the partitioning of americium between these organs (0.35 for skeleton and 0.50 for liver) is
reversed with respect to the partitioning of plutonium (0.51 for skeleton and 0.30 for liver). The
deposition of americium in the liver is generally higher than for plutonium. This is because the
partitioning of americium in blood favors the liver, as compared to plutonium. The clearance rates (rl)
of the two nuclides are nearly identical (Am: 0.00135d-1; Pu: 0.00114d-1).
A comparison with the ICRP biokinetic models for humans should be done with care, as they are
based on a model for the skeleton [9] which differs with regard to structure and parameter values from
the model for beagle dogs adopted here. However it could be worth pointing out that, as for beagles,
the ICRP models for humans have the same total fraction fskel + fl for americium and plutonium (0.56),
and the partitioning of americium among the skeleton and liver (0.21 and 0.35, respectively) is exactly
reversed with respect to plutonium [6].
For the other soft tissues the clearance rate is not significantly different to the clearance from liver,
even considering the small uncertainties associated to their values (Table II). This allows simplifying
the model by combining the liver and the other soft tissue compartments, at least in case of very low
intake levels, for which toxic effects with damage or alteration of hepatic tissues cannot be expected.
The clearance rate of americium from bone marrow (rm) is about 50% larger than for plutonium.
The affinity of americium to cortical bone surfaces is nearly three times lower than for trabecular
surfaces. For plutonium the cortical affinity was assumed to be 10 times lower than the trabecular
affinity. However qct, as it was defined in a previous publication [7] for both types of surfaces, is not
independent on qrf. One may calculate an affinity ratio cortical/trabecular for resting surfaces only.
Because qrf = 1 was assumed, the ratio for americium does not change but for plutonium it would
increase to 0.137, i.e. the affinity is about seven times higher for trabecular than for cortical bone. This
means that nuclide-specific differences remain for the ratios of americium and plutonium, even after
this correction.
Also the relatively lower affinity to trabecular surfaces and the lower affinity to forming surfaces
combine to reduce the total fraction to trabecular bone (0.26) compared to plutonium (0.47).
The excreted fraction fe = 0.10 is practically equal to the value for plutonium.
The biokinetic model for americium in beagles presented here provides a realistic description of the
experimental findings. The measured retention of americium in skeleton, liver and the other soft
tissues and the total cumulative excretion are shown in Fig. 2 together with the model calculations.
The calculations match the experimental data closely for each set of measurements over the whole
interval of observation.
4
Table II. Biokinetic parameters.
Parameter
Clearance from blood
Trab. surface to trab. marrow
Trab. volume to trab. marrow
Cort. surface to cort. marrow
Cort. volume to cort. marrow
Trab./cort. marrow to blood
Liver to blood
Soft tissue to blood
Fraction to skeleton
Fraction to liver
Fraction to soft tissue
Fraction to excretion
Fraction to trab. surface
Fraction to trab. volume
Fraction to cort. surface
Fraction to cort. volume
Affinity ratio cort./trab. bone
b
Value
r
rst
rvt
rsc
rvc
rm
rl
rs
fskel
fl
fs
fe
frt
fft
frc
ffc
qct
1.57
0.00362
0.00274
0.000140
0.000137
0.00294
0.00135
0.00134
0.35
0.50
0.050
0.1
0.21
0.052
0.083
0.00087
0.367
Stand. Dev. (%)
Source
14
–
–
–
–
18
6.5
8.0
2.8
2.0
2.1
14
4.5
4.5
2.8
2.8
15
Fit
Ref. (1)
Ref. (1)
Ref. (1)
Ref. (1)
Fit
Fit
Fit
Fit
Fit
Fit
calculated b
calculated b
calculated b
calculated b
calculated b
Fit
Rate constants are designated by letter r and have dimension d-1.
fe calculated as 1 – fskel– fl – fs. The fractions to bone volume and surface from expressions in [7]. The
uncertainty is estimated as quadratic combination of the single parameters uncertainties.
Fraction of activity
0.6
0,6
0.6
0,6
Skeleton
Fraction of activity
a
Symbol a
0.4
0,4
0.2
0,2
0
Liver
0,4
0.4
0.2
0,2
0
0
1000
2000
3000
4000
5000
0
1000
Days post injection
3000
4000
5000
Days post injection
0,2
0.2
0,1
0.1
Other soft
tissue
Fraction of activity
Fraction of activity
2000
0.05
0,05
0
Total cumulative
excretion
0,15
0.15
0,1
0.1
0,05
0.05
0
0
200
400
600
800
Days post injection
1000
1200
0
5
10
15
20
Days post injection
25
30
FIG. 2. Fractional retention in the skeleton, liver and other soft tissues and fractional total cumulative
excretion of 241Am. Circles: Measurements; Solid lines: Model calculations.
5
4. Conclusions
It has been demonstrated that this model of the biokinetics for 241Am in beagle dogs is in agreement
with measured retention data of a number of experimental studies. Significant analogies between the
biokinetic models for americium and plutonium in beagles and humans (ICRP Publication 67) were
discussed both with regard to the total deposition and the partitioning between the main deposition
organs skeleton and liver.
The set of measurements used in the fitting procedures was limited to the injection cases below to 3.7
kBq·kg-1, (0.1 µCi·kg-1). In previous investigations [3] it was pointed out that a more pronounced
decrease of the retention in liver occurs at higher injection levels, presumably due to release from
damaged cells. Correspondingly an increase of americium in the skeleton was observed, as some of
the americium lost from the liver redeposits in the skeleton. In terms of compartmental modelling this
could be taken into account by making the liver clearance dependent on the injection level.
Acknowledgements- This work was supported by ENEA/Italy, Forschungszentrum Karlsruhe/Germany
and NCI grant R01 CA66759/U.S.A.
References
1) Polig, E., Bruenger, F. W., Lloyd, R. D. and Miller, S. C., Biokinetic and dosimetric model of
plutonium in the dog. Health Phys. 78(2): 182-190, (2000).
2) Polig, E., Kinetic model of the distribution of 239Pu in the beagle skeleton. Health Phys. 57(3):
449-460, (1989).
3) Lloyd, R. D., Mays, C. W., Jones, C. W., Atherton, D. R., Bruenger, F. W., Shabestari, L. R. and
Wrenn, M. E., Retention and dosimetry of injected 241Am in beagles. Radiat. Res. 100:564-575, (1984).
4) Lloyd, R. D., Mays, C. W., Taylor, G. N. and Atherton, D. R., Americium-241 retention in beagles.
Health Phys. 18:149-156, (1970).
5) Lloyd, R. D., Mays, C. W., Taylor, G. N. and Atherton, D. R., Americium-241 retention in beagles.
In Report COO-119-241, “Retention and dosimetry of some injected radionuclides in beagles”, Lloyd,
R. D. (Ed.), Radiobiology Division of the department of Anatomy, University of Utah - College of
Medicine, (1970).
6) International Commission on Radiological Protection, Age-dependent doses to members of the
public from intake of radionuclides: Part 2 ingestion dose coefficients. Publication 67. Annals of
ICRP, 23, No. 3/4, Pergamon Press, Oxford and New York, (1994).
7) Polig, E., Labels of surface seeking radionuclides in the human skeleton. Health Phys. 72(1): 19-33,
(2000).
8) Bruenger, F.W., Stevens, W. and Stover, B. J., 241Am in the blood; in vivo and in vitro observations.
Research in Radiobiology, Report COO-119-237:135-152 (1968).
9) International Commission on Radiological Protection, Age-dependent doses to members of the
public from intake of radionuclides: Part 1. Publication 56. Annals of ICRP, 20, No. 2, Pergamon
Press, Oxford and New York, ICRP, (1989).
6