Evaluation of Cuprimine and Syprine for decorporation of (60)Co and (210)Po.

The acknowledged risk of deliberate release of radionuclides into local environments by terrorist activities has prompted a drive to improve novel materials and methods for removing internally deposited radionuclides. These decorporation treatments will also benefit workers in the nuclear industry, should an exposure occur. Cuprimine and Syprine are oral therapeutics based on the active ingredients D-penicillamine and N,N'-bis-(2-aminoethyl)-1,2-ethanediamine dihydrochloride, respectively. These therapeutic drugs have been used for several decades to treat Wilson's disease, a genetic defect leading to copper overload, by chelation and accelerated excretion of internally deposited copper. Studies were undertaken to evaluate these FDA-approved drugs for the in vivo decorporation of radioactive cobalt (Co) and polonium (Po) using male Wistar-Han rats. In these studies, Co or Po was administered to animals by IV injection, followed by oral gavage doses of either Cuprimine or Syprine. Control animals received the radionuclide alone. For Co studies, animals received a single dose of Cuprimine or Syprine, while for Po studies animals were repeatedly dosed at 24-h intervals for a total of 5 doses. Results show that Syprine significantly increased urinary elimination and skeletal concentrations of Co compared to controls. While Cuprimine had little effect on total excretion of Co, the skeletal, kidney, liver, muscle, and stomach tissues had significantly lower radioactivity compared to control animals. The low overall excretion of Po made it difficult to reliably measure urinary or fecal radioactivity and draw a definitive conclusion on the effect of Cuprimine or Syprine treatment on excretion. However, Cuprimine treatment was effective at reducing spleen levels of Po compared to controls. Similarly, Syprine treatment produced statistically significant reductions of Po in the spleen and skeletal tissues compared to control animals. Based on these promising findings, further studies to evaluate the dose-response pharmacokinetic profiles for decorporation are warranted.

Aminothiol receptors for decorporation of intravenously administered (60)Co in the rat.

This report provides a comparison of the oral decorporation efficacy of L-glutathione (GSH), L-cysteine (Cys), and a liposomal GSH formulation (ReadiSorb) toward systemic (60)Co to that observed following intravenous administration of GSH and Cys in F344 rats. Aminoacid L-histidine (His) containing no thiol functionality was tested intravenously to compare in vivo efficacy of the aminothiol (GSH, Cys) chelators with that of the aminoimidazole (His) chelator. In these studies, (60)Co was administered to animals by intravenous injection, followed by intravenous or oral gavage doses of a chelator repeated at 24-h intervals for a total of 5 doses. The results suggest that GSH and Cys are potent decorporation agents for (60)Co in the rat model, although the efficacy of treatment depends largely on the systemic availability of the chelator. The intravenous route of administration of GSH or Cys was most effective in reducing tissue (60)Co levels and in increasing excretion of radioactivity compared to control animals. Liposomal encapsulation was found to markedly enhance the oral bioavailability of GSH compared to non-formulated GSH. The oral administration of liposomal GSH reduced (60)Co levels in nearly all tissues by 12-43% compared to that observed for non-formulated GSH. Efficacy of oral Cys was only slightly reduced in comparison with intravenous Cys. Further studies to optimize the dosing regimen in order to maximize decorporation efficiency are warranted.

Investigation of chitosan for decorporation of 60Co in the rat.

With the increased threat of terrorist release of radioactive materials, there is a need for non-toxic decorporation agents to treat internal contamination with radionuclides. In this study, low molecular weight chitosan was evaluated for decorporation of radioactive cobalt (60Co). The affinity of chitosan for Co(II) was tested in vitro using spectrophotometric and potentiometric titration techniques. For in vivo studies, the effect of chitosan on ingested 60Co was evaluated using F344 rats administered a single dose followed by oral chitosan. Chitosan was also evaluated for systemic decorporation of 60Co following intravenous injection with repeated chitosan administration over 5 d. Control animals received 60Co without chelation treatment. Excreta and tissues were collected for analysis using gamma-counting techniques. Results from in vitro experiments confirmed the binding of Co(II) to chitosan, with the postulated formation of a mixed cobalt-chitosan-hydroxide complex species; a stability constant was calculated for this complex. For in vivo studies, oral administration of chitosan significantly reduced systemic absorption of orally administered 60Co as evidenced by an increase in fecal elimination and decrease in urinary elimination. However, oral administration of chitosan lactate slightly decreased fecal excretion of 60Co. Further, oral administration of chitosan significantly reduced 60Co levels in kidney, liver, and skeleton compared to control animals receiving 60Co alone. By the i.v. route, chitosan slightly reduced levels of 60Co in tissues compared to controls, although statistically significant reductions were only observed for blood and kidney. Overall, this commercially available chitosan oligosaccharide exhibited promising potential; further studies are warranted to evaluate the optimal dosing regimen and chemical modifications to increase effectiveness.

A real-time methodology to evaluate the nasal absorption of volatile compounds in anesthetized animals.

Nasal dosimetry models that combine computational fluid dynamics and physiologically based pharmacokinetic modeling incorporate information on species-specific anatomical differences, including nasal airflow, mucosal diffusion, clearance-extraction, and metabolism specific to different epithelial layers. As such, these hybrid models have the potential to improve interspecies dosimetric comparisons, and may ultimately reduce uncertainty associated with calculation of reference concentrations. Validation of these models, however, will require unique experimental data. To this end, a method for evaluating the uptake of a prototypical compound, methyl iodide (MeI), in the nasal cavity of the intact animal was developed. The procedure involved insertion of a small-diameter air-sampling probe in the depth of the nasal cavity to the nasopharynx region in anesthetized animals. The exterior portion of the probe was connected directly to a mass spectrometer to provide a continual real-time analysis of concentrations of MeI in the nasal cavity. A plethysmography system was used to monitor breathing parameters, including frequency and tidal volume for each animal. Animals were placed in a sealed glass chamber and exposed to MeI at initial chamber concentrations ranging from 1 to 50 ppm. Studies were conducted on n = 3 rabbits per exposure concentration for a total of nine animals and n = 6 rats at a single exposure concentration of 1 ppm. In the rabbit, the percent of MeI absorbed in the nasal cavity ranged from 57 to 92% (average 72 +/- 11) regardless of exposure concentration. Similarly, the percent of MeI absorbed in the nasal cavity of the rat ranged from 51 to 71% (average 63 +/- 8).

Development of a physiologically based pharmacokinetic model for chlorobenzene in F-344 rats.

A physiologically based pharmacokinetic (PBPK) model to describe the absorption, distribution, metabolism, and elimination of chlorobenzene in rats was developed. Partition coefficients were experimentally determined in rat tissues and blood samples using an in vitro vial equilibration technique. These solubility ratios were in agreement with previous reports. The in vivo metabolism of chlorobenzene was evaluated using groups of three F344 male rats exposed to initial chlorobenzene concentrations ranging from 82 to 6750 ppm in a closed, recirculating gas uptake system. An optimal fit of the family of uptake curves was obtained by adjusting Michaelis-Menten metabolic constants, K(m) (affinity) and Vmax (capacity), using the PBPK model. At the highest chamber concentration, the uptake curve could not be modeled without the addition of a first-order (Kfo) metabolic pathway. Pretreatment with pyrazole, an inhibitor of oxidative microsomal metabolism, had no impact on the slope of the uptake curve. The completed PBPK model was evaluated against real-time exhaled breath data collected from rats receiving either an intraperitoneal (i.p.) injection or oral gavage dose of chlorobenzene in corn oil. Exhaled breath profiles were evaluated and absorption rates were determined. Development of the chlorobenzene PBPK model in rats is the first step toward future extrapolations to apply to humans.

Use of real-time breath analysis and physiologically based pharmacokinetic modeling to evaluate dermal absorption of aqueous toluene in human volunteers.

Toluene is a ubiquitous chemical that is commonly used for its solvent properties in industry and manufacturing, and is a component of many paint products. Although human exposure to toluene is most likely to be through inhalation, toluene is also found in well and surface water. Therefore, an assessment of the dermal contribution to total toluene uptake is useful for understanding human exposures. To evaluate the significance of these exposures, the dermal absorption of toluene was assessed in human volunteers using a combination of real-time exhaled breath analysis and physiologically based pharmacokinetic (PBPK) modeling. Human volunteers wearing swimsuits were submerged in warm tap water to neck level in a stainless steel hydrotherapy tub containing an initial concentration of approximately 500-microg/l toluene. Volunteers were provided purified breathing air to eliminate inhalation exposures, and exhaled breath was continually analyzed before, during, and post exposure to track the absorption and subsequent elimination of the compound in real time. A PBPK model was used to estimate the dermal permeability coefficient (K(p)) to describe each set of exhaled breath data from 4-6 human volunteers. An average K(p) value of 0.012 +/- 0.007 cm/h was found to provide a good fit to all data sets. Volunteers also participated in a second study phase, in which the subject was allowed to breathe the room air during immersion, thus both dermal and inhalation exposures to toluene occurred. Exhaled breath analyses revealed that concurrent inhalation of volatilized toluene resulted in a transient increase in the peak exhaled-breath level by 100 ppb, or an approximate 50% increase over breath levels observed in dermal-only studies. For perspective, the total intake of toluene associated with oral consumption of 2 liters of water containing toluene at bath water concentrations were estimated to be more than 30 times greater than the dermal contribution due to bathing.

Development of a physiologically based pharmacokinetic model for methyl ethyl ketone in F344 rats.

A physiologically based pharmacokinetic (PBPK) model to describe the absorption, distribution, metabolism, and elimination of methyl ethyl ketone (MEK) in rats was developed. Partition coefficients were experimentally determined in rat tissues and blood samples using an in vitro vial equilibration technique. These solubility ratios were in agreement with previous human-based estimates that MEK is uniformly soluble within all tissues. The in vivo metabolism of MEK was evaluated using groups of three F344 male rats exposed to 100-2000 ppm MEK in a closed, recirculating gas uptake system. An optimal fit of a family of uptake curves was obtained by adjusting Michaelis-Menten metabolic constants, Km (affinity), and Vmax (capacity) using the PBPK model. At the highest chamber concentration, the uptake curve could not be modeled without the addition of a first-order (Kfo) metabolic pathway. Pretreatment with pyrazole, an inhibitor of oxidative microsomal metabolism, decreased the slope of the gas uptake curve but did not abolish metabolism. Optimal model fit to the gas uptake curve from pyrazole-pretreated animals required the apparent Km to be increased roughly 50 times the value determined in naive rats. The completed PBPK model was evaluated against real-time exhaled breath data collected from rats receiving an intravenous (iv) injection of MEK via a jugular vein cannula. Model simulation of the iv-treated animals required alveolar ventilation to be reduced 30% in order to match the data. Exhaled breath profiles from animals treated with MEK by oral gavage or intraperitoneal (ip) injection were evaluated and absorption rates was determined. Development of a comprehensive PBPK model for MEK in rats is the first step toward future extrapolations to apply to humans.

Route-of-entry and brain tissue partition coefficients for common superfund contaminants.

Various organic solvents may be encountered in contaminated water supplies at U.S. Environmental Protection Agency-designated Superfund sites. Human exposure to these environmental contaminants may occur by oral, dermal, or inhalation routes. The estimation of human health risk associated with exposure to these solvents can be improved through the use of physiologically based pharmacokinetic (PBPK) models to describe the absorption, tissue distribution, metabolism, and elimination of the compounds following any route of exposure. However, development of these PBPK models requires information on the relative solubility, or partition coefficient, of each compound in blood and various tissues. A number of investigators have provided partition coefficient information on different tissues in various species; however, the data for route of entry organs (i.e., skin, lung, stomach) and brain tissue are not complete. Therefore, the objective of this work was to replicate partition coefficient studies for several commonly encountered environmental contaminants using an in vitro gas-phase vial equilibration technique and to include tissues to evaluate brain, lung, stomach, and skin. A comparison of the partition coefficient values determined here with values reported in the literature, where available, showed good agreement in nearly all cases. An additional study was conducted to compare the liver-to-air partition coefficient values for toluene, benzene, and o-xylene introduced as single chemicals to partition coefficient values determined with the chemicals introduced as a mixture of all three compounds. The similarities of the resulting values suggest that both labor and laboratory resources may be reduced when partition coefficients are determined as chemical mixtures.

Evaluation of the dermal absorption of aqueous toluene in F344 rats using real-time breath analysis and physiologically based pharmacokinetic modeling.

Toluene is a ubiquitous chemical that is commonly used for its solvent properties in industry and manufacturing, and is a component of many paint products. Because of its widespread use, there is potential for both occupational and nonoccupational dermal exposure to toluene. To understand the significance of these exposures, the dermal bioavailability of toluene was assessed in F344 male rats using a combination of real-time exhaled breath analysis and physiologically based pharmacokinetic (PBPK) modeling. Animals were exposed to toluene at 0.5 or 0.2 mg/ml aqueous concentration (0.05% or 0.02%) using a 2.5-cm-diameter occluded glass patch system attached to a clipper-shaved area on the back of the rat. Immediately following exposure, individual animals were placed in a glass off-gassing chamber and exhaled breath was monitored as chamber concentration in real time using an ion-trap mass spectrometer (MS/MS). The real-time exhaled breath profile clearly demonstrated the rapid absorption of toluene, with peak chamber concentrations observed within 1 h from the start of exposure. The PBPK model describing the exposure and off-gassing chamber was used to estimate a dermal permeability coefficient (K(p)) to describe each set of exhaled breath data. Regardless of exposure level, a single K(p) value of 0.074 +/- 0.005 cm/h provided a good fit to all data sets. These rat studies using aqueous toluene will form the basis for comparing the dermal bioavailability of toluene in various paint products and may ultimately aid in understanding human health risk under a variety of exposure scenarios.