Study of plutonium workers at the Mayak complex in Russia.
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Two well characterised Pu inhalation cases show some remarkable similarities between substantially different types of Pu oxide. The circumstances of exposure, therapy, bioassay data, chemical solubility studies and dosimetry associated with these cases suggest that highly insoluble Pu may be more common than previously thought, and can pose significant challenges to bioassay programmes.
Linear octadentate spermine based 3,4,3-LI(1,2-HOPO) and the mixed ligand, 3,4,3-LI(1,2-Me-3,2-HOPO), are the most effective agents for decorporation of Pu prepared so far; they are effective at low dosage, orally active, and of low toxicity at effective injected dosage. Their pharmacological properties are favourable for in vivo Pu chelation--penetration of extracellular water, useful residence in the circulation, substantial hepato-biliary excretion, low but useful GI absorption, and transitory residence in the kidneys. Reductions of body Pu were significant, compared with controls, when oral administration to normally fed mice (30 or 100 micromol kg(-1)) was delayed as long as 24 h after i.v. Pu injection. The HOPO ligands (10-100 micromol kg(-1)) or CaNa3-DTPA (100 or 300 micromol kg(-1)) were given orally to normally fed mice starting at 4 h after an i.v. Pu injection and continued 5 d per week for 3 weeks. 3,4,3-LI(1,2-HOPO) (100 micromol kg(-1)) reduced Pu in skeleton, liver, and body, to 44 +/- 9, 18 +/- 8, and 38 +/- 7% of controls, respectively, reductions significantly greater than with the mixed HOPO ligand or with three times more CaNa3-DTPA.
This paper is a case study of the validity of different data sources and modelling approaches commonly used to build biokinetic models for radionuclides. The paper examines the basis and apparent predictive accuracy of each of the biokinetic models for Pu used over the years by the International Commission on Radiological Protection (ICRP), in view of recent improvements in the database. The results of this study and similar retrospective studies for other elements suggest the following five points. (1) Extrapolation of biokinetic data from laboratory animals to man is particularly uncertain for the liver due to qualitative differences among species in the handling of many elements by this organ. (2) As a 'default' approach to biokinetic modelling or model assessment, biokinetic data for unhealthy human subjects should be given higher weight than information extrapolated across animal species or chemical families, but there are counter-examples. (3) Little confidence can be placed in biokinetic model predictions for long times after exposure based solely on curve fits to short-term biokinetic data. (4) Bioassay and dosimetry models for a radionuclide should not be developed separately. (5) Where feasible, the systemic biokinetic model for a radionuclide should be developed within a physiologically realistic model structure, because this allows biokinetic data from experimental studies to be supplemented with physiological information, provides a basis for extrapolation of data across animals species or chemical families, results in models that can be used for both bioassay interpretation and dosimetry, and provides a logical basis for extrapolation of data to subgroups of the population (e.g., various ages) or to times outside the period of observation.
Radiation-induced lung cancer risk is currently estimated based on epidemiological data from populations exposed either to relatively uniform, low-LET radiation, or from uranium miners who inhaled radon and its progeny. Inhaled alpha-emitting radionuclides (e.g. Pu and Am) produce distinctive dose patterns that may not be adequately modelled at present. Thus the distribution of Pu is being measured in formalin-fixed autopsy lung tissue from former workers at the Mayak Production Association, and which is maintained in a tissue archive at SUBI. Lungs are sampled using contemporary stereological techniques and Pu particle activities and locations are determined using quantitative autoradiography and morphological identification of lung structures. To date, > 80% of Pu particles have been observed in parenchymal lung tissues with higher concentrations being found in scar tissue. Concentrations of Pu particles in conducting airways are uniformly low, thus indicating that long-term-retained Pu particles are non-uniformly distributed in human lung, mostly in the parenchyma.
Chelators are used to promote excretion of actinides and some other metals, but few are orally effective. The relative efficacies of orally administered triethylenetetraminepentaacetic acids (TT) with varying lipophilic properties on the removal of 241Am and 239Pu and comparison with parenteral Zn-DTPA was determined. The actinides were administered to adult rats 2 weeks prior to initiation of 30 d of chelation treatment. The TT compounds were given orally while Zn-DTPA was given twice weekly by injection. Total body content of 241Am was measured before and during the treatment period and organ contents of 241Am and 239Pu were measured at the end of the study. Significant reductions in 241Am occurred within the first week, with Zn-DTPA being the most effective. By 3 weeks, the most lipophilic chelator, C22TT was as effective as Zn-DTPA. After 30 d, reductions in organ content of 239Pu and 241Am directly correlated with increasing lipophilicity of the TT chelators. Oral C22TT was as effective as injected Zn-DTPA in liver and bone, the major organs of actinide deposition. The removal of 239Pu from the liver and reduction of redeposition of 239Pu in newly formed bone by C22TT was confirmed by neutron-induced autoradiographs. The amphipathic TT chelators may be useful as orally administered alternatives to current parenteral DTPA for the removal of actinide elements from the body, particularly for longer-term therapeutic applications.
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