Scientific basis for the development of biokinetic models for radionuclide-contaminated wounds.

Radionuclide-contaminated wounds are of radiological concern because the wound provides a portal of entry of the radionuclide to the systemic circulation, and can also be a tissue at risk if sufficient dose is deposited at the wound site. Accordingly, a scientific committee established jointly by the US National Council on Radiation Protection and the International Commission on Radiological Protection has been developing an approach to describing the biokinetics of radionuclides deposited in wounds and calculating dose to the wound site. This paper focuses on the analyses, performed principally using experimental animal data, that have led to the development of a biokinetic model for deposited soluble radionuclides as well as more insoluble forms, such as colloids, particles and fragments. The available data for injected soluble materials have provided a basis for categorising 48 different elements (from Be to Cm and representing all of the chemical groups, except halogens and noble gases) into four distinct retention groups. In general, the data are adequate for developing a mechanistically based biokinetic model, whose application is exemplified for soluble radionuclides.

Octadentate hydroxypyridinonate (HOPO) ligands for plutonium (i.v.): pharmacokinetics and oral efficacy.

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.

Competitive binding of Pu and Am with bone mineral and novel chelating agents.

Effective direct removal of actinides such as Pu and Am from bone in vivo has not been accomplished to date, even with the strong chelating agents CaNa3DTPA or ZnNa3DTPA. This study, using an established in vitro system, compared removal of Pu and Am bound to bone mineral by ZnNa3DTPA and 10 chelating agents designed specifically to sequester actinides, including Pu and Am. Ligands tested were tetra, hexa, and octadentate, with linear or branched backbones containing sulfocatechol [CAM(S)], hydroxycatechol [CAM(C)], hydroxipyridinone (1,2-HOPO, Me-3,2-HOPO), or hydroxamate functional groups. The wide range of Pu and Am removal exhibited by the test ligands generally agreed with their metal coordination and chemical properties. The most effective agents for Pu (100 microM concentration, 24-48 h contact) are all octadentate as follows: 3,4,3-LICAM(S) (54% unbound); 3,4,3-LICAM(C) (6.2%); 3,4,3-LI(1,2-HOPO) (3.8%); H(2,2)-(Me-3,2-HOPO) (2.2%) and DFO-(1,2-HOPO) (1.8%). The other ligands removed less than 1% of the bound Pu; and ZnNa3DTPA removed only 0.086%. The most effective ligands for Am removal (100 microM, 24-48 h contact) are as follows: octadentate H(2,2)-(Me-3,2-HOPO) (21% unbound); 3,4,3-LI(1,2-HOPO) (14.5%) and 3,4,3-LICAM(C) (5.9%); hexadentate TREN-(Me-3,2-HOPO) and TREN-(1,2-HOPO) (9.6%); and tetradentate 5-LIO(Me-3,2-HOPO) (5.2%). Am removal by ZnNa3DTPA was about 1.4%. Among the ligands presently considered for possible human use, only 3,4,3-LI(1,2-HOPO) removed potentially useful amounts of both Pu and Am from bone mineral.

Chelating agents for uranium(VI): 2. Efficacy and toxicity of tetradentate catecholate and hydroxypyridinonate ligands in mice.

Uranium(VI) (UO2(2+), uranyl) is nephrotoxic. Depending on isotopic composition and dosage, U(VI) is also chemically toxic and carcinogenic in bone. Several ligands containing two, three, or four bidentate catecholate or hydroxypyridinonate metal binding groups, developed for in vivo chelation of other actinides, were found, on evaluation in mice, to be effective for in vivo chelation of U(VI). The most promising ligands contained two bidentate groups per chelator molecule (tetradentate) attached to linear 4- or 5-carbon backbones (4-LI, butylene; 5-LI, pentylene; 5-LIO, diethyl ether). New ligands were then prepared to optimize ligand affinity for U(VI) in vivo and low acute toxicity. Five bidentate binding groups--sulfocatechol [CAM(S)], carboxycatechol [CAM(C)], methylterephthalamide (MeTAM), 1,2-hydroxypyridinone (1,2-HOPO), or 3,2-hydroxypyridinone (Me-3,2-HOPO)--were each attached to two linear backbones (4-LI and 5-LI or 5-LIO). Those ten tetradentate ligands and octadentate 3,4,3-LI(1,2-HOPO), an effective actinide chelator, were evaluated in mice for in vivo chelation of 233U(VI) (injection at 3 min, 1 h, or 24 h or oral administration at 3 min after intravenous injection of 233UO2Cl2) and for acute toxicity (100 micromol kg(-1) injected daily for 10 d). The combined efficacy and toxicity screening identified 5-LIO(Me-3,2-HOPO) and 5-LICAM(S) as the most effective low-toxicity agents. They chelate circulating U(VI) efficiently at ligand:uranium molar ratios > or = 20, remove useful amounts of newly deposited U(VI) from kidney and bone at molar ratios > or = 100, and reduce kidney U(VI) levels significantly when given orally at molar ratios > or = 100. 5-LIO(Me-3,2-HOPO) has greater affinity for kidney U(VI) while 5-LICAM(S) has greater affinity for bone U(VI), and a 1:1 mixture (total molar ratio = 91) reduced kidney and bone U(VI) to 15 and 58% of control, respectively--more than an equimolar amount of either ligand alone.

Multidentate hydroxypyridinonate ligands for Pu(IV) chelation in vivo: comparative efficacy and toxicity in mouse of ligands containing 1,2-HOPO or Me-3,2-HOPO.

PURPOSE: To identify the most effective multidentate 1,2-HOPO and Me-3,2-HOPO ligands for chelation of Pu(IV) in vivo. MATERIALS AND METHODS: Two sets of ligands with four identical backbones were prepared containing two, three or four bidentate 1,2-HOPO or Me-3,2-HOPO groups, and 3,4,3-LI(1,2-HOPO) was resynthesized in a higher yielding procedure. They were evaluated in mouse for acute toxicity and reduction of tissue 238Pu, in comparison with CaNa3-DTPA (30 micromol kg(-1)). RESULTS: Nine HOPO ligands, promptly injected or given orally or injected at low dosage, are superior to CaNa3-DTPA for reducing 238Pu retention in mouse. Five, given by delayed injection or promptly injected or orally administered as ferric complexes, are superior to CaNa3-DTPA or FeNa2-DTPA respectively. The Me-3,2-HOPO ligands are more effective than their structural 1,2-HOPO analogues, demonstrating the greater affinity of Me-3,2-HOPO for Pu(IV) in vivo. CONCLUSIONS: The most efficacious ligand, 3,4,3-LI(1,2-HOPO), contains the less stably binding 1,2-HOPO group; therefore, its linear spermine backbone must confer advantages for Pu(IV) binding (greater solubility, more favorable arrangement of ligating groups, more flexible backbone). Effective low toxicity tetradentate 5-LIO(Me-3,2-HOPO) and hexadentate TREN-(Me-3,2-HOPO) and highly effective but moderately toxic 3,4,3-LI(1,2-HOPO) (LD50 approximately 300 micromol kg(-1) in mouse) are recommended for further investigation.

The hexadentate hydroxypyridinonate TREN-(Me-3,2-HOPO) is a more orally active iron chelator than its bidentate analogue.

Bidentate hydroxypyridinone chelators effectively complex and facilitate excretion of trivalent iron. To test the hypothesis that hexadentate chelators are more effective than bidentate chelators at low concentrations, urinary and biliary Fe excretions were determined in Fe-loaded rats before and after administration of a bidentate chelator, Pr-(Me-3,2-HOPO), or its hexadentate analogue, TREN-(Me-3,2-HOPO). The bidentate chelator slightly increased biliary Fe excretion in Fe-loaded rats after IV (90 micromol/kg) and PO (90 or 270 micromol/kg) administration, but chelation efficiency did not exceed 1%. The hexadentate chelator markedly increased biliary Fe excretion, achieving overall chelation efficiencies of 14% after IV administration of 30 micromol/kg and 8 or 3% after PO (30 or 90 micromol/kg) administration. The hexadentate chelator was significantly more effective than the bidentate chelator after IV injection and oral dosing. In chelator-treated Fe-loaded or saline-injected rats, >90% of the excreted Fe was in the bile. Oral TREN-(Me-3,2-HOPO), given to non-Fe-loaded rats, did not appreciably change Fe output, indicating that there was little Fe depletion in the absence of Fe overload. These results support the hypothesis that greater Fe chelation efficiency can be achieved with hexadentate than with bidentate chelators at lower, and presumably safer, concentrations. The results also demonstrate that TREN-(Me-3, 2-HOPO) is a promising, orally effective, Fe chelator.

Efficacy of 3,4,3-LIHOPO for reducing neptunium retention in the rat after simulated wound contamination.

PURPOSE: The ligand 3,4,3-Li(1,2-HOPO) was tested for Np removal after intramuscular injection of 237Np nitrate in rats. MATERIALS AND METHODS: Two experiments were performed, one with simultaneous injection of neptunium and LIHOPO at dosages ranging from 3 to 200 micromol kg(-1) and the other with delayed administration of LIHOPO 30 micromol kg(-1) from 5 min to 30 min after Np injection. RESULTS: The data obtained after simultaneous injections showed that the ligand dosage effectiveness was not linear and depended on the tissues being considered. For bones, the best results were obtained with 200 micromol kg(-1) LIHOPO, where retention was reduced to 11% of controls. Maximum efficacies for removal in liver and kidney were obtained with 30 micromol kg(-1) LIHOPO, where retention was reduced to 39% and 1.6% of controls, respectively. At higher dosages, LIHOPO seemed to have a reverse effect on these tissues, demonstrated by a significant accumulation of the radionuclide. The delayed administration of LIHOPO dramatically decreased its efficacy. When administered 5 min after Np, LIHOPO was still efficient (60%, 37%, 7% of controls in bone, liver, kidneys, respectively) but not when treatment was delayed to 30 min. CONCLUSIONS: These results demonstrated that LIHOPO was able to complex Np at the wound site but not after translocation to blood.

237Np: oxidation state in vivo and chelation by multidentate catecholate and hydroxypyridinonate ligands.

Chemically, 237Np(V) is as toxic as U(VI), and radiologically, about as toxic as 239Pu. Depending on redox conditions in vivo, 237Np exists as weakly complexing Np(V) (NpO2+) or as Np(IV), which forms complexes as stable as those of Pu(IV). Ten multidentate catecholate (CAM) and hydroxypyridinonate (HOPO) ligands with great affinity for Pu(IV) were compared with CaNa3-DTPA for in vivo chelation of 237Np. Mice were injected intravenously with 237NpO2Cl: those in a kinetic study were killed 1 to 2880 min; in ligand studies, fed mice were injected intraperitoneally with a ligand 5, 60, or 1440 min after 237Np(V) (molar ratio 5.6 to 73), mice fasted for 16 h were gastrically intubated with a ligand 3 min after 237Np(V) (molar ratio 5.6 to 274), and all were killed 24 h after ligand administration; tissues and excreta were radioanalyzed. Rapid plasma clearance and urinary excretion of 237Np(V) resemble U(VI); deposition and early retention in skeleton and liver resemble Pu(IV). The x-ray absorption near edge structure spectroscopy (XANES) spectra of femora of 237Np(V)-injected mice, compared with spectra of Np(V) and Np(IV) from reference solids, showed predominantly Np(IV). Significant in vivo 237Np chelation was obtained with all of the HOPO and CAM ligands injected at molar ratio 22; the HOPO ligands reduced 237Np in skeleton, liver, and other soft tissue, on average, to 72, 25, and 25% of control, respectively, while CaNa3-DTPA was ineffective. Two HOPO ligands injected 60 min after 237Np (molar ratio 5.6) significantly reduced body and liver 237Np, and three HOPO ligands given orally (molar ratio > or = 73) significantly reduced body and liver 237Np, compared with controls. Combined with earlier work, these results indicate that: the dominant neptunium species circulating and excreted in urine is Np(V), while that in bone and liver deposits is Np(IV); Np(V) must be reduced to Np(IV) before it can be stably chelated; efficient decorporation of neptunium requires multidentate ligands that form exceptionally stable actinide(IV) chelates and facilitate Np(V) reduction.

Evaluation and modification of a physiologically based model of lead kinetics using data from a sequential isotope study in cynomolgus monkeys.

Endogenous (predominantly bone) and exogenous lead were differentially labeled in two 11-year-old female cynomolgus monkeys (Macaca fascicularis) to establish the contributions of the two sources to blood lead. The monkeys had been administered a common lead isotope "mix" at the rate of about 1300 micrograms Pb/kg body wt/day from age 10 months until the start of the study. On day 0, common lead was replaced in sequence by mixes artificially enriched in 204Pb, 206Pb, and 207Pb, given for periods of from 50 to 281 days. Total lead ingestion rate was held constant except during administration of the 207Pb-enriched mix to one of the monkeys, when it was reduced to 650 micrograms/kg/day. Blood and bone were sampled at intervals and analyzed for their content of each of the isotope mixes. A physiologically based model of human lead kinetics was scaled to the cynomolgus monkey and fit to the data to test the correctness of the model structure and to assist with interpretation of study results. Fractional absorption was varied to achieve the best visual fits of the scaled model to blood and bone concentration data for each monkey. The model failed to reproduce the sharp drop in isotope concentrations in blood observed after each exchange of isotope mix. Consequently, it was revised to include a rapid-turnover trabecular bone compartment and a slow-turnover cortical bone compartment, using estimates of trabecular and cortical bone turnover rates from histomorphometric studies in adult cynomolgus monkeys. The revised model fit most of the sets of bone and blood concentrations well. About 17% of the blood lead originated from bone after 11 years of exposure, at blood lead concentrations in excess of 50 micrograms/dl. The rate of return of common lead from bone, as estimated from the model, was 28 micrograms/day just before termination of controlled common lead exposure on day 0. Based on the success of the scaled human model in fitting these data and on the absolute and relative values of bone and blood lead concentrations, the metabolism of lead in the cynomolgus monkey appears to be similar to human lead metabolism.

New agents for in vivo chelation of uranium(VI): efficacy and toxicity in mice of multidentate catecholate and hydroxypyridinonate ligands.

Soluble uranyl ion [UO2(2+), U(VI)] is a kidney poison. Uranyl ion accumulates in bone, and the high specific activity uranium isotopes induce bone cancer. Although sought since the 1940's, no multidentate ligand was identified, until now, that efficiently and stably binds U(VI) at physiological pH, promotes its excretion, and reduces deposits in kidneys and bone. Ten multidentate ligands patterned after natural siderophores and composed of sulfocatechol [CAM(S)], carboxy-catechol [CAM(C)], or hydroxypyridinone [Me-3,2-HOPO] metal-binding units have been tested for in vivo chelation of U(VI). Ligands were injected intraperitoneally (i.p.) into mice 3 min after intravenous (i.v.) injection of 233U or (232+235)U as UO2Cl2 [ligand-to-metal molar ratio 75 to 92]. Regardless of backbone structure, denticity, or binding unit, all 10 ligands significantly reduced kidney U(VI) compared with controls or with mice given CaNa3-DTPA, and four CAM(S) or CAM(C) ligands also significantly reduced skeleton U(VI). Several ligands removed U(VI) from kidneys, when injected at 1 or 24 h. Injected at molar ratios > or = 300, 5-LIO(Me-3,2-HOPO) and TREN-(Me-3,2-HOPO) reduced kidney U(VI) to about 10% of control. Given orally to fasted mice at molar ratios > or = 300, those ligands significantly reduced kidney U(VI). In mice injected i.v. with 0.42 micromol kg(-1) of 235U and given 100 micromol kg(-1) of one of those Me-3,2-HOPO ligands i.p. daily for 10 d starting at 1 h after the U(VI)) loss of kidney U(VI) was greatly accelerated, and the kidneys of treated mice showed no microscopic evidence of renal injury. Crystals of uranyl chelates with linear tetradentate ligands containing bidentate Me-3,2-HOPO groups demonstrate a 1:1 structure. Considering low toxicity, effectiveness, and reasonable cost, the structurally simple linear tetradentate ligands based on the 5-LI backbone (diaminopentane) offer the most promising approach to a clinically acceptable therapeutic agent for U(VI). Work is in progress to identify the most suitable CAM or HOPO binding unit(s).

Circulatory kinetics of intravenously injected 238Pu(IV) citrate and 14C-CaNa3-DTPA in mice: comparison with rat, dog, and reference man.

New ligands for in vivo chelation of Pu(IV) are being synthesized and evaluated in mice for efficacy and toxicity. Biokinetic studies of the new ligands, CaNa3-DTPA, and Pu(IV) are major components of those investigations. Young adult female mice were injected intravenously (iv) with 3H-inulin, 14C-CaNa3-DTPA, or 238Pu(IV) citrate to provide baseline data for plasma clearance, tissue uptake, and excretion rates and to determine the dilution volume (VOD) and renal clearance rate (RC) of filterable substances. Published plasma clearance data for iv-injected 14C-CaNa3-DTPA and Pu(IV) citrate in Reference Man, dog, and rat were collected. Based on combined data for 3H-inulin and 14C-CaNa3-DTPA, VOD = 17% of body weight and RC = 18 mL kg(-1) min(-1) for mice. Retention of 14C-CaNa3-DTPA in the four species is proportional to body weight and inversely proportional to RC: Integrals of the retention of 14C-CaNa3-DTPA from R(t) = 1.0 to R(t) = 0.05 are 108, 43, 28, and 10 DF min, respectively, for Reference Man, dog, rat, and mouse. Clearances of iv-injected Pu(IV) citrate from plasma are in the same order: The plasma curve integrals from injection to 1440 min are 840, 640, 280, and 67 DF min, respectively, for Reference Man, dog, rat, and mouse. In mice, a large fraction of newly injected Pu(IV) is rapidly transferred to the interstitial water of bulk soft tissue (excluding liver and kidneys), from which it is cleared at the same rate as from the plasma. Rapid plasma clearance, escape into interstitial water (22%ID at 20 min), significant early urinary excretion (8%ID in 12 h), and prompt deposition in liver and skeleton (complete in 12 h) are evidence of inefficient binding to plasma protein (mainly transferrin) of newly injected Pu(IV) in mice. Conversely, slow plasma clearance, little early urinary excretion, and delayed deposition in liver and skeleton reflect more efficient binding by transferrin of newly injected Pu(IV) in Reference Man and dog. Pharmacokinetic parameters (effective dosage, effective concentration) of CaNa3-DTPA, alone or combined with plasma Pu(IV) integrals, yielded only qualitative predictions of the relative efficacies of CaNa3-DTPA therapy in four species. The need for improved models of Pu(IV) and ligand biokinetics and the suitability of the three animals for predicting chelation therapy outcomes in humans are discussed.

Chelation therapy by DFO-HOPO and 3,4,3-LIHOPO for injected Pu-238 and Am-241 in the rat: effect of dosage, time and mode of chelate administration.

The effectiveness of the siderophore analogues DFO-HOPO (a hydroxypyridone derivative of desferrioxamine) and 3,4,3-LIHOPO (a linear tetrahydroxypyridinone) for the decorporation of 238Pu and 241Am from rat was studied. (1) Dosage-effect relationship. A similar treatment effect on Pu was achieved by single s.c. injection of 30 mumol kg-1 or by oral administration of 100 mumol kg-1 of either of the two ligands, provided the oral dose was administered earlier. In general, LIHOPO was more effective than DFO-HOPO: retention of Pu in the liver and bones was reduced by LIHOPO to < 10% of control values. No increase in renal retention of the actinides was observed. Whilst DFO-HOPO did not affect Am retention, a substantial reduction was achieved by LIHOPO. Removal effectiveness for injected LIHOPO on Pu was higher than that on Am, especially in the bones and after low ligand doses. Orally administered small doses of LIHOPO, however, mobilized more Am than Pu, both from the liver and the bone. (2) Time-effect relationship. The effectiveness of the injected ligands for Pu decreased exponentially with the time between exposure and treatment. With DFO-HOPO, the calculated half-times for decrease of mobilized fractions of Pu from the bone and liver were 5 and 12 h respectively. The effect of LIHOPO on Pu decreased much more slowly, with a half-time of 3-4 weeks. For instance, a single injection of 30 mumol kg-1 LIHOPO at 10 days post-Pu removed 30 and 50% activity from the bone and liver respectively. The removal effect of LIHOPO for Am in the liver decreased with time in the same way as for Pu but the mobilized fractions of skeletal and renal Am decreased from the first day with a half-time of only 8 and 4 days respectively.

Treatment with 3,4,3-LIHOPO of simulated wounds contaminated with plutonium and americium in rat.

The effect of a siderophore analogue 3,4,3-LIHOPO has been investigated in rat after intramuscular injection of 238Pu, 239Pu and 241Am simulating puncture wounds. Various treatment regimens were used to remove the radioactivity from its injection site and to reduce its retention in body tissues. The local deposits could be reduced to 9% of that in untreated controls by a single local injection of 30 mumol kg-1 3,4,3-LIHOPO administered 1 day after the actinides. Tissue retention of radioactivity was most effectively reduced (to 3% of controls) by continuous subcutaneous infusion of 3,4,3-LIHOPO (3 mumol kg-1 day-1), starting immediately after the injection of actinides and continuing for 2 weeks. The administration of 3,4,3-LIHOPO in drinking water was least effective. Treatment efficacy was substantially higher with 238Pu than with an equal activity of 239Pu (the 238Pu mass, however, was almost 300 times lower than that of 239Pu). Accordingly, the biokinetics and removal of 241Am changed when it was injected with 239Pu instead of 238Pu. Continuous infusion of 3,4,3-LIHOPO (3 mumol kg-1 day-1), starting 4 and 30 days after intramuscular injection of 238Pu and 241Am reduced their femoral retention after 1 month to 20 and 60% of controls respectively; whole-body retention of 241Am was reduced to 20 and 70% of controls respectively.

Octadentate catecholamide ligands for Pu(IV) based on linear or preorganized molecular backbones.

Nine new octadentate ligands based on cyclic, spermine (3,4,3-LI), desferrioxamine (DFO), or H-shaped tetrakisamine (penten) molecular backbones were prepared containing catecholamide (CAM), carboxycatecholamide (CAM(C)), or terephthalamide (TAM) chelating units. Mice were injected intravenously with 239Pu(i.v.) citrate, treated with 30 mumol kg-1 of a ligand by intraperitoneal injection at 1 h or by gastric intubation at 3 min, and Pu retention in tissues and Pu transfer to excreta were measured at 24 h. Given by injection, three soluble ligands composed of MeTAM (3,4,3-LIMeTAM, DFO-MeTAM, H(2,2)-MeTAM) reduced Pu retention in the body to 27-28% of control compared with 32 and 37% of control obtained in mice similarly treated with 3,4,3-LICAM(C) or CaNa3-DTPA, respectively. The MeTAM ligands reduced Pu retention in the skeleton as much as an equimolar amount of CaNa3-DTPA, while Pu retention in the liver (on average, 16% of control) was significantly less than was obtained with CaNa3-DTPA (35% of control). Given orally, H(2,2)-MeTAM reduced Pu retention in the whole body to 58% of control compared with reductions to 62 and 94% of control achieved with 3,4,3-LICAM(C) or CaNa3-DTPA, respectively. Penten is both partially preorganized for metal binding and spatially suitable for encapsulation of actinide(IV), and ligands with the penten backbone are easier and less costly to prepare than those based on spermine or DFO. The biological results confirmed that penten is a suitable as well as practical structural backbone for new octadentate ligands. In agreement with the great stability of the ferric complex with MeTAM, as determined in vitro, the small, simple, soluble penten-based octadentate ligand, H(2,2)-MeTAM, was shown to be, overall, the most effective catecholamide ligand for enhancing Pu excretion. Either combined in H(2,2)-MeTAM or separately, the penten backbone and the MeTAM chelating unit are potentially useful additions to the set of backbones and binding units of multidentate ligands identified as effective for in vivo chelation of the actinides.

Efficacy of 3,4,3-LIHOPO for enhancing the excretion of plutonium from rat after simulated wound contamination as a tributyl-n-phosphate complex.

The siderophone analogue 3,4,3-LIHOPO, referred to hereafter as LIHOPO, has been examined for its ability to remove 238Pu in a tributyl-n-phosphate (TBP) complex from rat after intramuscular (i.m.) or subcutaneous (s.c.) contamination. The chelating agent was administered at a dosage of 30 mumol.kg-1, 30 min after the contamination, either by intravenous (i.v.) or local injection. By day 7 after exposure, local (i.m.) administration of LIHOPO reduced the amounts of i.m.-injected 238Pu in the would site, skeleton and liver to 75, 20 and 25% respectively of those in untreated animals. At the i.m. Pu would site, local treatment was superior to i.v. treatment; both ligands were equally effective. At the s.c. Pu would site, local and systemic treatments were equally effective and LIHOPO was superior to DTPA. After translocation, LIHOPO was the most effective treatment for enhancing Pu excretion, whatever the route of contamination and treatment: the administration of LIHOPO and DTPA reduced whole-body Pu retention by a factor of 1.8 and 1.4 respectively. All these results are encouraging for the use of LIHOPO in the future but more studies are needed, concerning both the toxicity of the compound and its use in man.

Specific sequestering agents for the actinides. 28. Synthesis and initial evaluation of multidentate 4-carbamoyl-3-hydroxyl-1-methyl-2(1H)-pyridinone ligands for in vivo plutonium(IV) chelation.

A new family of chelating agents based on 4-(substituted-carbamoyl)-3-hydroxy-2-pyridinones is reported. These have optional terminal substituents on the nitrogens, and the hydroxypyridonate (HOPO) rings are attached to molecular backbones through amide linkages. A very important feature of the methyl-substituted ligand derivatives (Me-3,2-HOPOs) is that, similarly to the catechoylamide complexes of the siderophore enterobactin and its analogs, these HOPO derivatives form strong hydrogen bonds between the amide proton and the adjacent oxygen of the phenolate in the metal complex; this enhances the stability of the complex. This rigidity helps to explain the great affinity of the Me-3,2-HOPO ligands for plutonium(IV), as observed here under physiological conditions. All 13 compounds studied significantly enhanced Pu excretion from mice compared with Pu-injected controls. Eight of the ligands studied promoted significantly more Pu excretion than an equal molar amount of CaNa3-DTPA (the compound in present clinical use). Five injected and two orally administered Me-3,2-HOPO ligands promoted as much or slightly more Pu excretion than an equal molar amount of the octadentate 3,4,3-LI(1,2-HOPO), the previously most effective in vivo ligand. Surprisingly, although plutonium has an eight-coordination requirement, tetra- and hexadentate Me-3,2-HOPO ligands were essentially as effective as the one octadentate ligand studied. These observations suggest that even the tetradentate Me-3,2-HOPO ligands compete with mammalian transferrin for Pu(IV). For the three most promising compounds, there is no acute toxicity seen up to the highest dose administered, which was 1000 mumol/kg. One compound, the hexadentate TREN-(Me-3,2-HOPO), is particularly effective, either injected or orally, and an exceptionally good in vivo chelator of several actinides in addition to Pu(IV). Three of these compounds studied have low toxicity and are relatively simple and inexpensive to prepare. They are promising therapeutic agents.

Decorporation of thorium-228 from the rat by 3,4,3-LIHOPO and DTPA after simulated wound contamination.

1. With DTPA as a comparison, the siderophore analogue 3,4,3-LIHOPO has been examined for its ability to remove 228Th nitrate from the rat after subcutaneous (sc) and intramuscular (im) injection to simulate wound contamination. The commencement of treatment was delayed 30 min, 6 h or 1 d and the animals killed at 7 d. 2. In all cases 3,4,3-LIHOPO was appreciably more effective than DTPA although the efficacy of treatment and the relative effectiveness of the ligands decreased rapidly with their delay in administration. 3. Optimum removal with both ligands occurred when initial local administration at 30 min after exposure was followed by repeated intraperitoneal injection at 6 h, 1, 2 and 3 d. Under these conditions the body content of 228Th was reduced to 20% of controls after sc injection and 15% after im injection. The corresponding values using repeated DTPA administration were 80% and 54%. 4. It is concluded that 3,4,3-LIHOPO represents, potentially, a considerable advance on DTPA, the current agent of choice for the treatment of wounds contaminated by 228Th.

Efficacy of 3,4,3-LIHOPO for reducing the retention of 238Pu in rat after inhalation of the tributyl phosphate complex.

The efficacy of 3,4,3-LIHOPO, a siderophore analogue, has been tested for removing 238Pu from rat after inhalation of plutonium as the tri-N-butylphosphate (TBP) complex. The amounts of Pu retained in the lung of untreated rat, 7 days after exposure ranged from 0.86 to 37 kBq. The results have been compared with DTPA, the current therapy of choice for man. The ligand 3,4,3-LIHOPO was more effective than DTPA for removing Pu from the body when repeated treatment began 1 h after inhalation. This observation was independent of the mass of Pu deposited in the lungs. The efficacy of 3,4,3-LIHOPO was mainly due to the decrease of Pu retention in lung, 1.5 times less than after DTPA administration; in liver and skeleton, retention was about four times less. Seven days after internal contamination, < 10% of the activity was found in organs other than lung when rat was treated with 3,4,3-LIHOPO. As this ligand showed an apparent lack of irreversible toxicity, it is likely to be of interest in the development of new decorporation treatments after inhalation of Pu as a TBP complex.

Comparative efficacies of 3,4,3-LIHOPO and DTPA for enhancing the excretion of plutonium and americium from the rat after simulated wound contamination as nitrates.

With DTPA as a comparison, the siderophore analogue 3,4,3-LIHOPO has been examined for its ability to remove 238Pu and 241Am from the rat after subcutaneous (s.c.) and intramuscular (i.m.) injection of about 200 Bq of each actinide (0.3 ng Pu, 1.6 ng Am). After the s.c. deposition of 238Pu and 241Am, both ligands were more effective after local administration than (in decreasing order) their repeated interperitoneal (i.p.) injection, single i.p. injection and continuous infusion. Dosages of 3 mumol kg-1 of 3,4,3-LIHOPO were at least as effective as 30 mumol kg-1 DTPA after each mode of administration. The most effective regimen of those investigated for s.c. 238Pu and 241Am involved local administration of 30 mumol kg-1 of 3,4,3-LIHOPO at 30 min followed by i.p. injections at 6 h, 1, 2 and 3 day. By day 7 after exposure, the amounts of 238Pu and 241Am retained in the body were 2 and 7% of those in controls, respectively and 10 and four times less than when DTPA was administered using the same regimen. The ligand 3,4,3-LIHOPO was more effective for 238Pu and 241Am after their i.m. injection. This was attributed to the greater retention of these actinides at the wound site (97 versus 67%) when treatment commenced. After a single local injection of 30 mumol kg-1 at 30 min, the amounts of 238Pu and 241Am retained in the body at 7 day were 0.9 and 0.8% of controls. These values were 34 and 27 times less than after local and repeated i.p. injections of DTPA at dosages of 30 mumol kg-1. It is concluded that the administration of 3,4,3-LIHOPO represents potentially a most significant advance in the treatment of wound contamination by 238Pu and 241Am by chelating agents.

Early chelation therapy for injected Pu-238 and Am-241 in the rat: comparison of 3,4,3-LIHOPO, DFO-HOPO, DTPA-DX, DTPA and DFOA.

Chelating agents were tested for removal of simultaneously injected Pu-238 and Am-241 from the rat. The effectiveness of early single chelate injections of Pu-238 retention in tissues decreased in the order 3,4,3-LIHOPO > DFO-HOPO > DTPA > DTPA-DX, and for Am-241 in the order 3,4,3-LIHOPO > DTPA-DX > DTPA >> DFO-HOPO. DTPA-DX showed a special ability to remove Am-241 from the liver. Injected 3,4,3-LIHOPO decreased the contents of Pu-238 in bone and liver to 9 and 3%, respectively, of those in untreated controls. Corresponding values for Am-241 in bone and liver were 30 and 6%, respectively, which indicates that 3,4,3-LIHOPO (unlike DFO-HOPO) is not a plutonium-specific chelator. The effectiveness of prompt single oral treatment with 3,4,3-LIHOPO and DFO-HOPO in reducing retention of actinides was comparable with that of those chelators injected with 1 h delay and at one-third of the oral dose. When 3,4,3-LIHOPO was administered by continuous infusion, a superior effect was achieved with total chelate amounts only slightly exceeding that given as single injection. The retention of PU-238 and Am-241 in bones was reduced to < 5 and 10% of controls, respectively; the contents in the liver were < 2% of controls.