212Pb: Production Approaches and Targeted Therapy Applications

Over the last decade, targeted alpha therapy has demonstrated its high effectiveness in treating various oncological diseases. Lead-212, with a convenient half-life of 10.64 h, and daughter alpha-emitter short-lived ²¹²Bi (T_1/2 = 1 h), provides the possibility for the synthesis and purification of complex radiopharmaceuticals with minimum loss of radioactivity during preparation. As a benefit for clinical implementation, it can be milked from a radionuclide generator in different ways. The main approaches applied for these purposes are considered and described in this review, including chromatographic, solution, and other techniques to isolate ²¹²Pb from its parent radionuclide. Furthermore, molecules used for lead’s binding and radiochemical features of preparation and stability of compounds labeled with ²¹²Pb are discussed. The results of preclinical studies with an estimation of therapeutic and tolerant doses as well as recently initiated clinical trials of targeted radiopharmaceuticals are presented.

The effects of ionizing radiation on domestic dogs: a review of the atomic bomb testing era

Dogs were frequently employed as laboratory subjects during the era of atomic bomb testing (1950–1980), particularly in studies used to generate predictive data regarding the expected effects of accidental human occupational exposure to radiation. The bulk of these studies were only partly reported in the primary literature, despite providing vital information regarding the effects of radiation exposure on a model mammalian species. Herein we review this literature and summarize the biological effects in relation to the isotopes used and the method of radionuclide exposure. Overall, these studies demonstrate the wide range of developmental and physiological effects of exposure to radiation and radionuclides in a mid‐sized mammal.

Strontium-90 Induced Bone Tumours in Beagle Dogs: Effects of Route of Exposure and Dose Rate

Bone tumours from beagles exposed by inhalation to 90SrCl2 at the Inhalation Toxicology Research Institute (ITRI), by chronic ingestion of 90Sr at the Laboratory of Energy-Related Health Research (LEHR), and by injection of 90Sr citrate at the University of Utah were analysed to determine if the bone tumour characteristics differed among the three studies. The range of average skeletal doses at which the bone tumours occurred was similar in all three studies, but differences in the skeletal distribution, histological phenotype, and time to death were observed. The differences observed were attributed to the difference in dose-rate pattern obtained in the chronic ingestion study, in contrast to the inhalation and injection studies. In general, however, the differences noted in bone tumour characteristics were subtle, and would be unlikely to make an impact on models developed to assess the risk of human exposure to 90Sr.

Ancestry of beagles in lifespan studies of radionuclide toxicity at the University of Utah

Analysis of the ancestry of the 1,262 lifespan beagles (LSB) entered into lifespan studies at the Radiobiology Division, University of Utah, beginning in 1952 and ending in 1980, indicated that about 97% of ancestor citations in the various pedigrees were of only 10 breeding animals (breeders) among breeders within the beagle colony. In turn, just 18 AKC-registered "champion" beagles from outside of this colony (founders) accounted for about 98% of all ancestor citations among founders for the LSB. We conclude from this study that the animals used in the lifespan radionuclide experiments can be considered to be somewhat genetically interrelated.

Two-mutation models for bone cancer due to radium, strontium and plutonium

Data from beagle experiments and radium dial painters were used to derive two-mutation carcinogenesis models for bone cancer induced by the bone-seeking radionuclides radium, strontium and plutonium. For all data, the model fits indicate that at low doses both mutation rates depend linearly and equally strongly on dose rate. For the high-LET alpha-particle emitters, a cell killing term reduces the second mutation rate at high dose rates. In all cases, the combined effect of both mutation rates is a linear-quadratic dose-effect relationship for cancer at low doses. This behavior may lead to experimental data that could be mistaken as showing a threshold below which no cancers are induced. Derived parameters such as toxicity ratios and tumor growth times compare well with values reported in the literature. Furthermore, results for plutonium indicate that rapid burial of the nuclide in the growing bones of juvenile beagles leads to a significant reduction of its toxicity, as was suggested previously. The results for radium in beagles compare well with those for humans and suggest that the models derived for strontium and plutonium in beagles may be translated to humans. The significant model parameters for the accurate animal data could then also be used to fit human epidemiological data.

Does low dose internal radiation increase lifespan?

No significant differences in lifespan could be established between control dogs and dogs given 241Am, 228Th, 90Sr, 228Ra, 226Ra or monomeric 239Pu at low dosage levels that induced less than 10% skeletal malignancies (low dose animals) in the Utah beagle colony when all dogs surviving at least 1 y were included in the analysis and dogs given individual radionuclides were considered separately or together. Censoring or exclusion of dogs from these groups that were diagnosed with skeletal malignancies or that died in a gran mal epileptic seizure made no important difference to these results. Therefore, an enhanced lifespan of low dose dogs as compared with controls could not be established. It is concluded that low doses from internal (mainly skeletal) deposits of these radionuclides probably do not benefit the survival of individuals so exposed.

Does malignant hematopoietic disease result from internal exposure to 239Pu?

Neither Fisher's Exact Test nor linear regression analysis (dose-response relationship) using average skeletal dose from 239Pu as a surrogate for marrow dose could establish a statistically significant association between malignant hematopoietic disease (MHD including leukemia and aleukemia) and exposure to 239Pu among dogs in the Utah beagle colony. In addition, when dog-years at risk was the parameter used to estimate the rate of expected malignant hematopoietic tumors in dogs exposed to 239Pu, the estimate was within the range of the 95% confidence limits of the control dogs for six cases (and also within the 95% confidence limits for five cases when the single case of lymphoma was omitted). It was concluded that if there was an effect of 239Pu exposure on the occurrence of MHD in this beagle experiment, it was not very meaningful. These data could not establish a significant susceptibility to neoplasia of either myeloid or lymphoid tissue of 239Pu exposure.

Survival and bone tumor hazard from internal deposition of 226Ra in beagles

The survival of 132 young adult control beagles and 117 beagles receiving graded injections of 226Ra ranging from 0.27 kBq kg-1 to 384.2 kBq kg-1 body mass was analyzed. The hazards of natural deaths, all deaths in injection groups, and deaths by bone tumors were assumed to follow a Weibull distribution with a common shape factor of 6.3. Only the scale factors of the Weibull distributions depend on the injection level. There were no significant sex differences. The relative risk with respect to controls for all causes of death increases up to 6,925 for 384.2 kBq kg-1. The dependence of the scale factors for all deaths and death by bone tumors on injected activity was fitted to an empirical regression model, which also contains a term representing radiation-caused deaths other than bone tumors. The risk of bone tumors increases nearly as the square of the injected activity, whereas the risk of the other radiation caused deaths increases approximately in proportion to the injected activity. By means of the regression model, it is possible to predict median survival times for all deaths, bone tumor and non-bone tumor deaths for an arbitrary intake level. Also, simple expressions for the fraction of animals with bone tumors and other radiation-caused deaths can be derived. The empirical model of bone tumor induction, which was based on results from a single injection design, can be generalized to an arbitrary systemic intake schema. This is achieved by using the average dose and dose rate to the skeleton as indices of detriment. Applying the generalized model, it was confirmed that no significant differences in survival can be expected for two groups of beagles receiving multiple injections, if compared to the corresponding single injection groups of about the same total activity. The general model also predicts that even extensive protraction of the intake would increase the survival times only to a very limited extent.

Does soft tissue cancer affect the development of radionuclide-induced skeletal malignancy?

Data in the archives of the beagle project at the University of Utah were used to determine whether there might have been an association between the occurrence of soft tissue cancer and bone tumor (skeletal malignancy) among the dogs injected with bone-seeking radionuclides. In addition to comparing the numbers of animals with and without bone tumors and with and without soft tissue cancers, data on bone sarcomas and soft tissue sarcomas were compared. Longevity (postinjection survival) of the dogs was investigated. It appeared that survival (time between radionuclide injection and death) was important in the eventual expression of malignancies. There could not be established a possible susceptibility to skeletal malignancy associated with the occurrence of soft tissue cancer (or soft tissue sarcoma). We concluded that, although the literature reports an increased susceptibility for subsequent malignancies for individuals with naturally-occurring or radiation-induced cancer, the data derived from this study do not confirm the concept that the appearance of soft tissue cancer is strongly associated with an increased risk of developing a radionuclide-induced skeletal malignancy.

Review of 239Pu and 226Ra effects in beagles

A long term biological study has been completed that was designed to assess the predicted effects in humans of internally deposited 239Pu by comparison with 226Ra in beagles. Herein we summarize for the first time results of several previous reports about the effects of these two radionuclides in our beagles in an attempt to elucidate what has been learned since the beginning of the study in the early 1950's. Perhaps the most important finding was that bone surface-seeking plutonium is more toxic at equal mean skeletal radiation doses (<3 Gy for 239Pu, <20 Gy for 226Ra) than bone volume-seeking radium for the induction of skeletal malignancy by about a factor of 16 for a single intravenous injection of monomeric 239Pu. In addition, ancillary studies have shown that when plutonium transfers continuously onto bone surfaces from a depot of particulate 239Pu in phagocytic cells, its relative toxicity per Gy average skeletal dose is enhanced by about a factor of 2. Juvenile animals or dogs injected as mature adults were only about half as sensitive for equal mean skeletal doses as dogs injected as young adults. Male and female dogs were about equally sensitive to radiation of the skeleton by either radionuclide. Findings about radiation-induced fractures are summarized as well as data on the induction of soft-tissue malignancies by 239Pu or 226Ra. Natural survival was not affected at the lower dosage levels of either 226Ra or 239Pu as compared with control dogs given no radioactivity, but the survival of animals at higher levels was reduced. No additional life-shortening effects beyond those attributable to occurrence of radiation-induced malignancies or other radiation-induced effects were suggested by analysis of data for low dosage levels.

Does longevity in beagles injected with bone-seeking radionuclides depend upon radiation dose in the absence of known radiation effects?

Regression analyses of longevity as a function of skeletal radiation dose among groups of beagles injected with 226Ra, 228Ra, 228Th, 241Am, 90Sr or monomeric 239Pu suggested that at low doses and dose-rates (those at which induced effects are low), age at death seems to be independent of dose when animals dying with specific radiation effects were excluded, although longevity does appear to be a function of dose when animals dying with established radiation effects and at all doses were included. We conclude tentatively that, for mammals receiving skeletal dose from bone-seeking radionuclides at low doses and low dose-rates, longevity may not be dependent upon skeletal radiation dose in the absence of radiation-induced malignancies or other radiation effects.

Effective thresholds for induction of skeletal malignancies by radionuclides

Our analysis of data from the beagle project completed at the University of Utah has provided some comparisons that appear to be useful in testing the model proposed by Raabe of effective thresholds for induction of skeletal malignancy by bone-seeking radionuclides in beagles. Raabe's model predicted that cumulative skeletal doses of less than about 0.9 to 1.4 Gy from alpha emitters or 28 to 70 Gy from beta emitters deposited in the skeleton require a long enough time for bone cancer expression that the dog's natural lifespan would be exceeded before the tumor appeared. Results from the Utah beagle project seem to confirm these projections for 226Ra, 228Ra and, perhaps, for 90Sr. The lowest doses at which malignant bone tumors were observed in animals injected with these radium isotopes were about 0.9 Gy (226Ra) and 3 Gy (228Ra). For the beta emitter, 90Sr, the lowest doses at which bone tumors were seen were about 18, 50, and 70 Gy with an expectation for naturally occurring tumor of about one. Twenty-six of the two hundred and thirty-three Utah beagles given monomeric 239Pu that developed skeletal malignancies had doses between 0.02 and 0.51 Gy (80 of these dogs had skeletal doses of less than 0.9 Gy). Three dogs of 54 given 241Am with doses lower than 0.9 Gy had bone tumors at 0.23, 0.56, and 0.88 Gy with the expectation of about one naturally occurring case. For 25 animals injected with 228Th at skeletal doses below 0.9 Gy, one bone tumor dog had a dose of about 0.4 Gy, and the expectation of a dog with natural tumor among the group was only about 0.38. Five beagles of 74 given 224Ra with resulting doses of less than 0.9 Gy died with skeletal malignancy at 0.32 Gy or less with an expectation for non 224Ra induced tumor of about one. It appears that Raabe's proposal might be confirmed for some but not all of the radionuclides used in the Utah studies. Models presented in earlier papers by Raabe provide results that are somewhat different from his recent abstract and compare more favorably with those cited herein for Utah dogs. Re-examination of our data for these analyses has suggested a novel concept for calculation of carcinogenic dose to endosteal bone surfaces.

Does body size contribute to sensitivity of bone tumor induction by radionuclide exposure?

Investigation of a possible increase in sensitivity to occurrence of radionuclide-induced skeletal malignancy with increasing body size was analyzed among 358 beagles injected as young adults with either 226Ra or monomeric 239Pu and maintained for their lifespans. Corresponding analyses were performed for about 240 other beagles injected as young adults with 90Sr, 228Ra, or 228Th. Body masses at the time of injection ranged between about 5.6 and 16 kg. Logistic regression analysis using body mass and cumulative skeletal radiation dose as the independent variables indicated that there could not be established a dependency of tumor occurrence upon body mass, although skeletal dose was found to be significantly correlated with occurrence of bone cancer. Regression analysis indicated that for any dosage group there could not be established a correlation between body mass and skeletal dose. Each dosage group having similar injected kBq kg(-1) for each nuclide was divided into 2 subgroups of equal size, one containing the less massive dogs and the other containing the more massive dogs. These subgroups within a roughly uniform value of skeletal dose-rate were compared by Fisher's Exact Test, and the less massive subgroups were combined within each nuclide for an additional, separate analysis against the combined more massive subgroups using the same method. In only one instance (the dosage group given 3607 kBq 90Sr kg(-1)) was there indicated a substantially greater tumor occurrence among dogs in the more massive subgroup (p = 0.061). However, for the group given 0.382 kBq 239Pu kg(-1) there was indicated a significant difference between subgroups, but the effect was exactly opposite to that found for the highest level 90Sr dogs in that the less massive subgroup had a higher relative tumor occurrence than the most massive (p = 0.042). For all groups with a p-value < 0.10, a possible correlation was investigated between survival and body mass at injection (since bone tumor occurrence might be a function of longevity), but a significant relationship could not be determined. No significant differences could be established between the combined more massive and the combined less massive subgroups for any radionuclide. We conclude that, for the conditions in our experiment, relative size within a species does not contribute importantly to the sensitivity (lifetime occurrence) for induction of skeletal malignancy.

Radium-induced eye melanomas in dogs

The intraocular radiotoxicity of intravenously injected 226Ra and 228Ra was studied in beagle dogs. Approximately 0.071% of injected radium was retained in each eye of beagles following intravenous administration. The retention was principally in the tapetum and the intraocular pigmented structures where significant pigmentary lesions were produced. These included melanotic plaques on the iris, melanosis of the ciliary body, varying degrees of tapetal degeneration, and intraocular melanomas. The tumors occurred principally in the ciliary body and to a much lesser extent in the iris. They appeared to arise from the pigment epithelium layer of the ciliary body. Thus, unlike melanomas arising in other sites, they are apparently not of neural crest origin. In addition to bone cancer, they represent another radium-induced neoplasm in beagles. Radium-induced intraocular melanomas have not been reported in people.

Fracture occurrence from radionuclides in the skeleton

Because skeletal fractures were an important finding among persons contaminated with 226Ra, experience with fractures among dogs in our colony was summarized to determine the projected significance for persons contaminated with bone-seeking radionuclides. Comparison by Fisher's Exact Test of lifetime fracture occurrence in the skeletons of beagles injected as young adults suggested that for animals given 226Ra, 228Ra, 228Th, or 239Pu citrate, there was probably an excess over controls in fractures of the ribs, leg bones, spinous processes, and pelvis (os coxae) plus the mandible for dogs given 226Ra and the scapulae for dogs given 228Ra or 228Th. Regression analysis indicated that significantly elevated fracture occurrence was especially notable at the higher radiation doses, at about 50 Gy average skeletal dose for 239Pu, 140 Gy for 226Ra, about 40 Gy for 228Ra, and more than 15 Gy for 228Th. The average number of fractures per dog was significantly elevated over that noted in controls for the highest radiation doses of 239Pu and 226Ra and for the higher doses of 228Ra and 228Th. For those dogs given 90Sr citrate, there was virtually no important difference from control beagles not given radionuclides, even at group mean cumulative skeletal radiation doses up to 101 Gy. Because of a large proportion of dogs with fractures that died with bone malignancy (even at dosage levels lower than those exhibiting an excess average number of fractures per dog), we conclude that fracture would not be an important endpoint at lower levels of plutonium contamination in humans such as would be expected to occur from occupational or environmental exposure.

Biokinetic and dosimetric model of plutonium in the dog

A biokinetic model of the systemic distribution and dosimetry of 239Pu in the beagle dog is presented. To achieve maximum consistency with experimental data, known histomorphometric parameters and results of autoradiographic studies were adopted directly. The remaining parameters were determined from retention and excretion measurements by optimization procedures. The beagle model attempts to parallel the human model as much as possible, but only one liver compartment and one compartment representing other soft tissues were needed to describe the data adequately. The salient features and differences of the biokinetic behavior of 239Pu beagles and humans are compared. Generally the organ retention of the beagle in relation to the lifetime is longer than in humans. This is particularly pronounced in the skeleton. Trabecular deposits of plutonium are gradually shifted to cortical sites. For the dosimetric model some additional features disregarded in the human model were employed. These relate to bone volume labels, a gradation of concentrations in marrow, the energy-dependence of absorbed fractions, and the self-absorption in marrow. The model predicts that the contribution of surface deposits to the endosteal dose still exceeds the contributions from bone volume and marrow labels. The average endosteal dose is about eight times and the marrow dose about two times larger than the average skeletal dose. The model provides the basis for the analysis of survival and relative risks.

Characterization of endosteal bone-lining cells from fatty marrow bone sites in adult beagles

Bone-lining cells cover the majority of trabecular bone surfaces in adult long-lived mammals. The morphology, ultrastructure, and population density of bone-lining cells was investigated in several fatty marrow trabecular bone sites in adult beagles of different ages. Although there is a low population density of bone-lining cells on bone surfaces, their total numbers greatly exceed the numbers of osteoblasts and osteoclasts found on these bone surfaces. In one of the bone sites studied, there are significantly fewer bone-lining cells in very old beagles (12-16 years old) when compared to young adult beagles (1.5-3 years old), otherwise there are no differences in the cell population that could be attributed to aging. Bone-lining cells are flattened against bone surfaces and have flat or sometimes ovoid-shaped nuclei which are often located adjacent to areas in the fatty marrow where capillaries are found. When viewed in the electron microscope, bone-lining cells contain few organelles, and the attenuated cytoplasm of these cells is well extended over bone surfaces. The bone-lining cell cytoplasm does not appear to form a continuous layer over the bone surface, as numerous gaps and spaces are seen. Bone-lining cell processes are frequently joined by junctions morphologically similar to gap junctions. Between the lamina limitans of the bone matrix and the bone-lining cell, as well as between the bone-lining cell and the adjacent fat cells, there are layers of connective tissue containing collagenous fibers and other amorphous material. These findings are discussed in relation to the possible role of bone-lining cells in the regulation of mineral homeostasis.

Influence of 226Ra and 239Pu on the dental root canal of the dog

Radium 226 and plutonium 239 were injected into dogs and the rate of decrease in diameter of the pulp chambers of the lower first molar of the left side of the mandible was studied by periodic radiograms. No significant change in the rate of decrease occurred as the result of irradiation.