Bone cancer from radium: canine dose response explains data for mice and humans.

Analysis of lifetime studies of 243 beagles with skeletal burdens of radium-226 shows that the distribution of bone cancers clusters about a linear function of the logarithms of radiation dose rate to the skeleton and time from exposure until death. Similar relations displaced by species-dependent response ratios also provide satisfactory descriptions of the reported data on deaths from primary bone cancers in people and mice exposed to radium-226. The median cumulative doses (or times) leading to death from bone tumors are 2.9 times larger for dogs than for mice and 3.6 times larger for people than for dogs. These response ratios are well correlated with the normal life expectancies. The cumulative radiation dose required to give significant risk of bone cancer is found to be much less at lower dose rates than at higher rates, but the time required for the tumors to be manifested is longer. At low dose rates, this time exceeds the normal life-span and appears as a practical threshold, which for bone cancer is estimated to occur at an average cumulative radiation dose to the skeleton of about 50 to 110 rads for the three species.

Gut reactions of radioactive nitrite after intratracheal administration in mice.

Intratracheal administration to mice of radioactive nitrite labeled with nitrogen-13 (13NO2-) (half-life, 9.96 minutes) in dosages that do not cause pharmacological perturbation reveals that oxidative and reductive reactions occur in different organs. Oxidation of 13NO2- to radioactive nitrate (13NO3-) predominates in the blood and liver. Reduction of 13NO2- occurs in those mice that harbor intestinal microflora; this reduction does not occur in germ-free mice. The intestinal reduction products include ammonium, glutamate, glutamine, and urea. With a detection limit of about 0.01 percent of the instilled nitrogen-13, no labeled nitrosamines were detected within 30 minutes. Reduced nitrogen-13 is transported out of the intensive into the circulatory system and appears in the urine along with 13NO3-. The biological half-period for 13NO2- destruction is about 7 minutes, and both oxidation and reduction products are formed.

Nitrogen-13-labeled nitrite and nitrate: distribution and metabolism after intratracheal administration.

Radioactive nitrogen-13 from nitrite (NO2-) or nitrate (NO3-) administered intratracheally or intravenously without added carrier to mice or rabbits was distributed evenly throughout most organs and tissues regardless of the entry route or the anion administered. Nitrogen-13 from both anions was distributed uniformly between plasma and blood cells. We found rapid in vivo oxidation of NO2- to NO3- at concentrations of 2 to 3 nanomoles per liter in blood. Over 50 percent oxidation within 10 minutes accounted for the similar nitrogen-13 distributions from both parent ions. The oxidation rates were animal species-dependent. No reduction of 13NO3- to 13NO2- was observed. A mechanistic hypothesis invoking oxidation of 13NO2- by a catalase-hydrogen peroxide complex accounts for the results. These results imply a concentration dependence for the in vivo fate of NO2- or nitrogen dioxide.

Skeletal uptake and lifetime retention of 90Sr and 226Ra in beagles.

Skeletal uptake and retention of graded doses of ingested or injected 90Sr and injected 226Ra have been studied in 863 beagles; measurements of skeletal burden were made up to a maximum lifetime of 18.5 years. Doses ranged from 0 in 162 controls to levels that markedly reduced life span. Skeletal uptake of the administered doses averaged 2 to 2.3% for 90Sr fed to 388 beagles from midgestation to age 540 days, 33 to 35% for 45 dogs that were given single intravenous injections of 90Sr at age 540 days, and 37 to 45% for 226Ra given in eight fortnightly intravenous injections to 253 dogs from age 435 to 540 days. Skeletal retention was evaluated from the time when uptake ended until death, which occurred, on the average, at 14 to 14.5 years for the lower levels. Simple two-parameter power functions of the form SB(t) = at-b, with SB the skeletal burden, t the time after beginning of intake, and a and b fitted parameters, but corrected for radioactive decay, were used to describe the whole-skeleton retention of deposited 90Sr or 226Ra, as well as in 17 skeletal subgroups. The negative logarithmic slope, b, of these power functions for whole skeleton was about the same for both 90Sr and 226Ra, with an average value of 0.30 +/- 0.05 SD, indicating a common clearance mechanism. The lifetime average cumulative absorbed dose to irradiated skeleton varied from 0.38 to 107 Gy for beta rays in the 90Sr studies and from 0.94 to 167 Gy for alpha particles in the 226Ra studies. Daily dose rates to the skeleton for singly injected 90Sr fell rapidly after injection and declined to about 10% of the peak values late in life. Rates declined more slowly to 40-50% of peak values in other treatment groups. The time-weighted average dose rate for fed 90Sr and injected 226Ra was a robust measure that declined only about 20% late in life compared to peak values. The lifetime average dose rate varied from 0.08 to 133 mGy day-1 for the 90Sr studies and from 0.21 to 162 mGy day-1 for the 226Ra studies. Lifetime doses to mandible and cervical vertebrae for the intermediate dose levels of fed 90Sr were calculated to be about 40% higher than the skeletal average.

Tissue and subcellular distribution of bismuth radiotracer in the rat: considerations of cytotoxicity and microdosimetry for bismuth radiopharmaceuticals.

The whole-body clearance, organ distribution, and subcellular distribution of no-carrier-added and carried-added intraperitoneally administered bismuth radiotracers (205Bi-206Bi) has been determined in Sprague-Dawley rats. Differences in clearance rate kinetics were observed for this study with the administration of neutral solutions of tracers in a carbonate buffer compared to other studies with other chemical forms. The final organ distribution was not strongly dependent on administered chemical form. We provide definitive evidence that bismuth does indeed enter subcellular organelles such as the nucleus and the mitochondria, which had 30-50% and 10-25%, respectively, of activity in kidney tissue. The kidneys were the main sink for radiotracer with uptake ranging from 20 to 50% of total body activity. The calculated energy deposition by recoil nuclei after alpha emission of potentially therapeutically useful 212Bi was found to equal or exceed the alpha energy deposition per organelle if the source is inside the cell nucleus or mitochondria.

Squamous cell carcinoma in the jaws of beagles exposed to 90Sr throughout life: beta flux measurements at the mandible and tooth surfaces and a hypothesis for tumorigenesis.

We present the first detailed dose-response measurements for 90Sr-induced soft tissue tumors other than hemopoietic dyscrasias in chronically exposed beagles. Twenty-four of 387 dogs exposed to 90Sr beginning in utero and by continuous ingestion to 540 days of age developed squamous cell carcinoma of the jaw during their lifetime. Eleven of the 24 tumors were observed in dogs ingesting 12 microCi/day and receiving cumulative average doses of 6500-12,000 rad. None of these tumors was observed in dogs ingesting less than 1.25 microCi/day and receiving cumulative skeletal average doses of 2100-3900 rad, but four were observed at this level. The teeth of these animals acquired a 90Sr burden that is not removed by skeletal remodeling. Measurements of the radiation dose to soft tissue adjacent to the mandible and teeth of dogs chronically fed 90Sr indicated the first 10 micron of soft tissue adjacent to teeth received a radiation dose initially about the same as the average skeletal doses. By 2000-3000 days, these tissues received about two to three times that calculated for the average skeletal dose, or about four to six times the mean marrow dose. We suggest that these tumors arise from epithelial rests, which are embryonic tissue trapped in the periodontal membrane between teeth and bone.

Bone sarcoma characteristics and distribution in beagles injected with radium-226.

A total of 155 primary bone sarcomas were found in 131 of the 246 beagles injected with 226Ra and 5 primary bone sarcomas were found in 4 of the 158 unexposed controls. Of these 155 bone sarcomas, 146 (94%) were osteosarcomas and 9 were non-osteosarcomas. An additional 31 primary bone sarcomas (28 osteosarcomas) developed in 44 dogs terminated from the main study because of limb amputation for bone sarcoma. Non-osteosarcomas predominated in both the controls and the second lowest of six logarithmically increasing dose levels (there were no bone sarcomas in the lowest dose group). Osteosarcomas predominated at the higher dose levels, and incidence tended to increase as dose increased. The 146 osteosarcomas were distributed quite evenly between males and females (72:74). Of the 9 non-osteosarcomas, 6 occurred in males and 3 in females. The ratio of bone sarcomas of the appendicular skeleton to those in the axial skeleton was 110:45, with osteosarcomas occurring more often in the appendicular skeleton (108:38). Cases of multiple primary bone sarcomas in dogs injected with 226Ra were found only in the four highest dose groups. Amputations were performed on 44 of the 96 dogs (94 injected and 2 unexposed) that developed appendicular bone sarcomas. A statistical study of the distribution of bone sarcomas among 16 separate bone groups showed a statistically significant correlation to cancellous skeletal surface, but the variability among bone groups was too large for this relationship to be of real predictive value. It is postulated that the distribution of bone sarcomas reflects primarily the relative cell division rates in the bone groups and secondarily the radiation dose distribution, with the highest occurrence of bone sarcoma in the humeri, pelvis, femora and tibiae/fibular tarsal, and no occurrence in the coccygeal vertebrae, sternum, forepaws or hindpaws.

Bone sarcoma characteristics and distribution in beagles fed strontium-90.

A total of 66 primary bone sarcomas were diagnosed in 47 beagles; 43 of these dogs were part of the 403 beagles fed 90Sr and 4 were part of the 162 controls. Multiple primary bone sarcomas were found in 15 of the 47 beagles (32%). The incidence of multiple primary bone sarcoma was restricted to the two highest dose groups, except for a single control dog which developed two bone sarcomas. A threshold-like radiation dose response was observed; no sarcomas were observed in the lowest three dose groups, but the number of primary bone sarcomas increased rapidly in the higher dose groups. Of the 66 primary sarcomas, 49 were osteosarcomas (74%). As the dose increased, the proportion of osteosarcomas increased sharply, 4/10 (40%), 26/29 (90%), and 16/18 (89%), in the three highest dose groups. Thirteen of the bone sarcomas of other types occurred in males, and 4 in females, whereas 21 osteosarcomas occurred in males, and 28 in females. The ratio of bone sarcomas of the appendicular skeleton to those in the axial skeleton was 40:26, with osteosarcomas occurring more often in the appendicular than the axial skeleton (32:17), whereas nonosteogenic tumors showed no predilection (8:9). A statistical study of the distribution of bone sarcomas among 16 separate bone groups showed a correlation only with the distribution of cancellous bone volume-to-surface ratio and not with either skeletal mass distribution or dose distribution. The highest occurrence of sarcomas was in the humeri, femora, and mandible, and no occurrence in the coccygeal vertebrae, paws, or sternum. It is postulated that the distribution of bone sarcomas reflects a critical combination of the osteosarcoma precursor cell population, their cell division rate, and the radiation dose absorbed by these cells.

In vitro killing of melanoma by liposome-delivered intracellular irradiation.

To better understand and optimize the mechanism of alpha particle killing of tumors, an in vitro model utilizing liposomes as carrier vehicles was developed to study the killing of melanoma via intracellular alpha-irradiation. The radionuclide 212Pb (lead), with its 10.6-hour half-life and alpha-emitting daughter 212Bi (bismuth), was encapsulated in liposomes to achieve the intracellular irradiation of melanoma cells in culture. In dose-response experiments, B16F10 mouse melanoma cells were incubated with liposomes 212Pb/212Bi bound to dextran 70. Plating efficiency and growth of the melanoma cells cultured on gridded petri dishes after incubation were compared with controls at 24 and 48 hours. Greater than 85% cell killing occurred by 48 hours, with administered radioactivity levels of 1.6 dpm/mumol of lipid/cell, which corresponds to intracellular delivery of five to seven alpha particles per cell. These alpha doses can be exceeded in vivo with recirculation or in a perfusion circuit, and more efficient cytotoxic action may be possible.

Localization of bismuth radiotracer in rat kidney following exposure to bismuth.

It has been proposed that alpha emitting 212Bi (t1/2 = 60 min) coupled to tumor-specific antibodies may be a useful radiotherapeutic agent. However, since Bi can accumulate in the kidney, it is necessary to characterize the factors influencing localization of Bi within this tissue in order to evaluate the potential for radiation damage to the renal system. In this study, the localization of Bi radiotracers was determined in kidneys of rats previously exposed and not exposed to mumole quantities of Bi. Following repeated injection of Bi (4 x 14 mumols (3 mg Bi)/kg bw) the element accumulated mainly in the kidney followed by liver, spleen, pancreas, bone, and brain. Kidney copper and liver zinc concentrations were higher in Bi-exposed rats than in non-exposed rats. Within the cytosol, in Bi-exposed rats, Bi radiotracer in the kidney was associated with a metallothionein-like protein (Mt). In contrast, non-exposed rats contained no detectable metallothionein-like proteins in the kidney and the Bi tracer was associated with the hemoglobin fraction of the cell. Thus, when Bi is administered in tracer quantities such as that incorporated for use as a radiopharmaceutical, no induction of, and association with, metallothionein-like proteins should occur. These results suggest that the potential nephrotic effects of 212Bi will be influenced by the individual's previous exposure to Bi-containing drugs, or other metallothionein-inducing insults.

Radionuclide distribution dynamics in skeletons of beagles fed 90Sr: correlation with injected 226Ra and 239Pu.

Data for the bone-by-bone redistribution of 90Sr in the beagle skeleton are reported for a period of 4000 d following a midgestation-to-540-d-exposure by ingestion. The partitioned clearance model (PCM) that was originally developed to describe bone-by-bone radionuclide redistribution of 226Ra after eight semimonthly injections at ages 435-535 d has been fitted to the 90Sr data. The parameter estimates for the PCM that describe the distribution and clearance of 226Ra after deposition on surfaces following injection and analogous parameter estimates for 90Sr after uniform deposition in the skeleton as a function of Ca mass are given. Fractional compact bone masses per bone group (mi,COM) are also predicted by the model and compared to measured values; a high degree of correlation (r = 0.84) is found. Bone groups for which the agreement between the model and experimental values of mi,COM was poor had tissue-to-calcium weight ratios about 1.5 times those for bones that agreed well. Metabolically defined "surface" in PCM is initial activity fraction per Ca fraction in a given skeletal component for intravenously injected alkaline earth (Sae) radionuclides; comparisons are made to similarly defined "surface" (Sact) values from 239Pu injection studies. The patterns of Sae and Sact distribution throughout the skeleton are similar.

Life span dynamics of intra-skeletal radionuclide distribution in radium-injected beagles.

The distribution of 226Ra activity among the bone groups of the skeleton has been measured as a function of time after exposure for 12 beagles. The dogs were given 8 semi-monthly injections of radium totalling 0.37 microCi/kg, 1.11 microCi/kg, or 3.33 microCi/kg between the ages of 435 and 535 days. They were then sacrificed from 27 to 2764 days after the last injection. The fractional contribution of individual bone groups to the initial skeletal radioactivity distribution changed during the time of this study; the maximum decrease was a factor of 0.54 and the maximum increase was a factor of 1.58. Using a partitioned clearance model, the whole-body retention function characteristic of beagles exposed to radium levels less than or equal to 3.33 microCi/kg was partitioned into two functions representative of cancellous and compact bone. The functions were incorporated into a mathematical description of the 226Ra distribution data in a manner that provides estimates for the fraction of cancellous and compact bone in each skeletal component. Comparison of dosimetric estimates and site-specific tumor occurrence for these beagles revealed an apparent 90-fold variation of dose-response among the bone groups studied.

Consideration of age-dependent radium retention in people on the basis of the beagle model.

This paper examines in humans the proposition emanating from studies in beagles that initial retention of radium varies in proportion to the calcium addition rate at the time of intake. Human calcium addition rates were scaled from those in beagles, the relative calcium accretion rates in the two species at equivalent stages of skeletal growth providing the scaling factor. The variation of radium retention with age was determined by fitting a modified power function to data on the retention of radium from about 30 to 15000 days following a series of therapeutic injections of 226Ra in humans ranging in age from 18 to 63 yr. The fractional retention R at t days following a single injection of 226Ra was described by R = (1 + t/d)-0.44. The age-dependent parameter d in the retention function was found to be proportional to the calcium addition rate at the time of injection in subjects receiving less than 200 micrograms 226Ra.

Lifetime bone cancer dose-response relationships in beagles and people from skeletal burdens of 226Ra and 90Sr.

The life-time tumor dose-response relationships observed in beagles injected with 226Ra or fed 90Sr at the University of California, Davis, provide a basis for understanding the induction of bone cancer for these bone-seeking radionuclides and for scaling to people. In these studies 385 dogs were exposed to graded dosage levels of 90Sr and 243 dogs were exposed to graded dosage levels of 226Ra with a total of 159 unexposed controls. The results show different dose-response relationships for bone cancer for the two radionuclides based upon the gravimetric average dose rates and cumulative doses to bone. These relationships were found to be well represented by three-dimensional log-normal dose-response surfaces that yield risk as a function of average dose-rate and time after beginning of exposure. All dose-rates suggested a 100% risk at some later time post-exposure but the time required to reach a given level of risk was long for low dose rates so that there exists a practical threshold in that at lower dose rates individuals may die spontaneously from causes associated with natural aging prior to the expected appearance of radiogenic cancer. The risks to people at various 226Ra body burdens (average skeletal dose rates) are estimated based on the model.

Comparative pathogenesis of radium-induced intracortical bone lesions in humans and beagles.

Morphologic changes resulting from the effects of chronic radionuclide toxicity (226Ra) in the skeletons of workers in the radioluminescent dial painting industry with preterminal body burdens ranging from about 1.5 to 0.042 muCi were compared with the pathologic alterations in the skeletons of a group of 38 beagle dogs injected with 1.12 muCi/kg. Similarities observed in the skeletal responses of the two species were the presence of (1) dead bone tissue with delayed resolution, (2) a chronic disturbance in the remodeling mechanism of bone tissue, and (3) radiation-induced bone sarcomas. A detailed analysis of sequential changes in radiographic lesions arising in the beagle skeletons, complemented by histopathologic evaluation at the time of limb amputation or at necropsy, has enabled us to examined the disturbance in the bone remodeling process. The perturbation of critical importance in the generation of primary bone tumors appears to lie in the bone tissue formation and deposition phase of the bone remodeling process and gives rise to a spectrum of histologic patterns which we have termed "radiation osteodystrophy." While some of the newly generated patterns demonstrate indolent behavior with fibrous tissue replacement and bone marrow refill, other sites of bone resorption are replaced by a unique fibro-osseous tissue response resembling fibrous dysplasia or osteoblastoma. Some of these proliferative lesions may undergo progressive malignant degeneration. While the more indolent part of the spectrum was also seen in human skeletal tissues, only premalignant and early sarcomatous stages were seen in canine tissues.

Experimental validation of radiopharmaceutical absorbed dose to mineralized bone tissue.

Therapeutic and palliative uses of bone-seeking radiopharmaceuticals are undergoing clinical trials for human subjects. Radiation dosimetry for these applications is based on the Medical Internal Radiation Dosimetry (MIRD) schema. An experimental method for dosimetry of bone tissue based on electron paramagnetic resonance (EPR) spectrometry is described. Preliminary results for beagle bone exposed to radiopharmaceuticals under clinical conditions have indicated that the EPR dose measurements give approximately the calculated dose, but suggest that the dose distribution may be non-uniform.