Potential of a Cetuximab-based radioimmunotherapy combined with external irradiation manifests in a 3-D cell assay.

Targeting epidermal growth factor receptor (EGFR)-overexpressing tumors with radiolabeled anti-EGFR antibodies is a promising strategy for combination with external radiotherapy. In this study, we evaluated the potential of external plus internal irradiation by [(90) Y]Y-CHX-A″-DTPA-C225 (Y-90-C225) in a 3-D environment using FaDu and SAS head and neck squamous cell carcinoma (HNSCC) spheroid models and clinically relevant endpoints such as spheroid control probability (SCP) and spheroid control dose 50% (SCD50 , external irradiation dose inducing 50% loss of spheroid regrowth). Spheroids were cultured using a standardized platform. Therapy response after treatment with C225, CHX-A"-DTPA-C225 (DTPA-C225), [(90) Y]Y-CHX-A"-DTPA (Y-90-DTPA) and Y-90-C225 alone or in combination with X-ray was evaluated by long-term monitoring (60 days) of spheroid integrity and volume growth. Penetration kinetics into spheroids and EGFR binding capacities on spheroid cells were identical for unconjugated C225 and Y-90-C225. Spheroid-associated radioactivity upon exposure to the antibody-free control conjugate Y-90-DTPA was negligible. Determination of the SCD50 demonstrated higher intrinsic radiosensitivity of FaDu as compared with SAS spheroids. Treatment with unconjugated C225 alone did not affect spheroid growth and cell viability. Also, C225 treatment after external irradiation showed no additive effect. However, the combination of external irradiation with Y-90-C225 (1 µg/ml, 24 hr) resulted in a considerable benefit as reflected by a pronounced reduction of the SCD50 from 16 Gy to 9 Gy for SAS spheroids and a complete loss of regrowth for FaDu spheroids due to the pronounced accumulation of internal dose caused by the continuous exposure to cell-bound radionuclide upon Y-90-C225-EGFR interaction.

Flow cytometric cell-based assay to preselect antibody constructs for radionuclide conjugation.

Radiolabeled antibodies (Abs) are an attractive tool for targeting and delivering particle emitters for therapy or imaging applications. The labeling of Abs with metal radionuclides requires chelating agents and can cause loss of binding to their ligands. The aim of the present approach was to design an easy-handling flow cytometric cell-based assay to evaluate Ab-binding capacity of conjugates of the therapeutic Ab Cetuximab and to verify the most promising candidate in a competitive radioactive binding experiment. The final setup for flow cytometric assessment of cellular binding capacities of epidermal growth factor receptor (EGFR)/ErbB1-directed Ab conjugates is based on (a) the selection of a robust cell line model (b) the definition of nonsaturated staining concentrations for the unconjugated reference Ab Cetuximab plus implementation of a reasonable isotype control, and (c) the calculation of relative Ab affinities based on the flow cytometric data. Two (FaDu, SAS) out of the three cell lines with different total and cell surface expression levels of EGFR turned out to be adequate models but the application of one cell line was sufficient to estimate reduced binding capacities of conjugates relative to Cetuximab. Only 1/11 conjugate Abs exhibited a fluorescence signal comparable to unconjugated Cetuximab and was applied for radiolabeling with Yttrium-90. Unaltered binding affinity of this conjugate was proven in a competitive radioactive Ab-binding study. We conclude that the flow cytometric assay is reliable and that the relative binding capacity of Cetuximab is neither affected by covalent modification with CHX-A"-DTPA (N-[(R)-2-Amino-3-(p-isothiocyanato-phenyl) propyl]-trans-(S,S)-cyclohexane-1,2-diamine-N,N,N',N",N"-pentaacetic acid) with a final chelator-to-Ab ratio of 5 nor by subsequent radiolabeling. [(90)Y]Y-CHX-A"-DTPA-Cetuximab thus qualifies for preclinical treatment testing as a prerequisite for therapeutic application.

[99mTc reduces clonogenic survival after intracellular uptake in NIS-positive cells in vitro more than 131I]. / 99mTc reduziert nach intrazellulärer Aufnahme in NIS-positiven Zellen in vitro das klonogene Uberleben stärker als 131I.

AIM: In addition to gamma radiation of 140 keV 99mTc emits during the transition to 99Tc electrons of low energy and tiny path-lengths. These Auger electrons cannot be utilized in diagnostic procedures. However, they were discussed frequently for therapeutic application. Hitherto proof of effect of the Auger electrons from 99mTc is missing which is supplied now in an in vitro-system in comparison to beta-emitter 131I. METHODS: The thyroid cell line PCCl3 (sodium iodide symporter (NIS)-positive) was incubated with 131I-sodium iodide (131I) or 99mTc-pertechnetate (99mTc) in presence or absence of perchlorate. For comparison the amount of radioactivity was adjusted to obtain the same dose from extracellular irradiation for both radionuclides. The colony forming assay detects the clonogenic cell survival as surviving fraction. In addition, intracellular radionuclide uptake was quantified. RESULTS: Dose effect curves were established for 131I and 99mTc for variable extra- and intracellular distribution of the radioactivity. In presence of perchlorate no cellular uptake of radioactivity was detectable. Survival curves were largely comparable confirming the dosimetric calculations. In absence of perchlorate cellular radiotracer uptake varied from 1.39% (131I) to 1.90% 99mTc). Effects on survival were twice for the beta-emitter and ten-fold higher for 99mTc. CONCLUSIONS: Intracellular uptake of 131I and 99mTc increases DNA-damage compared to strict extracellular radiotracer distribution which was demonstrated by means of colony forming assay. Increasing radiotoxicity from intracellular 99mTc is explained most likely by increased dose deposition in cellular structures due to Auger- and conversion-electrons of low range and high local energy deposition.

[DNA damage in lymphocytes after irradiation with 211At and 188Re]. / DNA-Schäden von Lymphozyten nach Bestrahlung mit 211At und 188Re - Quantifizierung mit dem alkalischen und neutralen Komet-Assay.

AIM: Ionising radiation produces many types of DNA lesions of different complexity. High linear energy transfer (LET) types of radiation are biological more effective than low LET radiation. In the present work we applied the single cell gel electrophoreses (comet assay) to study the induction of initial DNA damage, efficiency of repair and residual DNA damage in lymphocytes after treatment with 211At and 188Re. METHODS: Peripheral blood mononuclear cells (PBMC) were isolated from heparinized blood of healthy donors and irradiated with 211At and 188Re at different doses. The comet assay was performed under alkaline and neutral conditions in order to detect the initial DNA damage and its repair. The measure of damage was % tail DNA (percentage of DNA in the tail). RESULTS: After treatment of cells with 188Re the initial DNA damage (% tail DNA) detected with the alkaline comet assay was higher than the damage measured for 211At. The neutral comet assay estimated higher tail intensities for 211At in contrast to 188Re. Compared with the complete repair (10%) after irradiation with 188Re, the radiotoxicity of alpha particles indicated reduced rejoining of DNA strand breaks (60-80% residual damage). Rejoining of DNA damage measured by the neutral comet method detected about 70% unrepaired strand breaks for 211At and 188Re. CONCLUSIONS: There are major differences between the repair of strand breaks caused by 188Re and 211At detected by the alkaline comet assay. The DNA-damage induced by the high LET Emitter 211At remains nearly unrepaired detected by both alkaline and neutral comet assay. Represented data following irradiation of lymphocytes with alpha and beta particles demonstrated higher biological effectiveness of 211At by factors of 2.0-2.5.

[Cellular damage in vitro]. / Einfluss der mittleren Betaengergie und der intrazellulären Radionuklidaufnahme auf die Zellschädigung in vitro.

AIM: The cellular damage of ionising radiation depends on dose, physical radiation quality (e. g. LET) and intracellular radionuclide uptake. The influence of two beta emitters (188Re and 131I) on the thyroid cell line PCCl3 was studied. Furthermore, we analysed the effect of intracellular accumulation. METHODS: The thyroid cell line PCCl3 was irradiated with 188Re-perrhenate or 131I-sodium iodide in presence or absence of perchlorate. The initial DNA-damage was measured in the comet assay as olive tail moment (OTM). The colony forming assay detects the clonogenic cell survival as surviving fraction. Additional the intracellular radionuclide uptake was quantified. RESULTS: Dose response curves were established for irradiation with 188Re-perrhenate or 131I-iodine under various extra- and intracellular activity distribution conditions. In the presence of perchlorate DNA-damage and clonogenic cell survival for both radionuclides were comparable. In the absence of perchlorate radionuclide uptake of 1.39% (131I) and 4.14% (188Re) were measured causing twofold higher radiotoxicity. Although 131I uptake was lower than 188Re uptake the OTM values were higher und surviving fractions were lower. CONCLUSIONS: 131I, compared to 188Re, has lower mean beta energy and a higher LET, and therefore, it induced a higher DNA-damage even at lower intracellular uptake. An additional explanation for the higher radiotoxicity of 131I could be the higher dose exposition caused by cross-fire through neighborhood cells.

Carcinogenicity test of acetoxime in MRC-Wistar rats.

Acetoxime was tested for carcinogenicity by chronic administration in the drinking water to male and female outbred MRC-Wistar rats. The dose of 1.0 g/liter was administered 5 days/week for 18 months (total dose, 6.2--7.0 g/rat). The test compound induced benign hepatocellular adenomas in 80% of the males but did not produce tumors in the females. This is the first report that oximes, which are fairly widely used in industry, are tumorigenic.

Study of the carcinogenicity of large doses of dimethylnitramine, N-nitroso-L-proline, and sodium nitrite administered in drinking water to rats.

Large doses of dimethylnitramine (DMNM), N-nitroso-L-proline (NPRO), and sodium nitrite were administered in the drinking water to MRC Wistar rats for at least 1 year, and the rats were maintained for life. DMNM (total dose, 20 g/kg) produced liver tumors in 25 (69%) of the 36 rats and nasal cavity tumors in 9 (25%) of the rats. NPRO (total dose, 36 g/kg) induced no tumors in 37 treated rats. In the group receiving NaNO2 (3.0 g/liter drinking water; total dose, 63 g/kg), 8 (18%) of 45 rats had forestomach squamous papillomas. The tumor incidence in the NaNO2-treated group was significantly greater than that of 2% in a control group started 11 months earlier, which suggested that the NaNO2 was tumorigenic in this experiment.

Carcinogenicity of N-nitrosobis(2-hydroxypropyl)amine and N-nitrosobis(2-oxopropyl)amine in MRC rats.

Weekly sc injections of equitoxic doses of N-nitrosobis(2-hydroxypropyl)amine (BHP) and N-nitrosobis(2-oxopropyl)amine (BOP) to Wister-derived MRC rats induced tumors. The incidence, latency, multiplicity, morphologic type, and distribution of these tumors varied according to the compound given. The esophagus was the main target organ for BHP (100%), followed by the respiratory tract (87%), pharynx (80%), colon and liver (each 73%), kidneys (20%), thyroid gland (20%), and urinary bladder and urethra (each 7%). BOP was ineffective in the esophagus and pharynx but induced a higher incidence of tumors in the kidneys (27%), thyroid gland (60%), urinary bladder (33%), and urethra (73%) and fewer neoplasms in the respiratory tract (20%), colon (67%), and liver (53%). In addition, BOP caused a few, apparently primary, prostate squamous cell carcinomas. The results are compared with results of BHP treatment in Sprague-Dawley rats and with results of BHP and BOP treatment in Syrian golden hamsters.

New model for the induction of osteoblastic bone metastases in rat.

BACKGROUND: We have investigated different models for osteoblastic lesions. Currently, there are two models using MatLyLu R-3327prostate cancer cells: tumor cell application in the left heart ventricle and intravenous application with concomitant transient surgical clamping of the lower caval vein. MATERIALS AND METHODS: Thirty male Copenhagen rats (age 9+/-2 months, mean weight 323+/-21 g) were each injected with 200,000 R-3327 prostate cancer cells. In 10 rats the left ventricle route was used (group 1), in an other 10 rats the intravenous route (group 2), while in the third group of 10 rats a new model of a direct intra-osseous route was applied (group 3). Additionally, a control group of 5 rats underwent the same procedure as in group 3, but only saline without tumor cells was administered. A 99mTc-HMDP bone scan and histological examination of bone and lung were performed for follow-up. RESULTS: In the bone scan, bone lesions could be not visualized in groups 1 and 2, but in group 3 osteoblastic lesions were observed in both femora in 9 out of 10 rats. Upon histological examination, there were lung metastases in animals from groups 1 and 2, but not in group 3. Clinical signs for bone metastases in the lumbar spine (motor disablement of the hind legs) were found in groups 1 and 2. CONCLUSION: The intra-osseous administration of MatLyLu R-3327 prostate cancer cells represents a useful and effective model for osteoblastic bone lesion, and allows further autoradiographic evaluation of bone uptake using bone-seeking radiopharmaceuticals.

Pancreatic ductulitis in Syrian golden hamsters bearing homologous transplantable pancreatic adenocarcinomas.

A highly specific pancreatitis primarily affecting the intralobular and intrainsular ductules has been demonstrated in Syrian golden hamsters bearing homologous, non-syngeneic, transplantable pancreatic adenocarcinomas induced by N-nitrosobis(2-oxopropyl)amine (BOP). The ductulitis provides further evidence that induced pancreatic neoplasms originate from ductules.

Blood group specificity of pancreatic tumor mucin.

N-nitrosobis(2-oxopropyl)amine (BOP)-induced pancreatic adenocarcinomas in Syrian hamsters produce considerable amounts of mucin. The mucin is immunogenic in rabbits and is predominately of A blood group antigenic specificity, as determined by immunodiffusion in agar gel. Indirect immunofluorescence studies with human anti-A typing sera demonstrated bright tumor-mucin fluorescence. The mucin produced by transplantable pancreatic tumors is also of A blood group specificity. Eppley colony Syrian hamsters, randomly examined, demonstrated red cells of blood group O. They lacked the anti-A and anti-B isoagglutinins in their serum. It is emphasized that the antigenic specificity of tumor-mucin in induced pancreatic cancer might be useful for early clinical detection, and perhaps therapy, of the disease.

Ductular origin of pancreatic cancer and its multiplicity in man comparable to experimentally induced tumors. A preliminary study.

The histologic features of 3 randomly selected pancreatic cancer cases are compared with those found in Syrian hamsters after treatment with N-nitrosobis(2-oxopropyl)amine (BOP). The 3 human cases all exhibited hyperplastic, preneoplastic and malignant changes which were markedly multicentric, and which arose predominantly from ductules, as well as from small ducts. The findings were comparable to those in the hamster mode. Proliferation and malignant alterations of the intrainsular ductules were commonly seen in both human and experimental tumors. The data is consistent with the concept that cells of the small ducts and especially of the ductules represent a potential source of human, as well as experimental, tumors. The small number of human cases studied does not allow generalization, but the marked resemblances in all 3 randomly selected pancreatic cancer cases were remarkable.

The morphologic and biologic patterns of chemically induced pancreatic adenocarcinoma in Syrian golden hamsters after homologous transplantation.

A pancreatic adenocarcinoma induced by N-nitrosobis(2-oxopropyl)amine in the Syrian golden hamster was successfully transplanted to a homologous host by subcutaneous inoculation through 10 successive passages. The rate of 'tumor take' increased progressively with each generation from 60% to 100%, and the latency period after inoculation was reduced simultaneously from 6 weeks to 1 week in the second and following passages. The tumors grew rapidly, ulcerated the overlying skin, and metastasized to the regional lymph nodes and lungs. The animals usually died with multiple lung metastases between the 5th and 20th weeks. All transplanted tumors and their metastases retained the pattern of the original, well-differentiated adenocarcinomas.

Selective induction of pancreatic ductular tumors by single doses of N-nitrosobis(2-oxopropyl)amine in Syrian golden hamsters.

The selectivity of response by the Syrian hamster pancreas to the neoplastic effect of N-nitrosobis(2-oxopropyl)amine (BOP) was demonstrated by single subcutaneous injections of the compound. Only occasional tumors were induced simultaneously with those of the pancreas in the biliary ducts, kidneys and lungs of a few hamsters. In the pancreas, the ductules, especially those of an intrainsular location, were the cells primarily affected. The responded to doses as low as 1/40 of the LD50, whereas ductal lesions were found only in some hamsters treated with doses above 1/5 of the LD50. Hence it is concluded that hamster ductular cells are the most responsive to the carcinogenic action of BOP.

Chromogranin A: an additional tumor marker for postoperative recurrence and metastases of medullary thyroid carcinomas?

In this study, plasma concentrations of chromogranin A, calcitonin and carcinoembryonic antigen (CEA) were measured in 40 healthy volunteers as well as in 129 patients with recurrences and/or metastases of neuroendocrine tumors and of medullary thyroid carcinomas (MTCs). A double antibody assay was employed using polyclonal rabbit antibodies to a C-terminal fragment of the protein for detection of human chromogranin A. Using ROC analysis, a cutoff at 22 U/l chromogranin A was calculated. In patients with neuroendocrine tumours, much higher serum concentrations of chromogranin A than for patients with MTC (80% vs. 46%) were measured. The following sensitivities were found: chromogranin A; 46%, calcitonin 100%, CEA 52%. Furthermore, the mean values of chromogranin A concentrations correlated with the tumour mass and/or number of metastases in MTC and neuroendocrine tumours. Evaluation of follow-up studies remains to be completed; however, preliminary results showed similarities regarding the behaviour of chromogranin A and calcitonin. Despite the findings of this study, the observations could not confirm chromogranin A as a reliable marker for metastazing or recurrent MTC.

Rhenium-188-HEDP in the palliative treatment of bone metastases.

INTRODUCTION: Rhenium-188-HEDP (188Re-HEDP) is a new and attractive radiopharmaceutical for the treatment of bone pain due to metastases. As a product of a 188W/188Re generator it is convenient for clinical use. With a short physical half life of 16.9 hours and a maximal beta-energy of 2.1 MeV, it is suitable for therapy. METHODS: We investigated the influence of 188Re-HEDP on pain relief, analgesic intake and impairment of bone marrow function in 15 patients. All patients were interviewed using standardized questions before, and 1, 2, 3, 4, 8, and 12 weeks after therapy. Blood samples were drawn weekly for 12 weeks, and a blood count was performed. Patients underwent gamma camera imaging to determine the radionuclide accumulation 4, 20, and 28 hours after therapy. The patients were treated with 1600 to 3459 MBq of 188Re-HEDP. RESULTS: Patients showed an improvement of the Karnofsky performance index from 74 +/- 8% to 84 +/- 11% 12 weeks after therapy. This improvement was statistically significant (p = 0.001). Eighty percent of the patients described pain relief and reduction of analgesics. Twenty percent of the patients could discontinue their analgesics. Mean platelet count decreased from (284 +/- 84)*10(3)/microliter to (205 +/- 62)*10(3)/microliter, and mean leukocyte count from (7.5 +/- 1.5)*10(3)/microliter to (5.9 +/- 2.1)*10(3)/microliter after therapy. The maximal differences between the values of platelets and leukocytes before and after therapy were not statistically significant (p = 0.021 and p = 0.094). Prostate specific antigen decreased from 95 +/- 83 ng/ml to 41 +/- 21 ng/ml, the difference was not statistically significant (p = 0.443). The bone accumulation 4, 20, and 28 hours after therapy was 1.3 +/- 0.5%, 0.6 +/- 0.3%, and 0.45 +/- 0.2% of the injected dose of a single metastasis, and 57 +/- 17%, 15.5 +/- 2% and 11 +/- 3% in the whole body, respectively. The effective half-life of 188Re-HEDP was 15.3 +/- 3.0 hours in the bone metastases, and 11.4 +/- 2.8 hours in the whole body. This corresponds to a residence time of 0.22 +/- 0.25 hours in the bone metastases, and of 10.54 +/- 2.59 hours in the whole body. CONCLUSION: In a small patient population, 188Re-HEDP therapy for bone pain palliation was effective and was associated with minimal toxicity.

[Comparison of rhenium-188, rhenium-186-HEDP and strontium-89 in palliation of painful bone metastases]. / Vergleich von Rhenium-188-HEDP, Rhenium-186-HEDP und Strontium-89 für die palliative Schmerztherapie von Skelettmetastasen.

AIM: Several radiopharmaceuticals were compared previously with regard to the efficiency in pain palliation of bone metastases. Furthermore, first results were reported on the suitability for such kind of therapy of the generator produced radionuclide rhenium-188. METHOD: Influence of Rhenium-188-HEDP (Re-188), Rhenium-186-HEDP (Re-186) and Strontium-89 (Sr-89) on pain symptoms and bone marrow function were obtained in 44 patients (pts). These were 16 pts. with Re-188 (2943 +/- 609 MBq), 13 pts. with Re-186 (1341 +/- 161 MBq) and 15 pts. with Sr-89 (152 +/- 18 MBq) (6 woman with breast cancer and 38 men with prostata cancer). RESULTS: 81 of pts. after Re-188, 77% after Re-186 and 80% after Sr-89 reported relief of pain. The Karnofsky-Index established by pts. increased from 74 +/- 9% to 85 +/- 11% after Re-188, from 70 +/- 11% to 76 +/- 11% after Re-186 and from 62 +/- 10% to 69 +/- 10% after Sr-89. However, the difference between the pre- and the post-therapeutic value is only statistically significant in the case of Re-188 therapy (p = 0.001). A decrease of platelets of 30 +/- 14% after 2.8 +/- 0.7 for pts. treated with Re-188, of 39 +/- 20% after 3.7 +/- 1.0 weeks for pts. treated with Re-186 and of 34 +/- 26% after 4.4 +/- 1.0 weeks for pts. treated with Sr-89 compared to the value before therapy was observed. The difference was not significant between the 3 groups of pts. (p = 0.125 to 0.862). CONCLUSION: All tried radiopharmaceuticals were effective in pain palliation. The various radionuclides had no significant difference in the pain relief or the bone marrow impairment. If only the Karnofsky-Index after Re-188 HEDP seems to be a little more increase.

On the dose calculation at the cellular level and its implications for the RBE of (99m)Tc and ¹²³I.

PURPOSE: Based on the authors' previous findings concerning the radiotoxicity of(99m)Tc, the authors compared the cellular survival under the influence of this nuclide with that following exposure to the Auger electron emitter (123)I. To evaluate the relative biological effectiveness (RBE) of both radionuclides, knowledge of the absorbed dose is essential. Thus, the authors present the dose calculations and discuss the results based on different models of the radionuclide distribution. Both different target volumes and the influence of the uptake kinetics were considered. METHODS: Rat thyroid PC Cl3 cells in culture were incubated with either(99m)Tc or (123)I or were irradiated using 200 kV x-rays in the presence or absence of perchlorate. The clonogenic cell survival was measured via colony formation. In addition, the intracellular radionuclide uptake was quantified. Single-cell dose calculations were based on Monte Carlo simulations performed using Geant4. RESULTS: Compared with external radiation using x-rays (D37 = 2.6 Gy), the radionuclides (99m)Tc (D37 = 3.5 Gy), and (123)I (D37 = 3.8 Gy) were less toxic in the presence of perchlorate. In the absence of perchlorate, the amount of activity a37 that was necessary to reduce the surviving fraction (SF) to 0.37 was 22.8 times lower for (99m)Tc and 12.4 times lower for (123)I because of the dose increase caused by intracellular radionuclide accumulation. When the cell nucleus was considered as the target for the dose calculation, the authors found a RBE of 2.18 for (99m)Tc and RBE = 3.43 for (123)I. Meanwhile, regarding the dose to the entire cell, RBE = 0.75 for (99m)Tc and RBE = 1.87 for (123)I. The dose to the entire cell was chosen as the dose criterion because of the intracellular radionuclide accumulation, which was found to occur solely in the cytoplasm. The calculated number of intracellular decays per cell was (975 ± 109) decays/MBq for (99m)Tc and (221 ± 82) decays/MBq for (123)I. CONCLUSIONS: The authors' data indicate that extra-nuclear targets to Auger electrons exist, which is obvious from our dose calculations. When considering the dose to the cell nucleus, the authors found an enhanced RBE for(99m)Tc and (123)I relative to acute x-ray irradiation and pure extracellular irradiation with both radionuclides. Surprisingly, the authors did not find any radionuclide accumulation in the cell nucleus, indicating that there are additional radiosensitive targets besides the DNA. In addition, the authors demonstrated the necessity of cellular dose calculations in radiobiological experiments using unsealed radionuclides and identified the relevant parameters.

Preincubation with Sn-complexes causes intensive intracellular retention of (99m)Tc in thyroid cells in vitro.

UNLABELLED: Technetium radiopharmaceuticals are well established in nuclear medicine. Besides its well-known gamma radiation, (99m)Tc emits an average of five Auger and internal conversion electrons per decay. The biological toxicity of these low-energy, high-LET (linear energy transfer) emissions is a controversial subject. One aim of this study was to estimate in a cell model how much (99m)Tc can be present in exposed cells and which radiobiological effects could be estimated in (99m)Tc-overloaded cells. METHODS: Sodium iodine symporter (NIS)-positive thyroid cells were used. (99m)Tc-uptake studies were performed after preincubation with a non-radioactive (cold) stannous pyrophosphate kit solution or as a standard (99m)Tc pyrophosphate kit preparation or with pure pertechnetate solution. Survival curves were analyzed from colony-forming assays. RESULTS: Preincubation with stannous complexes causes irreversible intracellular radioactivity retention of (99m)Tc and is followed by further pertechnetate influx to an unexpectedly high (99m)Tc level. The uptake of (99m)Tc pertechnetate in NIS-positive cells can be modified using stannous pyrophosphate from 3-5% to >80%. The maximum possible cellular uptake of (99m)Tc was 90Bq/cell. Compared with nearly pure extracellular irradiation from routine (99m)Tc complexes, cell survival was reduced by 3-4 orders of magnitude after preincubation with stannous pyrophosphate. CONCLUSIONS: Intracellular (99m)Tc retention is related to reduced survival, which is most likely mediated by the emission of low-energy electrons. Our findings show that the described experiments constitute a simple and useful in vitro model for radiobiological investigations in a cell model.

Sodium-iodide symporter positive cells after intracellular uptake of (99m)Tc versus α-emitter 211At. Reduction of clonogenic survival and characterization of DNA damage.

PURPOSE: We evaluated the DNA damaging potential of Auger electrons emitted in the decay of (99m)Tc compared to α-particles of 211At. MATERIAL AND METHODS: The impact of (99m)Tc and 211At was monitored in a NIS-expressing rat thyroid cell model PCCl3 with varying, yet defined intra- and extracellular radionuclide distribution (using ± perchlorate). The radiotoxicity of (99m)Tc and 211At was studied by the comet assay under neutral and alkaline conditions and colony formation. RESULTS: In the presence of perchlorate, the radioactivity yielding 37% cellular survival, A37, was estimated to be (0.27 ± 0.02) MBq/ml and (450 ± 30) MBq/ml for 211At and (99m)Tc, respectively. In absence of perchlorate, cellular radiotracer uptake was similar for both radionuclides (2.2%, 2.7%), yet the A37 was reduced by 82% for the α-emitter and by 95% for (99m)Tc. Cellular dose increased by a factor of 5 (211At) and 38 (99mTc). Comet assays revealed an increased DNA damage after intracellular uptake of both radiotracers. CONCLUSIONS: The data indicate damage to the cell to occur from absorbed dose without recognizable contribution from intracellular heterogeneity of radionuclide distribution. Comet assay under alkaline and neutral conditions did not reveal any shift to more complex DNA damage after radionuclide uptake. Cellular uptake of (99m)Tc and 211At increased cellular dose and reduced clonogenic survival.