Egr-IFNγ基因治疗联合125I-脱氧尿嘧啶核苷治疗抑瘤效应的实验研究

目的 探讨Egr-IFNγ基因治疗联合放射性核素 ¹²⁵I -脱氧尿嘧啶核苷治疗方案在荷H22肝癌细胞小鼠体内抑瘤效应及机制。方法 小鼠肿瘤局部注射脂质体包裹的质粒,注射后48 h,肿瘤局部注射370kBq ¹²⁵I -UdR。观察各组小鼠治疗后不同时间肿瘤生长率;治疗后第3 天,检测肿瘤胞浆蛋白中IFNγ的表达和脾脏CTL细胞毒活性。结果 基因-放射核素治疗后第6～15天,pcDNAEgr-IFNγ+ ¹²⁵I -UdR组肿瘤生长率明显低于对照组、 ¹²⁵I -UdR组及pcDNAEgr-1+ ¹²⁵I -UdR 组;基因-放射性核素治疗后第3天,pcDNAEgr-IFNγ+ ¹²⁵I -UdR组肿瘤胞浆蛋白中可检测到IFNγ的表达,其余组肿瘤胞浆蛋白中未检测到IFNγ的表达;pcDNAEgr-IFNγ+ ¹²⁵I -UdR组小鼠脾脏CTL细胞毒活性明显高于其余组 (P<0.01)。结论 pcDNAEgr-IFNγ基因治疗联合放射性核素 ¹²⁵I -UdR治疗抑瘤效应明显优于单纯 ¹²⁵I -UdR放射性核素治疗。     

3H-TdR与125I-UdR掺入淋巴细胞增殖的比较

目的 比较 ³H-TdR与 ¹²⁵I-UdR掺入淋巴细胞的增殖效应。方法 用 ³H-TdR与 ¹²⁵I-UdR掺入法测定淋巴细胞和Daudi淋巴瘤细胞的增殖效应。结果 ³H-TdR和 ¹²⁵I-UdR在正常淋巴细胞中的掺入率分别为20.95%±1.06%和1.00%±0.04%,在Daudi淋巴瘤细胞中的掺入率分别为29.94%±4.10%和6.02%±0.73%。 ³H-TdR在细胞中的掺入率明显高于 ¹²⁵I-UdR;且 ³H-TdR和 ¹²⁵I-UdR在淋巴瘤细胞中的掺入率高于正常淋巴细胞。结论 就淋巴细胞而言,作为示踪剂 ¹²⁵I-UdR不能替代 ³H-TdR;但对于淋巴瘤细胞,能否代替 ³H-TdR有待于进一步研究。     

Cell loss from viable and necrotic tumor regions after local gamma irradiation measured by 125I-UdR

Using a tracer technique, loss of cells from perivascular and average tumor cells of the syngeneic mammary adenocarcinoma EO 771 in male C57Bl/6J mice may be measured in the living animal, by the use of 125-labelled 5-iodo-2'-deoxyuridine (125I-UdR). It was the purpose of this paper to compare measurements in vivo with those made in vitro following local 60Co-gamma irradiation in the absorbed dose range from 10 to 27.5 Gy, incorporation of radioactivity into DNA of tumor cells and activity loss from labelled tumor cells were measured externally by a special scintillation counter device. In addition, by injecting the vital dye "light green" into the mice the I-125-activity of the stained viable and unstained necrotic regions were separately measured for loss of activity following gamma irradiation. A comparison was made between radiation induced growth delay and the depression of 125I-UdR incorporation into DNA of the proliferating tumor cells. After local tumor irradiation with a dose of 27.5 Gy 60Co gamma rays an enhancement of the activity loss by 0.5% per hour was externally observed for the perivascular tumor cell population. A lower enhancement of 0.4% per hour was externally registered in the average tumor cell population. Both values were evaluated relative to sham-irradiated control tumors. The measurements on isolated tumors were in comparatively good agreement with the external values. The activity loss rate from the viable, euoxic tissue increased by 0.4% per hour after 27.5 Gy 60Co gamma rays and by 0.3% per hour in the average cell population, the latter representing a mixture of euoxic and hypoxic cells. The results demonstrate, that the external measurements are a good indicator for radiation effects under in vivo-conditions.     

125I-脱氧尿嘧啶核苷对淋巴瘤细胞Raji和Daudi的杀伤作用

目的 探讨¹²⁵I-脱氧尿嘧啶核苷(¹²⁵I-UdR)在淋巴瘤细胞Raji和Daudi中的特异性摄取及其杀伤效应。方法 测量Raji、Daudi细胞和细胞核在含不同放射性浓度¹²⁵I-UdR的培养液中培养不同时间后的活度;用噻唑蓝(MTT)实验和碘化丙啶(PI)染色周期分析评价¹²⁵I-UdR对Raji和Daudi细胞的杀伤作用。结果 Raji和Daudi细胞摄取¹²⁵I-UdR的量明显高于Na¹²⁵I对照组(P<0.05),在100kBq/ml浓度时,Raji和Daudi细胞摄取¹²⁵I-UdR的量分别为(14 414±95)和(6916±53.69)Bq/10⁶细胞,而对Na¹²⁵I的摄取量分别为(68±3.8)和(324±32.8) Bq/10⁶细胞;细胞和细胞核中¹²⁵I-UdR的量随培养基中¹²⁵I-UdR放射性浓度以及培养时间的增加而增加;¹²⁵I-UdR组的存活分数明显低于Na¹²⁵I对照组(P<0.05),以500kBq/ml浓度培养48h时, Raji和Daudi细胞¹²⁵I-UdR组的存活分数分别为(19.78±1.39)%和(43.17±2.69)%,而Na¹²⁵I组的存活分数分别为(79.10±1.79)%和(80.36±6.12)%;细胞存活分数有随培养基中放射性浓度增加而降低的趋势。结论 ¹²⁵I-UdR可被Raji和Daudi细胞特异性摄取并进入细胞核中,进而杀死细胞,其作用具有明显的时间-效应和剂量-效应关系。     

^125I-脱氧尿苷治疗膀胱癌的实验与临床研究

放射性核素通过电子俘获和（或）内转换衰变产生低能电子（〈1 keV）的俄歇效应,掺入细胞DNA后具有显著的细胞毒性。^125I-脱氧尿苷（^125I-UdR）是将^125I引入细胞核的有效载体,能特异性掺入S期细胞DNA。一系列研究表明,^125I能更多地被肿瘤细胞吸收,而不是正常的分裂细胞,从而有效地治疗恶性病变。由于膀胱为一天然囊腔,具有独特的易灌注及观察性,膀胱灌注^125I-UdR治疗膀胱癌,能高效、选择性杀伤肿瘤细胞,明显降低膀胱癌复发率,故可作为外科手术的辅助治疗手段,有希望成为一种安全、高效、不良反应小的治疗膀胱癌的新疗法。     

125I-UdR壳聚糖纳米微粒对肝癌细胞的内照射生物学效应

目的 评估¹²⁵I-UdR壳聚糖载药纳米微粒(¹²⁵I-UdR-CS-DLN)对肝癌细胞的内照射生物学效应.方法 采用激光共聚焦显微镜观察¹²⁵I-UdR-CS-DLN在肝癌细胞HepG2和人正常肝组织细胞HL-7702内的聚积和分布;通过MTT实验、流式细胞仪和单细胞凝胶电泳技术,评价内照射细胞生物学效应;采用TUNEL染色法观察兔肝原位肿瘤细胞经¹²⁵I-UdR-CS-DLN靶向治疗后的细胞凋亡.结果 纳米微粒作用30 min后,其在HepG2细胞质内的聚积大于HL-7702;当¹²⁵I-UdR-CS-DLN浓度大于37 kBq/ml时,HepG2细胞在纳米微粒作用后24、48 h的存活率显著低于HL-7702细胞(t=-4.46~6.31,P<0.05),且细胞周期G₁期阻滞明显, G₂/M期细胞明显受损;¹²⁵I-UdR-CS-DLN造成细胞DNA双链断裂的程度明显高于¹²⁵I-UdR,HepG2细胞的DNA损伤后修复能力显著低于HL-7702(Olive尾矩:t=2.94,P<0.05;彗尾DNA%:t=10.64,P<0.01);兔肝原位癌模型经介入被动靶向治疗后的TUNEL染色结果表明,¹²⁵I-UdR-CS-DLN可使兔肝原位肿瘤细胞产生明显的凋亡,而相同剂量¹²⁵I-UdR作用后肿瘤并未出现明显的凋亡.结论 ¹²⁵I-UdR-CS-DLN进入肝癌细胞的能力明显强于¹²⁵I-UdR,引起的DNA辐射损伤效应更强,可明显加剧肝癌细胞的凋亡,阻止DNA损伤修复.     

Changes in Cellularity Induced by Radiation in a Solid Tumour

The growth and cellular responses of Morris hepatoma 3924 A to a locally-administered dose of 3750 R X-rays were studied using the following parameters: (1) relative tumour volume changes; (2) tritiated thymidine (³H-TdR) incorporation into DNA; (3) tumour DNA content and (4) cellular analysis, including ³H-TdR labelling index, mitotic index, aberrant mitotic frequency and relative cell density. Before depression of tumour growth, cell proliferation is temporarily interrupted. As proliferation is reinitiated, a short-lived synchrony and prolongation of cell-cycle traverse are reflected in (a) the labelling index and mitotic index, (b) the relative cell density, and (c) the rate of incorporation of ³H-TdR into DNA. Within 4 days after radiation, cell proliferation and ³H-TdR incorporation are significantly depressed. Simultaneously there are reductions in both the relative cell density and tumour DNA contents, and these remain depressed as the tumours initiate regression. From these studies, it is apparent that the cellular responses to radiation insult occur well in advance of measurable volume changes and are observed both in tumours that continue to regress and in those that initiate regrowth.     

Selective uptake of toxic nucleoside (125IUdR) by resistant cancer

We report uptake of a thymidine analogue 125-Iodine-5-iodo-2'-deoxyuridine (125IUdR) by nude mice bearing human xenografts of choriocarcinoma or colonic cancer. When 125IUdR was given alone, uptake by intestinal tissues was 5-10 times greater than by the tumours as measured by tissue gamma counting. This ratio was reversed when hydroxyurea or cytosine arabinoside were used as inhibitors of ribonucleotide reductase and were given in combination with 5-fluorouracil or methotrexate to inhibit thymidine synthesis shortly before injecting 125IUdR. Counting the radioactivity in tissues removed 24 hours after 125IUdR gave tumour to highest normal tissue ratios of up to 15:1, but the corresponding nuclear grain counts, which is probably a more reliable indicator of selective uptake into DNA, were in excess of 100:1. The addition of unlabelled IUdR to the regimen only reduced the uptake of 125IUdR when given in relatively large amounts. For this approach to be exploited it is concluded that the tumour must be resistant at the cell level to the inhibitor of DNA synthesis either de novo or as a result of prior exposure to it. This inhibitor can then be used to block uptake of the potentially toxic nucleoside analogue by normal renewal tissues while it is taken up by the resistant cancer cells. By inhibiting synthesis of the corresponding normal nucleosides with inhibitors to which the cancer cells are not resistant, incorporation of the toxic analogues into tumour DNA was enhanced. Although 125IUdR is a convenient agent for exploring this approach and is highly cytotoxic when incorporated in DNA, the clinical potential of reverse role chemotherapy probably lies with the development of toxic non-radioactive nucleoside analogues.     

125IUDR labelling efficiency and cytotoxicity for murine tumours in vivo and in vitro

We have reviewed the literature on ¹²⁵IUDR (5-iodo-2-deoxyuridine), the DNA label of choice for cell distribution studies. In previous studies, the cytotoxicity in relation to labelling efficiency has often been inadequately investigated or reported. We have studied four syngeneic mouse tumours and compared in vitro with in vivo labelling procedures. Ascites tumours could be effectively labelled in vivo by IP injections of 60–80 Ci per mouse during 24 h, without apparent cytotoxicity. Comparable in vitro labelling was also effective, but caused a dose-related cytotoxicity, as measured by growth rate and transplantability. Comparison with cold IUDR disclosed that the toxicity was not chemical, but radiological. Our attempt to label a solid lymphoma in vivo was unsuccessful, obviously because the tumour grows too slowly. We conclude that ¹²⁵IUDR-labelling is suitable for cell distribution studies, but in view of labelling efficiency versus cytotoxicity the procedure has to be adjusted to each new tumour individually.     

3H-TdR与125Ⅰ-UdR掺入淋巴细胞增殖的比较

目的 比较3H-TdR与125Ⅰ-UdR掺入淋巴细胞的增殖效应.方法 用3H-TdR与125Ⅰ-UdR掺入法测定淋巴细胞和Daudi淋巴瘤细胞的增殖效应.结果 3H-TdR和125Ⅰ-UdR在正常淋巴细胞中的掺入率分别为20.95%±1.06%和1.00%±0.04%,在Daudi淋巴瘤细胞中的掺入率分别为29.94%±4.10%和6.02%±0.73%.3H-TdR在细胞中的掺入率明显高于125Ⅰ-UdR;且3H-TdR和125Ⅰ-UdR在淋巴瘤细胞中的掺入率高于正常淋巴细胞.结论 就淋巴细胞而言,作为示踪剂125Ⅰ-UdR不能替代3H-TdR;但对于淋巴瘤细胞,能否代替3H-TdR有待于进一步研究.     

Uptake mechanisms of L-3-[125I]iodo-alpha-methyl-tyrosine in a human small-cell lung cancer cell line: comparison with L-1

The radiolabelled amino acid analogue L-3-[125I]iodo-alpha-methyl-tyrosine (IMT) is under evaluation in brain tumours, where it reflects amino acid transport activity, but is also taken up in many other tumour types. This study investigated the uptake mechanism of IMT in tumour cells not derived from brain tumours, in comparison with the native amino acid 14C-tyrosine (Tyr) from which IMT is derived. Human GLC4 small-cell lung cancer cells in log-phase were incubated with IMT and Tyr. Tracer uptake was determined in various buffers, incubation periods, concentrations of specific amino acid transport blockers, pH and temperature. IMT uptake was very fast, reaching a plateau within 5 min, while Tyr kept on accumulating for > 60 min. Based on steady-state experiments, > 90% of IMT uptake could be attributed to amino acid transport activity. The L transport system was the most important, both for IMT and Tyr. IMT uptake into GLC4 tumour cells is almost completely the result of amino acid transport activity (especially the L system) and is very similar to Tyr uptake. Therefore, also outside the brain, IMT is a metabolic tracer that may reflect the increased amino acid transport that is characteristic for malignant tumours.     

An in vitro 125IUdR-release assay for measuring the kinetics of cell death.

A radionuclide release assay for measuring the in vitro kinetics of cell death has been developed. CHO cells were labelled for 24 h with 3.0 hBq/ml of [125I] iododeoxyuridine (125IUdR) and the fate of the labelled cells and their progeny was monitored at daily intervals by measuring the rate of 125I release. Prelabelling with 125IUdR did not alter the plating efficiency, the doubling time or the selection of mitotic cells. The rate of 125I release from labelled (but otherwise untreated) CHO cells was approximately equal to 4% day. Treatment with a lethal dose of X-rays (30 Gy), heat (46 degrees C, 1 h), cold (-90 degrees C, 1 h) or the antibiotic Geneticin (300 micrograms/ml, continuously) resulted in the release of greater than 99% the 125I activity associated with the cells. Cell death was rapid after heating or freezing, and delayed after treatment with X-rays or Geneticin. The results illustrate the efficacy of the 125I release assay for measuring the kinetics of cell death in mammalian tissue culture cells.     

Impact of increased cell loss on the repopulation rate during fractionated irradiation in human FaDu squamous cell carcinoma growing in nude mice

PURPOSE: To determine the impact of increased necrotic cell loss on the repopulation rate of clonogenic cells during fractionated irradiation in human FaDu squamous cell carcinoma in nude mice. MATERIALS AND METHODS: FaDu tumours were transplanted into pre-irradiated subcutaneous tissues. This manoeuvre has previously been shown to result in a clear-cut tumour bed effect, i.e. tumours grow at a slower rate compared with control tumours. This tumour bed effect was caused by an increased necrotic cell loss with a constant cell production rate. After increasing numbers of 3-Gy fractions (time intervals 24 or 48 h), graded top-up doses were given to determine the dose required to control 50% of the tumours (TCD50). All irradiations were given under clamp hypoxia. RESULTS: With increasing numbers of daily fractions, the top-up TCD50 decreased from 37.9 Gy (95% CI: 31; 45) after single dose irradiation to 14.1 Gy (8; 20) after irradiation with 15 fractions in 15 days. Irradiation with 18 daily 3-Gy fractions controlled more than 50% of the tumours without a top-up dose. After irradiation with six fractions every second day, the top-up TCD50 decreased to 26.9 Gy (22; 32). No further decrease of the TCD50 was observed after 12 and 18 irradiations every second day. Assuming a constant increase of TCD50 with time, the calculated doubling time of the clonogenic tumour cells (Tclon) was 7.8 days (4.4; 11.3). The Tclon calculated for FaDu tumours growing in pre-irradiated tissues was significantly longer (p=0.0004) than the Tclon of 5.1 days (3.7; 6.5) determined under the same assumptions in a previous study for FaDu tumours growing in normal subcutaneous tissues. CONCLUSIONS: Increased necrotic cell loss by pre-irradiation of the tumour bed resulted in longer clonogen doubling times during fractionated radiotherapy of human FaDu squamous cell carcinoma. This implies that a decreased necrotic cell loss might be the link between reoxygenation and repopulation demonstrated previously in the same tumour model.     

An in Vitro 125IUdR-release Assay for Measuring the Kinetics of Cell Death

Summary

A radionuclide release assay for measuring the in vitro kinetics of cell death has been developed. CHO cells were labelled for 24 h with 3·0 hBq/ml of [¹²⁵I] iododeoxyuridine (¹²⁵IUdR) and the fate of the labelled cells and their progeny was monitored at daily intervals by measuring the rate of ¹²⁵I release. Prelabelling with ¹²⁵IUdR did not alter the plating efficiency, the doubling time or the selection of mitotic cells. The rate of ¹²⁵I release from labelled (but otherwise untreated) CHO cells was ≈ 4% day. Treatment with a lethal dose of X-rays (30 Gy), heat (46°C, 1 h), cold (? 90°C, 1 h) or the antibiotic Geneticin® (300 μg/ml, continuously) resulted in the release of > 99% the ¹²⁵I activity associated with the cells. Cell death was rapid after heating or freezing, and delayed after treatment with X-rays or Geneticin. The results illustrate the efficacy of the ¹²⁵I release assay for measuring the kinetics of cell death in mammalian tissue culture cells.     

125I-脱氧尿嘧啶核苷-壳聚糖载药纳米微粒的研制和鉴定

目的 研制直径100～200 nm的¹²⁵I-脱氧尿嘧啶核苷-壳聚糖载药纳米微粒( ¹²⁵I-UdR-CS-DLN),并进一步分析其药物缓释性能和肿瘤靶向性。方法 采用离子交联法制备CS纳米微粒,以单因素分析和正交试验优化制备条件和工艺;用动态透析法分析其释放特性;激光共聚焦显微镜观察其肿瘤靶向性。结果 按照CS浓度1 g/L,搅拌速度600 r/min,TPP浓度2 g/L,相对分子质量为3×10³的条件下得到平均粒径(70.39±5.12) nm的纳米微粒(PDI为0.16±0.012)。透射电镜观察其外观为规整的球形,大小均匀,分散度较好。在投药量为2.96 MBq/ml、pH值为5的条件下, ¹²⁵I-UdR-CS-DLN的载药量1253.55 MBq/g,包封率42.35%,具有明显的缓释作用。激光共聚焦显微镜观察结果证明肿瘤细胞在2 h内摄入的纳米粒子明显多于正常细胞。结论 成功制备了直径为(127.81±15.25) nm (PDI 为0.240±0.035)的 ¹²⁵I-UdR-CS-DLN,确定了最佳工艺条件。所制备的纳米粒子具有典型的长效缓释制剂特性,并具有肿瘤细胞被动靶向性,为 ¹²⁵I-UdR应用于肿瘤内照射治疗提供了更有效的途径。     

Impact of preirradiation of the tumour bed on cell production rate and cell loss of human FaDu squamous cell carcinoma growing in nude mice.

PURPOSE: To explore whether the tumour bed effect (TBE) in FaDu squamous cell carcinoma growing in nude mice is caused by a reduced tumour cell production rate and/or by increased tumour cell loss. MATERIALS AND METHODS: Human FaDu tumours were studied in NMRI nude mice. The volume doubling time (VDT) between 100 and 400 mm3 was determined for tumours in unirradiated subcutaneous (sc) tissues (group 1), tumours in sc tissues preirradiated with 12.5 Gy (group 2), tumours irradiated in situ with 12.5 Gy (group 3), and tumours from group 3 re-transplanted into unirradiated sc tissues (group 4). Labelling index (LI), potential doubling time (Tpot), relative necrotic area and apoptotic index (AI) were evaluated in tumours from groups 1 and 2. RESULTS: The median VDT were 2.6 days (95% CI 2-4) in group 1 and 7.0 days (4-15) in group 2 (p<0.001). The VDT were not significantly different between groups 2 and 3, and group 1 and 4. In groups 1 and 2, the Tpot values (3.1 +/- 0.6 days (SD) versus 2.9 +/- 0.5 days) and the LI were identical (10 +/- 1.5%). The median relative necrotic area was significantly larger in group 2 (37% [23-42]) compared with group (6% [0.3-27]). The apoptotic index was low (0.2%) and did not differ between groups 1 and 2. CONCLUSIONS: The results indicate that the TBE in FaDu squamous cell carcinoma is not caused by a reduced cell production rate in the viable tumour compartment. Rather, the TBE reflects a decreased viable tumour cell compartment due to increased cell loss. Necrosis appears to be the major component of the tumour bed induced cell loss in FaDu tumours, whereas apoptosis has no impact on the TBE in this model.     

Impact of preirradiation of the tumour bed on cell production rate and cell loss of human FaDu squamous cell carcinoma growing in nude mice

Purpose: To explore whether the tumour bed effect (TBE) in FaDu squamous cell carcinoma growing in nude mice is caused by a reduced tumour cell production rate and/or by increased tumour cell loss. Materials and methods: Human FaDu tumours were studied in NMRI nude mice. The volume doubling time (VDT) between 100 and 400mm3 was determined for tumours in unirradiated subcutaneous (sc) tissues (group 1), tumours in sc tissues preirradiated with 12.5Gy (group 2), tumours irradiated in situ with 12.5Gy (group 3), and tumours from group 3 re-transplanted into unirradiated sc tissues (group 4). Labelling index (LI), potential doubling time (T pot), relative necrotic area and apoptotic index (AI) were evaluated in tumours from groups 1 and 2. Results: The median VDT were 2.6 days (95% CI 2-4) in group 1 and 7.0 days (4-15) in group 2 (p < 0.001). The VDT were not significantly different between groups 2 and 3, and group 1 and 4. In groups 1 and 2, the T pot values (3.1 0.6 days (SD) versus 2.9 0.5 days) and the LI were identical (10 1.5%). The median relative necrotic area was significantly larger in group 2 (37% [23-42]) compared with group (6% [0.3-27]). The apoptotic index was low (0.2%) and did not differ between groups 1 and 2. Conclusions : The results indicate that the TBE in FaDu squamous cell carcinoma is not caused by a reduced cell production rate in the viable tumour compartment. Rather, the TBE reflects a decreased viable tumour cell compartment due to increased cell loss. Necrosis appears to be the major component of the tumour bed induced cell loss in FaDu tumours, whereas apoptosis has no impact on the TBE in this model.     

Proliferation kinetic examinations on an experimental renal tumour with high portion of hypoxic tumour cells (author's transl)

By autoradiographic examination by means of pulse labelling with 3H-thymidine cell cycle times were determined in solid Ehrlich carcinomas (early carcinomas) in order to obtain comparative values for irradiated tumours. The subcells of the initially labelled cells near the capillaries were found to distribute themselves in the course of 48 h over the whole tumour and to emigrate to hypoxic tumour areas.     

Cytotoxicity of 125 I-oestrogen decay in non-oestrogen receptor-expressing human breast cancer cells,MDA-231 and oestrogen receptor-expressing MCF-7 cells

Purpose : To compare the cytotoxicity of 125 I-oestrogen (E-17 α [ 125 I]iodovinyl-11 β methoxyoestradiol or 125 IVME2) decay accumulation in human breast adenocarcinoma cells that do not express oestrogen receptor (ER) (MDA-231 cells) with human breast adenocarcinoma cells that do express ER (MCF-7 cells). Materials and methods : MDA-231 cells were labelled with 125 IVME2 or [ 125 I]iododeoxyuridine (125 IdU), frozen for decay accumulation, thawed and then plated for colony formation. γ-irradiation survival was also determined. A whole-cell 3 H-oestrogen-binding assay and a specific-binding assay were used to detect ER. Results : No MDA-231 cell killing by accumulated 125 IVME2 decays (up to 440 dpc) was observed but ER-positive MCF-7 cells were killed by 125 IVME2 (D o =28 dpc). MDA-231 cells were not significantly more radioresistant to γ-rays (D o =1.7Gy for MDA-231 cells; 1 Gy for MCF-7 cells) or to 125 IdU decays (D o = 44dpc for MDA-231 cells; 30 dpc for MCF-7 cells). No ER were detected in MDA-231 cells. Conclusions : ER-negative cells, MDA-231, are not killed by 125 IVME2 decay accumulation. It is speculated that without ER (required to translocate the 125 IVME2 to its nuclear target), formation of the 125 IVME2-ER-DNA oestrogen-response element (ERE) complex and subsequent specific irradiation of the DNA at the ERE cannot occur. These results support the hypothesis that the nuclear genome is a critical target for radiation-induced cell death.