Post-irradiation Administration Research Articles

Nicaraven Exerts a Limited Effect on Radiation-Induced Inhibition of Tumor Growth in a Subcutaneous Murine Tumor Model.

Nicaraven selectively protects normal tissue from radiation-induced injury. To further develop the clinical application of nicaraven for mitigating the side effects of cancer radiotherapy, we investigated the potential effect of nicaraven administration in radiation-induced inhibition of tumor growth. A subcutaneous tumor model was established in mice by the injection of Lewis lung cancer cells at the back of the chest. X-ray radiation was delivered to the thoracic area and different doses of nicaraven (0, 20, 50, 100 mg/kg) were administrated intraperitoneally pre- or post-irradiation. The tumor size was measured every other day. Mice were euthanized on day 30, and the tumor weight and the levels of cytokines in tumor tissue were measured. Pre- or post-irradiation administration of nicaraven up to a dose of 100 mg/kg did not significantly diminish the radiation-induced inhibition of tumor growth, but post-irradiation administration of 20 and 50 mg/kg nicaraven resulted in relatively lower tumor weight. The levels of IL-1β, IL-6, IL-10, MCP-1, MIP-2a, TGF-β1, VEGF, p53, p21, cyclin D1 and caspase-3 in tumor tissue did not change by nicaraven administration and were not significantly associated with the tumor weights. According to our experimental data, nicaraven will not significantly diminish the radiation-induced inhibition of tumor growth, even with pre-irradiation administration at a high dose.

Optimization on the dose and time of nicaraven administration for mitigating the side effects of radiotherapy in a preclinical tumor-bearing mouse model.

Radiation-induced lung injury (RILI) is one of the serious complications of radiotherapy. We have recently demonstrated that nicaraven can effectively mitigate RILI in healthy mice. Here, we further tried to optimize the dose and time of nicaraven administration for alleviating the side effects of radiotherapy in tumor-bearing mice. A subcutaneous tumor model was established in the back of the chest in C57BL/6N mice by injecting Lewis lung cancer cells. Therapeutic thoracic irradiations were done, and placebo or different doses of nicaraven (20, 50, 100 mg/kg) were administrated intraperitoneally pre-irradiation (at almost 5-10 min before irradiation) or post-irradiation (within 5 min after irradiation). Mice that received radiotherapy and nicaraven were sacrificed on the 30th day, but control mice were sacrificed on the 15th day. Serum and lung tissues were collected for evaluation. Nicaraven significantly decreased the level of CCL8, but did not clearly change the levels of 8-OHdG, TGF-β, IL-1β, and IL-6 in serum. Besides these, nicaraven effectively decreased the levels of TGF-β, IL-1β, and SOD2 in the lungs, especially by post-irradiation administration with the dose of 20 mg/kg. Although there was no significant difference, the expression of SOD1, 53BP1, and caspase 3 was detected lower in the lungs of mice received nicaraven post-irradiation than that of pre-irradiation. According to our data, the administration of nicaraven at a relatively low dose soon after radiotherapy will be recommended for attenuating the side effects of radiotherapy.

Experimental Study of the Efficacy of Sodium Deoxyribonucleate Used in Combination with Co-Transplantation of Mesenchymal and Hematopoietic Stem Cells after Exposure to γ-Radiation.

We studied the possibility of using sodium deoxyribonucleate (Derinat) for improving the efficiency of co-transplantation of mesenchymal (MSC) and hematopoietic stem cells (HSC) to female F1(CBA×C57BL/6) mice with bone marrow aplasia caused by exposure to γ-radiation. It was found that immunomodulator Derinat enhanced the effect of co-transplantation, in particular, triple post-irradiation administration of Derinat accelerated hematopoiesis recovery judging from the parameters of peripheral blood, total cellularity of the bone marrow and spleen, and animal survival. Single or double administration of Derinat prior to irradiation was ineffective. The optimal result was obtained when the following scheme was applied: MSC→HSC with an interval of 48 h starting during the first hours after irradiation and triple administration of Derinat (in 10-15 min, 3 and 7 days after irradiation) in a dose of 3 mg/mouse.

Influence of common dietary supplements (curcumin, andrographolide, and d-limonene) on the radiobiological responses of p53-competent colonic cancer epithelial cells

Purpose The main goal of the research was to determine whether commercially available common dietary phytochemical supplements (curcumin, andrographolide, and d-limonene) have radiomodulatory effects on p53-competent human colonic epithelial cells. Methods Clonogenic survival assays were used to characterize effects of the phytochemicals on cultured colonic epithelial cells (HCT116 p53+/+) in direct irradiation or upon receipt of irradiated-cell conditioned media (for bystander effects). In direct irradiation, feeding regimen experiments included compound administration pre- and post-irradiation, which was used as a basis to define effects as radioprotective and radiomitigative, respectively. In the bystander effect experiments, either donor or recipient cell cultures were fed with the phytochemicals and bystander-induced clonogenic cell death was quantitatively evaluated. Dose challenge was in the range of 0.5 − 5 Gy using the gamma source (Cs-137). Results Curcumin, andrographolide, and d-limonene appeared to not exhibit radioprotective and radiomitigative properties in HCT116 p53+/+ cells. D-limonene was found to induce radiosensitization in post-irradiation administration. All three compounds appeared not to modulate the radiation-induced bystander signal production and response in HCT116 p53+/+ cells. Conclusions Curcumin, andrographolide, and d-limonene are known to have many chemoprotective benefits. This work shows that they, however, did not protect colonic epithelial HCT116 p53+/+ cells from radiation killing. As HCT116 p53+/+ cells are tumourigenic in nature, this finding implies that these three dietary compounds would not reduce the killing efficacy of radiation in gastrointestinal tumorigenesis. The post-irradiation radiosensitizing effect of d-limonene was an intriguing observation worth further investigation.

Экспериментальное исследование эффективности использования дезоксирибонуклеата натрия в сочетании с котрансплантацией мезенхимных и гемопоэтических стволовых клеток после гамма-облучения

We studied the possibility of using sodium deoxyribonucleate (Derinat) for improving the efficiency of co-transplantation of mesenchymal (MSC) and hematopoietic stem cells (HSC) to female F1(CBA×C57BL/6) mice with bone marrow aplasia caused by exposure to γ-radiation. It was found that immunomodulator Derinat enhanced the effect of co-transplantation, in particular, triple post-irradiation administration of Derinat accelerated hematopoiesis recovery judging from the parameters of peripheral blood, total cellularity of the bone marrow and spleen, and animal survival. Single or double administration of Derinat prior to irradiation was ineffective. The optimal result was obtained when the following scheme was applied: MSC→HSC with an interval of 48 h starting during the first hours after irradiation and triple administration of Derinat (in 10-15 min, 3 and 7 days after irradiation) in a dose of 3 mg/mouse.

Pharmacological activation of the EDA/EDAR signaling pathway restores salivary gland function following radiation-induced damage.

Radiotherapy of head and neck cancers often results in collateral damage to adjacent salivary glands associated with clinically significant hyposalivation and xerostomia. Due to the reduced capacity of salivary glands to regenerate, hyposalivation is treated by substitution with artificial saliva, rather than through functional restoration of the glands. During embryogenesis, the ectodysplasin/ectodysplasin receptor (EDA/EDAR) signaling pathway is a critical element in the development and growth of salivary glands. We have assessed the effects of pharmacological activation of this pathway in a mouse model of radiation-induced salivary gland dysfunction. We report that post-irradiation administration of an EDAR-agonist monoclonal antibody (mAbEDAR1) normalizes function of radiation damaged adult salivary glands as determined by stimulated salivary flow rates. In addition, salivary gland structure and homeostasis is restored to pre-irradiation levels. These results suggest that transient activation of pathways involved in salivary gland development could facilitate regeneration and restoration of function following damage.

Effect of Tirapazamine and Mild Temperature Hyperthermia on the Recovery from Radiation-Induced Damage in Pimonidazole-Unlabeled Quiescent Tumor Cell Population

The aim in this study is to examine the effect of tirapazamine (TPZ) and mild temperature hyperthermia (MTH) on the repair of radiation-induced damage in pimonidazole-unlabeled quiescent (Q) tumor cells. Labeling of proliferating (P) cells in C57BL/6J mice bearing EL4 tumors was achieved by continuous administration of 5-bromo-2-deoxyuridine (BrdU). Tumors were irradiated with γ-rays at 1 h after the administration of pimonidazole followed by TPZ treatment or MTH. Twenty-four hours later, assessment of the responses of Q and total (= P + Q) cells were based on the frequencies of micronucleation and apoptosis using immunofluorescence staining for BrdU. The response of the pimonidazole-unlabeled tumor cell fractions was assessed by means of apoptosis frequency using immunofluorescence staining for pimonidazole. With γ-rays only, the pimonidazole-unlabeled cell fraction showed significantly enhanced radio-sensitivity compared with the whole cell fraction more remarkably in Q cells than total cells. However, a significantly greater decrease in radio-sensitivity in the pimonidazole-unlabeled than the whole cell fraction, evaluated using a delayed assay, was more clearly observed in Q cells than total cells. Post-irradiation MTH more remarkably repressed the decrease in radio-sensitivity in the Q cell than the total cells. Post-irradiation TPZ administration produced a large radio-sensitizing effect on both total and Q cells, especially on Q cells. On the other hand, in pimonidazole-unlabeled cell fractions in both total and Q cells, TPZ suppressed the reduction in sensitivity due to delayed assay much more efficiently than MTH, whereas no radio-sensitizing effect was produced. Not only through suppressing the recovery from radiation-induced damage but also through radio-sensitizing effect, post-irradiation TPZ administration is very useful for repressing the increase in the difference in radio-sensitivity due to the delayed assay not only between total and Q tumor cells but also between the pimonidazole-unlabeled and the whole cell fractions within the total and Q tumor cells.

Protection of Mice against X-ray Injuries by the Post-irradiation Administration of Inosine-5'-monophosphate

The aim of the present study was to investigate the radiation modulating properties of inosine-5'-monophosphate (IMP). Mice injected introperitoneally (i.p.) with IMP 15 minutes after irradiation with a lethal irradiation dose of 7 Gy have better survival rates comparative to irradiated mice non treated with IMP. The dose reduction factor of the IMP is 1.22. Using a hematologdical test we demonstrated that administration of IMP alleviates the symptoms of radiation-induced leukopenia and thrombocytopenia. The DNA damage in bone marrow and thymus cells of irradiated mice was measured by flow cytofluorometry and micronucleus test (MN-test). The tests show that i.p. administration of IMP to irradiated animals leads to a significant reduction of the DNA damage level. In this paper we show that IMP substantially modulates the damaging effects of ionizing radiation protecting irradiated mice and it is a promising agent for a treatment of leukopenia.

POLY MVA – a dietary supplement containing α-lipoic acid palladium complex, enhances cellular DNA repair

POLY MVA, a commercially available health supplement containing α-lipoic acid palladium complex, was evaluated for its potential to enhance the repair of radiation-induced DNA damage. Treatment of human peripheral blood leukocytes with POLY MVA immediately after radiation exposure enhanced the repair of cellular DNA as revealed by comet assay. Post-irradiation administration of POLY MVA to mice resulted in faster cellular DNA repair, as revealed from the increased Cellular Repair Index (CRI) of blood leukocytes and bone marrow cells. Exposure of mice to 2 Gy whole-body gamma radiation resulted in the formation of strand breaks of cellular DNA of bone marrow cells, and the unrepaired strand breaks eventually lead to the formation of micronuclei in the reticulocytes. Administration of POLY MVA under post-irradiation conditions showed a decrease in the number of micronucleated reticulocytes. Thus, the studies under ex vivo and in vivo conditions revealed that the enhancement of DNA repair could be one of the mechanisms underlying radioprotection by POLY MVA.

Acceleration of Regeneration of Mucosa in Small Intestine Damaged by Ionizing Radiation Using Anabolic Steroids

Damage to intestine is a serious problem after accidental radiation exposure. To examine substances to ameliorate damage by postirradiation administration, we focused on the regeneration process after irradiation of the intestine. Using experimental systems, the effects of clinically used sex hormones on regeneration were compared. An anabolic steroid, nandrolone (19-nortestosterone), stimulated proliferation in IEC-6 epithelial cells. A single injection of 19-nortestosterone ester with prolonged action into mice 24 h after abdominal irradiation at a lethal dose of 15.7 Gy showed significant life-saving effects. Regeneration indicators such as microcolonies of BrdU-incorporated cells at day 5 and c-myb mRNA expression levels at day 4 were enhanced by 19-nortestosterone administration. In contrast, high concentrations of estradiol inhibited growth of IEC-6 cells. Treatment of abdominally irradiated mice with estradiol ester decreased levels of regeneration indicators and survival. These results suggest the effectiveness of the anabolic steroid as well as the importance of manipulation of steroid receptors in the recovery of mucosa damaged by radiation.

Effect of silver nanoparticle and glycyrrhizic acid (SN-GLY) complex on repair of whole body radiation-induced cellular DNA damage and genomic instability in mice

Silver nanoparticles (SN) were redispersed in aqueous solution of Pluronic F127 and complexed with the phytoceutical, glycyrrhizic acid (GLY) to obtain SN-GLY complex. The ability of the SN-GLY complex to offer protection against ionising radiation in post-irradiation scenarios was evaluated in ex vivo and in vivo models using Swiss albino mice. Treatment of mouse blood leucocytes with SN-GLY immediately after 4 Gy gamma radiation exposure, ex vivo, enhanced the rate of repair of cellular DNA damages as revealed by comet assay. Exposure of mice to 4 Gy whole body gamma radiation induced formation of strand breaks in cellular DNA and the unrepaired double strand breaks eventually caused the formation of micronuclei. The post-irradiation administration of SN-GLY resulted in a faster decrease in the comet parameters indicating enhanced cellular DNA repair process and reduction in micronucleus formation. Thus the studies showed effective radiation protection by SN-GLY in post-irradiation conditions.

Protection of mice against X-ray injuries by the post-irradiation administration of guanosine and inosine

Purpose: To examine the radioprotective action of guanosine (Guo) and inosine (Ino) administered to mice after irradiation with X-rays.Materials and methods: Survival of mice exposed to lethal and sublethal doses of X-rays was studied. Peripheral blood cells were counted using a light microscope. The damage to bone marrow cells was assessed by micronucleus (MN) test. Damage and repair of DNA in blood leukocytes were estimated using the comet assay.Results: Mice injected intraperitoneally (i.p.) with Guo or Ino (∼30 μg g−1, i.e., ∼0.6 mg per 20-g mouse) 15 min after acute whole-body irradiation with 7 Gy recovered from X-ray injury. On the 30th day after irradiation, 50 and 40% of mice injected with Guo and Ino, respectively, remained alive. The dose reduction factor (DRF) was 1.23 for Guo and 1.15 for Ino. The protective effect gradually decreased as the time interval between the irradiation and injection was increased to 3, 5, 8 h. Guo and Ino facilitated the restoration of peripheral blood cell counts. These compounds protected bone marrow cells from damage and normalized erythropoiesis. Guo and Ino contributed to a more rapid and complete repair of DNA in mouse leukocytes irradiated both in vitro and in vivo.Conclusion: Guo and Ino introduced shortly after irradiation reduce leukopenia and thrombocytopenia and offer promise as therapeutic agents for treatment of radiation injuries.

The myeloprotective effect of medroxyprogesterone acetate in an irradiated animal model.

In this study we evaluated the effect of medroxyprogesterone acetate (MPA) and its major ingredients on protection of the hematopoietic organs against radiation damage. One group of mice was given saline as placebo and the other groups were given MPA. Mice were injected with MPA (10 mg/kg) or saline 10 days before or after a single 8 Gy whole body cobalt irradiation. On day 14 the mice were sacrificed and their bone marrow transplanted to recipient mice. Ten days after the transplantation, spleen colony formation was investigated in mice. Administration of MPA with irradiation increased the formation of the spleen colony. Statistically significant enhancement of the spleen colony formation was found in mice treated with MPA repeatedly, as compared with those treated with placebo (P < 0.001). No significant difference in Spleen Colony Forming Unit (CFU-S) numbers was observed between pre-and post-radiotherapy administration of MPA (P = 0.216). It is an important observation that no significant difference was observed in CFU-S numbers between pre- and post-irradiation administration of MPA.

Mechanism of protection against radiation-induced DNA damage in plasmid pBR322 by caffeine

Purpose : Caffeine (1,3,7-trimethyl xanthine), a dietary component, has been shown to have widely varying effects on DNA damage induced by UV and ionizing radiation, depending upon pre- or post-irradiation administration and its concentration. Caffeine administered post-UV irradiation is known to inhibit enzymatic repair of DNA lesions, leading to potentiation of damage, whereas its presence before or during irradiation elicits protection in a wide range of test systems: bacteria, cultured human cells, plant seeds and mouse. The purpose of this study is to test whether caffeine present during γ-irradiation of plasmid DNA, a system devoid of replication and repair, could elicit protection by scavenging free radicals. Materials and methods : Plasmid pBR322 DNA was exposed to γ-radiation in the presence or absence of caffeine at a dose-rate of 1.20 Gy min -1 and damage measured as single-strand breaks. To understand the mechanisms of the observed protection, especially under oxic conditions, reaction of caffeine with superoxide radical (O 2 -) , hydrogen peroxide (H 2 O 2) and the deoxyribose peroxyl radical (ROO •) were studied. Results : Irradiation of pBR322 was observed to induce a dose-dependent increase in single-strand breaks. Caffeine itself did not induce strand breaks but reduced radiation-induced strand breaks at micromolar to millimolar concentrations. Caffeine has been shown to react with the radiation-derived oxidants. The reaction rate constants observed were 7.5 ×10 1 M -1 s -1 with O 2 - 1.05 ×10 8 M -1 s -1 with ROO •and 8.8 ×10 1 M -1 s -1 with H 2 O 2. Conclusions : Caffeine effectively protects DNA against ionizing radiation in a system devoid of repair and replication machinery. Thus, DNA protection shown by caffeine is possibly due to the scavenging of radiation-derived primary as well as secondary reactive oxygen species, and this physicochemical protective pathway possibly pre-empts any subsequent inhibitory effect of caffeine on the enzymatic repair of DNA.

The Prophylactic and Therapeutic Effects of Glutamine- and Arginine-Enriched Diets on Radiation-Induced Enteritis in Rats

Background. Recent studies indicated that glutamine and arginine support the mucosal barrier in several ways. This experimental study hypothesized that administration of glutamine- and arginine-enriched diets before abdominal radiation therapy would provide a radioprotective effect on intestinal mucosa, and this would augment the therapeutic effectiveness provided by postirradiation administration.Materials and methods. A rat model of radiation enteritis was designed with a single dose of 1100 cGy to the abdomen. Thirty-five rats were randomized into five groups of seven. A 7-day glutamine-enriched diet for Group I and a 7-day arginine-enriched diet for Group II were administered both pre- and postradiation. For Groups III and IV, the same glutamine and arginine diets were given, respectively, postradiation only. Group V was fed a glutamine- and arginine-free diet and was the control group. The rats underwent laparotomy for culture of mesenteric lymph nodes and removal of segments of ileum, jejenum, and colon for microscopic examination.Results. Bacterial translocation was significantly higher in Group V (P < 0.05), while intestinal villus count and villus height were significantly higher in all of the groups fed glutamine and arginine when compared with the control group (P < 0.0001 and P < 0.05, respectively).Conclusion. Both arginine- and glutamine-enriched diets have protective effects on gut mucosa in the postirradiation state; however, pre- and postirradiation administration together does not provide superior protection versus postradiation administration alone.

Effects of Bioreductive Agents, Tirapazamine and Mitomycin C, on Quiescent Cell Populations in Solid Tumors, Evaluated by Micronucleus Assay

Mice bearing transplantable solid tumors received 10 intraperitoneal administrations of 5‐bromo‐2′‐deoxyuridine (BrdU) to label the proliferating (P) tumor cells, and were then irradiated with 60Cogamma‐rays or injected with cis‐diamminedichloroplatinum (II) (cisplatin). The tumor cells were isolated and incubated with cytochalasin‐B (a cytokinesis blocker). The micronucleus (MN) frequency in the cells without BrdU labeling, which were regarded as quiescent (Q) cells in the tumor, was determined using immunofluorescence staining for BrdU. The MN frequency in the total (P+Q) tumor cell population was determined from tumors that were not pretreated with BrdU. Pretreatment with tirapazamine, a bioreductive agent, could enhance the sensitivity of tumor cells, including Q cells, to radiation more markedly than mitomycin C pretreatment as judged from an in vivo assay immediately after irradiation. Post‐irradiation administration of tirapazamine produced a large post‐irradiation radiosensitizing effect on both the total and Q tumor cell populations in vivo. Cisplatin treatment combined with tirapazamine demonstrated that tirapazamine also has a chemosensitizing potential for both the total and Q tumor cell populations. We confirmed that the sensitivity of Q cell populations to radiation and chemotherapy using cisplatin can be enhanced by combined treatment with tirapazamine.

Effects of postirradiation carboxymethylglucan administration in mice

The hemopoiesis-enhancing ability of a soluble glucan derivative, i.e. carboxymethylglucan (CMG), was investigated in gamma-irradiated mice. Attention was focused on the usefulness of its single or repeated postirradiation administration. CMG was administered i.p. at (a) single dose of 6 mg 2 h postirradiation, (b) four 6 mg doses in the first 4 days postirradiation, (c) four 1.5 mg doses at the same time intervals. Indices of granulopoiesis and inflammatory side effects (liver weight increase and hepatic granulomas) were investigated in mice irradiated with a sublethal dose of 7 Gy. All three CMG-treated groups of mice were found to exhibit enhanced hemopoietic recovery in comparison with the controls. Although the mice repeatedly given the 6 mg CMG doses showed the most rapid recoveries of all the evaluated parameters of granulopoiesis, the most pronounced hepatic side effects were found in these mice, too. When survival of mice was recorded in lethally (9 Gy) irradiated animals, the best protective response were obtained following the repeated administration of the 1.5 mg CMG dose, the survival by day 30 in this group being significantly higher not only in comparison with the controls but also with the mice repeatedly given the 6 mg dose of CMG. The results suggest that the postirradiation CMG administration can be useful for enhancing radiation suppressed hemopoiesis. However, repeated larger CMG doses may produce side effects which compromise the overall survival of irradiated mice.

Postirradiation administration of adenosine monophosphate combined with dipyridamole reduces early cellular damage in mice

The administration of dipyridamole and adenosine 5′-monophosphate (AMP) to mice 5 to 25 min after 1 Gy of total-body gamma irradiation was found to decrease cellular damage, as indicated by the thymidine level in plasma and the amount of saline soluble polynucleotides in the thymus. The drug combination used did not influence similar cytotoxic effects of hydrocortisone. Furthermore, it was shown that the addition of dipyridamole and AMP to in vitro irradiated suspensions of thymocytes enhanced the rejoining processes of DNA strand breaks. Receptor-mediated action of extracellular adenosine may be responsible for the therapeutic effects observed.

Glutathione isopropyl ester (YM737) inhibits the progression of X-ray-induced cataract in rats.

The effect of glutathione (GSH) isopropyl ester on the progression of X-ray-induced cataract was investigated in rats. Intraperitoneal administration of 20 mg/kg GSH isopropyl ester, three times weekly, 1 day after a single irradiation dose delayed the progression of X-ray-induced cataracts significantly. The amount of non-protein SH groups and the Na+/K+ ratio in the lenses of drug-treated rats were maintained at the normal levels even 27 weeks after irradiation. Posttreatment with the drug resulted in a significantly lower level of malondialdehyde in the irradiated lenses than in the nontreated lenses. When 500 mg/kg GSH-isopropyl ester was administered by i.p. injection to normal rats, the GSH-ester was detected in plasma and aqueous humor after 15 min. In the lenses of the GSH-isopropyl ester-injected rats, the GSH level was 120% of that in the non-treated rats after 4 h, suggesting that GSH-isopropyl ester is transported from the aqueous humor to the lens and there converted to GSH after about 4 h. Our observations lead us to conclude that the delay of X-ray-induced lens opacity progression is due to maintenance of normal lenticular GSH levels achieved by post-irradiation administration of GSH-isopropyl ester. However, continuous administration of 100 mg/kg after irradiation had no effect on the progression of cataracts induced by X-rays.

Radiosensitization of cultured human colon adenocarcinoma cells by 5-fluorouracil: Effects on cell survival, DNA repair, and cell recovery

Although 5-fluorouracil (5-FU) is commonly used in conjunction with radiotherapy to treat gastrointestinal malignancies, the molecular mechanisms underlying the clinically observed therapeutic advantage of this combination of agents have not been clearly established. The present in vitro studies addressed the possibility that the radiosensitization of log-phase cultured human colon adenocarcinoma cells by postirradiation administration of 5-FU was accompanied by an interference either with the rejoining of radiation-induced DNA double-strand break (DSB's) or with recovery from potentially lethal damage (PLD). Significantly more killing was observed in cells exposed to γ-rays (1–6 Gy) and then treated with 5-FU (100 w μg/mL; 0.77 mM) for 1 hr at 37°C than in cells given γ-rays but not 5-FU; essentially, the survival curve shoulder was removed. DSB rejoining measured using the neutral filter elution method after exposure to 25 Gy was identical regardless of whether 5-FU (100 μg/mL) was present during the repair period; thus, radiosensitization by this high-concentration postirradiation 5-FU protocol does not appear to be a result of interference with the overall rate of ligation of γ-ray-induced DSB's. The effect of 5-FU on the ability of log-phase cells to recover from that sector of PLD that can be expressed by postirradiation incubation with hypertonic (0.5 M) salt solution (HSS) was also examined. When irradiated cells were treated with 5-FU during their recovery period and then incubated with HSS, no clonogenic cells survived. Therefore, although it was not possible to assess the actual kinetics of recovery from γ-ray-induced PLD in 5-FU-treated cells, the drug clearly altered the metabolism or structure of the cells such that their susceptibility to HSS was markedly enhanced.