A diarylpentanoid curcumin analog exhibits improved radioprotective potential in the intestinal mucosa.

PURPOSE: To best enhance the effects of radiotherapy, it is important to minimize adverse events, including free radical-induced intestinal cell damage. Given the threat of nuclear power plant accidents or nuclear terrorism, there is an urgent need for radioprotectants to counteract the radiation-induced toxicity and/or injuries. Curcumin exhibits protective effects against gamma irradiation; however, its in vivo efficacy is decreased due to the low bioavailability. We examined the radioprotective effect of a newly synthesized curcumin analog, GO-Y031, on 11-Gy X-ray-induced intestinal mucosal damage in mice. MATERIALS AND METHODS: The radioprotection experiments were conducted by using C57BL/6J or Jcl:ICR mice. Molecules related to radiation damage, including p53, Bax, Bcl-2, cleaved caspase-3, and reactive carbonyl species (RCS), were investigated immunohistochemically. RESULTS: GO-Y031 protected against crypt hypoplasia relative to a mock treatment at 0.5% (weight/weight); the number of crypts were 11.00 ± 2.00/circumference (mm) in treated versus 6.86 ± 0.99/mm in mock-treated C57BL/6 mice (p = 0.0079). GO-Y031 also reduced the levels of RCS, p53, and cleaved caspase-3 accumulation in the irradiated intestinal cells. CONCLUSIONS: GO-Y031 suppresses the accumulation of RCS and apoptosis-related molecules in irradiated cells. This compound may be a good primary radioprotective compound.

Gene expression profiling in rat liver after VMH lesioning.

It has been reported that ventromedial hypothalamic (VMH) lesions induce hepatic cell proliferation and apoptosis and metabolic changes in the body. In the present study, we identified genes of which expression profiles showed significant modulation in rat liver after VMH lesions. Total RNA was extracted, and differences in the gene expression profiles between rats at day 3 after VMH lesioning and sham-VMH lesioned rats were investigated using DNA microarray analysis. The results revealed that VMH lesions regulated the genes that were involved in various types of metabolisms and cell proliferations in the liver. Real-time PCR also confirmed that gene expressions of ELOVL6 and SPC24 were upregulated, and that of SERPINA7 was downregulated. VMH lesions may change the expressions of multiple metabolism genes and cell proliferation-related genes in rat liver.

Ventromedial hypothalamic lesions change the expression of neuron-related genes and immune-related genes in rat liver.

There are no reports that hypothalamus can directly affect the expression of neuron-related genes and immune-related genes in liver. We identified genes of which expression profiles showed significant modulation in rat liver after ventromedial hypothalamic (VMH) lesions. Total RNA was extracted, and differences in the gene expression profiles between rats at day 3 after VMH lesioning and sham-VMH lesioned rats were investigated using DNA microarray analysis. The result revealed that VMH lesions regulated the genes that were involved in functions related to neuronal development and immunofunction in the liver. Real-time PCR also confirmed that gene expression of SULT4A1 was upregulated, but expression of ACSL1 and CISH were downregulated at day 3 after VMH lesions. VMH lesions may change the expression of neuron-related genes and immune-related genes in rat liver.

Changes of neuron-specific and apoptosis gene expression levels after ventromedial hypothalamic lesions in rat intestine.

The intestinal epithelium is continuously renewed through a balance between cell proliferation and apoptosis. We identified genes of which expression profiles showed significant modulation, and we investigated the cellular mechanisms of this gene regulation in rat intestine after ventromedial hypothalamic (VMH) lesions. Total RNA was extracted, and differences in the gene expression profiles between rats at day 3 after VMH lesioning and in sham-VMH lesioned rats were investigated using DNA microarray analysis and real-time polymerase chain reaction (PCR) methods. DNA microarray analysis revealed that VMH lesions regulated the genes that were involved in functions predominantly related to neuronal development, cell proliferation and apoptosis. Real-time PCR also confirmed that gene expressions of Efnb1 were downregulated. Meanwhile, expression of Casp3 was similar. It is noted that the signaling networks of many gene families, including neuron-specific genes and apoptosis genes in the intestine were changed after VMH lesioning. VMH lesions may suppress mainly the caspase independent type II pathway for apoptosis and induce cell proliferation in the intestine.

Temporal change in brain natriuretic Peptide after radiotherapy for thoracic esophageal cancer.

PURPOSE: To investigate the relationships of plasma levels of brain natriuretic peptide (BNP) with abnormal (18)F-fluorodeoxyglucose (FDG) accumulation in the myocardium corresponding to irradiated fields and temporal changes in BNP, which is used as an index of heart remodeling, after radiotherapy for the mediastinum. MATERIALS AND METHODS: Brain natriuretic peptide concentrations were measured before and after radiotherapy for thoracic esophageal cancer, and the change in BNP concentration after radiotherapy was investigated. Moreover, FDG accumulation in the myocardium was investigated in patients who had undergone FDG positron emission tomography less than 14 days before or after measurement of BNP concentration, and the Mann-Whitney U test was used to detect significant difference between BNP concentrations in patients with and without abnormal FDG accumulation corresponding to the irradiated field. RESULTS: There was significant difference between the levels of BNP in patients without abnormal FDG accumulation in the irradiated myocardium and in patients with abnormal FDG accumulation (p < 0.001). The levels of BNP in the 9-24 months after radiotherapy group and in the >24 months after radiotherapy group were significantly higher than the levels in the before radiotherapy group, immediately after radiotherapy group, 1-2 months after radiotherapy group, and control group. CONCLUSIONS: The level of BNP was significantly increased more than 9 months after the start of radiotherapy and was significantly higher in patients who had high FDG accumulation corresponding to the irradiated field. The results of this study indicate that BNP concentration might be an early indicator of radiation-induced myocardial damage.

Initial evaluation of dynamic human imaging using 18F-FRP170 as a new PET tracer for imaging hypoxia.

UNLABELLED: 18F-FRP170, 1-(2-fluoro-1-[hydroxymethyl]ethoxy)methyl-2-nitroimidazole, is a new hypoxia imaging agent for positron emission tomography. This compound was synthesized by 18F-labeling of RP170, which was developed as a new hydrophilic 2-nitroimidazole analog. In the present study, we analyzed dynamic whole-body imaging in healthy volunteers and dynamic tumor imaging in three patients with lung cancer. METHODS: Four healthy male volunteers and three lung cancer patients were enrolled in this study. Volunteers underwent dynamic whole-body scans just after injection of 18F-FRP170 for about 90 min, while the lung cancer patients underwent dynamic tumor imaging for about 60 or 120 min. Data are expressed as standardized uptake values (SUV). Regions of interest were placed over images of each organ or tumor to generate time-SUV curves. RESULTS: The series of dynamic whole-body scans showed rapid elimination of 18F-FRP170 from the kidneys following elimination from the liver. Very low physiological uptake was observed above the diaphragm. 18F-FRP170 uptake in the lung cancer lesion could be visualized clearly from early after injection. The changes of tumor SUV, tumor/blood ratio, or tumor/muscle ratio about 30 min after injection or later were small. CONCLUSIONS: Dynamic imaging using 18F-FRP170 demonstrated rapid elimination from the kidney, suggesting the high hydrophilicity of this imaging agent. The background activity above the diaphragm was very low, and patients with lung cancer showed clear tumor uptake of 18F-FRP170 early after injection.

The utility of 18F-fluorodeoxyglucose positron emission tomography for early diagnosis of radiation-induced myocardial damage.

PURPOSE: We evaluated the clinical significance of focal increased uptake in the basal myocardium on F-fluorodeoxyglucose positron emission tomography (FDG-PET) in patients with esophageal cancer after radiotherapy. METHODS AND MATERIALS: Between August 2004 and July 2005, a total of 64 patients who had been irradiated for thoracic esophageal cancer underwent FDG-PET at least three months after the completion of chemoradiotherapy. Some patients showed increased FDG uptake in the basal portion of the myocardium. To clarify the clinical significance of these findings, further examinations of hearts were performed. The dose distribution in the myocardium with high FDG uptake was also analyzed retrospectively. RESULTS: Thirteen (20.3%) of the 64 patients showed high FDG uptake in the basal myocardium corresponding to the irradiated fields compared with FDG uptake in the myocardium outside the irradiated fields. Eight of the 13 patients consented to undergo examinations of the heart. Five of those eight patients showed low 123I-BMIPP uptake and four showed low 201TlCl uptake in the myocardium corresponding with high FDG uptake regions. In two patients, delayed enhancement was found in some parts of the area with high FDG uptake on Gd-DTPA magnetic resonance imaging (MRI), and the delay-enhanced lesion showed hypokinesia on cine-MRI in one patient. CONCLUSIONS: FDG-PET often shows focal increased uptake in the basal myocardium after radiotherapy for esophageal cancer. This finding indicates the possibility of radiation-induced cardiac damage, and cardiac function and symptoms of such patients should be followed carefully.

S-1, an oral fluoropyrimidine, enhances radiation response of DLD-1/FU human colon cancer xenografts resistant to 5-FU.

S-1, a novel oral fluoropyrimidine, is increasingly used for the treatment of human cancer including gastrointestinal carcinomas. Using the 5-FU resistant DLD-1/FU human colon cancer cell xenografts, the present study investigated whether S-1 enhances the therapeutic efficacy of radiation and if so what are the underlying mechanisms. Nude mice bearing tumor xenografts were treated with radiation, S-1, or both. Tumor growth delay was the treatments' endpoint. To determine whether S-1 enhances intrinsic cell radiosensitivity, we performed clonogenic cell survival assay. Also we assessed the expression of thymidylate synthase (TS) using immunohistochemistry assay. While S-1 or 5 Gy were only slightly effective as single agents in delaying tumor growth, the combined treatment was highly effective. Clonogenic cell survival showed that S-1 strongly enhanced cell radiosensitivity. Immunohistochemistry showed that the expression of TS was down-regulated in tumors treated by S-1 plus radiation. Combined S-1 plus radiation treatment resulted in a synergistic effect in the therapy of 5-FU resistant human colon carcinoma xenografts (EF = 2.06). The effect could be attributed to the ability of S-1 to increase cell radiosensitivity (EF = 1.9) and to the down-regulation of TS involved in cellular processes leading to radio- and (or) chemo-resistance.

Expression of thymidine phosphorylase and VEGF in esophageal squamous cell carcinoma.

The purpose of this study was to determine the role of TP and VEGF in angiogenesis and its clinical significance in prognosis of patients with esophageal carcinoma. Expressions of TP and VEGF, microvascular density and cell proliferation activity were evaluated by using 40 immunohistochemically stained resected esophageal carcinoma tissues, and the survival rate of the patients was analyzed. Significant positive correlation and regression were found between the VEGF expression level of tumor and microvascular density (r=0.73, p<0.0001). Not statistically strong but significant positive correlation and regression were found between the TP expression level of tumor and microvascular density (r=0.32, p=0.046). No significant relationships were found between TP and VEGF expressions. Pathological T-factor and pathological N-factor were significant prognostic factors. Tumor length, site of lesion, gender, age, and Ki67 labeling index were not significant prognostic factors. The VEGF expression level was one of the unfavorable prognostic factors (risk ratio =1.035, 95% CI=1.007-1.065, p=0.01). The patients with high TP expression showed a tendency for unfavorable prognosis, but it was not statistically significant (RR=1.017, 95% CI=0.996-1.042, p=0.1). The prognosis of patients in the TP/VEGF[+/+] group was significantly poorer than that of the patients in the TP/VEGF[-/-] group and TP/VEGF[+/- or -/+] group (RR=0.488 for TP/VEGF[-/-] group, =0.717 for TP/VEGF[+/- or -/+] group, p=0.005). In conclusion, VEGF and TP expression seems to have a relationship with tumor angiogenesis, and co-expression of TP and VEGF seemed to be one of the unfavorable prognostic factors.

Results of radiation therapy combined with nedaplatin (cis-diammine-glycoplatinum) and 5-fluorouracil for postoperative locoregional recurrent esophageal cancer.

BACKGROUND: Although the effectiveness of radiotherapy with concurrent administration of several anti-tumor drugs for postoperative recurrent esophageal cancer has been demonstrated, the results are not satisfactory. The purpose of the present study was to evaluate the effectiveness and safety of radiotherapy combined with nedaplatin and 5-FU for postoperative locoregional (excluding hematogenous metastasis) recurrent esophageal cancer. METHODS: In June 2000, we started a phase II study on treatment of postoperative locoregional recurrent esophageal cancer with radiotherapy (60 Gy/30 fr/6 weeks) combined with chemotherapy consisting of two cycles of nedaplatin (70 mg/m2/2 h) and 5-FU (500 mg/m2/24 h for 5 days). The primary endpoint of the present study was overall survival rate, and the second endpoints were irradiated-field control rate, tumor response and toxicity. RESULTS: A total of 30 patients were included in this study. The 1-year and 3-year overall survival rates were 60.6% and 56.3%, respectively, with a median survival period of 39.0 months, and the 1-year and 3-year irradiated-field control rates were 86.4% and 72%, respectively. Complete response and partial response were observed in 13.3% and 60.0% of the patients, respectively. Grade 3 or higher leukocytopenia and thrombocytopenia were observed in 30% and 3.3% of the patients, respectively, but renal toxicity of grade 3 or higher was not observed. The regimen was completed in 76.7% of the patients. In univariate analysis, the difference between survival rate in preradiotherapy performance status, recurrent pattern (worse for patients with anastomotic recurrence) and age (worse for younger patients) were statistically significant. CONCLUSION: Radiotherapy combined with nedaplatin and 5-FU is a safe and effective salvage treatment for postoperative locoregional recurrent esophageal cancer.

Inhibition of DNA repair as a mechanism of enhanced radioresponse of head and neck carcinoma cells by a selective cyclooxygenase-2 inhibitor, celecoxib.

PURPOSE: Previously, we reported that inhibitors of cyclooxygenase-2 (COX-2) enzyme enhanced murine and human tumor cell response to radiation in vitro and in vivo. However, the molecular mechanisms mediating the effects of COX-2 inhibitors are not clear. The present study was designed to investigate the ability of celecoxib, a selective COX-2 inhibitor, to sensitize human head-and-neck cancer cell line, HN5, to radiation, and examine its effects on DNA repair, which may be a potential mechanism of radiosensitization. METHODS AND MATERIALS: Cells were assessed for the effect of celecoxib (5-50 microM), by 3-[4,5-dimethylthiozol-2-yl]-2,5-diphenyltetrazolium bromide assay for growth inhibition and by clonogenic cell survival assay for the radiosensitizing effect. Kinase assay and Western analysis were conducted to assess the effect of celecoxib on DNA-dependent protein kinase catalytic subunit (PKcs) and Ku proteins. Electrophoretic mobility shift assays (EMSA) were performed to determine the DNA-binding activity of Ku/DNA-PKcs protein complex and nuclear factor kappa B (NFkappaB). RESULTS: Celecoxib (10 and 50 microM, for 2 days) inhibited the HN5 cell growth and significantly enhanced the cell radiosensitivity in a dose-dependent manner. It also reduced the shoulder region on the radiation-survival curve, suggesting that inhibition of DNA repair processes may have occurred. Western blot analysis demonstrated that celecoxib downregulated the expression of Ku70 protein and inhibited the kinase activity of DNA-PKcs, which are involved in the double-stranded DNA-break repair machinery. By EMSA, it was further shown that celecoxib reduced DNA-binding activity of Ku/DNA-PKcs protein complex. In addition, celecoxib inhibited the constitutively active NFkappaB and the radiation-induced NFkappaB in HN5 cells, suggesting that NFkappaB may play a role in mediating the effects of celecoxib. CONCLUSIONS: Celecoxib strongly enhanced the sensitivity of HN5 carcinoma cells to radiation, which, mechanistically, can be attributed to the inhibition of DNA repair processes in radiation-damaged cells.

[Development of [18F]FRP-170 injection for imaging hypoxia by PET].

A novel [18F]FRP-170 injection for imaging hypoxia by PET was developed for clinical use. The preparation was based on the simple on-column basic-hydrolysis and the whole procedure was automated by detecting He flow change for transferring and evaporating liquids. [18F]FRP-170 was prepared in around 15-20% decay-corrected radiochemical yield within 60 min and stable in saline for more than 6 hr. Radiochemical purity was over 99% and specific activity at EOS was 40-60 GBq/micromol. The radiation-absorbed dose to the whole body was estimated to be 1.0 mSv/185 MBq. The [18F]FRP-170 injection proved to be suitable for clinical use without acute toxicity or mutagenicity.

Intrinsic radiosensitivity by metallothionein expression has no great influence on clinical radiosensitivity in esophageal carcinoma.

Several studies have shown that macroscopically infiltrative type of esophageal carcinoma generally has a poorer local response to radiation therapy than that of localized type. The aim of this study was to determine the role of metallothionein (MT) as a radioprotective agent in the difference of clinical radiosensitivities in esophageal carcinoma. A total of 45 surgically resected esophageal carcinoma tissues [20 macroscopically localized type without preoperative treatment (PT), 20 macroscopically infiltrative type without PT and 5 macroscopically infiltrative type with PT] were stained for MT by immunohistochemistry and were analyzed. MT expression level of macroscopically localized type of esophageal carcinoma was significantly higher than that of infiltrative type without PT (P=0.026) and was significantly higher than that of infiltrative type with PT (P=0.024). No significant difference was found between MT expression levels in infiltrative type of esophageal carcinoma without PT and that with PT. The results of this study suggest that there is less possibility that clinical radioresistance of the tumor is due to MT expression in esophageal carcinoma and also suggest that there is less possibility that MT synthesis is induced by fractionated irradiation of a moderate dose or by an anti-cancer agent during a course of treatment.

C225 antiepidermal growth factor receptor antibody enhances the efficacy of docetaxel chemoradiotherapy.

PURPOSE: C225 anti-EGFR (epidermal growth factor receptor) antibody has been shown to enhance tumor response to radiation and a number of chemotherapeutic agents. Because of increased use of concurrent chemoradiotherapy in cancer treatment, it is important to determine whether C225 enhances also the antitumor efficacy of radiation when combined with chemotherapy. This study assessed the effect of C225 on tumor response when combined with docetaxel plus single or fractionated radiation. METHODS AND MATERIALS: MDA468 human adenocarcinoma and A431 human epidermoid carcinoma cells growing as xenografts in the right hind leg of nude mice were used. Mice bearing 8-mm tumors were treated with C225 antibody at a dose of 1 mg given i.p. once, twice, or three times 3 days apart, 10 or 30 mg/kg docetaxel given i.v., and/or local tumor irradiation of 8 or 10 Gy single dose or fractionated irradiation consisting of 2 Gy daily for 5 days. When all three agents were combined, C225 was given 6 h before or 18 h after docetaxel, and radiation was given 24 h after docetaxel. The treatment end point was tumor growth delay. RESULTS: C225 enhanced the antitumor efficacy of docetaxel, local tumor irradiation, and docetaxel combined with radiation. The response of both MDA468 and A431 carcinomas was enhanced. The enhancement factors ranged from 1.19 to 8.52, the degree of the enhancement depending on experimental conditions such as administration of multiple vs. single dose C225 or single or fractionated irradiation. C225 given twice or 3 times was more effective than when administered as a single dose. The effect of C225 was more pronounced when combined with single than fractionated irradiation with or without docetaxel. The triple-agent therapy was more effective than a single agent or double combination therapies, expressed by both increased tumor growth delay and the rate of tumor cure. CONCLUSIONS: Our results show that C225 anti-EGFR antibody is a potent enhancer of tumor response to docetaxel or radiation as single agents, and to docetaxel when combined with radiation. Thus, these findings provide strong preclinical evidence in support of combination of anti-EGFR blockade with chemoradiotherapy.

Potentiation of tumor response to radiation or chemoradiation by selective cyclooxygenase-2 enzyme inhibitors.

Cyclooxygenase-2 (COX-2) is an enzyme expressed primarily in pathologic states, such as inflammatory disorders and cancer, where it mediates prostaglandin production. Its overexpression is associated with more aggressive biologic tumor behavior and adverse patient outcome. Increasing evidence shows that agents that selectively inhibit COX-2 enhance tumor response to radiation or chemotherapeutic agents. This article gives an overview of some of this evidence. In addition, we describe new results showing that celecoxib, a selective COX-2 inhibitor, enhanced response of A431 human tumor xenografts in nude mice to radiation by an enhancement factor (EF) of 1.43 and to the chemotherapeutic agent docetaxel by an EF of 2.07. Celecoxib also enhanced tumor response when added to the combined docetaxel plus radiation treatment (EF = 2.13). Further experiments showed that selective COX-2 inhibitors enhanced tumor cell sensitivity to ionizing radiation, involving inhibition of cellular repair from radiation damage and cell cycle redistribution as mechanisms for some cell types. The results show that selective COX-2 inhibitors have the potential to improve tumor radiotherapy or radiochemotherapy, and this therapeutic strategy is currently under clinical testing.

Flavopiridol, a cyclin-dependent kinase inhibitor, enhances radiosensitivity of ovarian carcinoma cells.

Flavopiridol, a cyclin-dependent kinase (cdk) inhibitor, can cause cell cycle arrest, induce apoptosis in cancer cells, and inhibit tumor cell growth in vivo. The present study investigated the in vitro radiosensitizing effect of flavopiridol and the underlying molecular mechanisms in a murine ovarian cancer cell line, OCA-I. Flavopiridol inhibited cell growth in a dose-dependent manner and enhanced cell radiosensitivity assessed by the clonogenic cell survival assay. A flavopiridol dose of 300 nM, given for 1 day, enhanced radiosensitivity by a factor of 2.1. Clonogenic cell survival after split-dose radiation showed that flavopiridol inhibited repair from radiation damage. In addition, flavopiridol treatment (300 nM, 1 day) resulted in decreased levels of Ku70 and Ku86 proteins that play a role in DNA repair processes, suggesting that DNA repair processes may have been disrupted by this agent. Flow cytometry analysis showed that flavopiridol (300 nM, 1 day) accumulated the cells in G(1) and G(2) phases, with a significant reduction in the S phase component. This cell cycle redistribution is likely another mechanism underlying flavopiridol-induced cell radiosensitivity. Flavopiridol down-regulated cyclin D1 and cyclin E protein levels and also inhibited phosphorylation of retinoblastoma protein, which is inconsistent with the observed cell cycle arrest. Among the cdks tested, cdk-9, the catalytic subunit of positive transcription elongation factor b, was significantly down-regulated by flavopiridol, suggesting that flavopiridol may modulate cellular transcription processes. Furthermore, flavopiridol on its own induced apoptosis in the OCA-I cells, whereas in combination with radiation, exerted no additional increase in apoptosis. Taken together, our data show that flavopiridol strongly augmented the response of ovarian carcinoma cells to radiation and that the underlying mechanisms included inhibition of sublethal DNA damage repair and cell cycle redistribution. At the molecular level, transcriptional regulation by flavopiridol may have been involved.

In vitro enhancement of tumor cell radiosensitivity by a selective inhibitor of cyclooxygenase-2 enzyme: mechanistic considerations.

PURPOSE: Selective cyclooxygenase-2 inhibitors have been reported to enhance the tumor response to radiation in vivo, but the cellular mechanisms underlying the radiosensitizing effect are not understood. In the present study, we investigated several possible mechanisms using a murine sarcoma cell culture system. METHODS AND MATERIALS: Cells derived from a murine sarcoma, designated NFSA, were cultured in vitro and exposed to different (either single or split) doses of radiation with and without a pretreatment of SC-236 (4-[5-(4-chlorophenyl)-3-(trifluoromethyl)-1H-pyrazol-l-yl] benzene sulfonamide), a selective cyclooxygenase-2 (COX-2) inhibitor. The cells were assayed for clonogenic survival to determine the radiosensitizing effect of SC-236. In addition, MTT assay and TUNEL assay were performed to determine the effects of SC-236 and radiation on the cell survival and cell cycle distribution. RNase protection assay was performed on the total RNA extract using probes that encoded for selected cell cycle regulatory proteins, such as cyclins and cyclin-dependent kinases. To monitor the extent of COX-2 activity and its role in radiosensitization, the cellular content of prostaglandin E2, a major metabolite of COX-2 activity on arachidonic acid, was also determined. RESULTS: The cell clonogenic survival assay showed that SC-236 significantly enhanced tumor cell radiosensitivity: 50 microM SC-236 increased it by a factor of 1.51 at the 0.1 cell survival level. Treatment with SC-236 (50 microM, 3 days) removed the "shoulder" region on the radiation survival curve, suggesting that the drug inhibited repair of sublethal radiation damage. The inhibition was confirmed by split-dose experiments where two doses (3 Gy each) of radiation were given 4 h apart. The cells exposed to radiation only repaired the damage by a factor of 1.44, whereas those treated with SC-236 plus radiation repaired it by a factor of 1.1 only. Whereas SC-236 induced apoptosis in these NFSA cells, radiation did not. No further increase in apoptosis was observed when the cells were exposed to both SC-236 and radiation, suggesting that SC-236 did not render tumor cells more susceptible to radiation-induced apoptosis. The RNase protection assay showed that SC-236 (50 microM, 3 days) inhibited the expression of cyclins A and B, as well as cyclin-dependent kinase-1. Inhibition of these cell cycle regulatory elements by SC-236 was associated with the arrest of cells in the radiosensitive G2-M phase (67%), determined by flow cytometry. CONCLUSIONS: SC-236 significantly enhanced radiosensitivity of tumor cells; the magnitude of sensitivity was dependent on the drug's concentration. The likely mechanisms involve accumulation of cells in the radiosensitive G2-M phase of the cell cycle and inhibition of repair from sublethal radiation damage.