Hypoxic Irradiation Research Articles

LET Dependence of 8-Hydroxy-2'-deoxyguanosine (8-OHdG) Generation in Mammalian Cells under Air-Saturated and Hypoxic Conditions: A Possible Experimental Approach to the Mechanism of the Decreasing Oxygen Effect in the High-LET Region.

One of the most distinguished features in biological effects of heavy ions would be the decrease of oxygen effect in the high-LET region. This feature has been referred to as the radiobiological basis for the control of hypoxic fraction in cancer radiotherapy. However, mechanisms to explain this phenomenon have not been fully understood. One of the explanations was given by the oxygen in the track hypothesis, which proposes that oxygen is produced along ion tracks even in the hypoxic irradiation condition. In the present study, we designed an experimental approach to support this hypothesis by using 8-hydroxy-2'-deoxyguanosine (8-OHdG) as DNA damage requiring oxygen to produce. The LET dependence of 8-OHdG under hypoxic condition revealed that with increasing LET 8-OHdG yield seems to increase, despite that the yield of OH radical, which is also required for the production of 8-OHdG, decreases in the high-LET region. This result is consistent with the explanation that the local generation of oxygen along ion tracks contributes to the increase of 8-OHdG yield.

Новые аспекты клеточной терапии ишемических заболеваний сердца (обзор литературы)

The authors reviewed the modern methods of cell therapy for ischemic heart disease. The experimental and clinical researches demonstrated that transplantation of hematopoietic progenitor cells and mesenchymal stem cells contributes to reduction of the postinfarction scar size, to increase of left ventricular ejection fraction, to decrease of cardiac remodeling, resulting in the reduction of end-diastolic and end-systolic volumes. This treatment promoted an increase of exercise tolerance, decrease of angina functional class and improved the quality of life. The best long-term results were observed for the early beginning of cell therapy and the implantation of cells directly into the myocardium. The cultures of resident cardiomyocyte progenitor cells significantly improved the results of treatment. Hypoxic preconditioning or low intensity laser irradiation enhanced the therapeutic potential of the cells cultured for transplantation, activated proliferation and reduced apoptosis. Good results were achieved with laser revascularization of myocardium before the cell transplantation. New non-invasive methods of treatment for ischemic heart disease, based on the increase in the concentration of stem cells in peripheral blood with G-CSF application or laser irradiation of the bone marrow localization areas were suggested.

Regulation of DNAzyme function by hypoxic irradiation that induces one-electron reduction of 2-oxoalkyl group on thymine base

ABSTRACT : We characterized the one-electron reduction of oligodeoxynucleotides with a complementary ODN 6 in the X2-oxopropyl group on a thymine base (d oxo T) and applied the reaction to the radiolytic activation of DNAzyme function. We designed a system in which the DNAzyme function of cleaving mRNA was suppressed by introduction of d oxo T into the strand of DNAzyme. Figure 2 shows a representative profile for the radiolytic reduction of Hypoxic X-irradiation led to recovery of the cleavage ability because the 2-oxopropyl group was removed to form unmodified DNAzyme. We characterized the DNAzyme function by monitoring of which increased as a function of increasing radiation dose. To the fluorescence change of fluorophore- and quencher-labeled target strands. We confirmed that the DNAzyme function could be regulated by hypoxic X-irradiation and the reaction of d oxo T. Stimuli-responsive DNAs offer favorable properties for biological applications such as gene analysis and therapy.

Regulation of hypoxia-induced mRNA expressions of HIF-1alpha?and osteopontin and in vitro radiosensitization by tirapazamine in human nasopharyngeal carcinoma HNE-1 and CNE-1 cells

Combined hypoxic cytotoxic drugs and chemoradiotherapy is an important mean of oncotherapy, and Tirapazamine (TPZ) is one of the most remarkable drugs. It has been shown that TPZ has a synergistic effect with radiotherapy on tumor cells, but whether TPZ would down-regulate the expression of the hypoxia-induced genes has not been reported. This study was to investigate the hypoxia-induced mRNA expressions of hypoxia inducible factor-1alpha (HIF-1alpha) and osteopontin (OPN) in human nasopharyngeal carcinoma HNE-1 and CNE-1 cells and the radiosensitization of TPZ, a hypoxia-specific drug, on HNE-1 and CNE-1 cells in vitro. The IC50 values of TPZ for HNE-1 and CNE-1 cells were measured using MTT assay, and the mRNA expressions of HIF-1alpha and OPN in HNE-1 and CNE-1 cells was determined using RT-PCR under aerobic and hypoxic conditions, respectively. The survival rates of HNE-1 and CNE-1 cells treated with or without TPZ at IC10 in the presence or absence of oxygen for 6 h were determined using colony formation assay following exposure to 1-6 Gy of 60Co radiation. The dose-survival curves were plotted and the values of D0, Dq and SER were calculated as a single-hit multitarget model. The IC50 values of TPZ were 34.81 μmol/L and 35.02 μmol/L in HNE-1 and CNE-1 cells under aerobic condition, and 30.20 μmol/L and 28.48 μmol/L under hypoxic condition, respectively. The expressions of HIF-1alpha and OPN mRNA were reduced by TPZ in HNE-1 cells, but not in CNE-1 cells under hypoxic condition. For the HNE-1 cells, the respective values of D0 and Dq were 0.89 Gy and 0.28 Gy following normoxic irradiation versus 1.47 Gy and 0.44 Gy following hypoxic irradiation. For the CNE-1 cells, the respective values of D0 and Dq were 0.72 Gy and 0.68 Gy following normoxic irradiation versus 0.95 Gy and 0.56 Gy following hypoxic irradiation. The values of D0 and Dq for HNE-1 and CNE-1 cells treated with TPZ under hypoxic condition following irradiation were 0.66 Gy, 0.21 Gy and 0.85 Gy, 0.79 Gy, respectively. TPZ can down-regulate hypoxia-induced expression of HIF-1alpha and OPN mRNA of HNE-1 cells and radiosensitize the HNE-1 cells but not CNE-1 cells, and act as a hypoxia modifier.

Radioprotection by DMSO in nitrogen-saturated mammalian cells exposed to helium ion beams

The contribution of OH radical-mediated indirect action by particle beams under hypoxic irradiation condition was investigated by using a radical scavenger. V79 cells were irradiated with 150 MeV/nucleon helium ions at an LET of 2.2 keV/μm in the presence or absence of DMSO, and their colony survivals were determined. The contribution of indirect action to cell killing under hypoxic condition was estimated to be 52±9%. We conclude that OH radical-mediated indirect action still has a half in total contribution on cell killing under hypoxic condition.

LET dependence of the formation of oxidative damage 8-hydroxy-2′-deoxyguanosine (8-OHdG) in 2′-deoxyguanosine aqueous solution irradiated with heavy ions

To evaluate the contribution of indirect action mediated by OH radicals in biological effects of high LET radiations, we examined the production of 8-hydroxy-2′-deoxyguanosine (8-OHdG) in 2′-deoxyguanosine aqueous solution using various ion species with an LET range from 20 to 300 keV/μm. The 8-OHdG yield decreased with increasing LET. In the hypoxic irradiation condition, the yield showed constant or rather increasing tendency above about 100 keV/μm, which is consistent with an oxygen-in-the-track hypothesis to explain the diminishment of oxygen effect.

The effect of post-irradiation tumor oxygenation status on recovery from radiation-induced damage in vivo: with reference to that in quiescent cell populations

To elucidate the effect of tumor oxygenation status on recovery from damage following gamma-ray or accelerated carbon ion irradiation in vivo, including in quiescent (Q) cells. SCC VII tumor-bearing mice were continuously given 5-bromo-2'-deoxyuridine (BrdU) to label all proliferating (P) cells. They received gamma-ray or accelerated carbon ion irradiation with or without tumor clamping for inducing hypoxia. Immediately after irradiation, cells from some tumors were isolated, or acute hypoxia-releasing nicotinamide was loaded to the tumor-bearing mice. For 9 h after irradiation, some tumors were kept aerobic or hypoxic. Then isolated tumor cells were incubated with a cytokinesis blocker. The response of Q cells was assessed in terms of the micronucleus frequency using immunofluorescence staining for BrdU. That of the total (=P + Q) tumor cells was determined from BrdU non-treated tumors. Clearer recovery in Q cells than total cells and after aerobic than hypoxic gamma-ray irradiation was efficiently suppressed with carbon ion beams. Inhibition of recovery through keeping irradiated tumors hypoxic after irradiation and promotion of recovery by nicotinamide loading were observed more clearly with gamma-rays, after aerobic irradiation and in total cells than with carbon ion beams, after hypoxic irradiation and in Q cells, respectively. Tumor oxygenation status following irradiation can manipulate recovery from radiation-induced damage, especially after aerobic gamma-ray irradiation in total cells. Carbon ion beams are promising because of their efficient suppression of the recovery.

Radiolytic Ligation of Oligodeoxynucleotides Possessing Disulfide Bond

Hypoxic X-radiolysis of diluted aqueous solutions was performed to generate hydrated electrons that induced one-electron reduction of oligodeoxynucleotides (ODNs) possessing a disulfide bond. Upon hypoxic irradiation, reductive ligation reaction between two types of ODNs occurred in the presence of a template ODN strand, resulting in the formation of a prescribed ODN in substantial amounts.

One-Electron Reductive Template-Directed Ligation of Oligodeoxynucleotides Possessing a Disulfide Bond

Hypoxic X-radiolysis of diluted aqueous solutions was performed to generate hydrated electrons that induced one-electron reduction of oligodeoxynucleotides (ODNs) possessing a disulfide bond. Upon hypoxic irradiation of dinucleotides, two forms of dinucleotides were produced via intermolecular exchange of the disulfides and ligation that proceeded with a multiple turnover. In contrast to the efficient reaction induced by hypoxic irradiation, the reaction efficiency was dramatically decreased when irradiation was performed under aerobic conditions, presumably due to capturing reactive hydrated electrons by molecular oxygen. We subsequently applied these unique reaction characteristics to template-directed ligation. In the presence of a complementary template ODN, two ODNs possessing a disulfide bond produced a prescribed ODN with high regioselectivity via interstrand crossing upon hypoxic irradiation.

Regulation of DNA duplex formation by hypoxic irradiation: one-electron reduction characteristics of oligodeoxynucleotides possessing 2-oxoalkyl functional group

We synthesized oligodeoxynucleotides (ODN) possessing 2-oxoalkyl group on thymidine (d(oxo)T) to characterize the radiolytic reduction in aqueous solution. Corresponding unmodified ODNs were generated from ODNs containing d(oxo)T upon hypoxic irradiation. Enzymatic digestion of the duplex consisting of the hypoxically irradiated ODN containing d(oxo)T and its complementary strand resulted in an efficient cleavage, but no cleavage was observed when a control duplex was digested without irradiation.

Post-irradiation hypoxic incubation of X-irradiated MOLT-4 cells reduces apoptotic cell death by changing the intracellular redox state and modulating SAPK/JNK pathways

To elucidate radiobiological effects of hypoxia on X-ray-induced apoptosis, MOLT-4 cells were treated under four set of conditions: (1) both X irradiation and incubation under normoxia, (2) X irradiation under hypoxia and subsequent incubation under normoxia, (3) X irradiation under normoxia and subsequent incubation under hypoxia, and (4) both X irradiation and incubation under hypoxia, and the induction of apoptosis was examined by fluorescence microscopy. About 28-33% apoptosis was observed in cells treated under conditions 1 and 2, but this value was significantly reduced to around 18-20% in cells treated under conditions 3 and 4, suggesting that post-irradiation hypoxic incubation rather than hypoxic irradiation mainly caused the reduction of apoptosis. The activation and expression of apoptosis signal-related molecules SAPK/JNK, Fas and caspase-3 were also suppressed by hypoxic incubation. Effects of hypoxic incubation were canceled when cells were treated under conditions 3 and 4 with an oxygen-mimicking hypoxic cell radiosensitizer, whereas the addition of N-acetyl-L-cysteine again reduced the induction of apoptosis. From these results it was concluded that hypoxia reduced the induction of apoptosis by changing the intracellular redox state, followed by the regulation of apoptotic signals in X-irradiated MOLT-4 cells.

Hypoxia-selective activation of 5-fluorodeoxyuridine prodrug possessing indolequinone structure: radiolytic reduction and cytotoxicity characteristics

We synthesized a 5-fluorodeoxyuridine ( 5-FdUrd) derivative possessing an indolequinone structure ( IQ-FdUrd) to characterize the radiolytic reduction in aqueous solution and the radiation-activated cytotoxicity against EMT6/KU cells under hypoxic conditions. IQ-FdUrd released antitumor agent 5-FdUrd upon hypoxic, but not aerobic, irradiation with the G value of 0.38 × 10 −7 mol J −1. Laser flash photolysis of IQ-FdUrd in Ar-purged aqueous solution with dimethylaniline as an electron donor gave rise to a transient absorption spectrum characteristic of semiquinone radical anion, which decayed via second order kinetics. It is most likely that bimolecular disproportionation of intermediate semiquinone radicals occurs to release 5-FdUrd. IQ-FdUrd showed enhanced cytotoxicity against EMT6/KU cells in a radiation dose-dependent manner upon hypoxic irradiation. IQ-FdUrd is potentially a prototype compound for new class of radiation-activated antitumor prodrugs that are useful for radiation treatment of hypoxic tumors.

In vitro and in vivo evaluation of novel antitumor prodrugs of 5-fluoro-2′-deoxyuridine activated by hypoxic irradiation

Purpose We previously developed a novel antitumor prodrug that has a 2-oxopropyl substituent at the N(1) position of 5-fluorouracil (5-FU) and releases 5-FU via one-electron reduction on hypoxic irradiation. Although the compound was effective in vivo, its activity against murine tumors was not high enough to warrant clinical studies. Therefore, we developed a similar family of radiation-activated prodrugs of 5-fluoro-2′-deoxyuridine (FdUrd), which is generally more potent than 5-FU, and investigated their radiation chemical reactivity and in vitro and in vivo effects. Methods and materials Compounds bearing various 2-oxoalkyl substituents at the N(3) position of FdUrd were synthesized and investigated. After aerobic or hypoxic irradiation to the prodrugs dissolved in water or culture medium, release of FdUrd was measured using high-performance liquid chromatography. To investigate in vitro cytotoxicity, SCCVII and EMT6 cells in culture were irradiated in the presence of the prodrug under aerobic or hypoxic conditions, and then kept with the compound for 24 h. Cell survival was then measured using a colony assay. To investigate in vivo effects, the drug was injected intraperitoneally at a dose of 100 or 300 mg/kg into Balb/c mice bearing EMT6 tumors 30 min before irradiation. The tumor growth delay-time was then assessed. Results In vitro, the prodrugs released FdUrd at G-values (molar numbers of molecules produced by 1 J of radiation energy) of 1.6–2.0 × 10 −7 mol/J after hypoxic irradiation. The G-values for FdUrd release with hypoxic irradiation were about 100-fold greater than those with aerobic irradiation. Among the prodrugs tested, OFU106 bearing a 2-oxocyclopentyl substituent released the highest amount of FdUrd in the culture medium, and it was subjected to further in vitro and in vivo assays. Although OFU106 administered alone showed no cytotoxicity up to a concentration of 0.2 mM, it produced an enhanced cytotoxic effect when administered before hypoxic irradiation and kept with the cells for 24 h. The enhancement ratios calculated at the surviving fraction of 1% were 1.35–1.4 at 0.04 mM and 1.45–1.5 at 0.2 mM. In vivo, however, administration of OFU106 (100 or 300 mg/kg) before 20 Gy of irradiation did not produce marked growth delays compared with 20 Gy of radiation alone. Conclusion On hypoxic irradiation in vitro, the prodrugs of FdUrd were activated as efficiently as were the prodrugs of 5-FU, but marked in vivo effects could not be detected. This strategy of prodrug design should be used in further development of radiation-activated prodrugs of more potent anticancer agents.

One-Electron reduction characteristics of N(3)-Substituted 5-fluorodeoxyuridines synthesized as radiation-Activated prodrugs

We designed and synthesized N(3)-substituted 5-fluorodeoxyuridines as radiation-activated prodrugs of the antitumor agent, 5-fluorodeoxyuridine (5-FdUrd). A series of 5-FdUrd derivatives possessing a 2-oxoalkyl group at the N(3)-position released 5-FdUrd in good yield via one-electron reduction initiated by hypoxic irradiation. Cytotoxicity of the 5-FdUrd derivative possessing the 2-oxocyclopentyl group ( 3d) was low, but was enhanced by hypoxic irradiation resulting in 5-FdUrd release.

Effect of a hypoxic cell sensitizer doranidazole on the radiation-induced apoptosis of mouse L5178Y lymphoma cells.

We investigated the sensitizing effect of the 2-nitroimidazole analogue doranidazole, a new hypoxic radiosensitizer, on radiation-induced apoptosis in L5178Y cells. Apoptosis was assessed by checking DNA ladder formation, the presence of sub-G1 peaks in flow cytometry, and chromatin condensation. A radiosensitizing effect of doranidazole was also confirmed by a soft-agar colony assay of surviving cells. In the assay of DNA ladder formation, DNA fragmentation was observed following irradiation under an aerobic or hypoxic condition with or without doranidazole. The proportions of the cells at the sub-G1 peak in a flow cytometric measurement was not very different among the irradiations at 5 Gy under the aerobic condition, 15 Gy under hypoxia, and 10 Gy with 1 mM doranidazole under hypoxia. The fraction of cells with chromatin condensation was found to be significantly increased with doranidazole up to 3 mM when applied under hypoxic irradiation, but did not increase even at 10 mM. The sensitizer enhancement ratio was estimated to be about 1.7 with a concentration of 1 mM. This enhancement ratio was not different from that observed by assaying cell survivals. On the other hand, doranidazole showed no radiosensitizing effect under aerobic conditions with 1 mM. In conclusion, the radiation-induced apoptosis of L5178Y cells was enhanced by doranidazole under hypoxia.

In vivo evaluation of a novel antitumor prodrug, 1-(2′-oxopropyl)-5-fluorouracil (OFU001), which releases 5-fluorouracil upon hypoxic irradiation

Purpose: We previously proposed that a prodrug of 5-fluorouracil (5-FU), OFU001, is activated through capturing of hydrated electrons produced by hypoxic irradiation. Because hydrated electrons are readily deactivated by oxygen, the 5-FU release occurs specifically upon hypoxic irradiation. In this study, we investigated the in vivo efficacy, pharmacokinetics, and toxicity of OFU001. Methods and Materials: Female 10-week-old C3H/He mice bearing SCCVII tumors were used. To measure release of 5-FU from OFU001 in vivo, the mice were given 100 mg/kg of OFU001 intraperitoneally and irradiated. Thereafter, 5-FU levels in the tumor and serum were measured by high-performance liquid chromatography. To evaluate in vivo efficacy, OFU001 was administered 30 min before irradiation, and radiation-potentiating effects were investigated by means of a tumor growth delay assay and a 50% tumor control dose (TCD-50) assay. The lethal dose of OFU001 was evaluated in the same mice. Results: Following administration of OFU001 and irradiation at 30 Gy, the average 5-FU levels in the tumor and serum were 179 ng/g and 83 ng/mL, respectively. Administration of OFU001 (100–200 mg/kg) to the tumor-bearing mice before a single dose of 15-Gy irradiation produced a mean tumor growth delay of 1–5 days as compared to radiation alone (although the delay was not significant). However, no additional growth delay was observed when OFU001 was combined with 5 radiation fractions of 4 Gy each. The enhancement ratio of OFU001 in the TCD-50 assay was 1.2. No mice died after administration of 0.6–1.2 g/kg of OFU001. Conclusions: OFU001 appears to work in vivo via the proposed mechanism of activation. Although the in vivo effect of this compound was not strong enough for clinical efficacy, these results should encourage further research on the development of prodrugs of more potent anticancer agents activated through the same mechanism.

Radiosensitivity of human pancreatic cancer cells in vitro and in vivo, and the effect of a new hypoxic cell sensitizer, doranidazole

Background and purpose: A clinical study of the new 2-nitroimidazole nucleoside analogue doranidazole (PR-350) in combination with intraoperative radiotherapy is ongoing in Japan for localized unresectable pancreatic cancer. However, few data have been reported on the radiosensitivity and hypoxic fraction of human pancreatic cancers, and the efficacy of doranidazole against them. This study was undertaken to address these issues. Materials and methods: In vitro, four established human pancreatic cancer cell lines (SUIT-2, PANC-1, MIA PaCa-2 and BxPC-3) and murine SCCVII tumor cells (for comparison) were used. These cells were treated with 0.4 or 1 mM doranidazole for 45 mm prior to and during aerobic or hypoxic irradiation, and the cell survival was determined using the colony assay. In vivo, Balb/c nude mice bearing the pancreatic cancers (about 200 mg) on their backs received whole-body irradiation either after cervical dislocation, without physical restraint or anesthesia, or 20 min after intravenous injection of 100 mg/kg (0.4 mmol/kg) or 250 mg/kg (1 mmol/kg) of doranidazole. Following irradiation, the in vivo–in vitro assay was performed. The hypoxic fraction was estimated by the paired survival curve method. Results: Regarding in vitro radiosensitivity, there were no characteristics common to the four pancreatic cancer cell lines. In vitro, doranidazole had no sensitizing effect under aerobic conditions, but under hypoxic conditions, its sensitizer enhancement ratio (SER) was 1.25–1.3 at 0.4 mM and 1.4–1.55 at 1 mM against the four pancreatic cancer cell lines. These SERs were similar to those obtained in SCCVII cells. In vivo, the hypoxic fraction was 20% (95% CI, 11–38%) in SUIT-2, 14% (6.5–28%) in PANC-1, 10% (5.9–16%) in MIA PaCa-2, and 27% (15–46%) in BxPC-3 tumors. The SER of doranidazole was 1.15–1.3 at the dose of 100 mg/kg and 1.35–1.45 at 250 mg/kg. Conclusions: The four xenografted human pancreatic cancers had hypoxic fractions of 10–27% (mean: 18%). Doranidazole had definite in vitro and in vivo effects on all pancreatic cancer cell lines.

A novel class of antitumor prodrug, 1-(2'-oxopropyl)-5-fluorouracil (OFU001), that releases 5-fluorouracil upon hypoxic irradiation.

We have been developing prodrugs of anticancer agents such as 5‐fluorouracil (5‐FU) that are activated by irradiation under hypoxic conditions via one‐electron reduction. Among them, OFU001 [1‐(2′‐oxopropyl)‐5‐fluorouracil] is a prototype radiation‐activated prodrug. In this study, we investigated the radiation chemical reactivity and the biological effects of OFU001. This prodrug is presumed to release 5‐FU through incorporation of hydrated electrons into the antibonding σ* orbital of the C(1′)‐N(1) bond. Hydrated electrons are active species derived from radiolysis of water, but are readily deactivated by O2 into superoxide anion radicals () under conditions of aerobic irradiation. Therefore, 5‐FU release occurs highly specifically upon irradiation under hypoxic conditions. OFU001 dissolved in phosphate buffer released 5‐FU with a G‐value (mol number of molecules that are decomposed or produced by 1 J of absorbed radiation energy) of 1.9×10−7 mol/ J following hypoxic irradiation, while the G‐value for 5‐FU release was 1.0×10−8 mol/J following aerobic irradiation. However, the G‐values for decomposition of OFU001 were almost the same, i.e., 3.4×10−7 mol/J following hypoxic irradiation and 2.5×10−7 mol/J following aerobic irradiation. When hypoxically irradiated (7.5–30 Gy) OFU001 was added to murine SCCVII cells for 1–24 h, a significant cell‐killing effect was observed. The degree of this cytotoxicity was consistent with that of authentic 5‐FU at the corresponding concentrations. On the other hand, cytotoxicity was minimal when the cells were treated with aerobically irradiated or unirradiated OFU001. This compound had no radiosensitizing effect against SCCVII cells under either aerobic or hypoxic conditions when the drug was removed immediately after irradiation. Since hypoxia is generally most marked in tumors and irradiation is applied at the tumor site, this concept of prodrug design appears to be potentially useful for selective tumor treatment with minimal adverse effects of anticancer agents.

RADIOSENSITIZATION WITH A 3-NITROTRIAZOLE (AK-2123)

We investigated to clarify the mechanism of action of AK-2123. V79 cells were used, and the effect of the substance was estimated with different end-points viz. induction of DNA breaks, micronuclei and clonogenic survival after hypoxic irradiation with different X-ray doses. After treatment with 0.1 mmol dm(-3) of the substance, sensitization by a factor of 1.8, 1.1 and 1.4 respectively, was calculated. By determining the enhancement ratio after treatment with the drug at 7-8 different concentrations varying between 0.001 and 50 mmol dm(-3), the concentration of the substance for half of the maximum sensitization (K-value) was estimated to be 40 mu mol dm(-3) (DNA breaks), 1200 mu mol dm(-3) (micronuclei) and 2000 mu mol dm(-3) (survival); the maximum sensitizer enhancement ratio was 2.4 (DNA breaks), 2.3 (micronuclei) and 2.4 (survival). No cytotoxic effects were found when the cells were treated with the drug in different concentrations for 30 minutes. However, after longer periods the compound was found to be cytotoxic both under aerobic and hypoxic conditions, in a concentration and time dependent manner. A post-irradiation enhancement of the sensitivity was noted when the drug was left in the medium for 24 hours after hypoxic as well as aerobic radiation exposure.

The in vitro interaction with DNA of several hypoxic radiosensitizers

The five 5-nitroimidazole derivatives and the four glyoxylic compounds tested in this paper for their interaction with DNA with and without irradiation had previously been reported to act as radiosensitizers at a cellular level. Our aim was to find if the radiosensitizing activity of these products could be due to their interaction with DNA. For this purpose the formation of a complex was tested by comparing the spectrophotometrical absorbances of (DNA-product) solutions with those of DNA and of the products, at the absorption maxima of the products. None of the products formed in vitro a complex with DNA. The irradiation of product solutions with and without DNA induced a molecular degradation of the product which was linearly correlated with the radiation dose. The G values characterizing this radiodegradation showed that the presence of DNA did not modify the radiosensitivity of the products, neither in aerobic nor under hypoxic irradiation conditions. These results proved that the radiosensitizing properties of these compounds cannot be attributed to their chemical interaction with DNA.