Strategizing the clone wars: pharmacological control of cellular sensitivity to radiation

The combined administration of ionizing radiation and systemic chemotherapy is an accepted standard of care for numerous cancers. Improved efficacy through the combination of therapies reflects several interrelated processes, including DNA damage, inhibition of DNA synthesis, alteration of cell cycle distribution, and impaired DNA repair. Insights into cellular responses to radiation have led to the use of drugs that target specific intracellular signaling pathways to sensitize cells to radiation. Combinations of chemotherapy and radiation continue to be optimized, based on preclinical and early-phase clinical data that indicate the ideal sequencing of therapies, the best combinations of agents (including radiosensitizers), and the most reliable biological markers for predicting patient responsiveness. This review summarizes our current understanding of radiosensitization as it relates to preclinical drug development and discusses the potential benefits of judiciously incorporating both traditional and targeted chemotherapy into radiation regimens.     

Modification of radiation response by metal complexes: a review with emphasis of nonplatinum studies

There is a need to develop compounds which alter the effects of radiation, particularly in the hypoxic radioresistant cell, following the limited success to date of the electron-affinic nitroimidazoles. The chemistry of transition metals is briefly outlined to point out certain aspects which might be exploited in the design of radiosensitizers. The best known clinical example of a metal complex which enhances the effect of radiation in hypoxic cells is the successful antineoplastic cisplatin. Past studies on enhancement of radiation damage by complexes of metals other than platinum, mainly in bacterial spores and bacterial and mammalian cells, have been summarized according to the metal used. The many mechanisms by which metal complexes could interact with radiation are outlined, and examples are given where possible. This survey emphasizes the need for a systematic study of the effect of metal/ligand variation on radiosensitization with regard to mechanisms of action to assess the potential of these compounds as radiosensitizers. Metal complexes offer many advantages, both for the study of mechanisms by which radiation kills cells and for drug development.     

Influence of anoxic sensitizers on the radiation damage in biologically active DNA in aqueous solution

The competition between biologically active single-stranded phiX174 DNA and the anoxic radiosensitizers metronidazole, misonidazole, paranitroacetophenone or nifuroxime for OH radicals is studied. The results are compared with experiments in which the protection of the DNA by t-butanol is determined. Also the effects of the sensitizers on the chemical nature of the damage (immediate and potential break, immediate and potential base damage) is studied. It is found that in diluted aqueous solutions of DNA these radiosensitizers do not sensitize with respect to the biological inactivation. The only effect observed is a shift from potential to immediate breaks with misonidazole and also nifuroxime.     

DNA lesions that signal the induction of radioresistance and DNA repair in yeast

DNA recombinational repair, and an increase in its capacity induced by DNA damage, is believed to be the major mechanism that confers resistance to killing by ionizing radiation in yeast. We have examined the nature of the DNA lesions generated by ionizing radiation that induce this mechanism, using two different end points: resistance to cell killing and ability of the error-free recombinational repair system to compete for other DNA lesions and thereby suppress chemical mutation. Under the various conditions examined in this study, the "maximum" inducible radiation resistance was increased approximately 1.5- to 3-fold and suppression of mutation about 10-fold. DNA lesions produced by low-LET gamma rays at doses greater than about 20 Gy given in oxygen were shown to be more efficient, per unit dose, at inducing radioresistance to killing than were lesions produced by neutrons (high-LET radiation). This suggests that DNA single-strand breaks are more important lesions in the induction of radioresistance than DNA double-strand breaks. Oxygen-modified lesions produced by gamma rays (low-LET radiation) were particularly efficient as induction signals. DNA damage due to hydroxyl radicals (OH.) derived from the radiolytic decomposition of H2O produced lesions that strongly induced this DNA repair mechanism. Similarly, OH. derived from aqueous electrons (e-aq) in the presence of N2O also efficiently induced the response. Cells induced to radioresistance to killing with high-LET radiation did not suppress N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-generated mutations as well as cells induced with low-LET radiation, supporting the conclusion that the type of DNA damage produced by low-LET radiation is a better inducer of recombinational repair. Surprisingly, however, cells induced with gamma radiation in the presence of N2O that became radioresistant to killing were unable to suppress MNNG mutations. This result indicates that OH. generated via e-aq (in N2O) may produce unusual DNA lesions which retard normal repair and render the system unavailable to compete for MNNG-generated lesions. We suggest that the repairability of these unique lesions is restricted by either their chemical nature or topological accessibility. Attempted repair of these lesions has lethal consequences and accounts for N2O radiosensitization of repair-competent but not incompetent cells. We conclude that induction of radioresistance in yeast by ionizing radiation responds variably to different DNA lesions, and these affect the availability of the induced recombinational repair system to deal with subsequent damage.     

The influence of oxygen on the induction of radiation damage in DNA in mammalian cells after sensitization by intracellular glutathione depletion

Treatment of mammalian cells with buthionine sulphoximine (BSO) or diethyl maleate (DEM) results in a decrease in the intracellular GSH (glutathione) and non-protein-bound SH (NPSH) levels. The effect of depletion of GSH and NPSH on radiosensitivity was studied in relation to the concentration of oxygen during irradiation. Single- and double-strand breaks (ssb and dsb) and cell killing were used as criteria for radiation damage. Under aerobic conditions, BSO and DEM treatment gave a small sensitization of 10-20 per cent for the three types of radiation damage. Also under severely hypoxic conditions (0.01 microM oxygen in the medium) the sensitizing effect of both compounds on the induction of ssb and dsb and on cell killing was small (0-30 per cent). At somewhat higher concentrations of oxygen (0.5-10 microM) however, the sensitization amounted to about 90 per cent for the induction of ssb and dsb and about 50 per cent for cell killing. These results strengthen the widely accepted idea that intracellular SH-compounds compete with oxygen and other electron-affinic radiosensitizers with respect to reaction with radiation-induced damage, thus preventing the fixation of DNA damages by oxygen. These results imply that the extent to which SH-compounds affect the radiosensitivity of cells in vivo depends strongly on the local concentration of oxygen.     

Hypoxia-selective radiosensitization of mammalian cells by nitracrine, an electron-affinic DNA intercalator

The radiosensitizing ability of the 1-nitroacridine nitracrine (NC) is of interest since it is an example of a DNA intercalating agent with an electron-affinic nitro group. NC radiosensitization was evaluated in Chinese hamster ovary cell (AA8) cultures at 4 degrees C in order to suppress the rapid metabolism and potent cytotoxicity of the drug. Under hypoxic conditions, submicromolar concentrations of NC resulted in sensitization (SER = 1.6 at 1 mumol dm-3). Sensitization was also seen under aerobic conditions but a concentration more than 10-fold higher was required. In aerobic cultures NC radiosensitization was independent of whether cells were exposed before and during, or after, irradiation. Postirradiation sensitization was not observed under hypoxic conditions. The time dependence of NC uptake and the development of radiosensitization were similar (maximal at 30 min at 4 degrees C under hypoxia) suggesting that sensitization, unlike cytotoxicity, is due to unmetabolized drug. NC is about 1700 times more potent as a radiosensitizer than misonidazole. This high potency is adequately accounted for by the electron affinity of NC (E(1) value at pH7 of -275 mV versus NHE) and by its accumulation in cells to give intracellular concentrations approximately 30 times greater than in the medium. However, concentrations of free NC appear to be low in AA8 cells, presumably because of DNA binding. If radiosensitization by NC is due to bound rather than free drug, it suggests that intercalated NC can interact very efficiently with DNA target radicals. This is despite a binding ratio in the cell estimated as less than 1 NC molecule/400 base pairs under conditions providing efficient sensitization. This work suggests a new approach in the search for more effective clinical radiosensitizers, and poses questions on the means by which intercalated drugs can interact with DNA damage.     

Radiosensitization by hypoxic pretreatment with misonidazole: an interaction of damage at the DNA level

Prolonged exposures to misonidazole (MISO) in vitro under hypoxic conditions result in radiosensitization which is characterized by a decrease in the size of the radiation survival curve shoulder for cells irradiated under hypoxic or aerobic conditions after drug removal. Although intracellular glutathione (GSH) was depleted during hypoxic exposures to MISO, this could not account for the dose-additive radiosensitization (decrease in shoulder size) since GSH depletion by diethylmaleate had no effect on the sensitivity of cells irradiated in air. The alkaline elution assay was used to measure DNA strand breaks and their repair after exposure to MISO, graded doses of X rays, and the combination of MISO pretreatment with X rays. The elution rate of DNA from irradiated cells increased linearly with X-ray dose, with and without MISO pretreatment. However, the DNA elution rates measured after MISO pretreatment were greater by a constant amount at all X-ray doses greater than 1 Gy. In terms of both cell survival and DNA elution rate, MISO-pretreated cells behaved as though they had received an extra 1.5 Gy. Although the initial damage after X rays was greater in MISO-pretreated cells, there was no effect of MISO pretreatment on the rate of repair of radiation-induced DNA strand breaks. The agreement between the differences in survival levels and DNA elution rates for irradiated control and MISO-pretreated cells and absence of an effect on DNA repair rates suggest that the pretreatment sensitization is due to an additive interaction of damage at the DNA level.     

Radiation damage to φX174 DNA and biological effects

Summary Dilute aqueous solutions of biologically active DNA can serve as a simplified model system of the cell. As a biological endpoint the survival of the DNA (after transfection to E. coli spheroplasts) is used. Damage in the DNA, irradiated in water with gamma rays, can be ascribed to reactions with primary waterradicals. By introducing additives in such solutions, which will scavenge the primary waterradicals, competition between a scavenger and DNA for such radicals can be studied. Comparison of different additives makes it possible to decide whether a compound behaves like a simple scavenger, radiosensitizer or like a radioprotector. In this context work has been done with the electron-affinic radiosensitizers metronidazole, misonidazole and nifuroxime. We have found that these wellknown cellular sensitizers do not enhance the inactivation of biologically active DNA. They act as simple competitive scavenger for waterradicals. However, if besides a sensitizer a trace of a metalloporphyrin containing compound (e.g. cyt. c) is present during irradiation an enhanced DNA inactivation, which can be interpreted as sensitization, is observed. Without sensitizer metalloporphyrins induce an enhanced protection of DNA.Apart from these effects the consequences of both chemical-(sulphy-dryl) and enzymatic-(excision; recombination) repair has been studied. It has been found that sulphydryl compounds are able to react with DNA radicals, modifying the radiation damage in such a way that e.g. breaks are prevented. Further in double-stranded DNA a considerable amount of OH and also H radical damage appeared to be reparable by the excision-repair mechanism. However, post-replication repair had only very small or no effect on the amount of damage.     

Radiation-induced damage to DNA in drug- and radiation-resistant sublines of a human breast cancer cell line.

An Adriamycin-resistant subline of a human breast cancer cell line, MCF-7 ADRR, has been shown to exhibit radioresistance associated with an increase in the size of the shoulder on the radiation survival curve. In the present study, damage to DNA of MCF-7 sublines WT and ADRR by 60Co gamma radiation was measured by filter elution techniques. The initial amount of DNA damage, measured by both alkaline and neutral filter elution, was lower in ADRR cells, suggesting that these cells are resistant to radiation-induced single- and double-strand DNA breaks. In the case of double-strand breaks the difference between WT and ADRR cells was significant only at the lower radiation doses studied (up to 100 Gy). In cells depleted of glutathione (GSH) by L-buthionine sulfoximine (BSO) treatment, ADRR cells were sensitized to radiation-induced DNA damage, while WT cells were unaffected. The rate of repair of single- and double-strand DNA breaks following radiation was the same for both sublines, and repair of radiation damage was not affected by BSO treatment in either cell line. The relative resistance of ADRR cells to initial DNA damage by radiation is the only difference so far detected at the molecular level which reflects radiation survival, and it is possible that other factors are involved in the resistance of ADRR cells to killing by radiation. Sensitization of ADRR cells to radiation-induced DNA damage by GSH depletion, although not likely to involve inhibition of GSH-dependent detoxification enzymes per se (irradiation was done at 4 degrees C), suggests that at the molecular level radioresponse in this subline is related to maintenance of GSH/GSSG redox equilibrium.     

Failla Memorial Lecture. Redox, radiation, and reductive bioactivation.

A brief review is presented of the background to, and the principles involved in, the development of redox-sensitive drugs for use in cancer therapy. The role of redox processes in the action of various types of radiosensitizers and in the activation of bioreductive drugs is described. The mechanisms by which many simple hypoxic cell radiosensitizers act are believed to involve fast electron transfer processes involving DNA. Some of these agents can also function as hypoxic cell cytotoxins, although the mechanisms involved are different. These "bioreductive drugs" are activated by intracellular metabolic reduction mediated through various cellular reductases. Usually, though not always, bioreduction is favored under hypoxic conditions, and this is why many of these compounds display differential cytotoxicity to hypoxic cells. This is one of the rationales for selectivity in solid tumors. The potencies of both hypoxic cell radiosensitizers and bioreductive drugs are strongly correlated with their electron affinities. Classes of bioreductive agents of current interest are described briefly. These include simple and dual-function nitroheterocycles including the highly potent compound RB-6145, quinone-based drugs including analogues of mitomycin C, and heterocyclic compounds containing N-oxide functions. The study of bioreductive agents for potential use as adjuncts for various approaches to cancer treatment is described.     

Monte carlo simulation of base and nucleotide excision repair of clustered DNA damage sites. II. Comparisons of model predictions to measured data

Clustered damage sites other than double-strand breaks (DSBs) have the potential to contribute to deleterious effects of ionizing radiation, such as cell killing and mutagenesis. In the companion article (Semenenko et al., Radiat. Res. 164, 180-193, 2005), a general Monte Carlo framework to simulate key steps in the base and nucleotide excision repair of DNA damage other than DSBs is proposed. In this article, model predictions are compared to measured data for selected low-and high-LET radiations. The Monte Carlo model reproduces experimental observations for the formation of enzymatic DSBs in Escherichia coli and cells of two Chinese hamster cell lines (V79 and xrs5). Comparisons of model predictions with experimental values for low-LET radiation suggest that an inhibition of DNA backbone incision at the sites of base damage by opposing strand breaks is active over longer distances between the damaged base and the strand break in hamster cells (8 bp) compared to E. coli (3 bp). Model estimates for the induction of point mutations in the human hypoxanthine guanine phosphoribosyl transferase (HPRT) gene by ionizing radiation are of the same order of magnitude as the measured mutation frequencies. Trends in the mutation frequency for low- and high-LET radiation are predicted correctly by the model. The agreement between selected experimental data sets and simulation results provides some confidence in postulated mechanisms for excision repair of DNA damage other than DSBs and suggests that the proposed Monte Carlo scheme is useful for predicting repair outcomes.     

Mechanism of radiosensitization by halogenated pyrimidines: effect of BrdU on cell killing and interphase chromosome breakage in radiation-sensitive cells

The effect of BrdU incorporation on cell radiosensitivity as well as on the induction of chromosome damage by radiation was studied in plateau-phase xrs-5 cells using the premature chromosome condensation (PCC) method. It is well known that xrs-5 cells are sensitive to ionizing radiation and defective in the repair of radiation-induced DNA double-strand breaks, chromosome damage, and potentially lethal damage (PLD). Compared to repair-proficient CHO 10B cells, a reduction was observed in the overall BrdU-mediated radiosensitization in plateau-phase xrs-5 cells for the same degree of thymidine replacement. This finding is interpreted with a model for BrdU-induced radiosensitization advanced previously, in which two distinct components act to produce the overall radiosensitization observed. One component involves processes associated with the increase in initial damage (DNA and chromosome) production per unit absorbed dose and causes an increase in the slope of the survival curve, while the second component involves enhanced fixation of radiation-induced damage (PLD) and causes a reduction in the width of the shoulder of the survival curve. It is suggested that in plateau-phase xrs-5 cells, the deficiency in the repair of radiation-induced damage compromises BrdU-mediated radiosensitization by leaving active only the radiosensitization component that is associated with an increase in damage induction. Enhancement of cell killing by BrdU in plateau-phase xrs-5 cells resulted in a decrease in D0, the relative value of which was similar to the relative increase in the production of chromosome damage as measured by the PCC method. The relative values for the change in D0 and the production of chromosome aberrations were similar in plateau-phase CHO 10B and xrs-5 cells, suggesting that the physicochemical and/or biochemical processes associated with this phenomenon are the same in the two cell lines. Radiosensitization of a magnitude similar to that observed in exponentially growing CHO 10B cells was induced by BrdU in exponentially growing xrs-5 cells. This effect is attributed to a partial expression of the repair gene (transiently during S phase in all cells, or throughout the cycle in a fraction of cells) that permits some repair of radiation-induced damage and which is compromised by BrdU.     

Radiosensitization, pharmacokinetics, and toxicity of a 2-nitroimidazole nucleoside (RA-263)

A 2-nitroimidazole nucleoside, 1-(2',3'-dideoxy-alpha-D-erythro-hex-2'-enopyranosyl)-2-nitroimida zole (RA-263), has been investigated for its radiosensitization, pharmacokinetics, and toxicity properties. The in vitro radiosensitization tests against hypoxic Chinese hamster (V-79) cells demonstrated that RA-263 was a more potent radiosensitizer than misonidazole and at 2 mM concentration approached the oxic curve. Significant in vitro radiosensitization activity was also observed in EMT6 mammary tumor cells. The in vitro cytotoxicity data suggested that RA-263 is considerably more toxic to hypoxic cells than misonidazole. The increased cytotoxicity may be related to its higher depletion of nonprotein thiols (NPSH) than misonidazole. The combined effects of radiosensitization and hypoxic cell toxicity were measured by preincubation of the V-79 cells for 4 h under hypoxic conditions before irradiation. The results demonstrated a synergistic response by causing a significant decrease in the extrapolation number with loss of shoulder of the radiation survival curves. The in vivo radiosensitization experiments conducted by the in vivo-in vitro cloning assay with the EMT6 mammary tumor indicate that RA-263 is an effective sensitizer. Pharmacokinetic data suggested that RA-263 was eliminated from plasma by a rapid alpha phase and a slower beta phase with T 1/2 of 36 and 72 min, respectively. The concentration in the brain was approximately one-sixth of tumor concentration, suggesting that RA-263 is excluded from the CNS. Moreover, RA-263 was two times less toxic than misonidazole on equimolar basis by acute LD50 tests. This agent was also significantly less mutagenic than misonidazole in a strain of Escherichia coli.     

DNA-targeted 2-nitroimidazoles: studies of the influence of the phenanthridine-linked nitroimidazoles, 2-NLP-3 and 2-NLP-4, on DNA damage induced by ionizing radiation

The nitroimidazole-linked phenanthridines 2-NLP-3 (5-[3-(2-nitro-1-imidazoyl)-propyl]-phenanthridinium bromide) and 2-NLP-4 (5-[3-(2-nitro-1-imidazoyl)-butyl]-phenanthridinium bromide) are composed of the radiosensitizer, 2-nitroimidazole, attached to the DNA intercalator phenanthridine by a 3- and 4-carbon linker, respectively. Previous in vitro assays showed both compounds to be 10-100 times more efficient as hypoxic cell radiosensitizers (based on external drug concentrations) than the untargeted 2-nitroimidazole radiosensitizer, misonidazole (Cowan et al., Radiat. Res. 127, 81-89, 1991). Here we have used a (32)P postlabeling assay and 5'-end-labeled oligonucleotide assay to compare the radiation-induced DNA damage generated in the presence of 2-NLP-3, 2-NLP-4, phenanthridine and misonidazole. After irradiation of the DNA under anoxic conditions, we observed a significantly greater level of 3'-phosphoglycolate DNA damage in the presence of 2-NLP-3 or 2-NLP-4 compared to irradiation of the DNA in the presence of misonidazole. This may account at least in part for the greater cellular radiosensitization shown by the nitroimidazole-linked phenanthridines over misonidazole. Of the two nitroimidazole-linked phenanthridines, the better in vitro radiosensitizer, 2-NLP-4, generated more 3'-phosphoglycolate in DNA than did 2-NLP-3. At all concentrations, phenanthridine had little effect on the levels of DNA damage, suggesting that the enhanced radiosensitization displayed by 2-NLP-3 and 2-NLP-4 is due to the localization of the 2-nitroimidazole to the DNA by the phenanthridine substituent and not to radiosensitization by the phenanthridine moiety itself.     

Radiosensitization of human colorectal tumor cells by 4-nitro-5-sulfonatoimidazoles

Misonidazole, a clinically-effective 2-nitroimidazole hypoxic cell radiation sensitizer, and 12 4-nitro-5-sulfonatoimidazoles were tested in cultured human SW1116 colorectal adenocarcinoma cells for radiosensitizing efficiency. Octanol-water partition coefficients and HPLC capacity factors were determined for all agents as measurements of lipophilicity, and an excellent correlation was found between the two measurements. Cytotoxicity, in vitro glutathione reactivity, and one-electron reduction potential were also determined for each compound to evaluate potential utility as macromolecularly transported radiosensitizers. Ten members of the set were found to be 40 to 300 times more radiotoxic than misonidazole, but no correlation was found between their radiosensitizing efficiencies and the chemical and physical parameters.     

The effect of single versus double-strand substitution on halogenated pyrimidine-induced radiosensitization and DNA strand breakage in human tumor cells

To better understand the mechanism underlying halogenated pyrimidine-mediated cytotoxicity and radiosensitization in human tumor cells, a study was undertaken to determine the influence of unifilar (one DNA strand) versus bifilar (both DNA strands) substitution of thymidine by the halogenated bases 5-iodo-2'-deoxyuridine (IdUrd) and 5-bromo-2'-deoxyuridine (BrdUrd) in HT29 human colon cancer cells. Unifilar labeling was obtained by incubating cells with IdUrd or BrdUrd for one doubling time. Cells were incubated for at least three doublings to approximate bifilar substitution. Only IdUrd caused significant cytotoxicity, which correlated with incorporation into DNA. Both BrdUrd and IdUrd were potent radiosensitizers. Radiosensitization was linearly correlated with incorporation of both bases regardless of the number of strands in which thymidine was substituted. In contrast, the relationship between radiosensitization and DNA double-strand breakage was critically dependent in the case of IdUrd, but not for BrdUrd, on whether substitution was unifilar or bifilar. These findings suggest that incorporation is the best predictor of radiation sensitivity, and that the induction of DNA double-strand breaks alone does not account for radiosensitization mediated by halogenated pyrimidines in these human tumor cells.     

Damage pattern as a function of radiation quality and other factors

An understanding of damage pattern in critical cellular structures such as DNA is an important prerequisite for a mechanistic assessment of primary radiation damage, its possible repair, and the propagation of residual changes in somatic and germ cells as potential contributors to disease or ageing. Important quantitative insights have been made recently on the distribution in time and space of critical lesions from direct and indirect action of ionizing radiation on mammalian cells. When compared to damage from chemicals or from spontaneous degradation, e.g. depurination or base deamination in DNA, the potential of even low-LET radiation to create local hot spots of damage from single particle tracks is of utmost importance. This has important repercussions on inferences from critical biological effects at high dose and dose rate exposure situations to health risks at chronic, low-level exposures as experienced in environmental and controlled occupational settings. About 10,000 DNA lesions per human cell nucleus and day from spontaneous degradation and chemical attack cause no apparent effect, but a dose of 4 Gy translating into a similar number of direct and indirect DNA breaks induces acute lethality. Therefore, single lesions cannot explain the high efficiency of ionizing radiation in the induction of mutation, transformation and loss of proliferative capacity. Clustered damage leading to poorly repairable double-strand breaks or even more complex local DNA degradation, correlates better with fixed damage and critical biological endpoints. A comparison with other physical, chemical and biological agents indicates that ionizing radiation is indeed set apart from these by its unique micro- and nano-dosimetric traits. Only a few other agents such as bleomycin have a similar potential to cause complex damage from single events. However, in view of the multi-stage mechanism of carcinogenesis, it is still an open question whether dose-effect linearity for complex primary DNA damage and resulting fixed critical cellular lesions translate into linearity for radiation-induced cancer. To solve this enigma, a quantitative assessment of all genotoxic and harmful non-genotoxic agents affecting the human body would be needed.     

Is Nitric Oxide (NO) the Last Word in Radiosensitization? A Review

As a short-lived radical that diffuses across membranes, rather than interacting with membrane-bound receptors, nitric oxide (NO) represents a significant departure from synthetically derived radiosensitizers. An endogenous compound, NO may equal or surpass its molecular cousin, oxygen, as a hypoxic radiosensitizer, through pleiotropic phenotypic effects on tumor perfusion, cell signaling, mitochondrial respiration, the fixation of radiation-induced damage, and the radioprotection of normal tissue. However, unlike oxygen, in the context of radiosensitization, the clinical role and utility of NO are poorly understood, with often contradictory and controversial reported effects: whether NO functions as a radiosensitizer may ultimately be contextual to the tumor microenvironment. This may make NO manipulation an ideal candidate for a personalized radiosensitization approach tailored to specific patient and tumor types/microenvironmental characteristics. Effective delivery of NO both systemically and directly to the tumor may be critical to the success of this approach. Compounds that release NO or NO precursors have the potential to drive innovation and result in a new fertile branch of the radiosensitizer tree.     

Mechanism of radiosensitization by halogenated pyrimidines: bromodeoxyuridine and beta-arabinofuranosyladenine affect similar subsets of radiation-induced potentially lethal lesions in plateau-phase Chinese hamster ovary cells

Chinese hamster ovary (CHO) cells were grown to plateau phase in the presence of various amounts of bromodeoxyuridine (BrdU) and treated after irradiation with beta-arabinofuranosyladenine (ara-A), an inhibitor of DNA and potentially lethal damage (PLD) repair, in order to investigate the importance of repair reactions in general and of PLD repair, in particular, on the mechanism of radiosensitization by halogenated pyrimidines. The degree of BrdU-mediated radiosensitization observed in ara-A-treated cells was compared to that of cells incubated after irradiation in the absence of ara-A. A substantial reduction in BrdU-mediated radiosensitization was observed in cells treated with ara-A at concentrations that, when given alone, produced maximum potentiation in cell killing (500-1500 microM). The residual BrdU-mediated radiosensitization observed at high levels of thymidine replacement could be explained by a BrdU-mediated increase in DNA and chromosome damage induction per gray. These findings are similar to those reported previously for a repair-deficient mutant of CHO cells, the xrs-5 cell line, and consistent with the hypothesis that BrdU-mediated radiosensitization has two distinct components, one that derives from an increase in damage induction per gray, and a second one that derives from an effect of BrdU on the repair of radiation-induced damage. It is proposed that the reduction in BrdU-mediated radiosensitization observed in ara-A-treated cells is the result of ara-A-mediated expression of radiation damage, the repair of which would have been otherwise modulated by BrdU. Since ara-A is known to act by fixing a form of radiation-induced PLD (alpha-PLD), we further propose that BrdU acts by fixing alpha-PLD. A synergistic effect in the potentiation of cell killing was observed between ara-A and BrdU when ara-A was given at concentrations below 100 microM. This result suggests that a benefit may be expected in the clinic from the combined application of halogenated pyrimidines with repair inhibitors, if administered at a carefully screened range of concentrations.