Synthesis of new hypoxia markers EF1 and [18F]-EF1.

We report on the preparation of a hypoxia marker 2-(2-nitroimidazol-1[H]-yl)-N-(3-fluoropropyl)acetamide (EF1) and its 18F analog, 2-(2-nitroimidazol-1[H]-yl)-N- (3-[18F]fluoropropyl)acetamide ([18F]-EF1). Two methods for the preparation of 3-fluoropropylamine, the EF1 side chain, are described. [18F]-EF1 was prepared with a radiochemical yield of 2% by nucleophilic substitution of bromine in 2-(2-nitroimidazol-1[H]-yl)-N-(3-bromopropyl)acetamide (EBr1) by carrier-added 18F in DMSO at 120 degrees C. Our results demonstrate the preparation of clinically relevant amounts of [18F]-EF1 for use as a non-invasive hypoxia marker with detection using positron emission tomography (PET).

Radioresponsiveness at low doses: hyper-radiosensitivity and increased radioresistance in mammalian cells.

The rationale for and importance of research on effects after radiation at "low doses" are outlined. Such basic radiobiological studies on induction of repair enzymes, protective mechanisms, priming, and hypersensitivity are certainly all relevant to treatment of cancer (see Section 1, Studies at low doses - relevance to cancer treatment). Included are examples from many groups, using various endpoints to address the possibility of an induced resistance, which has been compared to the adaptive response [M.C. Joiner, P. Lambin, E.P. Malaise, T. Robson, J.E. Arrand, K.A. Skov, B. Marples, Hypersensitivity to very low single radiation doses: its relationship to the adaptive response and induced radioresistance, Mutat. Res. 358 (1996) 171-183.]. This is not intended to be an exhaustive review--rather a re-introduction of concepts such as priming and a short survey of molecular approaches to understanding induced resistance. New data on the response of HT29 cells after treatment (priming) with co-cultured activated neutrophils are included, with protection against X-rays (S1). Analysis of previously published results in various cells lines in terms of increased radioresistance (IRR)/intrinsic sensitivity are presented which complement a study on human tumour lines [P. Lambin, E.P. Malaise, M.C. Joiner, Might intrinsic radioresistance of human tumour cells be induced by radiation?, Int. Radiat. Biol. 69 (1996) 279-290].It is not feasible to extrapolate to low doses from studies at high doses. The biological responses probably vary with dose, LET, and have variable time frames. The above approaches may lead to new types of treatment, or additional means to assess radioresponsiveness of tumours. Studies in many areas of biology would benefit from considerations of different dose regions, as the biological responses vary with dose. There may also be some implications in the fields of radiation protection and carcinogenesis, and the extensions of concepts of hyper-radiosensitivity (HRS)/IRR extended to radiation exposure are considered in Section 2, Possible relevance of IRR concepts to radiation exposure (space). More knowledge on inducible responses could open new approaches for protection and means to assess genetic predisposition. Many endpoints are used currently--clonogenic survival, mutagenesis, chromosome aberrations and more direct--proteins/genes/functions/repair/signals, as well as different biological systems. Because of scant knowledge of the relevant aspects at low doses, such as inducible/protective mechanisms, threshold, priming, dose-rate effects, LET within one system, it is still too early to draw conclusions in the area of radiation exposure. Technological advances may permit much needed studies at low doses in the areas of both treatment and protection.

Assessment of toxicity of bis-platinum complexes in hypoxic and aerobic cells.

There is considerable interest in the bis-platinum series of complexes as potential chemotherapeutic agents, due to their activity in cisplatin-resistant lines and in various tumor types. Our interest in their hypoxic selectivity stems from the fact that cisplatin exhibits greater cytotoxicity in hypoxic than aerobic cells. Unlike nitroaromatics, quinones, tirapazamine and many other hypoxia selective agents, a 'bioreductive' moiety cannot explain these observations. We hypothesized that DNA-protein cross-links (D-P) might play a role in the mechanism. Bis-platinum complexes have variable cross-linking potentials, and their toxicities were assessed in air or hypoxia in CHO cells. Of the three classes examined, only those from the 2,2/cis,cis series show greater hypoxic selectivity than cisplatin. These have greater potential for cross-links than cisplatin, being potentially bifunctional at each platinum, with the two leaving groups (X) in the cis position, and with variable distance (n) between the platinum centers: cis-[(PtX2(NH3))2H2N(CH2)nNH2]. Cellular platinum accumulation and DNA binding were also measured, and like cisplatin, results are consistent with a more toxic lesion formed in hypoxia. Lower hypoxic selectivity in the UV20 cell line may reflect an inability to excise the relevant lesion. These results support the D-P hypothesis. Further support comes from a 1,1/trans,trans complex which does not form D-P and which exhibited the reverse behavior to 2,2/cis,cis or cisplatin, i.e. higher toxicity in aerobic than in hypoxic cells. This study examines the possibility of an additional mechanism of selection for hypoxic toxicity involving DNA-protein cross-links.

Low dose hyper-radiosensitivity and increased radioresistance in mammalian cells.

This manuscript reviews the low-dose survival work using the DMIPS cell analyser that has been carried out at the Gray Laboratory in the U.K. and the British Columbia Cancer Research Centre in Canada. It describes low dose hyper-radiosensitivity (HRS) detected after single doses of X-rays less than approximately 0.3 Gy and the subsequent increased radioresistant response (IRR) seen as the dose increases up to 1 Gy. Work is summarized from studies in V79 cells, normal human and human tumour cell lines and mutant cell lines deficient in DNA repair. The data are considered in light of the hypothesis that hyper-radiosensitivity and increased radioresistance reflect the existence of an inducible protective mechanism, possibly triggered by DNA damage.

Hypersensitivity to very-low single radiation doses: its relationship to the adaptive response and induced radioresistance.

There is now little doubt of the existence of radioprotective mechanisms, or stress responses, that are upregulated in response to exposure to small doses of ionizing radiation and other DNA-damaging agents. Phenomenologically, there are two ways in which these induced mechanisms operate. First, a small conditioning dose (generally below 30 cGy) may protect against a subsequent, separate, exposure to radiation that may be substantially larger than the initial dose. This has been termed the adaptive response. Second, the response to single doses may itself be dose-dependent so that small acute radiation exposures, or exposures at very low dose rates, are more effective per unit dose than larger exposures above the threshold where the induced radioprotection is triggered. This combination has been termed low-dose hypersensitivity (HRS) and induced radioresistance (IRR) as the dose increases. Both the adaptive response and HRS/IRR have been well documented in studies with yeast, bacteria, protozoa, algae, higher plant cells, insect cells, mammalian and human cells in vitro, and in studies on animal models in vivo. There is indirect evidence that the HRS/IRR phenomenon in response to single doses is a manifestation of the same underlying mechanism that determines the adaptive response in the two-dose case and that it can be triggered by high and low LET radiations as well as a variety of other stress-inducing agents such as hydrogen peroxide and chemotherapeutic agents although exact homology remains to be tested. Little is currently known about the precise nature of this underlying mechanism, but there is evidence that it operates by increasing the amount and rate of DNA repair, rather than by indirect mechanisms such as modulation of cell-cycle progression or apoptosis. Changed expression of some genes, only in response to low and not high doses, may occur within a few hours of irradiation and this would be rapid enough to explain the phenomenon of induced radioresistance although its specific molecular components have yet to be identified.

Small doses of high-linear energy transfer radiation increase the radioresistance of Chinese hamster V79 cells to subsequent X irradiation.

Chinese hamster V79 cells show a complex X-ray survival response which is characterized by hypersensitivity followed by increased resistance as the dose increases to 1 Gy. This hypersensitivity can be eliminated by pretreating cells with X rays or hydrogen peroxide. Accordingly, the protective effect that results from the priming treatments could be considered analogous to the "adaptive response" induced by low-linear energy transfer (LET) radiation and some chemical agents in human lymphocytes. Indeed, no hyper-radiosensitive response after single treatments in V79 cells or adaptive response in human lymphocytes has been reported after exposure to high-LET radiation. To investigate this further, we measured the survival after X irradiation of V79-379A cells previously irradiated with small priming doses of high-LET radiation. After a 0.2-Gy priming dose of neutrons followed by a 1-Gy 250 kVp X-ray dose given 4 h later, survival was 1.08 +/- 0.04 compared to 0.73 +/- 0.03 when the doses were given concurrently. Increases in survival were also observed from 0.80 +/- 0.03 to 0.96 +/- 0.05 after a 0.2-Gy priming treatment with 250 kVp X rays and from 0.78 +/- 0.03 to 0.84 +/- 0.03 with a priming dose of Bragg-peak negative pi mesons. The results indicate that a protective effect, as measured by an increase in radioresistance, is induced by high-LET neutrons, as well as by Bragg-peak pi mesons and X rays, and that a threshold level of damage is required for adaptation to occur.

Response of V79 cells to low doses of X-rays and negative pi-mesons: clonogenic survival and DNA strand breaks.

Mammalian cells are hypersensitive to very low doses of X-rays (< 0.2 Gy), a response which is followed by increased radioresistance up to 1 Gy. Increased radioresistance is postulated to be a response to DNA damage, possibly single-strand breaks, and it appears to be a characteristic of low linear energy transfer (LET) radiation. Here we demonstrate a correspondence between the extent of the increased radioresistance and linear energy transfer of 250 kVp X-rays and plateau and Bragg peak negative pi-mesons. The results support our hypothesis since the size of the increased radioresistant response appears to correspond to the number of radiation induced single-strand breaks. Furthermore, since survival prior to the increased radioresistant response (< 0.2 Gy) was LET-independent, these data support the notion that the increased radioresistant response may dictate the overall survival response to higher doses. However, while these data provide further circumstantial evidence for the involvement of DNA strand breaks in the triggering of increased radioresistance, more direct conclusions cannot be made. The data are not accurate enough to detect structure in the single-strand break profiles, the production of single-strand breaks being apparently linear with dose.

Porphyrins and metalloporphyrins: potential hypoxic agents.

Synthetic water-soluble porphyrins and their metalloporphyrin derivatives with Co(III), Cu(II), Ru(II) and Pt(II), containing various functional groups within the meso-positions of the porphyrin, were synthesised and evaluated as hypoxic agents, especially as cytotoxins and radiosensitisers. Cobalt complexes of the porphyrins containing positively charged methylpyridinium groups showed selective toxicity toward hypoxic Chinese Hamster Ovary (CHO) cells. The Co(III) complexes of the cationic and the anionic porphyrins are all weak radiosensitisers toward hypoxic cells, the highest sensitisation enhancement ratio (SER = 1.22, at 50 muM) being with a porphyrin complex containing a cis-arrangement of two nitro and two methylpyridinium meso-substituents. A copper complex of a tetracationic porphyrin showed slight radiosensitisation activity with an SER value of about 1.1. The other metalloporphyrins showed no hypoxic selectivity or radiosensitisation activity. In total, over 50 porphyrin free bases have been synthesised, of which half are water-soluble and have been metallated; thus, the chemistry is now in place for further development of water-soluble hypoxic agents.

Effects of geometric isomerism and ligand substitution in bifunctional dinuclear platinum complexes on binding properties and conformational changes in DNA.

The DNA binding profile of a series of dinuclear platinum complexes [{trans-PtCl-(L)2}2H2N(CH2)nNH2]2+ (L = NH3 or py; 1,1/t,t/NH3 and 1,1/t,t/py, respectively) and [{cis-PtCl-(NH3)2H2N(CH2)nNH2]2+ (1,1/c,c/NH3) was examined to compare the effects of geometrical isomerism and the presence of ligands other than NH3 in the coordination sphere. Steric effects, because of the geometry of the leaving groups cis to the diamine bridge or the presence of planar pyridine ligands, result in diminished binding to calf thymus DNA for these isomers. In contrast, the pyridine derivative shows a distinct binding preference for poly(dG-dC).poly(dG-dC) in comparison to both NH3 isomers. Both NH3 complexes induced the B-->Z transition in poly(dG-dC).poly(dG-dC), but the presence of a pyridine ligand stabilized the B conformation. The bifunctional binding of the NH3 isomers results in unwinding of supercoiled pUC19 plasmid DNA equivalent to cis-DDP, while the unwinding of the pyridine derivative is approximately twice that of the mononuclear trans-[PtCl2(py)2]. DNA-DNA interstrand cross-linking is very efficient for all three agents, but sequencing studies indicated that only the 1,1/t,t/NH3 derivative is capable of forming a (Pt,Pt) intrastrand cross-link to the adjacent guanines of a d(GpG) sequence. The effects on DNA caused by bifunctional binding of dinuclear complexes are compared with those from the mononuclear [PtCl2(NH3)2] isomers. The results are discussed with respect to the antitumor activity of the dinuclear series.

Recognition of platinum-induced DNA damage by nuclear proteins: screening for mechanisms.

The association of high mobility group (HMG) proteins with cisplatin-damaged DNA has attracted considerable interest because of a possible relationship with drug-acquired resistance and the repair of DNA damage caused by this important chemotherapeutic agent. We have further characterized the binding of HMG proteins to cisplatin-damaged DNA using a modification of the damaged DNA affinity precipitation assay (DDAP) and proteins isolated from the nuclei of V79 cells. HMG proteins recognized cisplatin adducts only in double-stranded DNA sequences. Pre-treatment of cells with cisplatin (1 microM) prior to protein extraction decreased the apparent yield of HMG proteins. However, the pre-treatment of isolated protein did not prevent recognition of the DNA adducts. To investigate the possible role of HMG proteins in toxicity and resistance, we have extended the DDAP method to study other platinum agents, some of which are active in cisplatin-resistant cells. A comparison of trans- and cis-[PtCl2(NH3)quinoline] is presented as an example. HMG proteins recognized DNA damage caused by the cis, but not the trans isomer. However, the trans isomer is known to be significantly more toxic and is highly active in cisplatin-resistant cells, suggesting a mechanism of action different from cisplatin. Therefore, the toxicity of trans-[PtCl2(NH3)quinoline] appears to be unrelated to the recognition of damage by HMG proteins. The DDAP assay may provide an additional screen for new mechanisms of cytotoxic platinum agents.

Effects of fixation method on image cytometric measurement of DNA content and distribution in cells stained for fluorescence with propidium iodide.

We performed image cytometric measurements of DNA content and distribution on cycling human HCT-8 cells stained for fluorescence with propidium iodide (PI). Seven different fixation protocols were evaluated for stoichiometry of PI staining and for their ability to preserve in vivo chromatin structure. Bimodal integrated optical intensity (IOI) histograms were obtained with all fixation protocols. Increased accessibility of DNA to the dye was evident in increased values of the IOI at the GI peak. The fixatives studied, in order of increasing accessibility to DNA, were Regaud's Boehm-Sprenger, Carnoy's, air-drying, methanol, ethanol, and acetone/methanol. In general, the coefficient of variation of the IOI within the G1 peak was higher for fixatives where DNA is less accessible. Features describing the spatial distribution of stain exhibited dramatic changes for Boehm-Sprenger fixation, which were consistent with the observation that in vivo conformation of chromatin is best preserved with this method.

Enhanced radiation-sensitivity by preincubation with nitroimidazoles: effect of glutathione depletion.

PURPOSE: The mechanism of enhanced radiosensitization by nitroheterocyclics after a preincubation period under hypoxic conditions was investigated. The hypothesis that this phenomenon was caused by glutathione depletion was tested. METHODS AND MATERIALS: The phenomenon of enhanced radiosensitization by nitroheterocyclics after a preincubation period under hypoxic conditions is potentially of importance therapeutically because essentially nonlethal preradiation exposures to the electron affinic drugs cause a much larger radiation sensitization than would otherwise be expected. We have investigated this interesting property of several 2-nitroimidazoles to determine its possible cause and to test various hypotheses about maximizing its possible therapeutic benefit. In view of many observations that thiols are depleted by incubation of cells with nitroimidazoles under hypoxic conditions, we have specifically investigated this aspect of the preincubation effect. Depletion of glutathione was either enhanced by an overnight incubation with buthionine sulfoximine or minimized by preincubation with a 2-nitroimidazole which is sterically inhibited from causing thiol depletion. RESULTS: When conditions were chosen which minimized variations in cellular glutathione content during the preincubation period, no preincubation effect was observed. At low, therapeutically relevant radiation doses, where 2-nitroimidazoles are less efficient sensitizers, the preincubation effect may be even more important, but thiol depletion still minimizes its impact in this region of the dose-response curve. CONCLUSION: These results suggest that the preincubation effect is caused by a "self-sensitization" involving the known enhancement of radiation sensitization by thiol depletion.

Molecular, cellular, and genetic basis of radiosensitivity at low doses: a case of inducible repair?

Many proteins are induced by ionizing radiation, and genes are activated. We still do not know which, if any, are responsible for IRR, or what leads to the adaptive response seen at still lower doses. Are these the same responses? Are they related to apoptosis, repair of potentially lethal damage and other responses? Does the cell have a whole battery of responses, depending on the dose? I suspect this is the case. Can the responses be explained more simply, as effects on regulators of cell cycle or induction of fidelity, or is there induction of repair? Are there still other explanations for the apparent protection? The initial slope of the survival curve which was addressed earlier (1) must take on new meaning given the hyperradiosensitive portion. Similarly, we may have to change our thinking with respect to the LQ description of survival data. It is not surprising that this workshop, held at such an early stage primarily to address the phenomenon of increased radioresistance, produced more questions than answers. Single-strand breaks may trigger resistance, but additional lesions or classes of damage may be relevant. Some physicists expect the damage caused to be linear with dose; the biologists suggest that the response is nonlinear (e.g. saturation of an enzyme, induction of repair, cell cycle effects) and there is room for biochemistry which could also vary with dose (e.g. consumption of a protector or a sensitizer). Some biophysicists would argue that the observed structures in survival curves might be explained by change in the target cross section such as a large change in DNA conformation caused by a very low dose. There is some reluctance in the radiobiology community to accept that cells may respond to ionizing radiation by inducing or activating protective mechanisms, although the cell exhibits defensive responses to many other detrimental stimuli. If "the heart of the matter is in the shape of the survival curve" as suggested by Dr. Elkind in his summary of the 1974 "low doses" conference (p. 385 in ref. 1), then we are fortunate indeed that there are now additional methods to attack the question directly of what is turned on or activated. It is anticipated that there will be many further developments within the year, to be presented at related sessions at larger meetings, and at a closely related meeting to be held in June 1994 in Montreal, entitled "Gene Induction and Adaptive Responses in Irradiated Cells: Mechanisms and Clinical Implications."

The effect of radiosensitizers on the survival response of hypoxic mammalian cells: the low X-ray dose region, hypersensitivity and induced radioresistance.

It has been shown previously that the extent of chemical modification of the hypoxic radiation response is dependent on dose. Some types of sensitizer are more effective at low doses (to 4 Gy) than at higher doses. Since such drugs are possible adjuvants to radiotherapy, the mechanisms responsible for the variable response at clinical doses are of interest. Existing reports on sensitization at low doses are summarized, and the effects of cisplatin and buthionine sulfoximine on the purported induced response to radiation in hypoxic cells are presented. Cisplatin at a low, nontoxic concentration (1 microM) appears to abolish the increased radioresistant portion of the survival response. A role for high-mobility-group protein binding by platinum drugs is hypothesized to explain their interaction with radiation, and conversely, it is suggested that the heretofore unexplained different behavior of certain hypoxic sensitizers at low doses could be, at least in part, an effect on the induction of resistance.

The effect of oxygen on low-dose hypersensitivity and increased radioresistance in Chinese hamster V79-379A cells.

Chinese hamster V79 cells irradiated in air are hypersensitive to X-ray doses less than 0.5 Gy and show an increased radioresistance over the dose range 0.5-1 Gy (Marples and Joiner, Radiat. Res. 133, 41-51, 1993). Of considerable interest from both a mechanistic and clinical viewpoint is the response of hypoxic cells over this dose range. The data presented here indicate that hypoxic cells are also hypersensitive to low X-ray doses and exhibit an increased radioresistant response, albeit triggered at a somewhat higher dose (0.69 Gy, SEM +/- 0.18 Gy) than observed in oxygenated cells (0.5 Gy, SEM +/- 0.21 Gy). These data indicate that the triggering event for increased radioresistance may be independent of oxygen. As reported by others previously, the oxygen enhancement ratio was found to decrease with a decreasing X-ray dose.

The response of Chinese hamster V79-379A cells exposed to negative pi-mesons: evidence that increased radioresistance is dependent on linear energy transfer.

Chinese hamster V79-379A cells exhibit low-dose hypersensitivity to 250 kVp X rays followed by an increased radioresistant response over the dose range 0.5-1 Gy. This phenomenon is not seen with neutrons (Marples and Joiner, Radiat. Res. 133, 41-51, 1993). It was therefore postulated the induction of radioresistance might develop as a response to a cellular event(s) which predominates after low- and not high-LET radiation. To test this hypothesis, we measured the survival response of V79-379A cells exposed to pions. Clonogenic survival was assessed for cells irradiated in the Bragg peak (35 keV/microns) and plateau region (10-20 keV/microns) of the beam, using an automated microscope (DMIPS cell analyzer). As expected, peak pions were found to be more effective per unit of dose at killing cells than plateau pions. The survival curve for cells irradiated in the plateau of the pion beam was found to incorporate a region of low-dose hypersensitivity and increased radioresistance, the effective D0 was dose-dependent, ranging from 3.5-5. This was not seen with peak pions, where the effective D0 was, on average, constant reflecting a single-exponential survival curve. Fitting the data with an induced repair model indicates that the phenomenon of increased radioresistance is almost certainly dependent on LET.

Radiosensitizing and toxic properties of quinoline and nitroquinoline complexes of platinum [PtCl2(NH3)quinoline].

The assessment of drugs designed to be useful in the eradication of hypoxic (resistant) cells involves comparison of hypoxic and aerobic radiosensitization, cytotoxicities, as well as DNA binding and reduction potentials. Three pairs of isomers of quinoline complexes [amino dichloro quinoline platinum (II)] were studied in this context. For the cis 5- and 6-nitroquinoline complexes, the DNA binding and toxicity were higher with the 5-substituted ligand. Reduction potentials were similar (-260 and -280 mV). No selectivity for hypoxic toxicity was observed, but radiosensitizing ability by both complexes was greater in hypoxic (than oxic) Chinese hamster ovary (CHO) cells. The trans 5-nitroquinoline complex produced better sensitization in hypoxia than its cis isomer [enhancement ratio (ER) 1.7 at 10 microM versus 40 microM for cis]. However, this was accompanied by some aerobic sensitization. The trans isomer of the (unsubstituted) quinoline complex was considerably more toxic than its cis isomer. Neither showed selectivity for hypoxia, either as radiosensitizers or as cytotoxins, which may be attributable to the lack of a reducible (nitro) function. Four quinoline complexes showed high activity in cisplatin-resistant L1210 cells, with the lowest resistance factor being for the trans quinoline complex. Results suggest that trans complexes with one aromatic ring may have activity different from the cis geometry, which should be exploited with respect to cisplatin resistance and cross-resistance with radiation.

Low concentrations of nitroimidazoles: effective radiosensitizers at low doses.

PURPOSE: There have been various reports that nitroimidazole radiosensitizers are less effective modifiers of radiation response in the clinically relevant x-ray dose regions (0-4 Gy) than they are at doses used in classical in vitro experiments. Our studies at low concentrations of etanidazole led us to question this generalization, and our purpose was to further investigate low concentrations at low doses, using the microscopic location of cells to facilitate these experiments. The observations are compared with data on these drugs in the literature using other methods and systems. METHODS AND MATERIALS: Survival of V79 cells after irradiation in hypoxia +/- drug at various concentrations was assessed using the Cell Analyser (DMIPS) for low doses, in comparison with the standard clonogenic assay (higher doses). Enhancement ratios (ERs) were calculated at 80% and 2% survival, respectively. RESULTS: Etanidazole (SR-2508) consistently gave higher ERs at low doses (measured at 80% survival) than at high doses (survival 2%), when cells were exposed to drug concentrations below approximately 2 mM (e.g., at 1 mM, ER80% = 2.2, ER2% = 1.8 in CHO cells after 1 h preincubation at 37 degrees C). Preincubation of cells for 1 h or 15 min at 37 degrees C with etanidazole prior to irradiation increased the ERs at high and low doses but did not change the "cross-over" behavior (ER80% > ER2% at low concentrations, ER2% > ER80% above 2 mM, regardless of pretreatment at 37 degrees C or cell line, CHO and V79 cells), whereas the 5-nitroimidazole nimorazole consistently gave the same ERs whether determined at high or low radiation doses (e.g., at 1 mM, 1 h preincubation at 37 degrees C ER80% = ER2% = 1.3). This crossover behavior also occurred after irradiation/preincubation at 0 degrees C. The 2-nitroimidazole predecessor, misonidazole, shows the same cross-over behavior. CONCLUSION: Two nitroimidazoles at low concentrations appear to be as effective sensitizers (or better) at low doses (80% S); at high doses (2% S).

Radiosensitization by nickel lapachol.

The properties of interest in the radiosensitization of a metal complex, nickel lapachol, are compared with those of the 2-nitroimidazole, misonidazole. These very different compounds were found to be surprisingly similar in terms of their reduction potential (-370 mV), enhancement ratios for killing of hypoxic Chinese hamster ovary cells by X-irradiation, and enhancement of DNA breaks in hypoxia. For nitroimidazoles, the sensitization depends on 'electron affinity', reduction of the nitro group; for nickel lapachol it is the metal which is necessary for reduction, yet the sensitization efficiencies are remarkably close. However, the metal complex has additional activities (some sensitization in aerobic cells; increased sensitization with preincubation) which are as yet unexplained but are assumed to be related to the nature of the naphthoquinone ligand, rather than to the reduction of the metal.