Novel nitroimidazole alkylsulfonamides as hypoxic cell radiosensitisers

A novel class of nitroimidazole alkylsulfonamides have been prepared and evaluated as hypoxia-selective cytotoxins and radiosensitisers. The sulfonamide side chain markedly influences the physicochemical properties of the analogues: lowering aqueous solubility and raising the electron affinity of the nitroimidazole group. The addition of hydroxyl or basic amine groups increased aqueous solubility, with charged amine groups contributing to increased electron affinity. The analogues covered the range of electron affinity for effective radiosensitisation with one-electron reduction potentials ranging from -503 to -342mV. Cytotoxicity under normoxia or anoxia against a panel of human tumour cell lines was determined using a proliferation assay. 2-Nitroimidazole sulfonamides displayed significant hypoxia-selective cytotoxicity (6 to 64-fold), while 4- and 5-nitroimidazole analogues did not display hypoxia-selective cytotoxicity. All analogues sensitised anoxic HCT-116 human colorectal cells to radiation at non-toxic concentrations. 2-Nitroimidazole analogues provided modest sensitisation due to the relatively low concentrations used while several 5-nitroimidazole analogues provided equivalent sensitisation to misonidazole and etanidazole at similar molar concentrations.

Hypoxia-specific Inhibition of Recovery from Radiation Damage by a Novel 2-nitroimidazole with a Theophylline Side Chain

A novel 2-nitroimidazole with a theophylline side-chain, 7-(4'-(2-nitroimidazol-1-yl)-butyl)-theophylline, (NITP) was as efficient a hypoxic-cell radiosensitizer as misonidazole. However, if cells were irradiated with NITP under hypoxic conditions and then exposed to the drug under aerobic conditions, a much larger radiosensitizing effect was observed, partly because of a reduction in the size of the shoulder of the survival curve. There was little effect of NITP on the radiosensitivity of well oxygenated cells, even with post-irradiation drug contact. Split-dose survival curves showed that the drug inhibited recovery from radiation damage only when the cells were irradiated under hypoxia but not when irradiations were under oxic conditions. A reduction in the size of the shoulder of the survival curve should allow the hypoxic-cell radiosensitizing efficiency of NITP to be maintained with low doses of radiation used in multifraction cancer radiotherapy. Bifunctional drugs containing both electron-affinic and repair inhibiting groups may represent a new approach to the synthesis of hypoxic-cell targeted adjuncts to radiotherapy.

Radiation chemistry comes before radiation biology

PURPOSE

This article seeks to illustrate some contributions of radiation chemistry to radiobiology and related science, and to draw attention to examples where radiation chemistry is central to our knowledge of specific aspects. Radiation chemistry is a mature branch of radiation science which is continually evolving and finding wider applications. This is particularly apparent in the study of the roles of free radicals in biology generally, and radiation biology specifically. The chemical viewpoint helps unite the spatial and temporal insight coming from radiation physics with the diversity of biological responses. While historically, the main application of radiation chemistry of relevance to radiation biology has been investigations of the free-radical processes leading to radiation-induced DNA damage and its chemical characterization, two features of radiation chemistry point to its wider importance. First, its emphasis on quantification and characterization at the molecular level helps provide links between DNA damage, biochemical repair processes, and mutagenicity and radiosensitivity. Second, its central pillar of chemical kinetics aids understanding of the roles of 'reactive oxygen species' in cell signalling and diverse biological effects more generally, and application of radiation chemistry in the development of drugs to enhance radiotherapy and as hypoxia-specific cytotoxins or diagnostic agents. The illustrations of the broader applications of radiation chemistry in this article focus on their relevance to radiation biology and demonstrate the importance of synergy in the radiation sciences.

CONCLUSIONS

The past contributions of radiation chemistry to radiation biology are evident, but there remains considerable potential to help advance future biological understanding using the knowledge and techniques of radiation chemistry.

Measuring hypoxia and predicting tumor radioresistance with nuclear medicine assays

Tumor cells at low oxygen tension are relatively radioresistant. The hypoxic fraction of individual tumors before, during and after radiotherapy is likely to have prognostic value but its diagnosis still awaits an accurate and acceptable assay. The recent indications that hypoxia can also induce the expression of specific genes and promote a more aggressive tumor phenotype makes its diagnosis even more important. Over 15 years ago, misonidazole, an azomycin-based hypoxic cell radiosensitizer, was found to link covalently to cellular molecules at rates inversely proportional to intracellular oxygen concentration. The use of bioreducible markers to positively label zones of viable hypoxic cells within solid tumors and to predict for tumor radioresistance was proposed. Several hypoxic markers have now been identified and their selective binding within tumors has been measured by both invasive and non-invasive assays. Research from our laboratory has emphasized both mechanistic and preclinical studies associated with nuclear medicine procedures for measuring tumor hypoxia and predicting tumor radioresistance. This report updates radiation oncologists about the status of nuclear medicine hypoxic marker research and development as of mid-1997. While several potential imaging agents have been identified, their testing and validation in appropriate human tumors will require focused research efforts by individual academic departments and, possibly, by clinical trials performed through cooperative groups. Since the prediction of hypoxia in individual tumors could strongly impact radiotherapy treatment planning, the radiation oncology research community is best positioned to execute the validation studies associated with these markers.

Evaluation of the radiosensitizing effects of RK28 intravenous, intraarterial, and intratumoral injections on the rabbit VX2 tumor system

PURPOSE

To evaluate the differences in the radiosensitizing effects of intravenous (i.v.) injection, intraarterial (i.a.) injection, and intratumoral (i.t.) injections of the hypoxic cell radiosensitizer RK28 ([1-(4'-hydroxy-2'-butenoxy)methyl-2-nitroimidazole], a 2-nitroimidazole with an acyclic sugar analogue substituted at the N-1 position of the imidazole ring) using an animal experimental system.

METHODS AND MATERIALS

Rabbit VX2 tumors, which were implanted in the muscle of left hind legs and grown to 3 cm in diameter, were treated with RK28 (80 mg/kg x b.wt.) before 15 Gy of local x-ray irradiation. The auricular vein and the left saphenous artery were used for systemic injection and regional injection, respectively. For i.t. injection, a 21-gauge needle with three lateral holes was positioned in the central area of the tumor. Tumor regression was precisely evaluated by computed tomograpy (CT), and survival time was also studied. Using high-performance liquid chromatography (HPLC), pharmacokinetic studies for RK28 and its seven major metabolites were performed in tumor and serum at 0, 10, 20, 30, and 60 min after drug injection was completed.

RESULTS

Radiosensitizing effects of RK28 were considered present after i.a. injection (p < 0.05) and i.t. injection (p < 0.05) after analyzing tumor volumes on day 21 after treatment. Increased survival was not observed in any group with RK28 injection compared with survival in the group treated by x-ray irradiation alone. Pharmacokinetic studies showed the average concentration of RK28 in the tumor during x-ray irradiation was 1.3 times higher after i.a. injection and 3.5 times higher after i.t. injection than that after i.v. injection. The time modifying factor50 (TMF50: ratio of time for tumor to decrease by 50%, radiation alone vs. radiation plus drug) was calculated to be 1.5 after i.v. injection, 1.7 after i.a. injection, and 2.3 after i.t. injection. The values of TMF50 correlated to the average concentrations of RK28 in the tumor. As to metabolites of RK28, beta-glucuronated compound and cysteine conjugate were highly detected. The concentrations of cysteine conjugate were higher in the tumor than in serum via i.v. injection.

CONCLUSIONS

Radiosensitizing effects of RK28 were observed on the rabbit VX-2 tumor system after i.a. or i.t. injection. Pharmacokinetic studies proved that radiosensitizing effects depended on the concentration in the tumor, though the administration routes were different. Combined forms with nonprotein thiols were detected. However, survival benefits were not obtained by RK28. For clinical applications of RK28, i.a. or i.t. injection could facilitate better local control of cancer.

Mutagenicity of nitrobenzyl derivatives: potential bioreductive anticancer agents

Ortho-, meta- and para-nitrobenzyl bromides, alcohols, ethers and esters were synthesised and tested for their mutagenicity toward Salmonella typhimurium TA100, TA100NR (nitroreductase deficient) and TA98 in absence of S9 mix and in TA100 with S9 mix. Compounds of the ortho- and meta-series were non mutagenic with and without S9 mix. Except for the alcohol and ether, the compounds of the para-series were mutagenic in TA100 with activity sequence propionate > butyrate > benzoate > acetate > bromide and this specific activity was reduced considerably by S9 mix. The Ames Salmonella test system does not seem to be an appropriate model to evaluate mutagenicity of o-nitrobenzyls. However, further work is in progress to test all the compounds for mutagenicity in mammalian system.

Electron-affinic radiosensitizers possessing NPSH-reactive side chains: cytotoxicity and radiosensitizing activity towards hypoxic EMT6 cells in vitro

A new class of dual-function nitroazole derivatives that are composed of electron-affinic nitroazole rings and a thiol-reactive alpha, beta-unsaturated carbonyl side chain were synthesized to evaluate their physico-chemical properties, reactivity with glutathione (GSH), and cytotoxicity and radiosensitizing activity towards EMT6/KU cells in vitro. Among this class of nitroazole compounds, 2-nitroimidazole-derivative (1), 3-nitro-1,2,4-triazole derivative (2), and 2-methyl-4-nitroimidazole derivative (3) with a common side-chain structure of trans CH2CH = CHCO2CH3 readily reacted with GSH in phosphate-buffer solution (pH 7.2, 310 K). These compounds showed higher cytotoxicities to both aerobic and hypoxic EMT6/KU cells than the corresponding alpha, beta-saturated counterparts (6-8) with a side-chain structure of CH2CH2CH2CO2CH2CH3. The hypoxic cytotoxicity of 1 and 2 with similar electron affinities to that of misonidazole (9) was potentiated by the combined effects of depletion of non-protein thiols (NPSH) by the alpha, beta-unsaturated carbonyl side chains and bioreduction of the nitroazole rings. The sensitizer enhancement ratios in vitro (SERvitro) of 1 (2.80 +/- 0.20) and 2 (2.63 +/- 0.27) at a dose of 1.0 mmol dm-3 are comparable with the oxygen enhancement ratio (OER = 2.90 +/- 0.10) and are significantly larger than those of their respective counterparts 6 (1.28 +/- 0.06) and 7 (1.22 +/- 0.09). A less electron-affinic compound, 3, also gave a large SERvitro = 1.80 +/- 0.18, whereas the counterpart 8 was not effective (1.10) in radiosensitizing hypoxic cells. Compounds 1-3 not only altered the slope, but also reduced the shoulder of the dose-survival curve. These 'dual-function' nitroazole radiosensitizers show much lower levels of in vitro radiosensitization, as measured by the C1-6, than the previously reported 'anomalous' radiosensitizers such as 5-substituted 4-nitroimidazole radiosensitizers.

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.

Modification of the aerobic cytotoxicity of etanidazole

PURPOSE

To determine the feasibility of modifying the aerobic cytotoxicity of etanidazole without interfering with the tumoricidal action of radiation plus etanidazole.

METHODS AND MATERIALS

The aerobic cytotoxicity of etanidazole was studied using two different models: (1) Induction of apoptosis in EL4 cells: apoptotic DNA fragmentation was analyzed by agarose gel electrophoresis following 24 h treatment with etanidazole alone or in combination with various modifiers. (2) Spinal cord neuronal loss in organotypic roller tube cultures: Survival of acetylcholinesterase positive ventral horn neurons was analyzed morphometrically following 72 h treatment with etanidazole alone or in combination with vitamin E succinate.

RESULTS

Etanidazole (10 mM) induced apoptosis in EL4 cells. This effect was suppressed by 24 h treatment with TPA, IBMX, the free radical scavenger TEMPOL or vitamin E succinate. Vitamin E succinate also protected spinal cord cultures from etanidazole-induced neuronal loss.

CONCLUSION

These results suggest that it might be possible to modify the neurotoxicity of etanidazole with agents that would not be expected to interfere with the tumoricidal action of radiation plus etanidazole.

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.

Metabolism and radiosensitization of 4,5-dimethylmisonidazole, a ring-substituted analog of misonidazole

4,5-Dimethylmisonidazole (DMM) is a ring-substituted derivative of the 2-nitroimidazole, misonidazole. 2-Nitroimidazoles are able to sensitize radioresistant hypoxic cells, and to kill them outright through bioreductive metabolism. The toxic process is believed to reflect the consequences of reductively activated drugs forming adducts with cellular (macro)molecules. Both this process and the radiosensitizing activity are thought to correlate with the electron affinity of radiosensitizing agents. In the present study, methyl groups were added to the imidazole ring of misonidazole in order to hinder adduct formation with cellular molecules after reductive-activation of the compound. It was anticipated that this would substantially decrease the hypoxic-cell toxicity of the parent drug. The presence of the two methyl groups reduced the half-wave reduction potential of DMM by about 70 mV, so we expected that its radiosensitizing ability would also decrease. In direct comparison with misonidazole, DMM, at equimolar concentrations, showed dramatically reduced binding to cellular macromolecules under bioreductive conditions, both in vivo, using a liver perfusion system, and in vitro, using tissue culture cells incubated under extreme hypoxia. However, DMM was only moderately less toxic than the parent compound, and showed greatly diminished radiation sensitization capacity. Since the decrease in toxicity was much less than expected, and the decrease in radiosensitization was much more than expected, this compound may be an important drug for continuing studies on the mechanisms of radiation sensitization, binding and cytotoxicity caused by electron affinic drugs.

Sensitization of Chinese hamster ovary cells to melphalan by etanidazole under intermittent hypoxia

Incubation of Chinese hamster ovary cells with etanidazole under hypoxic conditions increases cell killing by subsequent treatment with melphalan under aerobic conditions. We report here that this sensitization can be achieved even if periods of hypoxia are interrupted by reoxygenation. Preincubation under hypoxia in the absence of etanidazole also results in sensitization to melphalan. Intermittent hypoxia is less effective than continuous hypoxia as a single sensitizing factor. Glutathione depletion does not appear to have more than an additive effect on chemosensitization by etanidazole. These results suggest that prolonged treatment with low doses of etanidazole may be a more effective strategy for clinical chemosensitization than a short exposure to a higher dose, in order to target intermittently as well as chronically hypoxic cells.

Vascular attack as a therapeutic strategy for cancer

The blood supply to all solid tumours consists of parasitized normal vessels and new vessels which have been induced to grow by the presence of the tumour. These vessels are inadequate in many respects, being tortuous, thin-walled, chaotically arranged, lacking innervation and with no predetermined direction of flow. The walls consist of a basement membrane lined with rapidly proliferating immature endothelial cells, and are more permeable than normal vessels. The spacing of the vessels and their average diameters are not optimal for nutrient provision. This paper focuses on the evidence that many existing therapies may already have, as part of their action, a vascular mediated process of killing tumour cells. This may result from local changes within individual vessels or from systemic alterations in blood pressure, viscosity, coagulability etc. The hallmarks of vascular injury are identified and the dangers of discarding useful anticancer agent by failing to understand their mechanism of action are highlighted.

Radiosensitization by metal complexes of 4(5)-nitroimidazole

Four closely-related cis-platinum (Pt) complexes of 4(5)-nitroimidazole have been examined with respect to properties of radiobiological interest, to test the hypothesis that targeting a nitroimidazole (NO2Im) to DNA could enhance its radiosensitizing ability: I [PtCl2(5-NO2Im)2]; II [PtCl2(4-NO2Im)2]; III [PtCl2(NH3)(5-NO2Im)]; IV [PtCl2(NH3)(4-NO2Im)]. The reduction potential was affected to the same extent on metal binding in all of the complexes (delta E1/2 = +200 mV, cf. ligand measured polographically). Higher sensitization by 5-NO2 complexes I, III (cf. II, IV) was found. Only the mono complexes III and IV bind to DNA (in an assay using inhibition of restriction endonuclease activity); these radiosensitize as well as, or better than, free ligand in hypoxic CHO cells, and better than the bis complexes (I and II). The toxicity of the mono complexes is higher than ligand, and parallels the binding (III, IV, mono bis analogues). The complexes are compared with 4-nitroimidazole complexes of ruthenium, with respect to toxicity, binding and radiosensitization.

Role of glutathione in the radiation response of mammalian cells in vitro and in vivo

Radiation interacts with biological systems to produce many types of molecular lesions. Much of the molecular damage is of little consequence with regard to cell killing. The lesions that are most likely to contribute to cell killing are DNA lesions produced by clusters of radicals. The formation of clusters of radicals is characteristic of ionizing radiation and accounts for its high efficiency as a cytotoxic agent. The mechanism by which these lesions kill cells is probably the formation of DNA double-strand breaks, ultimately resulting in chromosomal breaks. There is a possibility that some of the other types of molecular lesions produced by radiation may participate in more subtle mechanisms of cell damage. For instance, radiation induces a self-destructive process (apoptosis) in certain cell types, and the molecular lesions that initiate this process have not been identified. Glutathione (GSH) is a versatile protector. Several distinct mechanisms of radioprotection by GSH can be identified. These include radical scavenging, restoration of damaged molecules by hydrogen donation, reduction of peroxides and maintenance of protein thiols in the reduced state. Of these mechanisms, hydrogen donation to DNA radicals is probably the most important. Since competing reactions are very rapid, this mechanism requires a high concentration of GSH. Radioprotection by hydrogen donation to DNA radicals is not effective in oxygenated cells because the normal intracellular GSH concentration is not sufficient for effective competition with oxygen. Consequently, moderate depletion of GSH has no effect on the radiosensitivity of oxygenated cells. Under hypoxic conditions GSH becomes more competitive, and GSH depletion can markedly affect radiosensitivity. The radiosensitivity of hypoxic cells is most affected by GSH depletion in the presence of low concentrations of radiosensitizers. Since hypoxic cells are a characteristic feature of tumors, moderate depletion of GSH in combination with treatment with hypoxic cell radiosensitizers appears to be a promising strategy for selective tumor sensitization in radiation therapy. Oxidation of GSH can result in radiosensitization of both hypoxic and oxygenated cells. The mechanism of this effect appears to involve oxidation of protein thiols which are important for DNA repair. In principle, modification of DNA repair could have a greater impact on radiation therapy than modification of the number of lesions produced by radiation. However, a strategy for modification of GSH or protein thiol redox state in vivo has not yet been devised.

Aerobic radiosensitization by SR 4233 in vitro and in vivo

During the past 3 years, our laboratory has identified and characterized the drug SR 4233 (3-amino-1,2,4-benzotriazine 1,4-dioxide) as the lead compound in a series of benzotriazine di-N-oxides that are both potent and selective killers of hypoxic cells in vitro and in rodent tumors in vivo. Recently, we have identified a novel property of SR 4233: the ability of a pre- or post-irradiation drug treatment under hypoxic conditions to radiosensitize aerobic cells in culture. For the mouse cell lines RIF-1 and SCC VII in vitro, this radiosensitization took the form of a steepening of the slope of the acute dose radiation survival curve, although there was also reduced survival in the "shoulder region" of the curve. For both cell lines, the sensitization occurred whether the hypoxic drug exposure was given immediately before or after the irradiation under aerobic conditions. To determine whether radiosensitization could be demonstrated for RIF-1 and SCC VII mouse tumors in vivo, tumor-bearing animals were exposed to 4 daily dose fractions of 5 Gy of X rays either alone, or followed immediately by injections of SR 4233 and the vasoactive agent hydralazine, which increases tumor hypoxia and therefore can potentiate the effect of such hypoxiaspecific drugs. Although treatment with the SR 4233/hydralazine combination after each dose fraction reduced tumor cell survival to between 10(-5) and 10(-6), near the limits of resolution of the clonogenic survival assay, the effect appeared to be strictly additive, suggesting that with this fractionated protocol, aerobic radiosensitization could not be detected. This is likely to be a consequence of the exquisite direct cytotoxicity of the SR 4233 and hydralazine combination toward the hypoxic cells in tumors.

Pre- and post-irradiation radiosensitization by SR 4233

SR 4233 (3-amino-1,2,4-benzotriazine 1,4-dioxide) is a bioreductive agent which exhibits highly selective killing of hypoxic cells in a variety of mammalian cell lines in vitro and in murine tumors in vivo. The selective toxicity of the drug results from its one-electron reduction under hypoxic conditions to form a free radical intermediate capable of damaging DNA, through the formation of strand breaks. Using the neutral filter elution assay, SR 4233 was found to be more efficient at producing DNA double strand breaks in Chinese hamster ovary (CHO) cells than an equitoxic dose of gamma-rays. Drug and radiation sequencing experiments were also performed, with both cell survival and DNA strand break rejoining used as endpoints. As a result of these studies, we now describe two additional properties of SR 4233: (a) radiosensitization of aerobic cells in culture produced by hypoxic incubation with drug either before or after irradiation, and (b) the inhibition of subsequent rejoining of radiation-induced DNA double strand breaks after hypoxic pretreatment with drug. The magnitude of the radiosensitization produced did not vary for drug treatments which, when given alone, reduced cell survival over a range from 30% to 2%. The extent of DNA repair inhibition increased with increasing severity of the SR 4233 pretreatment, but was quite small for non-lethal drug exposures.

Keynote address: cellular reduction of nitroimidazole drugs: potential for selective chemotherapy and diagnosis of hypoxic cells

Nitroimidazole drugs were initially developed as selective radiosensitizers of hypoxic cells and, consequently, as adjuvants to improve the local control probabilities of current radiotherapies. Misonidazole (MISO), the prototype radiosensitizing drug, was found in Phase I clinical studies to cause dose-limiting neurotoxicities (mainly peripheral neuropathies). MISO was also found to be cytotoxic in the absence of radiation and to covalently bind to cellular molecules, both processes demonstrating rates much higher in hypoxic compared with oxygenated cells. It is likely that neurotoxicity, cellular cytotoxicity and adduct formation results from reactions between reduction intermediates of MISO and cellular target molecules. Spin-offs from radiosensitizer research include the synthesis and characterization of more potent hypoxic cytotoxins and the exploitation of sensitizer-adducts as probes for measuring cellular and tissue oxygen levels. Current developments in hypoxic cell cytotoxin and hypoxic cell marker research are reviewed with specific examples from studies which characterize the cellular reduction of TF-MISO, (1-(2-nitro-1-imidazolyl)-3[2,2,2-trifluoroethoxy]-2-propanol).

Enhancement of radiation-induced tumor cell killing by the hypoxic cell toxin SR 4233

SR 4233 (3-amino-1,2,4-benzotriazine 1,4-dioxide) is the lead compound in a series of benzotriazine di-N-oxides which exhibit high selective killing of hypoxic mammalian cells in vitro. Drug concentrations to produce equivalent levels of cell killing of SCC VII murine carcinoma cells under hypoxia were nearly 200-fold lower than under aerobic conditions. Following a one hour hypoxic incubation with drug, 20 microM SR 4233 killed 99.9% of SCC VII cells. The hypoxia-specific cytotoxicity of SR 4233 is due to bioreductive metabolism. For in vivo studies, pharmacokinetic measurements showed that drug concentrations well in excess of 20 microM were achievable in SCC VII tumors in mice for approximately one hour after a single injection of SR 4233. Under these conditions, cell killing was considerably enhanced in SCC VII tumors when SR 4233 was combined with a single X-ray dose of 20 Gy. The enhancement was seen whether SR 4233 was given for up to 2 h before or for up to an hour after radiation, and was comparable to the enhanced cell killing achievable with a single large dose of the radiosensitizer misonidazole. While this finding is consistent with the selective killing of at least some subset of hypoxic tumor cells by SR 4233, other interactions between the drug and radiation damage may contribute to the overall effect observed.

Is misonidazole binding to mouse tissues a measure of cellular pO2?

Misonidazole (MISO), a hypoxic cell radiosensitizer, forms covalently-linked adducts to cellular molecules as a result of bioreductive metabolism, a process which is strongly dependent upon oxygen concentration. MISO binding to liver tissue taken from air-breathing mice was three to five times greater than binding to other normal tissues. The relative binding of [14C]MISO to various mouse tissue cubes in vitro was measured by autoradiography as a function of defined oxygen concentrations, and standard curves (binding rate vs oxygen concentration) were generated. The oxygen concentration for half-maximum binding as well as the maximum and minimum binding rates (grains per 100 micron 2) observed for liver tissue were not significantly different from those measured for brain or heart tissue. These results, along with previously published data on MISO binding to isolated hepatocytes in vitro, suggest that the elevated binding to liver in vivo may result, in part, from the organ existing at a significantly lower pO2 than other normal tissues. They also suggest that this drug adduct procedure could be developed as a sensitive method for the quantitative measurement of tissue pO2 at the cellular level.

A comparison of the intracellular uptake and radiosensitization efficiency in different media of uncharged 2-nitroimidazoles of varying lipophilicity

The effect of varying octanol: water partition coefficients, P, (range 0.026-260) on the uptake of uncharged 2-nitroimidazoles into Chinese hamster V79 379A cells has been studied. Average intracellular concentrations were measured by high performance liquid chromatography after centrifuging cells through oil or an aqueous medium. The ratio of intracellular concentration of radiosensitizer to extracellular concentration (Ci/Ce) for misonidazole (P = 0.43) was 0.85 for the oil method and 0.68 for the aqueous method. For values of P less than about 0.05 uptake was initially very slow and Ci was always less than Ce. When P greater than or equal to 0.1 uptake was rapid and then remained unchanged for times up to 3 h; for P greater than or equal to 10, Ci/Ce increased rapidly as P increased. Ro 31-1405 (P = 260) concentrated by a factor of 7 inside the cell. Although uptake was identical for cells suspended in full growth medium and PBS, radiosensitization was greater for cells in PBS: 1 mmol dm-3 misonidazole produced an enhancement ratio of 1.6 in full growth medium and 1.9 in PBS. This increase in radiosensitization could not be accounted for by protein binding. However, measurements on cellular non-protein sulphydryl (NPSH) demonstrated the levels to be reduced to about 60 per cent for cells in PBS. Similar reductions in NPSH levels have previously been shown not to increase the radiosensitivity of control cells but to increase greatly the effectiveness of nitroimidazole radiosensitizers.

Radiolabeled hypoxic cell sensitizers: tracers for assessment of ischemia

Hypoxic, non-functional, but viable, tissue may exist in heart and brain following an arterial occlusion. Identification of such tissue in vivo is crucial to the development of effective treatment strategies. It has been suggested that certain compounds capable of sensitizing hypoxic tumor cells to killing by x-rays (i.e., misonidazole) might serve as in vivo markers of hypoxic tissue in ischemic myocardium or brain if properly radiolabeled. To this end we have radiolabeled two fluorinated analogs of nitroimidazole based hypoxic cell sensitizers with the 110 minute half-lived positron-emitting fluorine-18. The ability of these tracers to quantitate the presence of hypoxic tissue has been studied in a gerbil stroke model. The in vivo uptake of one of these tracers [F-18]-fluoronormethyoxymisonidazole is dependent on the extent of tissue hypoxia, and thus, appears to have potential as a diagnostic indicator of non-functional but viable tissue when the tracer is used in conjunction with positron emission tomography.

Radiosensitization of Chinese hamster cells by oxygen and misonidazole at low X-ray doses

The radiosensitization of Chinese hamster V79 cells in vitro by air and misonidazole at low X-ray doses (0.2-6.0 Gy) had been studied. These survival data, together with high-dose data, were fitted to the linear quadratic model ln S = -(alpha D + beta D2), deriving estimates of alpha and beta by six different methods to illustrate the influence of the statistical treatment on the values so derived. This in vitro study clearly demonstrated that the survival parameters alpha and beta are dependent to some degree on the method of analysis of the raw survival data; however, their ratios, the values of oxygen enhancement ratios (OERs) and radiosensitizer enhancement ratios (SERs) derived from the different methods, are similar. All methods of analysis give reduced OERs at low radiation doses for combined low- and high-dose X-ray data. However, the OERs are still appreciably high, ranging from 2.45 to 2.50 for an oxic dose of 2 Gy. All methods of analysis gave reduced SERs at low doses for combined low and high X-ray dose data for hypoxic cells irradiated in 1 mmol dm-3 misonidazole. At survival levels corresponding to doses of 2 Gy in the presence of 1 mmol dm-3 misonidazole and SERs ranged from 1.2 to 1.5.

Radiosensitization of hypoxic cells at low doses

We have measured the radiosensitization produced in hypoxic V79 cells in vitro by SR 2508, Ro-03-8799, and oxygen at conventional doses (0-30 Gy) as well as at low doses (0-4 Gy). Two techniques have been used: (a) FACS (Fluorescence Activated Cell Sorter) plating followed by the conventional colony formation assay and (b) microscopic scoring of surviving and non-surviving cells using the DMIPS (Dynamic Microscope Image Processing Scanner) system. The results show that the enhancement ratios (ER) for both drugs and for oxygen are dose dependent, decreasing somewhat at doses less than 4 Gy. The low dose sensitization measured at S = 0.8 for SR 2508, Ro-03-8799 and O2 was found to be approximately 73%, 61% and 75% of the high dose sensitization for these three agents, respectively. These values are not significantly different. Both the alpha- and beta-components of cell kill appeared to be sensitized by all three agents. The enhancement ratios obtained by the two different techniques are in general agreement, though some qualitative differences have been observed.

Direct effects of the hypoxic cell sensitizer misonidazole on colony formation in a human tumor cloning assay

The human tumor cloning assay as described by Hamburger and Salmon was utilized to study the direct antitumor effects of the hypoxic cell sensitizer misonidazole (MISO). Cells from 106 tumor specimens directly obtained from patients were exposed to MISO at clinically achievable drug concentrations (0.5 mM). Of 30 evaluable tumors, seven specimens (23%) showed a less than or equal to 50% decrease of TCFU's. In vitro sensitivity to MISO was noted in human breast cancer, renal cancer, non small-cell lung cancer, and adenocarcinoma of unknown primary site. A dose response relationship was demonstrated in a subset of experiments including 6 patient's tumors and one human breast cancer cell-line. An analysis relating MISO sensitivity or resistance to the results obtained with other, simultaneously tested standard anticancer drugs indicated that tumors exhibiting a less than or equal to 50% decrease of TCFU's in the presence of MISO were also likely to be sensitive to other cytotoxic drugs. In summary, our data suggest that the 'nitroimidazoles' may exert clinically significant direct antitumor effects in individual tumors. The human tumor cloning assay may have potential to evaluate these direct effects of MISO-analogues and other new radiosensitizers currently being tested in clinical trials.

The influence of thiols on the pre-irradiation incubation effect of nitroimidazoles in E. coli cells

The increase in the degree of radiosensitization of Escherichia coli cells following prolonged pre-irradiation incubation with nitroimidazoles is not correlated with the loss of intracellular non-protein thiols (NPSH) alone. The rates of reduction of the nitro compounds and the NPSH removal do not show strong dependencies on the lipophilicities of the nitroimidazoles whereas the highly lipophilic compound RGW-609 effects an increase in radiosensitization in a much shorter incubation time than the other nitroimidazoles. Exogenous dithiothreitol (DTT) increased the rate of reduction of misonidazole in the cells but did not alter the fraction converted to the amine. Added DTT (0.15 mmol dm-3) completely protected against the pre-irradiation incubation effect of misonidazole (2.5 mmol dm-3) when added at the start of the incubation but only partially protected when added before irradiation. It is suggested that NPSH can intercept metabolite(s) (or their precursors) of nitroimidazoles which can potentiate cell killing by radiation.

Changes in misonidazole binding with hypoxic fraction in mouse tumors

Binding of misonidazole (MISO) or a derivative to hypoxic cells in tumors has been proposed as a method for identifying tumors and measuring their level of hypoxia. We have recently shown that the hypoxic fraction of tumor cells can be altered over a wide range in vivo by acutely changing the hematocrit of the host animal by transfusion. The present study aimed to investigate the changes in binding by 14C MISO that accompanied this procedure. Tumor bearing mice were injected with 14C MISO, irradiated with a single dose of X rays (20 Gy) and their tumor excised and bisected. One half of each tumor was used to determine cell survival in vitro, the other was used for 14C scintillation counting. As previously described, tumor cell survival was dramatically increased in acutely anemic mice and this was accompanied by an increase in 14C MISO binding to the tumors. The relationship between clonogenic cell survival and binding was found to be linear on a log-log plot for each of the tumor lines studied, but the slopes of the lines were different tumor lines and generally steeper than the value of 1.0 expected for a 1:1 correspondence between cells binding radioactivity and radiobiological resistance. We attribute these differences to MISO binding to cells in the tumor which were not clonogenic.

Changes in ultrastructure and function of hypoxic V79 fibroblast cells after treatment with misonidazole

Morphologic and enzymatic changes due to exposure to the radiosensitizing chemical, misonidazole, have been identified in V79 cells grown in a system in which oxygen tensions and culture density have been controlled. Misonidazole prevented the increase in mitochondrial size normally seen during exposure of these cells to conditions of moderate hypoxia (2 X 10(3) ppm O2). Mitochondrial size was also significantly decreased in cells from exponential cultures exposed to 1 mmol/l misonidazole. Morphologic changes to the mitochondria that varied from data reported elsewhere were also noted. Misonidazole caused a significant initial decrease in cytochrome oxidase activity after 4 hours of exposure of aerobic and moderately hypoxic cultures that did not return to normal in chronically hypoxic cells during continued exposure to the drug.

The detection and measurement of hypoxic cells in solid tumors

The presence of viable hypoxic cells in human cancers has concerned oncologists for years. Cells in tissues that are deficient in oxygen are relatively resistant to radiation inactivation and may not be accessible to some systemic chemotherapy. The premise that hypoxic tumor cells do, indeed, control the radiocurability of some cancers is supported by some clinical evidence. The presence of hypoxic regions within tumors can be directly and indirectly inferred from invasive procedures such as oxygen electrode techniques and histologic study, respectively, but such information does not significantly contribute to current prescriptions given by oncologists for tumor treatment. Novel procedures (some of which are noninvasive) for detecting hypoxic regions within solid tumors have been proposed and are based upon two recent developments: (1) the discovery that some radiosensitizing drugs become selectively bound by metabolism to the molecules of viable hypoxic cells, and (2) the growing availability of new imaging procedures based upon positron-emission tomography, single-photon emission tomography, and nuclear magnetic resonance spectroscopy. Preliminary research results from these novel procedures are reviewed, and the potential clinical impact of each is discussed.

Pharmacology and toxicology of sensitizers: mechanism studies

Nitroimidazoles are being studied extensively as hypoxic cell radiosensitizers. Besides their ability to selectively sensitize hypoxic cells to radiation, which depends on the parent compound, nitroimidazoles have a variety of other effects in vitro, in vivo and clinically which appear to require reductive metabolism. These other effects include direct cytotoxicity to hypoxic cells, mutagenicity and antimicrobial effects. As a first step to suggesting possible mechanisms for these other biological effects, a summary has been made of the known oxidative and reductive products of the two most widely studied radiosensitizers, metronidazole and misonidazole. Focussing on reductive products, it is clear that a great variety exists which are or may be reactive with biological molecules. Knowledge about the reduction chemistry of nitroimidazoles is new and far from complete. As a second step to suggesting possible mechanisms for these biological effects, it is important to view the problem in terms of the in vivo situation where distribution and sites of metabolism of the drug and its reduction products will be important factors. Variables such as levels of tissue oxygenation and nitroreductase activity will be important to assess. Combining basic information about the reduction chemistry of nitroimidazoles with knowledge about the pharmacology of drugs and their reduced products should allow a better assessment of mechanism of action as well as a better implementation of these drugs clinically.

Misonidazole-glutathione conjugates in CHO cells

Misonidazole, after reduction to the hydroxylamine derivative, reacts with glutathione (GSH) under physiological conditions. The reaction product has been identified as a mixture of two isomeric conjugates. When water soluble extracts of CHO cells exposed to misonidazole under hypoxic conditions are subjected to HPLC analysis, misonidazole derivatives, having the same chromatographic properties as the GSH-MISO conjugates, were detected. The identity of the synthetic and cellular products was further confirmed by identification of the amine derivative of misonidazole after desulfurization with Raney Nickel. When CHO cells were incubated with misonidazole in the presence of added GSH, a substantial increase in the amount of the conjugate was detected. When extracts of CHO cells exposed to misonidazole under hypoxia were subsequently exposed to GSH, an increased formation of the conjugate was observed. A rearrangement product of the hydroxylamine derivative of misonidazole is postulated as the reactive intermediate responsible for the formation of the conjugate.

Radiosensitization of hypoxic cells in vivo by SR 2508 at low radiation doses: a preliminary report

We have used the RIF-1 tumor implanted intradermally in the lower dorsum of C3H mice to explore to what extent the radiosensitizer SR 2508 is capable of sensitizing hypoxic cells at clinically relevant doses of 1 and 2 Gy per fraction. We injected SR 2508 (1000 mg/kg) 45 min prior to each radiation dose in fractionated regimens of 2 or 4 doses/day for up to 5 days (1 or 2 Gy/fraction) given locally to the tumors, which were clamped to occlude the blood supply prior to each radiation exposure. This necessitated the design of clamps which totally occluded blood flow could be applied to nonanesthetized mice without obvious discomfort, and could be applied up to 20 times without compromising the tumor blood supply on removal of the clamps. We have performed various experiments which confirm the validity of these 3 requirements. The response of the tumor cells with and without clamping and with and without SR 2508 was determined by constructing multifraction cell survival curves using the in vivo-in vitro assay. The initial results demonstrate significant radiosensitization of artificially hypoxic tumor cells at 1 and 2 Gy/fraction by SR 2508 (1000 mg/kg). Using the ratio of the D0's of the exponential, multifraction survival curves, we obtained an SER for SR 2508 of 1.6 (3 experiments pooled) compared to an OER (D0 clamped/D0 air-breathing mice) of 2.3 (4 experiments pooled). These data suggest that SR 2508 (and presumably other electron-affinic sensitizers) can radiosensitize hypoxic cells at low radiation doses, and indicate that this and similar drugs may be useful in the radiotherapy of those tumors for which hypoxia limits curability.

Increased radiosensitivity of aerobic mammalian cells following pre-incubation with misonidazole in hypoxia

The effect of pre-incubation of CHO cells with misonidazole in hypoxia on the aerobic radiosensitivity of these same cells was observed and analyzed according to the concepts of independent and additive interaction. It was found that the sensitivity of cells irradiated in air, in the presence or absence of misonidazole, increased when the cells had been pre-incubated with misonidazole in hypoxia at 37 degrees C. This increase in aerobic radiosensitivity was evident as a loss of the shoulder of the survival curve, and the data were consistent with an additive, rather than an independent interaction mechanism. The effect was also observed when the cells were washed prior to irradiation. These data are consistent with our previous work with hypoxic cells, and support the hypothesis that a toxic metabolite of the sensitizer is produced during hypoxic incubation, whose effect is manifested as an additional increment of cell killing equivalent to an additional dose of radiation. Given the possibility of a diffusible toxic product of sensitizer nitroreduction, the hypoxic cell toxicity of nitroimidazoles, generally perceived to be of importance only in hypoxic cells, may be a factor in the radiosensitivity of non-hypoxic cells in tumours.

In vivo and in vitro mechanisms of radiation sensitization, drug synthesis and screening: can we learn it all from the high dose data?

The evidence for a decreased enhancement ratio of oxygen and an electron affinic radiosensitizer (misonidazole) at low doses is presented, and the mechanism of this effect is discussed. The factors which influence the magnitude of this effect, as well as the dose levels at which the effect will be significant, are identified. This will allow further characterization of this phenomenon in the future. An approach by which present and new hypoxic radiosensitizers could be made more effective at low doses is indicated.

The effect of prolonged high dose misonidazole on tumor response to radiation

WHFIB and SA F tumors were exposed to misonidazole (MISO) concentrations of 2.5 mM or more for up to 4 hours (SA F) or 6 hours (WHFIB). There was no increase in the MISO enhancement ratio (SER) in the SA F for a 4 hour exposure to MISO relative to that following a single injection. In the WHFIB tumor, the ER increased from 2.2 for a single MISO injection to 2.5 for a 4 hour contact with MISO for tumor growth delay, and from 2.1 to 2.3 for a cloning assay. (These differences may not be statistically significant) Prolonged contact with MISO was toxic and reduced the body temperature by 4 to 5 degrees C. For WHFIB cells in vitro, when the contact time (in hypoxia) with 2.5 mM MISO was increased from 0.5 to 2.5 hours, the ER increased from 2.1 to 2.9 at 37 degrees C and from 1.9 to 2.5 at 33 degrees C.

Metabolism induced binding of 14C-misonidazole to hypoxic cells: kinetic dependence on oxygen concentration and misonidazole concentration

Under conditions of extreme hypoxia, metabolic products of the metabolism of misonidazole bind to cellular molecules at a rate which is linear with time and proportional to the square root of misonidazole concentration. Very small amounts of oxygen reduce the overall rate of binding and cause a change in the dependence on misonidazole concentration from square root (half order) to linear (first order). Because of the known electron affinity of misonidazole, a model is presented whereby the nitro-group is reduced to a radical in a first order reaction. This radical binds to cellular molecules in a slow first order reaction and either disproportionates or dimerizes in a fast second order reaction. Based on the overall effect of oxygen on the kinetics of the rate of binding, the radical is tentatively assumed to be the 3 electron reduction product.

2-Hydroxylaminoimidazoles--unstable intermediates in the reduction of 2-nitroimidazoles

An unstable 2-hydroxylaminoimidazole (2-hydroxylamino-1-methylimidazole) was prepared by the reaction of 2-fluoro-1-methylimidazole with hydroxylamine. This substance was sufficiently stable (half-life of 1-2 days) in acid solutions to be observed and characterized by NMR spectroscopy; decomposition at neutrality was, however, rapid (half-life of 1-10 min). Radiochemical and electrochemical reduction experiments were carried out at pH 4 and pH 7 with 2-nitro-1-methylimidazole and misonidazole [1-(3'-methoxy-2'-hydroxypropyl)-2-nitroimidazole]. A four electron stoichiometry was found in every case. The pH 4 reduced product was identified as the 2-hydroxylamino derivative (greater than 80% yield). The pH 7 reduced solutions, on the other hand, showed no aromatic 1H NMR signals, suggesting that a simple imidazole ring was no longer present. A shift to pH 7 of the hydroxylamine produced at pH 4, however, resulted in very similar NMR spectra. The conclusion, therefore, is that the hydroxylamine was produced initially on reduction of the nitroimidazole, but it was not stable.

Effect of misonidazole on the radiosensitivity and repair of potentially lethal damage of L5178Y ascites tumor cells

The radiosensitizing effect of low concentrations of misonidazole was investigated by using L5178Y cells growing as an ascites tumor in DBA-2 mice. The cells were irradiated in vivo with graded doses of X-rays in the presence or absence of 0.1-0.5 mg/g body weight of misonidazole. Then cell survival was assayed in vitro by plating cells in soft agar medium. By analyzing the X-ray survival curves with or without misonidazole, the dose-modifying effects were determined. The results indicated that the slope and shoulder of the survival curves were greatly modified by the treatment with misonidazole. The dose-modifying factor in terms of the D0 ratio between the drug-treated and untreated control cells was increased as the drug concentration was increased. Further, it was revealed that the isoeffect dose ratios, estimated by the linear quadratic equation of Chadwick and Leenhouts, were higher at a low-radiation dose range. This is due to the suppression of the shoulder region of survival curves for the drug-treated cells. The inhibition of the X-ray-induced repair of potentially lethal damage was apparent with 0.1 mg/g body weight of misonidazole. The inhibition became more effective as the drug concentration increased.

A rapid-mix study on the effect of lipophilicity of nitroimidazoles on the radiosensitization of mammalian cells in vitro

A liquid flow rapid-mixing apparatus has been used to study the role of lipophilicity (octanol: water partition coefficient, P) in the sensitization of hypoxic V79 cells by nitroimidazoles. Sensitization by seven neutral 2-nitroimidazoles of similar reduction potential but widely differing partition (0.11-77) and one basic 2-nitroimidazole (pKa=8.9; p=8.5 (of free base)) was studied as a function of pre-irradiation contact time ca. 3-40 ms. With increasing P, sensitization occurs at increasingly shorter pre-irradiation contact times. The results suggest that even though factors other than passive diffusion control the sensitization observed with the base Ro 03-8799 it is able to diffuse to the target site faster than midonidazole.