Modification of radiation-induced chromosome damage and micronucleus induction in mouse bone marrow by misonidazole and hyperthermia

The effect of misonidazole (MISO), local hyperthermia (HT) and their combination on radiation-induced chromosome damage and micronucleus (MN) induction was studied in mouse bone marrow cells. It was found that MISO treatment did not enhance the clastogenic effect of radiation, which indicates a lack of radiosensitization of bone marrow chromosomes. But post-irradiation HT increased the frequency of aberrant cells and MN. A combination of MISO and HT produced a significant increase in the frequency of radiation-induced aberrant cells and MN at all the radiation doses as compared to radiation alone. The percentage of aberrant cells as well as the percentage of MN showed a linear quadratic increase with radiation dose in all the treatment groups. At higher radiation doses, cells with > 1 MN increased quadratically with a pronounced increase in cells bearing > 2 MN and severely damaged cells (SDCs) at radiation doses above 3.0 Gy in the HT and MISO+HT treated groups. Our results indicate that though MISO itself may not have a radiosensitizing effect on mouse chromosomes, a combination of MISO with HT can enhance the radiation damage in normal bone marrow.

Use of the comet assay to detect hypoxic cells in murine tumours and normal tissues exposed to bioreductive drugs

The alkaline comet assay was applied to individual cells from mice exposed to two bioreductive drugs, tirapazamine and RSU 1069, with the goal of comparing DNA damage to tumours and normal tissues. More DNA single-strand breaks (SSBs) and a greater heterogeneity in DNA damage were observed in tumour cells than in spleen and marrow cells of mice exposed to 10-100 mg/kg tirapazamine, consistent with the presence of hypoxic cells and the greater bioreductive capacity of tumours. In mice injected with 25-200 mg/kg RSU 1069, aerobic cells exhibited large numbers of SSBs while toxic DNA interstrand crosslinks were produced only in hypoxic cells. Cells from bone marrow and spleen showed extensive numbers of SSBs, but minimal crosslinking compared to tumours where 10-20% of cells were heavily crosslinked. DNA damage produced by these two bioreductive drugs may be useful in estimating the range of individual cell oxygen contents within tumours and normal tissues.

[Radiosensitizing effects of nitroimidazole derivative, KIN-804]

KIN-804(2-nitroimidazole-1-methylacetohydroxamate) is a new hypoxic cell radiosensitizer developed in Japan. It showed a high level of radiosensitizing effect in vitro experiments and is expected to have low neurotoxicity because of its hydrophilic side chain. In this paper, the in vivo characteristics of KIN-804 were studied. Acute toxicity, pharmacokinetics and radiosensitizing effect were studied using C3H/He mice and SCC VII carcinoma. Misonidazole was used as a standard comparison. LD50/7 was used for the evaluation of acute toxicity. The LD50/7 of KIN-804 and Misonidazole were 3200 mg/kg and 2000 mg/kg, respectively. Pharmacokinetics were studied using high performance liquid chromatography. The concentration of KIN-804 in the tumor peaked 20 min after administration and reached 62% of the maximum concentration in blood. The concentrations in brain and sciatic nerve were low. The radiosensitizing effect was evaluated using the growth delay method. The enhancement ratios of KIN-804 were 1.71, 1.50 and 1.22 at doses of 200, 100 and 50 mg/kg, respectively, compared with 1.36 for Misonidazole at a dose of 100 mg/kg. When irradiation was performed with double fractionation, the enhancement ratio of KIN-804 at a dose of 100 mg/kg decreased to 1.25. Based on these results, KIN-804 is considered a promising radiosensitizer.

Hyperthermia enhances the cytotoxicity against hypoxic cells of RP-170, a new 2-nitroimidazole nucleoside hypoxic cell sensitizer

The effect of the new hypoxic cell sensitizer, 1-[2-hydroxy-1-(hydroxymethyl)-ethoxy]methyl-2-nitroimidazole (RP-170), combined with heat against EMT6/KU cells, was determined under conditions of in vitro hypoxia. Heat-induced cytotoxicity for the EMT6/KU cells was increased to a greater extent under conditions of hypoxia and a normal pH of the medium. Hypoxia also reduced the surviving fraction of the cells treated either with RP-170 alone or with RP-170 plus heat. The concomitant treatment of RP-170 and heat inhibited the clonogenic activity of the EMT6/KU cells under conditions of in vitro hypoxia in all experimental groups, with a significant difference (p < 0.05). Therefore, RP-170 combined with exposure to heat may be an effective treatment for hypoxic cells in a solid tumor, as these cells are resistant to radiation and/or to many chemotherapeutic agents.

Radiosensitizing activity and pharmacokinetics of multiple dose administered KU-2285 in peripheral nerve tissue in mice

PURPOSE

In a clinical trial in which a 2-nitroimidazole radiosensitizer was administered repeatedly, the dose-limiting toxicity was found to be peripheral neuropathy. In the present study, the in vivo radiosensitizing activity of KU-2285 in combination with radiation dose fractionation, and the pharmacokinetics of cumulative dosing of KU-2285 in the peripheral nerves were examined.

METHODS AND MATERIALS

The ability of three nitroimidazoles, misonidazole (MISO), etanidazole (SR-2508) and KU-2285, to sensitize SCCVII tumors to radiation treatment has been compared for drug doses in the range 0-200 mg/kg. Single radiation doses or two different fractionation schedules (6 Gy/fractions x three fractions/48 h or 5 Gy/fractions x five fractions/48 h) were used; the tumor cell survival was determined using an in vivo/in vitro colony assay. The pharmacokinetics in the sciatic nerves were undertaken, when KU-2285 or etanidazole were injected at a dose of 200 mg/kg intravenously one, two, three or four times at 2-h intervals.

RESULTS

At less than 100 mg/kg, KU-2285 sensitized SCCVII tumors more than MISO and SR-2508 by fractionated irradiation. Evaluation of pharmacokinetics in the peripheral nerves showed that the apparent biological half-life of SR-2508 increased with the increases in the number of administrations, whereas that of KU-2285 became shorter.

CONCLUSION

Since most clinical radiotherapy is given in small multiple fractions, KU-2285 appears to be a hypoxic cell radiosensitizer that could be useful in such regimens, and that poses no risk of chronic peripheral neurotoxicity.

In vivo radiosensitizing effect of nitroimidazole derivative KIN-804

PURPOSE

In vivo characteristics of 2-nitroimidazole-1-methylacetohydroxamate (KIN-804), which is a newly developed hypoxic cell radiosensitizer, are presented.

METHODS AND MATERIALS

The toxicity, pharmacokinetics, and radiosensitizing effect of KIN-804 were studied by in vivo experiments using C3H/He mice bearing the SCC-VII tumor. Results were compared with misonidazole (MISO).

RESULTS

LD50(7) of KIN-804 and MISO were 3200 mg/kg and 2000 mg/kg, respectively. The peak of concentrations of KIN-804 in the tumor occurred 20 min after intraperitoneal injection and reached about 62% of the maximum concentration in the blood. The concentrations in brain and sciatic nerve were very low and clearance from sciatic nerve was rapid. Enhancement ratios of KIN-804 calculated using the growth delay method were 1.22, 1.50 and 1.71 at doses of 50, 100, and 200 mg/kg, respectively, compared with 1.36 for MISO at a dose of 100 mg/kg. In the TCD50 assay, enhancement ratios at a dose of 200 mg/kg were 1.69 for KIN-804 and 1.52 for MISO, respectively.

CONCLUSION

KIN-804 is a promising radiosensitizer since it shows less toxicity and higher radiosensitizing activity than MISO.

Free radical mechanism in enhancement of radiosensitization by SRSBR

The free radical mechanism of enhancing radiosensibility by the Synergic Recipe to Strengthen Body Resistance (SRSBR) consisting of 10 Chinese drugs was studied by electron spin resonance (ESR) with spin trapping reagent--Nitroso-tert-butane (NtB) following irradiation of the Deoxy thymidine (dT) solution system by 60Co 3.7 PBq. The results showed SRSBR cannot only enhance the generation of e- aq but also enhance the production of . OH and H . after irradiation with gamma ray in dT-NtB-SRSBR aqueous solution system. These result in damage to biological molecules, attacking and killing tumor cells radiosensitized by SRSBR. The characteristics of SRSBR for radiosensitization are concluded to be increasing production of . OH and H. in comparison with the well-known radiosensitizer-Misonidazole (Miso).

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.

Unusual oxygen concentration dependence of toxicity of SR-4233, a hypoxic cell toxin

Toxicity from drugs activated by bioreductive metabolism has been suggested as a means to eliminate the treatment resistance caused by hypoxic tumor cells. In general, drugs have been selected to maximize the hypoxic cytotoxicity ratio [exposure (drug concentration x time) in air:exposure in nitrogen] to cause equal toxicity. On this basis, two recently developed drugs have very similar characteristics; an aziridine derivative of misonidazole (RSU1069) and a benzotriazine di-N-oxide (SR4233). The oxygen dependence of the toxic response has not previously been characterized. This report shows that the toxicity from SR4233 extends over a much greater range of oxygen concentrations than does that of RSU1069. Furthermore, unlike all previous drugs studied, the toxicity of SR4233 does not level off at high oxygen concentrations, but continues to decrease as the oxygen concentration increases. For 1 mM oxygen (the solubility of oxygen in medium at 37 degrees C equilibrated with 100% oxygen and water vapor) the toxicity from SR4233 is at least 2000-fold less than that for hypoxia. Modeling the effect of oxygen on combined radiation and toxicity shows that radiation plus SR4233 should be much more effective in eliminating hypoxic cells than radiation plus RSU1069. The unusual oxygen dependence of toxicity by SR4233 may indicate a unique biochemical activation process.

Influence of nitrogen, oxygen, and nitroimidazole radiosensitizers on DNA damage induced by ionizing radiation

Oxygen and nitroaromatic compounds are known to enhance the sensitivity of cells to ionizing radiation. Employing calf thymus DNA and oligo(dA)12/poly(dT), we have examined the differences to DNA damage, in particular thymine glycols and the 3'-DNA termini at strand breaks, arising from irradiation under anoxic and oxic conditions and the presence and absence of misonidazole [1-(2-nitro-1-imidazoyl)-3-methoxy-2-propanol]. We show that (i) irradiation under nitrogen generates strand breaks almost exclusively with 3'-phosphate termini; (ii) irradiation under oxic conditions increases the yield of strand breaks 3-fold, and the 3' termini consist of 3'-phosphoglycolate and 3'-phosphate end groups in a ratio of approximately 1.6; (iii) the patterns of base and sugar damage detectable by a postlabeling assay [Weinfield, M., & Soderlind, K.-J. (1991) Biochemistry 30, 1091-1097] differ completely between DNA irradiated under oxic vs anoxic conditions; (iv) the presence of misonidazole under anoxic conditions does not increase the level of strand breakage but, like oxygen, significantly enhances the formation of 3'-phosphoglycolate end groups; (v) the presence of misonidazole during anoxic irradiation does not increase the yield of any other type of 'oxic' damage detectable by the postlabeling assay, such as thymine glycols; and (vi) misonidazole at concentrations greater than 50 microM affords significant protection to naked DNA, probably by OH radical scavenging, and both the nitroaromatic ring and methoxyisopropanol side chain contribute to this protective action.

Effect of hyperthermia on the activity of 1-[(4'-hydroxy-2'-butenoxy)methyl]-2-nitroimidazole, which is cytotoxic to hypoxic cells

The effect on EMT6/KU cells of a newly synthesized hypoxic cell sensitizer, 1-[(4'-hydroxy-2'-butenoxy)methyl]-2-nitroimidazole (RK28), combined with heat was determined in vitro under conditions of hypoxia. As compared with aerobic conditions, hypoxia produced a 1.30-fold increase in the cytotoxicity of the drug for mouse mammary EMT6/KU cells induced by 1 h heat treatment at 43 degrees C in medium with a normal pH. Hypoxia also reduced the surviving fraction of cells treated with both RK28 alone for 2 h and the same concentrations of RK28 and heat (43 degrees C) in combination. Those enhancement ratios corresponded to a 20.3- and > 345-fold increase, respectively. Moreover, concomitant treatment with RK28 and heat greatly inhibited the clonogenic activity of the EMT6/KU cells under conditions of in vitro hypoxia and in all experimental groups; there was a statistically significant difference in the time-response curves (P < 0.05). As hypoxic cells in a solid tumor are resistant to various anticancer drugs, RK28 combined with hyperthermia deserves further study for possible clinical applications.

Induction of mutations in V79-4 mammalian cells under hypoxic and aerobic conditions by the cytotoxic 2-nitroimidazole-aziridines, RSU-1069 and RSU-1131. The influence of cellular glutathione

Incubation of the 2-nitroimidazole-aziridine, RSU-1069 [1-(2-nitro-1-imidazolyl)-3-(1-aziridinyl)-2-propanol], and its monomethylaziridine analogue, RSU-1131 [1-(2-nitro-1-imidazolyl)-3-(1-(2-methylaziridinyl))-2-propanol], with V79-4 mammalian cells for 2 hr under aerobic or hypoxic conditions induces mutations as measured at the hypoxanthine phosphoribosyl transferase locus. The ability of these agents to induce mutations is increased by a factor of 12-14 under hypoxic conditions. The increased cytotoxicity of these agents under hypoxic conditions was confirmed following a 2 hr incubation period. Decreasing the glutathione (GSH) content of the cells with buthionine-(S,R)-sulphoximine to < 1% of the control generally results in an increase in the cytotoxicity and mutagenicity of these agents under both aerobic and hypoxic conditions. Since these agents do not modify the cellular GSH levels, it is inferred that the thiols partially detoxify through removal of a reactive metabolite of the agents, under hypoxic conditions, or removal of known DNA adducts, and not through their interaction with the agents themselves. Under aerobic conditions, the formation of mutations is consistent with the established monofunctional action of these agents whereas under hypoxic conditions the bifunctional action predominates for mutation induction, based upon the large differential aerobic:hypoxic effect. From a comparison of the number of mutations per lethal event, the effect of thiol depletion is more pronounced for cytotoxicity than for mutation induction by these agents. In summary, these agents are considered to be weak mutagens towards V79-4 cells under aerobic conditions when compared with other DNA alkylating agents, although they are more potent under anoxic conditions.

Effects of glutathione depletion using buthionine sulphoximine on the cytotoxicity in mammalian cells and human tumor cells in vitro

An inhibitor of glutathione biosynthesis, buthionine sulphoximine (BSO), was used to deplete the endogenous thiols in mammalian cells in vitro. In this study, the cytotoxicity of BSO and BSO combined with the hypoxic cell radiosensitizer misonidazole (MISO) was investigated. Both aerobic and hypoxic cytotoxicity of MISO was found to be increased. The concentration of BSO required to reduce the colony forming ability to 50% (Cc) for the chronic cytotoxicity on V79 cells was 0.03 mmol/L under aerobic condition, while the Cc for the acute cytotoxicity on V79 cells under hypoxic and aerobic conditions was 0.4 and 0.5 mmol/L. The growth inhibition rate of human tumor cells K562 and SGC-7901 by BSO was 6.89-26.06% and 12.01-55.69%, respectively. Enhanced cytotoxicity activity was observed when BSO was used in combination with cis-dichlorodiamino Pt(II) or 5-fluorouracil.

Protection against light-activated photofrin II killing of tumor cells by nitroimidazoles

Nitroimidazoles are good quenchers of triplet state porphyrins in chemical systems, thereby inhibiting singlet oxygen formation and type II photodynamic reactions. Photobiological studies were performed with EMT-6 tumor cells in vitro utilizing Photofrin II (PII) in combination with etanidazole (ETAN), misonidazole (MISO), and trifluoromisonidazole (TF-MISO). After short-term (1 h) exposure of cells to PII, 5 mM ETAN and MISO had no effect on photoinactivation while 5 mM TF-MISO had a small but significant protective effect. When the intracellular oxygen level was equilibrated with 0.3% oxygen in the gas phase, all three nitroimidazoles produced significant photoprotection at concentrations as low as 0.3 microM. After long-term (24 h) exposure of cells to PII, all three nitroimidazoles demonstrated large photoprotective effects under both aerobic and 0.3% oxygen conditions. At equal concentrations of nitroimidazole, photoprotection was greatest for the most lipophilic compound (TF-MISO) and least effective for the most hydrophilic compound (ETAN). These studies suggest that nitroimidazoles can quench triplet state porphyrins (within cells) to reduce intracellular concentrations of singlet oxygen, the putative toxin in PII photoinactivation. In addition, after long-term exposures to PII when porphyrins have partitioned into cellular membranes and lipid environments, the lipophilicity of this class of photoprotector correlates with effectiveness in these mammalian cells.

Neocarzinostatin-mediated DNA damage in a model AGT.ACT site: mechanistic studies of thiol-sensitive partitioning of C4' DNA damage products

Double-strand (DS) DNA damage caused by neocarzinostatin (NCS) has been studied in the trinucleotide AGT-ACT sequence in an AP-1 transcription factor binding site. There are strong similarities between bistranded lesions produced at AGT.ACT and AGC-GCT, including the fact that DS lesions outnumber SS lesions on the AGT and AGC strands, while SS exceed DS on the ACT and GCT strands. Structure-function studies revealed that a variety of different thiols produced bistranded lesions in this model by predominantly C4'-hydrogen atom abstraction (84-93%) at the T of AGT and C5'-hydrogen atom abstraction (87-91%) at the T of ACT. Single-strand (SS) lesions were found to represent a variable mixture of C4' and C5' chemistry. The C4'-hydroxylated abasic site occurred in both SS and DS lesions at both sites and accounted for most of the DS damage at AGT (60-83%); the remaining damage consisted of 3'-phosphoglycolate- and 3'-phosphate-ended fragments. The nature of the thiol was found to affect the partitioning of the breakdown products arising from C4' and, to a lesser extent, C5' hydrogen atom abstraction. Production of 3'-phosphoglycolate residues, restricted mainly to the T of AGT in bistranded lesions, correlated with the incidence of direct DS breaks in the AGT.ACT model and in plasmid DNA and appeared to be influenced by the reducing power of the thiol activator. Furthermore, hydrazine and sodium borohydride both inhibited the formation of glycolate, an effect that was exploited to determine the rate constant for 3'-phosphoglycolate formation: 0.06 min-1 at 0 degree C, pH 7.4. Under anaerobic conditions, the nitroaromatic radiation sensitizer misonidazole caused a large increase in glycolate production in both SS and DS lesions formed by NCS, which suggests that the formation of 3'-phosphoglycolate, like 3'-formylphosphate generated by C5' chemistry, involves an oxyradical intermediate. The pathways for DNA damage involving C4' and C5' hydrogen atom abstraction thus share many common features, several of which are consistent with a mechanism for the production of NCS-mediated bistranded lesions at AGT.ACT that involves a tetraoxide bridge joining the lesions on opposite strands of DNA.

Uptake and cytotoxicity of novel nitroimidazole-polyamine conjugates in Ehrlich ascites tumour cells

A number of tumour cells, including Ehrlich ascites tumour cells (EATC), possess a polyamine uptake system which selectively accumulates endogenous polyamines and structurally related compounds by an active energy dependent system(s). We suggest that it may be possible to utilize this uptake system to target certain cytotoxic agents to those tumour cells possessing this system. In an initial attempt to determine the feasibility of this suggestion, we have synthesized a series of 2- and 5-nitroimidazoles linked to polyamines and determined their ability to utilize the polyamine uptake system. Within the limited series of compounds synthesized, 2-nitroimidazole-polyamine conjugates were more potent inhibitors of spermidine uptake into EATC than the 5-nitroimidazole conjugates. It has been assumed partly based on the competitive nature of this inhibition, that the ability of these compounds to inhibit spermidine uptake is also a measure of their ability to be accumulated by EATC. A greater than 700-fold variation was observed in the ability of different analogues to inhibit spermidine uptake. The most potent inhibitors retained certain structural characteristics similar to those of spermidine. Those compounds linked to polyamines were much more potent inhibitors of polyamine uptake than the parent nitroimidazoles i.e. metronidazole and misonidazole. The toxicity of the parent compounds and their polyamine conjugates in control and polyamine-depleted EATC was assessed by measuring inhibition of tritiated thymidine incorporation. Polyamine depletion, by prior exposure to difluoromethylornithine, results in a compensatory increase in the uptake of polyamines and related structures which may result in an increase in toxicity. Whilst many of the novel conjugates showed only little or moderate toxicity to control cells, the toxicity of several of the conjugates but not the parent nitroimidazoles increased in the polyamine-depleted cells. A clear distinction was also observed between the ability to inhibit spermidine uptake (and hence affinity for the uptake system) and toxicity, e.g. compound 430, a dinitroimidazole-polyamine conjugate, was the best inhibitor of spermidine uptake studied but showed no toxicity. These results support the hypothesis that linking polyamines to nitroimidazoles facilitates the entry of the latter into cells, such as EATC, which possess the polyamine uptake system and may therefore have therapeutic application in the delivery of polyamine-linked cytotoxics to certain tumours.

Comparative study of nitroimidazoles on the bioelectric properties of frog skin as a membrane model

The effects of misonidazole (MISO) and two other nitroimidazoles (5-NO2 and 4,5-NO2) on the bioelectric parameters of ion transport (potential difference and short circuit current) across frog skin as a membrane model, were studied in vitro. The nitroimidazoles investigated caused structure dependent effects on the sodium transport function of the membrane. MISO induced a biphasic action following administration on the external side of the membrane: after an initial enhancement, the potential difference and short circuit current signals both decreased. The other imidazole derivatives, 5-NO2 and 4,5-NO2, showed only one phase, whether administered on the external or internal membrane surface. All the nitroimidazoles investigated decreased sodium transport after internal or external surface administration. It was found that the 4,5-NO2 imidazole derivative irreversibly decreased the bioelectric membrane parameters.

Modification of radioresponse of sublethally irradiated mouse jejunum by misonidazole

The radiation-induced changes in a critically radiosensitive tissue, jejunum of BALB/c mice and their modification by a hypoxic cell sensitizer, misonidazole (MISO), were studied. Adult mice were exposed to 1.5, 3.0 and 5.0 Gy of 60Co gamma radiation 30 min after MISO injection and quantitative studies in jejunal cross sections were carried out at different post-treatment times from 3 h to 28 d. Radiation-induced damage to the jejunal mucosa was dose-dependent and reached a maximum on day 1 post-treatment in all the groups. MISO pretreated animals did not exhibit any change in the crypt survival or crypt and villous cellularity compared to the radiation alone group. However, there was a significant suppression of mitotic activity by MISO at early intervals after irradiation. This reduction in mitosis in the drug pretreated animals appeared to affect the rate of recovery from radiation damage, which was slower and prolonged compared with that in the radiation alone group.

Modification of radiation-induced carcinogenesis in mice by misonidazole and WR-2721

The effects of the radiosensitizer misonidazole (MISO) and the radioprotector WR-2721 on radiation-induced carcinogenesis in C3Hf/Kam mice were investigated. The right hind legs were exposed to graded single doses of gamma-rays. MISO and WR-2721 were given i.p. 30 min before irradiation at a dose of 1 mg/g and 0.4 mg/g, respectively. The RCD50, or radiation dose inducing tumors in 50% of the irradiated legs, was determined 650 days after treatment. The same animals were also checked for the effect of these drugs on hair loss and radiation-induced leg contractures. MISO enhanced radiation carcinogenesis by a factor of 1.43, whereas WR-2721 reduced it by a factor of 1.75. These effects on carcinogenesis correlated well with the modifying effects of the two agents on radiation-induced hair loss (early damage) and leg contractures (late damage).

Cytotoxicity of dual function nitrofurans in rodent and human tumor cells

The efficacy and selective hypoxic cell cytotoxicity of four dual function nitrofurans and two nitroimidazole-aziridines was determined in human (A549, HT-29) and rodent (KHT/iv) tumor cells. All bioreductive compounds were found to be less effective at killing human than mouse tumor cells (approximately 2-6-fold). This reduced cytotoxicity in the human tumor cells occurred irrespective of the state of oxygenation. In addition, the degree of selective toxicity toward hypoxic cells or the cytotoxicity factor (CF), defined as the ratio of the surviving fraction in air to that under hypoxic conditions, was (a) greater for the nitroimidazole-aziridines than for the nitrofurans and (b) less in the human than the rodent tumor cell lines investigated. For example, CF values in A549 or HT-29 cells typically were 2-4-fold lower than those determined in KHT/iv cells. This reduction in the CF in the human cells resulted from a greater loss in the hypoxic toxicity than in the aerobic toxicity when compared with the rodent cells.

Enhancement of alkylating agent activity in vitro by PD 128763, a potent poly(ADP-ribose) synthetase inhibitor

The ability of DNA repair inhibitors to potentiate alkylating agent cytotoxicity was explored with PD 128763, a dihydroisoquinolinone known to effectively inhibit poly(ADP-ribose) synthetase. The cytotoxic activity of streptozotocin in L1210 leukemia cells was maximally potentiated (7-fold decrease in IC50) under conditions of 24 hr exposure to PD 128763 following treatment with the alkylating agent for 1 hr. Similar treatment conditions resulted in a much greater effect (36-fold enhancement in activity) for the 2-nitroimidazole RSU 1069. In contrast, 3-aminobenzamide was only weakly effective at enhancing activity of either streptozotocin or RSU 1069 (2-3 fold potentiation). However, PD 128763 was ineffective at potentiating the cytotoxicity of the bifunctional alkylating agents carmustine (BCNU) and lomustine (CCNU). Our results are consistent with a role for (poly-ADP) ribosylation in the repair of monofunctional alkylating agent damage. This study supports further exploration of the combination of PD 128763 and RSU 1069 as a potentially useful chemotherapeutic regimen.

Tumor blood flow changes induced by chemical modifiers of radiation response

This study was designed to investigate blood flow changes induced by chemical agents that are commonly used in combination with radiation therapy. The hypoxic cell cytotoxin RSU 1069 at a dose of 100 mg/kg was shown to reduce blood flow by 80% in both SCCVII and C3H mammary tumors. Blood flow reductions of 30-40% were also observed in both tumors following administration of the radiation sensitiser pimonidazole (Ro-03-8799) at a dose of 500 mg/kg. Moreover studies in the C3H mammary tumor indicated that this effect was tumor size-dependent, being more profound and of longer duration in 500 mg than in 100 mg tumors. Blood flow decreases were also observed in the SCCVII tumor after administration of adriamycin at a dose of 15 mg/kg. Blood flow decreased by over 80% immediately after drug administration but recovered to remain 20% below control values 1 hr after drug administration. In contrast to the other agents studied, cis platinum at a dose of 2 mg/kg produced a small 20-30% increase in blood flow which persisted over the 1 hr observation period. The potential implications of these findings are discussed.

Potentially lethal damage repair by quiescent cells in murine solid tumors

We investigated potentially lethal damage repair by quiescent tumor cells in vivo. SCC VII tumor-bearing C3H/He mice were irradiated after being given 10 injections of 5-bromo-2'-deoxyuridine (BUdR) to label all the proliferating cells in their tumors, and the tumors were then excised and trypsinized. The tumor cell suspensions thus obtained were incubated with cytochalasin-B (Cyt-B, a cytokinesis blocker), and the micronucleus frequency in cells without BUdR labelling was determined using immunofluorescence staining to BUdR. The micronucleus frequency was then used to determine the surviving fraction of unlabelled cells on the basis of the regression line obtained for the micronucleus frequency and the surviving fraction of all tumor cells not labeled by BUdR, which can be regarded as the quiescent cells in a tumor for all practical purposes. Assessment performed 0, 3, 6, 9, and 24 hr after irradiation showed that quiescent cells had more potentially lethal damage repair capacity than the tumor cell population as a whole. Assays were also performed immediately after irradiation alone, 24 hr after the injection of cis-diamminedichloroplatinum(II) (CDDP), mitomycin C (MMC), or misonidazole [1-(2-nitro-1-imidazolyl)-3-methoxy-2-propanol] (MISO) following irradiation, and 24 hr after irradiation alone. It was found that CDDP and MISO (especially the latter) inhibited potentially lethal damage repair more strongly in quiescent cells than in the tumor cell population as a whole. This assay method thus appears to be quite useful for detecting the responses of quiescent tumor cells to various chemical agents.

The repair of DNA damage induced in V79 mammalian cells by the nitroimidazole-aziridine, RSU-1069. Implications for radiosensitization

The induction and repair of single (ssb) and double (dsb) strand breaks in DNA under aerobic or hypoxic conditions have been determined using sucrose sedimentation techniques following incubation of V79 mammalian cells with RSU-1069 or misonidazole, representative of a conventional 2-nitroimidazole radiosensitizer, for 1-1.5 hr at either 293 or 277 degrees K and subsequent irradiation at 277 degrees K. In all cases, the dose dependences for the induction of strand breaks are linear and consistent with an enhancement in the yield of DNA damage induced by the 2-nitroimidazoles under hypoxic conditions. With RSU-1069 at 293 degrees K, the dose dependence of ssb is displaced reflecting DNA damage induced during pre-incubation. From these dependences, it is evident that the enhanced radiosensitization by RSU-1069 may not be accounted for in terms of accumulation of the agent at DNA. From the repair studies, DNA breaks induced by RSU-1069 in the absence of radiation have been shown to persist for at least 3 hr. With a combination of RSU-1069 and radiation under hypoxic conditions, the repair timescale of the induced breaks is significantly longer and an increase in the residual yields of both ssb and dsb (at 2-3 hr) was observed when compared with the observation in the presence of misonidazole or oxygen. From these studies, it is inferred that the enhanced radiosensitization of RSU-1069 at 293 degrees K is a consequence of the formation of non-repairable DNA damage together with a modification of the repairability of the radiation-induced DNA breaks.