Effect of hyperthermia on differential cytotoxicity of a hypoxic cell radiosensitizer, Ro-07-0582, on mammalian cells in vitro

A phase I study of the nitroimidazole hypoxia marker SR4554 using 19F magnetic resonance spectroscopy

Background:

SR4554 is a fluorine-containing 2-nitroimidazole, designed as a hypoxia marker detectable with ¹⁹F magnetic resonance spectroscopy (MRS). In an initial phase I study of SR4554, nausea/vomiting was found to be dose-limiting, and 1400 mg m⁻² was established as MTD. Preliminary MRS studies demonstrated some evidence of ¹⁹F retention in tumour. In this study we investigated higher doses of SR4554 and intratumoral localisation of the ¹⁹F MRS signal.

Methods:

Patients had tumours ⩾3 cm in diameter and ⩽4 cm deep. Measurements were performed using ¹H/¹⁹F surface coils and localised ¹⁹F MRS acquisition. SR4554 was administered at 1400 mg m⁻², with subsequent increase to 2600 mg m⁻² using prophylactic metoclopramide. Spectra were obtained immediately post infusion (MRS no. 1), at 16 h (MRS no. 2) and 20 h (MRS no. 3), based on the SR4554 half-life of 3.5 h determined from a previous study. ¹⁹Fluorine retention index (%) was defined as (MRS no. 2/MRS no. 1)*100.

Results:

A total of 26 patients enrolled at: 1400 (n=16), 1800 (n=1), 2200 (n=1) and 2600 mg m⁻² (n=8). SR4554 was well tolerated and toxicities were all ⩽grade 1; mean plasma elimination half-life was 3.7±0.9 h. SR4554 signal was seen on both unlocalised and localised MRS no. 1 in all patients. Localised ¹⁹F signals were detected at MRS no. 2 in 5 out of 9 patients and 4 out of 5 patients at MRS no. 3. The mean retention index in tumour was 13.6 (range 0.6–43.7) compared with 4.1 (range 0.6–7.3) for plasma samples taken at the same times (P=0.001) suggesting ¹⁹F retention in tumour and, therefore, the presence of hypoxia.

Conclusion:

We have demonstrated the feasibility of using ¹⁹F MRS with SR4554 as a potential method of detecting hypoxia. Certain patients showed evidence of ¹⁹F retention in tumour, supporting further development of this technique for detection of tumour hypoxia.

Cytotoxicity and probable mechanism of action of sulphimidazole

Sulphimidazole (1-methyl-2((4-aminophenyl)-sulphonyl)-amino-5-nitroimidazole) is a new compound in which a p-aminobenzenesulphonamide radical has been attached at position 2 of the 5-nitroimidazole ring. It possesses a useful spectrum of activity in vitro against various anaerobic microorganisms and its action against aerobic and facultative bacteria is synergically enhanced in association with trimethoprim. In the present study, we determined the cytotoxicity in vitro of sulphimidazole and trimethoprim, both alone and in combination, and analysed the viability of Vero cells and the protein content of their cell lysate in the presence of increasing concentrations of these drugs. Also, in order to verify the hypothesis that the action of sulphimidazole against aerobic and facultative bacteria is mediated by the sulphonamide component of the molecule, while that against anaerobic bacteria depends on the action of the nitro group of the 5-nitroimidazole ring, we studied the mechanism of action of the new compound both indirectly, by means of microbiological techniques, and directly, by determining its oxidoreduction potential with respect to that of metronidazole. The results show that sulphimidazole is only slightly toxic in vitro for Vero cells, either alone or in association with trimethoprim, and that the combination of the two functional groups in a single molecule not only maintains its structure-activity relationship intact but also broadens its antibacterial spectrum.

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.

Dose-dependent increase in the frequency of micronuclei and chromosomal aberrations by misonidazole in mouse bone marrow

Cytogenetic effects produced in bone marrow of mice by various doses of misonidazole (MISO, 100 to 1000 mg/kg bodyweight) were studied by assaying the induction of micronuclei (MN) and chromosomal aberrations. Misonidazole increased the frequency of both micronuclei and chromosomal aberrations over normal at 24 h after treatment, however, the values were statistically significant from normal only at drug doses above 250 mg/kg. The increase was proportional to the drug dose with a best fit to linear quadratic model for MN induction. For chromosomal aberrations the data fitted equally well to linear as well as linear quadratic models.

Comparative study of thermoradiosensitization by misonidazole and metronidazole in vivo: antitumour effect and pharmacokinetics

Tumour control by local hyperthermia (43.5 degrees C, 30 min) and radiation (20 Gy) given in combination with misonidazole (MISO) or metronidazole (METRO) was studied using FSa-II murine fibrosarcoma. When MISO or METRO (5 mmol/kg) was given 30 min before heat and subsequently treated with radiation, tumour regression was observed for both agents. Radiation dose-response curves for MISO and METRO with heating at 43.5 degrees C for 30 min were identical. Mouse foot reaction was used to evaluate local toxicity following combined heat, a nitroimidazole and radiation treatment. MISO enhanced the magnitude of foot reaction and prolonged the recovery time compared with heat plus radiation controls. There were no observable differences of foot reaction between animals treated with heat plus radiation and those animals treated with heat, radiation and METRO. Pharmacokinetics of the nitroimidazoles heated at 43.5 degrees C for 30 min in FSa-II tumours were investigated as a possible mechanism of thermal sensitization. Local hyperthermia did not alter the pharmacokinetics of METRO. Tumour concentration and tumour/plasma ratio of MISO were slightly decreased during heating. Since the hypoxic metabolism of the nitroimidazoles did not increase significantly during the heat treatment, the thermal enhancement of MISO or METRO radiosensitization cannot be explained by the increase in hypoxic cytotoxicity of the nitroimidazoles at elevated temperature alone. The two nitroimidazoles also were not accumulated in the tumour after heating. Therefore, alternation of pharmacokinetics is not the major mechanism for the thermal enhancement of nitroimidazole radiosensitization. The METRO radiosensitization effect became identical to that of MISO at elevated temperatures is of particular importance in clinical radiosensitization. The very low local and systemic toxicity together with the high efficacy of METRO at elevated temperatures will make it an attractive candidate as a future clinical radiosensitizer.

A flow cytometric study of the effect of heat on the kinetics of cell proliferation of Chinese hamster V-79 cells

Two methods involving labelling cells with bromodeoxyuridine (BrdUrd) have been used to study by flow cytometry the effect of hyperthermia (43 degrees C for up to 1 h) on Chinese hamster V79 cells. One method involved the use of an antibody to BrdUrd after pulse-labelling the cells either before or at time intervals after treatment. In the second method, the cells were incubated continuously in BrdUrd after heat treatment, and the components of the cell cycle were then visualized by staining with a combination of a bis-benzimidazole and ethidium bromide. All three methods showed that heating at 43 degrees C stopped DNA synthesis which, at 37 degrees C, subsequently recovered reaching the normal rate 8-12 h later. The cells in S phase at the time of treatment then progressed to G2 where they were further delayed. Cells heated in G1. after the recommencement of synthesis, progressed around the cycle, albeit slower than in unheated cells. The difference between the cells in G1 and S phases at the time of treatment may account for the greater sensitivity of S phase cells to hyperthermia.

Effects on tumour microcirculation in mice of misonidazole and tumour necrosis factor plus hyperthermia

We examined the effects of misonidazole (MISO) and recombinant human tumour necrosis factor (rh-TNF) on tumour blood flow in mice given hyperthermic treatments. MISO (500 mg kg-1) or rh-TNF (6 x 10(4) unit kg-1) was administered intraperitoneally (i.p.) prior to hyperthermia to nude mice bearing a xenoplanted human gastric cancer and tumour blood flow was measured by a hydrogen diffusion method based on polarographic determinations. MISO plus hyperthermia produced a temperature-dependent decrease in blood flow and, at 43.5 degrees C, the flow decreased to 15-30% of control and remained low for up to 24 h. Blood flow following rh-TNF plus hyperthermia was less than that at the same temperatures following MISO plus hyperthermia, and, at 43.5 degrees C, the flow decreased to 10-20% of control and remained low for up to 48 h. Tumour growth delay was closely related to the duration of the decrease in blood flow. Thus, the profound decrease in tumour blood flow following hyperthermia plus MISO or rh-TNF and the consequential tumour regression may well be of potential clinical significance.

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.

Effect of environmental conditions (pH, oxygenation, and temperature) on misonidazole cytotoxicity and radiosensitization in vitro and in vivo in FSaIIC fibrosarcoma

The effect of pH on misonidazole-induced cell killing at normal and elevated temperatures and on radiosensitization by misonidazole at 37 degrees C was assessed in FSaIIC fibrosarcoma cells in vitro. At doses of 5-500 microM for 1 hr, misonidazole was 1.5- to 2-fold more toxic toward hypoxic versus euoxic cells at 37 degrees C and pH 7.40. At 42 degrees C and 43 degrees C at pH 7.40, a less than 2-fold increase in cytotoxicity was observed in both normally oxic and hypoxic cells as compared with 37 degrees C. At pH 6.45 and 37 degrees C, misonidazole was less cytotoxic toward both euoxic and hypoxic cells than at pH 7.40. Unexpectedly, exposure to misonidazole at 42 degrees C or 43 degrees C and pH 6.45 caused no significant increase in cytotoxicity over that attributable to hyperthermia alone. Similarly, the dose modifying effect of misonidazole on single radiation fractions in vitro was also reduced at pH 6.45 versus pH 7.40 (2.60 versus 2.40, p less than 0.01). In vivo, treatment of the FSaIIC tumor with misonidazole (1 g/kg) and/or local hyperthermia (43 degrees C for 30 min to the tumor-bearing limb) in conjunction with radiation (10, 20, or 30 Gy) yielded a radiation dose modifying factor for misonidazole of 1.32, for hyperthermia of 1.38, and for the combination of 2.06 (probably additive). Analysis of the cytotoxicity achieved by these treatments in Hoechst 33342 dye-selected tumor subpopulations demonstrated that, whereas radiation was more toxic toward bright (presumably euoxic) cells, misonidazole, hyperthermia, and the combination were significantly more toxic toward dim (presumably hypoxic) cells. The addition of both hyperthermia and misonidazole to radiation more than overcame the relative resistance of the dim subpopulation to 10 Gy. These results indicate that misonidazole is a reasonable drug for use with hyperthermia and radiation to increase killing of hypoxic cells, but the decrease in cytotoxicity and radiosensitizing abilities of this agent observed under acidotic conditions could reduce the effectiveness of this treatment.

Effect of pH, oxygenation, and temperature on the cytotoxicity and radiosensitization by etanidazole

The effect of etanidazole was examined in vitro and in vivo in the FSaIIC tumor system. At pH 7.40 and 37 degrees C, etanidazole at 5-500 microM for 1 hr was minimally cytotoxic. At 42 degrees C and 43 degrees C, however, the cytotoxicity of etanidazole increased. Etanidazole was more cytotoxic at pH 6.45 and 37 degrees than at pH 7.40 by about 1 log. Increasing the temperature to 42 degrees C or 43 degrees C at pH 6.45 during drug exposure, however, caused little increase in drug killing above the lethality of hyperthermia. When the radiosensitizing abilities of etanidazole were tested in vitro, there was a radiation dose modifying factor of 2.40 at pH 7.40, but only 1.70 at pH 6.45. In vivo, etanidazole (1 g/kg) produced a radiation dose modifying factor of 1.47, whereas 43 degrees C for 30 min produced a radiation dose modifying factor of 1.38. The combination resulted in a radiation dose modifying factor of 2.29. When the cytotoxicities of hyperthermia (43 degrees C x 30 min), etanidazole (500 mg/kg or 1 mg/kg), and radiation (10 Gy) combinations were assayed by Hoechst 33342 dye selected tumor subpopulations, 43 degrees C x 30 min increased the killing of irradiated dim cells by approximately 9.2-fold but by only 2.9-fold in bright cells. Etanidazole (1 g/kg) increased radiation killing of bright cells by about 3-fold and dim cells by about 4.3-fold. The combination of hyperthermia and etanidazole increased the killing of both dim and bright cells exposed to radiation by approximately 10-fold versus 10 Gy alone.

The effect of ultrasound on the cytotoxicity of adriamycin

The effect of continuous wave ultrasound exposures on the cytotoxicity of adriamycin has been studied. It has been found that 2.6 MHz, 2.3 Wcm-2 (spatial average) ultrasound can enhance the cell killing potential of adriamycin both in suspensions of single V79 chinese hamster fibroblast cells and in spheroids formed from these cells. The ratio of the slopes of the survival curves for single cell suspensions is 1.5. For spheroids, the growth delay is increased by 1.3 days by simultaneous ultrasound exposure. Flow cytometric studies of the intracellular concentration of adriamycin following ultrasound exposure reveals that this is increased when compared with that measured when the cells are only exposed to adriamycin. Evidence is presented to suggest that this is a non-thermal effect of ultrasound.

Cytocidal effects of misonidazole, Ro 03-8799, and RSU-1164 on euoxic and hypoxic BP-8 murine sarcoma cells at normal and elevated temperatures

Euoxic and hypoxic BP-8 murine sarcoma cells were exposed for up to 3 hours to various concentrations of three nitroimidazole derivatives (misonidazole, Ro 03-8799, RSU-1164) at normal or elevated incubation temperatures. Cell survival was monitored with the iodine 125 (125I)-iododeoxyuridine prelabeling assay. When cell lethality was evaluated as a function of drug molarity, the three nitroimidazoles displayed widely different toxicities, but when expressed in terms of toxicity ratio between euoxic and hypoxic cells, all three drugs showed nearly identical toxicity differentials of 16 to 18 in 1-hour drug incubation experiments. Prolonging the treatment period to 3 hours did not change the euoxic/hypoxic toxicity ratio for misonidazole and Ro 03-8799, but with RSU-1164 the toxicity ratio was increased significantly from 16 (1 hour) to 73 (3 hours). This increase was attributed to the bifunctional action of RSU-1164 as a combined electron-affinic and alkylating agent, with the alkylation component of cell killing becoming more pronounced after prolonged drug incubation under hypoxic conditions. Combined administration of hyperthermia and nitroimidazoles increased drug-induced cell lethality for all three agents, but did not materially change the relative toxicity differential between euoxic and hypoxic cells. In short, based on cellular toxicity data, Ro 03-8799 appears to offer no advantage over misonidazole as a selective cytocidal agent for hypoxic cells, but RSU-1164 does provide a moderate therapeutic advantage.

High efficiency of ferricenium salts as radiosensitizers of V79 cells in vitro and the KHT tumor in vivo

Ferricenium salts (Fc+X-) can act as radiosensitizers of hypoxic cells in vitro and of KHT tumor cells in vivo. Sensitization is manifest by a removal of the shoulder of the hypoxic radiation survival curve in vitro. This results in a dose modification factor (DMF) of 2.0 when measured at a surviving fraction of 0.1, for 10 mumol dm-3 FcPF6. In comparison, little sensitization is seen in air. The toxicity of these metal complexes, but not their radiosensitizing ability, can be decreased by including protein (Bovine serum albumin) in the medium. This may be related to the fact that the ferricenium complexes react readily with nucleophiles to generate OH radicals. Ferricenium trichloracetate, when given at a dose of 200 mg/kg to mice with the KHT tumor gives an enhancement ratio of 1.3.

NLP-1: a DNA intercalating hypoxic cell radiosensitizer and cytotoxin

The 2-nitroimidazole linked phenanthridine, NLP-1 (5-[3-(2-nitro-1-imidazoyl)-propyl]-phenanthridinium bromide), was synthesized with the rationale of targeting the nitroimidazole to DNA via the phenanthridine ring. The drug is soluble in aqueous solution (greater than 25 mM) and stable at room temperature. It binds to DNA with a binding constant 1/30 that of ethidium bromide. At a concentration of 0.5 mM, NLP-1 is 8 times more toxic to hypoxic than aerobic cells at 37 degrees C. This concentration is 40 times less than the concentration of misonidazole, a non-intercalating 2-nitroimidazole, required for the same degree of hypoxic cell toxicity. The toxicity of NLP-1 is reduced at least 10-fold at 0 degrees C. Its ability to radiosensitize hypoxic cells is similar to misonidazole at 0 degrees C. Thus the putative targeting of the 2-nitroimidazole, NLP-1, to DNA, via its phenanthridine group, enhances its hypoxic toxicity, but not its radiosensitizing ability under the present test conditions. NLP-1 represents a lead compound for intercalating 2-nitroimidazoles with selective toxicity for hypoxic cells.

Enhancement of DNA damage in mammalian cells upon bioreduction of the nitroimidazole-aziridines RSU-1069 and RSU-1131

The induction of DNA double-(dsb) and single-(ssb) strand breaks by RSU-1069, RSU-1131 and misonidazole in V79 mammalian cells has been investigated using sedimentation in isokinetic sucrose gradients after incubation for various times (1-3 hr) at 310 K under both hypoxic and aerobic conditions. Double strand breaks are produced by RSU-1069 and RSU-1131 predominantly under hypoxic conditions. Comparison of the cellular DNA damage induced by these agents leads to the following facts: (1) the yield of ssb induced by these agents is substantially increased under hypoxia, (2) RSU-1069 and RSU-1131 are much more effective than misonidazole, on a concentration basis, at causing strand breakage both under hypoxic and aerobic conditions; and (3) RSU-1069 is more efficient on a concentration basis than RSU-1131 at inducing both ssb and dsb under both conditions. From these findings and molecular studies it is suggested that these 2-nitroimidazole aziridines act as monofunctional alkylating agents under aerobic conditions, a factor that governs their aerobic cytotoxicity. Under hypoxic conditions, it is suggested that the induction of dsb and crosslinks by these agents (bifunctional character) may play a major role in determining the ability of such agents to act as hypoxia-selective cytotoxins.

Misonidazole reduces blood flow in two experimental murine tumours

The effects of single doses of misonidazole (MISO) on blood flow and vascular volume in the SaFA and CaNT tumours and normal tissues of the mouse have been studied. MISO was administered in the dose range 250-1,000 mg kg-1 and blood flow measured at different times after MISO by the 86RbCl extraction technique. Vascular volume was assessed by the distribution of 51Cr-labelled red blood cells. MISO at doses of 500 mg kg-1 or greater decreased flow in both tumours by up to 60% within 2 h. Flow remained reduced for up to 24 h. Similar but less profound changes were seen in the skin, although flow had recovered by 24 h. Only slight changes were seen in muscle, and none in kidney. The apparent loss of flow in tumours seen after large single doses of MISO may have important implications for its use as a chemosensitizer.

Enhancement of misonidazole chemopotentiation by mild hyperthermia (41 degrees C) in vitro and selective enhancement in vivo

Experiments were designed to test the hypothesis that mild heat treatment would selectively increase misonidazole (MISO) chemopotentiation of CCNU toxicity in hypoxic versus aerobic cells in vitro and in tumours in vivo via an augmentation of nitroreduction. EMT-6 cells were exposed to CCNU +/- 1.0 mM MISO under aerobic or hypoxic conditions for 4 h either at a constant 37 degrees C or at 41 degrees C for the first hour followed by 37 degrees C for the remaining 3 h. Chemopotentiation was not observed under aerobic conditions and heat treatment did not modify CCNU toxicity. Co-incubation with MISO and CCNU under hypoxic conditions resulted in enhanced toxicity (i.e. chemopotentiation) with either incubation protocol; however, the magnitude of the enhancement was significantly larger (P less than 0.025) when 41 degrees C incubation was included. Systemic heat treatment produced a similar enhancement of chemopotentiation in KHT tumours in C3H/HeN mice treated with MISO (0.5 mg g-1) and whole body hyperthermia (41 degrees C, 1 h) prior to administration of CCNU (15 mg kg-1). Heating had no effect on CCNU response but doubled the median growth delay produced by the CCNU-MISO combination. Heat treatment did not enhance myelosuppression of the combination. Both the in vitro and in vivo data indicate that mild hyperthermia can selectively enhance the magnitude of MISO chemopotentiation.

The effects of whole body hyperthermia on the pharmacokinetics and toxicity of the basic 2-nitroimidazole radiosensitizer Ro 03-8799 in mice

We have investigated the effects of 50 min whole-body hyperthermia (WBH; 15 min equilibration followed by 41 degrees C for 35 min) on the toxicity and pharmacokinetics of the radiosensitizer Ro 03-8799 in mice. WBH markedly reduced Ro 03-8799 LD50/7d from 779 to 259 micrograms g-1 (P less than 0.001). Pharmacokinetics were studied at 175 micrograms g-1 (approximately 0.6 WBH LD50/7d) with and without heat and 437 micrograms g-1 (approximately 0.6 control LD50/7d) without heat. WBH increased Ro 03-8799 plasma concentrations and prolonged its elimination t1/2 by 26% (P less than 0.01). Total plasma area under the curve (AUC0-infinity) was increased by 22%, but was still less than 50% of the unheated high-dose value. Ro 03-8799 concentrated 300-400% in tumour and brain relative to plasma. Absolute tumour and brain levels were unaltered by WBH, giving reduced tissue/plasma ratios. WBH greatly inhibited glomerular filtration (51Cr EDTA clearance) during heating, contributing to the increased plasma Ro 03-8799 concentrations. WBH increased peak plasma concentrations of the Ro 03-8799 N-oxide metabolite Ro 31-0313 by 61% and the beta-phase AUC of i.v. administered Ro 31-0313 by 36%. Since Ro 31-0313 levels were increased to a greater extent after Ro 03-8799 and WBH than Ro 31-0313 and WBH, WBH must both increase metabolite production and decrease its plasma clearance. WBH had no effect on Ro 31-0313 tumour concentrations or its exclusion from brain. These complex effects of WBH on Ro 03-8799 pharmacokinetics may contribute to the enhanced toxicity, possibly through hyperthermia-stimulated bioreductive drug activation, but do not wholly explain it.

The effects of melphalan and misonidazole on the vasculature of a murine sarcoma

A method for estimating both structural and functional vascular volumes in murine sarcomas is described. Intact vessels were demonstrated by the presence of laminin, a basement membrane-associated antigen, using an immunofluorescent technique, and functional vessels in the same sample by prior injection with the DNA binding dye Hoechst 33342. No significant vascular effects were seen after melphalan but a very pronounced decrease in both functional and structural vascular volume was seen after MISO. Combined chemotherapy of a murine sarcoma with melphalan and MISO induced a rapid decrease in the functional vascular volume, and there was a resumption of blood flow prior to measurable regrowth. The fully regrown tumour retained the vascular characteristics of untreated tumours of similar size.

The differential cytotoxicity of RSU 1069: cell survival studies indicating interaction with DNA as a possible mode of action

The hypoxic cell radiosensitizer RSU 1069 (1-(2-nitro-1-imidazolyl)-3-(1-aziridinyl)-2-propanol) shows, on a concentration basis, a 100-fold greater toxicity towards hypoxic relative to aerobic cells. This toxicity is substantially greater than that of misonidazole, a compound of similar electron affinity. Reductive processes are important for hypoxic toxicity; this is demonstrated by the fact that misonidazole, in excess, can protect against the hypoxic but not aerobic toxicity of RSU 1069. The importance of the interaction of RSU 1069 with DNA, suggested initially by molecular studies, is supported by the fact that cells containing 5-bromodeoxyuridine (5-BUdR) incorporated into their DNA show greater sensitivity towards the lethal effects of RSU 1069 both in air and nitrogen, compared to cells not treated with 5-BUdR. Experiments with RSU 1069 and 3-aminobenzamide (3-AB) show the latter compound to potentiate aerobic toxicity, consistent with monofunctional alkylation by RSU 1069. In contrast, 3-AB has no effect on the hypoxic cytotoxicity of RSU 1069, which would be predicted if RSU 1069 is functioning as a bifunctional agent under these conditions. It is our contention that in air, RSU 1069 functions as a typical monofunctional alkylating agent, presumably due to the presence of the aziridine group whereas, in hypoxia, reduction of the nitro group provides an additional alkylating species, converting the compound into a bifunctional agent.

The interaction between radiation and complexes of cis-Pt(II) and Rh(II): studies at the molecular and cellular level

A range of Rh(II) carboxylates and cis-Pt(II) complexes have been examined for their ability to increase the radiation sensitivity of aerobic and hypoxic V79 cells in vitro. The transition metal complexes sensitize in both air and nitrogen, with the greater effect generally occurring in nitrogen. The cis-Pt(II) complexes only show small levels of sensitization with dose modification factors (DMFs) of no more than 1.2. In contrast, the Rh(II) complexes can give DMFs of 2.0. Radiation chemical experiments show the transition metal complexes to have substantially lower redox potentials than metronidazole and, in addition, neither type of complex undergoes electron transfer reaction or adduct formation on interaction with radicals derived from DNA bases. Thus, the inorganic complexes do not operate by mechanisms similar to those occurring with electron affinic or stable free radical sensitizers. The increase in radiation sensitivity for cells treated with the Rh(II) carboxylates, but not the cis-Pt(II) complexes, is attributed to the ability of the Rh compounds to deplete intracellular thiols. Further, the efficiency of sensitization by the Rh(II) complexes and their ability to interact with cellular thiols depends upon the nature of the carboxylate ligand and follows the order butyrate greater than propionate greater than acetate greater than methoxyacetate. The differences between the carboxylates may be due to differences in drug uptake. A combination of the Rh(II) complexes with misonidazole given to hypoxic cells irradiated in vitro gives an additive response. However, it was not possible to demonstrate a similar effect in tumours in mice given the combination of Rh(II) methoxyacetate and the misonidazole analogue RSU 1070.

Effects of glutathione depletion using buthionine sulphoximine on the cytotoxicity of nitroaromatic compounds in mammalian cells in vitro

The inhibitor of glutathione biosynthesis, buthionine sulphoximine (BSO) has been used to deplete endogenous thiols in mammalian cells in vitro. The effect of such depletion on the toxicity of nitroaromatic compounds has been investigated. Substantial enhancement of both aerobic and hypoxic toxicity of the 2-nitroimidazole, misonidazole is observed in thiol-depleted cells; the hypoxic toxicities of metronidazole, nitrofurantoin and nimorazole are also increased by thiol depletion. These data of significance for the potential combined use of BSO with nitroaromatic radiosensitizers to increase their radiosensitizing efficiency in radiotherapy, and as a potential method for enhancing the efficiency of anti-protozoal nitroaromatic drugs.

Reaction of the vascular system in the trachea of the rabbit exposed to fractionated irradiation with and without the addition of misonidazole

As the radiation field used in the radiation therapy of malignancies in the thoracic cavity often exposes the trachea to ionizing irradiation, it is important to ascertain the effects of radiation on this tissue either as a single therapy or in combination with radiosensitizers. In the study reported here the vascular area in the subepithelial layer of the trachea has been calculated in 160 rabbits treated in four ways: (1) 10 rabbits received no treatment and served as controls; (2) 50 rabbits were given 100 mg misonidazole daily on consecutive days, with the individual total dose ranging from 100 to 1000 mg; (3) 50 rabbits were treated with misonidazole in the same way, but were also exposed to radiation (2 Gy/F) at 15-30 min later; (4) 50 rabbits received only fractionated radiation (2 Gy/F). The total radiation dose in the irradiated animals ranged from 2 to 20 Gy. In the treated groups, an oedema was observed in both the ciliary cell layer and in the subepithelial area. In the group given only irradiation, this oedema was dose-dependent, but no such dose-dependency was observed in the two groups treated with misonidazole. The vascular area in the groups treated with misonidazole was significantly increased as compared with the group given only irradiation and the control group; this was valid both with and without correction for the oedema. There was a significant correlation between the oedema and the vascular area in the groups treated with misonidazole, which was not found in the group irradiated without the drug.

Hyperthermia for cancer: a practical perspective

A causal relationship between hyperpyrexia and tumor regression was first suggested in 1866, when Busch reported the cure of a histologically diagnosed sarcoma in a middle-aged woman, following a bout of erysipelas. Over the years, interest in the effect of heat on cancer has remained alive, but this interest has increased dramatically in recent years. The literature on this subject is broadly reviewed and the clinical results discussed. It is apparent from clinical studies thus far that it is a relatively simple undertaking to treat superficial neoplasms with hyperthermia. However, the major challenges in clinical thermotherapy pertain to patients with deeply situated tumors. The lack of safe and reliable methods of monitoring temperature in deep tissues is a major impediment to a thorough understanding of thermal dosimetry in clinical hyperthermia, and routine thermal dosimetry in clinical hyperthermia will have to await the development of reliable noninvasive thermometry. As responses have been reported with modest levels of hyperthermia, the need for thermometry is somewhat lessened, given that invasive monitoring is imperfect and somewhat risky when used in deeply seated tumours. The eventual place of thermotherapy in the treatment of malignant tumours in man is as yet unclear and must be rigourously and thoroughly assessed in well-designed, prospective, randomized patient trials.

The relevance of tumour pH to the treatment of malignant disease

The wide range of tumour pH values that have been determined in human tumours is shown in Fig. 4. It can be seen that tumour pH values may be very low, or may fall in the same range as the values found in normal tissues. This means that pH-mediated modification of therapeutic effectiveness will be patient specific, rather than a general phenomenon. That the pH of the cellular environment might influence the effectiveness of various therapeutic agents is not a new idea. The data published in this field to date concerning such effects have been discussed extensively and are summarized in Table IV. Here we can see that low pH leads to decreased cell survival following treatment with hyperthermia, radiotherapy combined with hyperthermia, radiosensitizers and various chemotherapeutic agents. Conversely, low pH affords some protection against radiation and some drugs. Most of these data were, of necessity, derived from in vitro studies. In vivo studies are in most cases not feasible due to the difficulty of isolating the effect of one selected factor. Low tumour pH is, in vivo, generally assumed to be closely interlinked with tissue hypoxia and low blood-flow levels, each of which may individually influence the experimental outcome. Moreover, most of the aforementioned in vitro studies were conducted under well-oxygenated conditions. As previously mentioned, euoxic cells can, under certain conditions, maintain a pH gradient over the cell membrane. This collapses with the onset of hypoxia, leading to intracellular acidification. Low oxygen levels have been shown to be characteristic of many tumours. Within these limitations it is thus evident that tumour pH values could have far-reaching consequences for therapy. If the in vitro findings should prove to be relevant to the clinical situation various applications are possible. Pre-selection of patients less likely to respond to certain (toxic) chemotherapeutic agents, or conversely selection of agents that are more likely to be effective in the pH range of the tumour to be treated are two examples. Alternatively, the exploitation of low tumour pH values is a possibility. Agents that form or release toxic derivatives in areas of low pH, e.g., pH-sensitive liposomes, will work selectively in such areas. Tumour selective therapy may also be possible in patients with higher tumour pH values if the tumour pH can be lowered. This has been achieved experimentally by the administration of hyperthermia at temperatures above 42 degrees C, or by the administration of glucose.(ABSTRACT TRUNCATED AT 400 WORDS)

Chemopotentiation by CB 1954: the importance of postincubations and the possible involvement of poly(ADP-ribosylation)

CB 1954 potentiates the cytotoxic action of the bifunctional alkylating agent melphalan (L-PAM). In vitro, this potentiation does not require the preincubation in hypoxia normally needed for other nitroaromatic compounds such as misonidazole. Chemopotentiation is observed when cells are held in CB 1954 in air after treatment with L-PAM. This may reflect an inhibition of DNA repair process(es). Structural considerations suggested that CB 1954 might be acting as an inhibitor of poly(ADP-ribosylation). However, an inhibition of the drop in NAD levels consequent on exposure to melphalan was not obtained. Furthermore, unlike the known poly(ADP-ribose) inhibitor, 3-aminobenzamide, CB 1954 does not potentiate the cytotoxicity of the monofunctional alkylator N-methyl-N nitro N-nitrosoguanidine, or inhibit NAD depletion caused by this agent. Therefore the evidence suggests that CB 1954 is not an inhibitor of poly(ADP ribosylation).

Radiosensitization of mammalian cells by transition metal complexes containing nitroimidazole ligands

Various transition metal complexes containing nitroimidazoles as ligands have been shown to act as radiosensitizers of hypoxic cells in vitro. Sensitization by cis-Pt(II)Cl2 (nitroimidazole)2 complexes is no greater than that which can be demonstrated by the free nitroimidazole ligand alone. These results differ from those previously described for the compound FLAP where an ER of 2.4 was obtained at a concentration of 50 microM. We report that, even using the same treatment technique, the maximum ER that can be achieved is 1.2. Further, the sensitizing efficiency of FLAP cannot be improved when cells are kept in contact with the compound for 12 hours in air prior to deoxygenation and irradiation. In contrast, Rh(II) complexes show much greater sensitization than can be obtained with the free ligand and these compounds are in turn more efficient sensitizers than the comparable Pt(II) complexes.

The effect of nitroimidazoles on the oxygen consumption rate and respiratory control ratio of beef heart mitochondria

The neurotoxic effect of the nitroimidazole radiosensitizers misonidazole (MISO) and desmethylmisonidazole (DMM) has seriously compromised their clinical effectiveness. We compare here the effect of MISO and DMM on oxygen consumption in purified beef heart mitochondria. MISO has been found to significantly increase the oxygen consumption rate and decrease the respiratory control ratio in isolated mitochondria when incubated in the presence of the NAD+ dependent substrate, beta-hydroxybutyrate. DMM has a similar but less pronounced effect than MISO on these respiratory parameters. When mitochondria were incubated in the presence of these radiosensitizers for 8, 15, 30, 45, and 60 minutes, the oxygen consumption rate was decreased when succinate, a FAD dependent substrate, was added following the incubation. This decrease, which is both time and dosage dependent, is equivalent for MISO and DMM.

A study of the effects of prior heat treatment on the skin reaction of mouse feet after heat alone or combined with X-rays: influence of misonidazole

The skin of mouse feet was used to study the effects of hyperthermic treatment, either alone or combined with irradiation. The present experiments show that a priming heat treatment induces resistance both to a subsequent heat treatment and to a subsequent combined irradiation-heat treatment. The development of resistance to a combined irradiation-heat treatment after a priming heat treatment (30 min at 43 degrees C) was relatively slow (18-24 h) compared to development of resistance to a heat treatment without irradiation (6 h). Misonidazole, when administered prior to heat treatment only, did not influence the heat-induced skin reaction. However, when misonidazole was administered prior to combined irradiation-heat treatment, a slight but significant increase of the skin reaction was observed. Also, in combination with misonidazole resistance to combined treatment was observed by a priming heat treatment.

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.

Combination of misonidazole and lipoic acid on the radiation sensitivity of Bacillus megaterium spores

The radiation sensitizing action of a combination of misonidazole and lipoic acid was investigated in Bacillus megaterium spores at various oxygen concentrations. Lipoic acid and misonidazole were applied in two concentrations (0.1 and 1.0 mmol/l) and four combinations were prepared from them. No uniform correlation was found, neither to the combination of the compounds nor to the gas conditions. The combination of two radiation sensitizing compounds with in all probability different action mechanisms does not unequivocally enhance the radiation sensitizing effect under anoxic and hypoxic conditions.

Inhibition of cell-cycle progression by acute treatment with various degrees of hypoxia: modifications induced by low concentrations of misonidazole present during hypoxia

The effect on cell-cycle progression in various phases of the cell cycle caused by an acute exposure to hypoxia in absence and presence of misonidazole (MISO) was investigated. Exponentially growing and synchronized cells of the human line NHIK 3025 were exposed to different degrees of hypoxia for a short period (1.5 or 3 h). The cell-cycle progression was studied both during and after hypoxia by flow-cytometric recording of DNA-histograms from treated and untreated cells. The rate of cell-cycle progression was reduced during hypoxia only if the O2-concentration was below 1000 ppm. The inhibition was phase specific with a strong effect in S (reduced DNA-synthesis), and a specific cell-cycle inhibition in late G1, probably at the G1/S-border. For cells inhibited (or arrested for extreme hypoxia) at the G1/S-border, the cell-cycle progression changed back to normal shortly after aerobic conditions were re-established. For cells rendered hypoxic and inhibited during S, hypoxia exerted a lasting effect expressed by a low cell-cycle progression rate even after aerobic conditions were re-established. This effect was strongly dependent on both the degree and the duration of the hypoxic treatment. The presence of a low concentration of MISO (0.05 mM) during hypoxia did not affect the cell-cycle progression during hypoxia at any O2-concentration. For cells rendered hypoxic during S, however, MISO (0.05 mM) counteracted the lasting effect of hypoxia for all concentrations of O2 where this lasting effect was observed.

Basic principles in hyperthermic tumor therapy

Literature on hyperthermic tumor therapy in the past 10 years has grown exponentially. Since 1975 three international symposia on cancer therapy by hyperthermia have been held. Hyperthermia is of clinical interest in the temperature range of 40 degrees-43 degrees C. Higher temperatures of 44 degrees-46 degrees C are not clinically realizable. With local heat application a higher elevation of tissue temperature is possible. Whole-body hyperthermia in men is limited physiologically, as the rate of complications increases exponentially above 42 degrees C. The heat dose normally is defined by temperature degree and time of temperature elevation. Hyperthermia has several effects on tumor cells. It influences proliferation activity; within the mitotic cycle, preferentially the M-phase cells and S-phase cells are thermosensitive. It is possible to synchronize tumor proliferation by heat. Hyperthermia inactivates tumor cells in hypoxic condition as well. This was demonstrated in vitro with tumor cells under varying oxygenation and with spheroid experimental tumors. Experiments with solid tumors in animals had the same effect. Hyperthermia enhances the effect of radiation on tumors. In solid human tumors only 3%-5% of cells are in growth fraction; 95% of tumor cells are hypoxic or prenecrobiotic. Only well-oxygenated cells are sensitive to a sparsely ionizing radiation and can be killed. This selective radiosensitivity is the reason why other radiation qualities for radiotherapy, which are also effective on hypoxic cells, are examined. Neutrons and heavy ions are densely ionizing radiations, which inactivate hypoxic radioresistant cells. Hyperthermia in combination with sparsely ionizing radiations--e.g., X-rays or gamma rays--could be an alternative to neutrons or heavy ions. The main problem with heat application in clinical radiotherapy is the lack of heating methods which are able to heat the entire volume of a large solid tumor homogeneously. In small experimental animals there is a TER of about 1.5-2.0. The therapeutic gain of additional heat in radiotherapy is greatly dependent on localization of the tumor (skin, extremities) and on cooling of the skin. Hyperthermia enhances cytostatic drugs. Many investigations have been done on the interaction of heat and cytostatics; in vitro experiments evaluated three types. First, the activity of many drugs increases slightly with temperature; no special effects are observed above 42 degrees C. Examples of drugs of that pattern are the hypoxic sensitizer Ro-07-0582 and the alkylating agents thio-TEPA and CCNU. A second type of mechanism is seen with cytostatic drugs which exhibit greatly increased effectiveness at temperatures above 42 degrees C; adriamycin and bleomycin belong to this type.(ABSTRACT TRUNCATED AT 400 WORDS)

Enhancement of the cytotoxicity of radiosensitizers by modest hyperthermia: the electron-affinity relationship

The cytotoxicity of 3 electron-affinic radiosensitizers has been studied in Chinese hamster V-79 cells as a function of pH and modest hyperthermia. When equitoxic concentrations were used and temperature was increased from 34 to 41 degrees C metronidazole, the compound with the lowest electron affinity showed the greatest enhancement of hypoxic-cell toxicity, and nitrofurantoin, the compound with the highest electron affinity, the least. The results can be explained if the mechanisms of toxicity involves a redox reaction, since it would be expected that the least toxic compound (lowest electron affinity) would have the largest activation energy and hence the greatest temperature effect. This appears to hold for these 3 compounds. Experiments also showed that nitrofurantoin which exhibits no increase in toxicity when the temperature was increased from 37 to 41 degrees C at pH 7.4, showed an increase in toxicity for the same temperature change at the pH of 7.0 and 6.6. Under aerobic conditions only metronidazole showed significant toxicity at 41 degrees C, where the differential between aerobic and hypoxic cell toxicity was minimal, both at pH 7.4, and at the low pH values of 7.0 and 6.6. In the clinical setting there is evidence that tumour cells are at a lower pH than their surrounding normal tissues. Hypoxic-cell cytotoxicity is enhanced at low pH, and even further enhanced at low pH in combination with a temperature of 41 degrees C. However, this finding correlates conversely with electron affinity. Thus, the radiosensitizer (and trichomonicide) metronidazole is most influenced by low pH and high temperature with the nitroimidazole, misonidazole, demonstrating a smaller enhancement due to higher temperatures.

The radiosensitizing effect of ornidazole in hypoxic mammalian tissue: an in vivo study

In this study the sensitizing effect of ornidazole is investigated in vivo. The selected test system is the acute killing effect of radiation within 4-6 days after abdominal irradiation ranging from 9 to 24 Gy, in groups of C57 black mice. Ornidazole is given intraperitoneally in 500 mg/kg, 100 mg/kg, 20 mg/kg doses prior to irradiation of animals breathing air, oxygen or nitrogen. A decrease of LD50 dose is observed from 24.39 +/- 5.66 to 16.38 +/- 1.86 and 18.04 +/- 2.48 Gy, respectively, in nitrogen breathing animals. No sensitizing effect was observed in doses of 20 mg/kg. Enhancement Ratio (ER) was found to be 1.48 +/- 0.25 and 1.35 +/- 0.27; relative sensitizing efficiency (RSE) was 40% and 29% respectively. No sensitizing effect was observed in animals irradiated in oxic conditions. These results showed that ornidazole (Ro-7-0207) has a sensitizing effect on hypoxic cells in vivo. It is worthwhile to try this drug in a clinical study.

Effect of hyperthermia and misonidazole on the radiosensitivity of a transplantable murine tumor: influence of factors modifying the fraction of hypoxic cells

Hypoxia has been demonstrated to play an important role in the effect of hyperthermia on tumors. We have studied the influence of different factors modifying the oxygenation status of a transplantable murine mammary adenocarcinoma (tumor volume and pentobarbital sodium anesthesia). The effect of hyperthermia alone on the tumor is not significantly influenced by the change in oxygenation status during the growth of the tumor. Also, the large increase of the acutely hypoxic cell fraction, as a result of anesthesia, does not change the effect of hyperthermia alone. In the combined irradiation-heat treatment there is a clear influence of the chronically hypoxic cell fraction on the response to hyperthermia: an increase in tumor size, resulting in a larger hypoxic cell fraction, leads to an increase in thermal enhancement ratio. However, the increased acutely hypoxic cell fraction, resulting from anesthesia, did not lead to an increase in thermal enhancement ratio; in fact the enhancement ratio apparently decreased. In spite of the fact that hyperthermia was applied immediately after irradiation no potentiation of radiation effects was found. The thermal enhancement of the radiation response was never larger than the enhancement as a result of misonidazole. All thermal enhancement could be explained by effects of heat on the chronically hypoxic cell fraction. Misonidazole had no effect on the response of tumors to heat alone, but greatly enhanced the effect of heat combined with irradiation. Anesthesia of the animals did not influence these effects of misonidazole.

Enhancing effect of pre-treatment of cells with misonidazole in hypoxia on their response to melphalan in air

Pre-treatment of hypoxic cells with misonidazole (MISO) can render these cells more sensitive to a subsequent treatment with melphalan. Results in this paper show that this enhancement (or chemopotentiation) depends on the contact time and concentration of MISO, on the melphalan concentration and also on the cultural history of the cells. Damage due to hypoxic pre-incubation in MISO can be repaired if cells are subsequently aerated at 37 degrees C. In contrast, for cells washed free of MISO and then held under N2 at 37 degrees C, repair is much slower. No repair occurs when cells are held in air at 0 degrees C. The kinetics and extent of repair were dependent on the cells prior culture. Thus for exponential cells repair was complete after approximately 4 h, whereas for plateau-phase cells and cells with prior chronic hypoxia, repair was only partially complete after this time. Dithiothreitol was shown to protect partially against the enhancement of melphalan toxicity. Increased cell killing is also obtained if cells are given high concentrations of MISO (50 mM) in air during exposure to melphalan.

A comparison of the cytological effects of three hypoxic cell radiosensitizers

Misonidazole has entered Phase III clinical trials as a hypoxic cell radiosensitizer. Neurotoxocity is the major dose-limiting factor and has prompted the development of two further compounds with reduced lipophilicity and shorter half-life in vivo. Aside from the short-term problem of neurotoxocity, other potential long-term consequences should be considered. Such is the purpose of this investigation where the cytological effects of three radiosensitizers upon oxic and hypoxic Chinese hamster V-79 cells have been examined. Two newer compounds, desmethylmisonidazole and Stanford Research compound 2508, were compared with their clinically used predecessor, misonidazole. Under aerated conditions, cell killing was increased with SR-2508 in a concentration and time dependent manner, so as to exceed by more than three times the level produced by the other two drugs at 5 mM for 72 hours. Cell progression into mitosis was also markedly reduced by as much as 1/10,000 of control values. However, as the three compounds induced similar frequencies of sister chromatid exchange (SCE) and chromosome aberration, the enhanced cytotoxic effect of SR-2508 appears to be mediated via an interphase rather than a post-mitotic cell death. Cells were made hypoxic and treated with the three drugs for 4 hr, then mitoses sequentially collected for 16 hr. The three compounds produced similar levels of cell killing, slowing of cell cycle progression, SCE's and chromosome aberrations, with cycle-specific effect on S and G-1 phase cells for SCE induction. These results indicate that desmethylmisonidazole and misonidazole have similar cytotoxic and clastogenic properties under oxic and hypoxic conditions. SR-2508 is relatively more toxic to aerated cells and may deserve close clinical observation for toxicity to normal tissues; further, all three agents may enhance DNA damage and mutagenesis in tissues that are normally hypoxic.

Nitroimidazole inhibition of lactate production in CHO cells

Misonidazole (MISO) and desmethylmisonidazole (DMM) inhibit lactate production in CHO cells under aerobic conditions. This inhibition is time and dose dependent. At the 20 mM level DMM is stronger in producing this inhibition of lactate than is MISO. This inhibiton of lactate production suggests that these compounds are capable of inhibiting the glycolytic pathway. The resultant energy deficiency may be an explantation for the toxicity of these compounds under hypoxic as well as aerobic conditions. The increased dependency on glycolysis of hypoxic cells may correspond to the analogous increase in toxicity of hypoxic cells to nitroimidazoles.

Chemosensitization in vitro: potentiation of melphalan toxicity by misonidazole, metronidazole and nitrofurazone

Misonidazole, metronidazole and nitrofurazone can enhance the cytotoxicity of melphalan in vitro if the cells are subjected to a hypoxic pretreatment with the drug prior to exposure of melphalan in air. Potentiation of melphalan is dependent upon the concentrations of the nitro compound and the duration of the hypoxic pretreatment. On a concentration basis, nitrofurazone was most effective at enhancing melphalan toxicity and metronidazole was the least effective. This potentiating activity correlates with the electron affinity of each drug.