No abstract

Summary.-There is now evidence that several classes of nitro compounds which have been used as radiosensitizers also function as cytotoxic agents specific for hypoxic cells. The 2-nitroimidazole, Ro-07-0582, (1-(2-nitroimidazol-1-yl)-3-methoxy-2-propanol) is a compound of this type, and its effectiveness as a cytotoxic agent is dependent on drug concentration, contact time and temperature. In vitro, Ro-07-0582 in air at 37°C does not cause loss of cell viability at concentrations up to 2 mM, even when in contact for several days. In contrast, hypoxic cells do not tolerate much lower concentrations of drug, even if the contact time is only a few hours. When the temperature is raised above 37°C, there is a pronounced increase in the slope of the survival curves; for example, at 41°C (for 1 mM Ro-07-0582, (200 ,ug/ml), the slope changes by a factor of 2-0 relative to that for 37°C. For cells in air at 41°C, as at 37°C, there is no toxic effect at the concentration of drug tested. In the absence of drug, there is no cytotoxic effect of hyperthermia alone under these conditions. These results are discussed in terms of Arrhenius parameters.

alpha-[(1-Aziridinyl)methyl]-2-nitro-1H-imidazole-1-ethanols, of general formula ImCH2CH(OH)CH2NCR1R2CR3R4, where Im = 2-nitroimidazole and R1, R2, R3, R4 = H, Me, are radiosensitizers and selective bioreductively activated cytotoxins toward hypoxic tumor cells in vitro and in vivo. Treatment of the aziridines with hydrogen halide in acetone or aqueous acetone gave the corresponding 2-haloethylamines of general formula ImCH2CH(OH)CH2(+)-NH2CR1R2CR3R4X X-, where R1, R2, R3, R4 = H, Me, and X = F, Cl, Br, I. These 2-haloethylamines were evaluated as prodrugs of the parent aziridines. The rates of ring closure in aqueous solution at pH approximately 6 were found to increase with increasing methyl substitution and to depend on the nature of the leaving group (I approximately Br greater than Cl much greater than F). A competing reaction of ImCH2CH(OH)CH2+NH2CH2CH2X X- (X = Cl, Br) with aqueous HCO3- ions gives 3-[2-hyroxy-3-(2-nitro-1H-imidazol-1-yl)propyl]-2-oxazolidinone. The activities of these prodrugs as radiosensitizers or as bioreductively activated cytotoxins were consistent with the proportion converted to the parent aziridine during the course of the experiment. alpha-[[(2-Bromoethyl)amino]methyl]-2-nitro-1H-imidazole-1- ethanol (RB 6145, 10), the prodrug of alpha-[(1-aziridinyl)methyl]-2-nitro-1H-imidazole-1-ethanol (RSU-1069, 3), is identified as the most useful compound in terms of biological activity and rate of ring closure under physiological conditions.

Mammalian cells (V79-379A) in suspension culture rendered chronically hypoxic showed greater resistance to Adriamycin than exponentially growing aerobic cells. Resistance to Adriamycin increased as a function of the time cells were held under hypoxic conditions, with maximal resistance after 6 h. Chronically hypoxic cells retained their resistance when reoxygenated, and did not return to their original sensitivity until they had been in air for 24 h. Uptake of Adriamycin was similar for chronically hypoxic and exponentially growing aerobic cells, but much more than for plateau-phase cells. These findings suggest that chronically hypoxic cells in tumours may be resistant to this drug.

Summary The vaso-active drug hydralazine causes a considerable increase in the cytotoxic effect of melphalan towards the KHT tumour in mice. The enhancement in response, measured as the concentration of melphalan required to achieve a given tumour response, is 3.0 and 2.35 when determined using the regrowth delay assay and the technique for determining surviving fraction in vitro following treatment in vivo respectively. In contrast, measurement of systemic toxicity shows that the addition of hydralazine only causes a small increase (ER= 1.15) in melphalan damage. This suggests that the drug combination may have some therapeutic benefit. The tumour specificity for the action of hydralazine is supported by the finding that binding of 3H-misonidazole is increased in tumours but not in other tissues when mice are treated with hydralazine. Increased binding of labelled misonidazole is associated with an increase in the level and duration of hypoxia, which will occur as a consequence of changes in tumour blood flow brought about by hydralazine. However, hypoxia per se is not responsible for the enhanced effect of melphalan, since the agent BW12C, which also induces substantial tumour hypoxia as a result of changing the 02 affinity of haemoglobin, has no effect on melphalan tumour cytotoxicity.There have been various reports showing that vasoactive drugs can significantly affect the nature of blood flow in both experimental rodent and human tumours (Algire & Lagallais, 1951;Cater et al., 1962;Kruuv et al., 1967; Vorhees & Babbs, 1982;Knapp et al., 1985). Reduced blood flow in tumours can cause lowering of the oxygen status of the tumour, thereby causing radiation resistance (Kruuv et al., 1967). This so-called 'stealing' effect has recently been exploited by Chaplin and Acker (1987) in order to increase the anti-tumour effect of the bio-reductive agent, RSU 1069 (Adams et al., 1984) a compound which is activated under hypoxic conditions to give a species 100 x more toxic than the parent compound (Stratford et al., 1986).Reduction of blood flow in tumours may also be potentially useful for enhancing the effects of some anti-cancer drugs. The rationale for this is that, administration of the vaso-active drug at the time at which the chemotherapeutic agent has reached its maximum tumour concentration, will inhibit loss of active drug from the tumour. This could increase the overall exposure of the tumour cells to the cytotoxic drug. This paper describes the results of a study of the effect of the vasoactive agent hydralazine on the cytotoxic action of melphalan (L-phenylalanine mustard, L-PAM) towards the KHT sarcoma in mice. Materials and methodsMice and tumours Eight to 12 week old male Category IV C3H/He mice, obtained from NIMR, Mill Hill, London in 1984 and subsequently bred 'in-house', were used in the present experiments. The KHT sarcoma (Kallman et al., 1967), provided by Dr P. Twentyman, MRC, Cambridge in 1983, was maintained by inoculation of a tumour brei into the gastrocnemius muscle of female mice. Generall...