Cytotoxicity and radiosensitization in mouse and man

The effects of anesthetics and misonidazole on the development of radiation-induced lung damage in mice

The measurement of breathing frequency as a functional end-point of radiation-induced lung injury in mice allowed two phases of damage to be discerned; the first was manifest at 12-20 weeks after irradiation, the second beyond 28 weeks. Anesthesia by pentobarbitone sodium or steroids gave significant radioprotection of the lung during the early pneumonitic phase. Addition of the hypoxic cell sensitizer misonidazole removed the protective influence of the anesthetics but did not sensitize the lungs of unanesthetized mice. No anesthetic protection was detected for the late response, showing evidence for dissociation between early and late lung damage. The degree of epilation was measured on the dorsal thoracic region of the same mice. Protection by anesthetics and its reversal by misonidazole was also demonstrated. These results provide a warning of potential hazards in the laboratory evaluation of chemical radiosensitizers. The use of anesthetics at the time of irradiation could lead to an exaggerated enhancement of normal tissue damage.

Enhancement of chemotherapy agents

The interaction in vivo between misonidazole and cytotoxic drugs is reviewed. In general MISO interacts best with bifunctional alkylating agents and nitrosoureas when there is generally a therapeutic gain at large MISO doses. The interaction is unlikely to be a result of pharmacokinetic effects of MISO, but may reflect its ability to inhibit recovery from drug induced damage. It is important to extend these studies to clinically relevant MISO doses simulating human pharmacokinetics.

Enhancement of the effect of cytotoxic drugs by radiosensitizers

Misonidazole (MISO) potentiates the action of cyclophosphamide (CY) and melphalan in the WHFIB culture-adapted fibrosarcoma, whether assayed by cell survival or tumour-growth delay. In the case of CY, MISO also inhibited recovery from potentially lethal drug damage. The optimum effect was seen when MISO was given 1 h before CY, though it was also effective when given 6 h before or 1 h after the drug. Other radiosensitizers also potentiated the action of CY. There was only a small effect of MISO on the LD50 of CY and no effect on CY toxicity as assayed by changes in blood counts or damage to bladder epithelium. However, mice bearing multiple lung tumours were less able to cope with the combined treatment than those bearing s.c. tumours.

Low concentrations of misonidazole counteract effects of extreme hypoxia on cells in S

Populations of NHIK 3025 cells synchronized by mitotic selection were exposed at 37 degrees C to extreme hypoxia in absence and presence of misonidazole (MISO). Cells in G1, S or G2 and mitosis were treated for 3 h. Inhibition of cell-cycle progression by this treatment was measured by flow cytometry of DNA histograms and cell inactivation was measured by colony formation. The exposure to hypoxia alone of cells in G1 or in G2 and mitosis led to only minor cell-cycle inhibition, and hardly reduced cell survival. However, the exposure of cells in S to hypoxia alone had a strong inhibitory effect on cell-cycle progression, and cell survival was only 40% of untreated cells. Low concentrations of MISO (0.05-0.4 mM) during exposure of cells in S to hypoxia, produced less cell-cycle inhibition than after hypoxia alone, and cell survival was restored to 100%. The presence of MISO during the 3h exposure to hypoxia of cells in G1 or in G2 and mitosis only increased the effects of hypoxia alone. MISO at concentrations greater than 0.8 mM during hypoxia produced cell inactivation, for all phases of the cell cycle, comparable to that already known from the literature.

In vivo interaction of anti-cancer drugs with misonidazole or metronidazole: methotrexate, 5-fluorouracil and adriamycin

I have studied the effects on growth of two tumours in mice and on host toxicity, of combining Misonidazole (MISO) or Metronidazole (METRO) with Methotrexate (MTX)-5-fluorouracil (FU) or Adriamycin (ADR). The nitroimidazoles alone had no effect on the growth of either tumour, but MISO (1 mg/g) led to a small increase in delay to regrowth of the 16/C mammary carcinoma but not the KHT fibrosarcoma, when given after X-irradiation. MTX was active only against the KHT tumour, and growth delay was not increased by the addition of MISO or METRO. FU delayed growth of both tumours, and growth delay was increased slightly by single-dose MISO. ADR was active only against the 16/C tumour, and delay to regrowth was increased by adding MISO. Host toxicity assessed by death and loss of body weight was much greater when MISO or METRO were added to MTX, and a little greater when they were added to FU. ADR plus MISO caused no deaths and no greater loss of body weight than ADR alone. The addition of MISO to treatment with anticancer drugs led to a slightly greater and more prolonged myelosuppression. METRO and MISO increase the anti-tumour effects of some anti-cancer drugs, but may also increase host toxicity. Nitroimidazoles should be used with caution in combination with chemotherapy.

In vivo interaction of anti-cancer drugs with misonidazole or metronidazole: cyclophosphamide and BCNU

The addition of misonidazole (MISO) or metronidazole (METRO) to treatment with cyclophosphamide (CY) increased delay to regrowth of 2 experimental tumours. The effect was observed for large an small tumours, was present for doses of MISO that are ineffective for killing hypoxic cells, and required that it be given with, or shortly before CY. Mice receiving combined treatment had more weight loss and myelosuppression than those receiving CY alone, and the Therapeutic Index was lower. MISO caused a marked increase in growth delay when combined with BCNU to treat the KHT sarcoma. This effect was observed for small and large tumours, required simultaneous administration of drugs, and also led to increased host toxicity. There was no therapeutic advantage from combined treatment. Survival of aerobic or anoxic Chinese hamster ovary (CHO) cells was assessed after exposure in vitro to serum from mice that had received CY or BCNU alone. MISO alone, or combined treatment. Results of these experiments suggest that (1) MISO delays the excretion or breakdown of active metabolites of CY, and (2) at a dose that does not kill hypoxic cells, it may selectively "sensitize" hypoxic cells (but not aerobic cells) to the action of BCNU. The presence of other undetermined interactions of BCNU and MISO is inferred from the increased toxicity to (aerobic) normal tissue. Misonidazole or metronidazole should be used with caution in patients who are receiving BCNU or cyclophosphamide.

Misonidazole in fractionated radiotherapy: are many small fractions best?

Computer simulations have been made of four radiotherapy fractionation regimes either in current use or proposed for clinical trials of misonidazole. A variety of cell-survival parameters and reoxygenation patterns have been used. The models allow the relative importance of repair capacity, reoxygenation rate, and dose per fraction to be assessed for these four schedules in the presence or absence of misonidazole. Unlike hyperbaric oxygen, the dose of misonidazole and the fractionation scheme to be used are critically interdependent, because the total drug dose is limited to 12 g/m2 by its neurotoxicity, regardless of the extent to which it is fractionated. The largest sensitizing effect is always demonstrated with six fractions, each given with 2 g/m2 of misonidazole. In the absence of reoxygenation a sensitizer enhancement ratio of 1.7 is predicted, but this falls to 1.1--1.2 if extensive reoxy-generation occurs. Less sensitization is observed with 30 fractions, each with 0.4 g/m2 of drug. However, for clinical use, the important question is which treatment kills the maximum number of tumour cells. Many of the simulations predict a marked disadvantage of reducing the fraction number for X rays alone. The circumstances in which this disadvantage is offset by the large SER values with a six-fraction schedule are few. The model calculations suggest that many small fractions, each with a low drug dose, are safest unless the clinician has some prior knowledge that a change in fraction number is not disadvantageous.

Is tumour radiosensitization by misonidazole a general phenomenon?

The response of 14 mouse tumour sub-lines to the radiosensitizing action of a large single dose of misonidazole (MISO) has been assessed by regrowth delay. In 13 of these, significant enhancement of radiation effect occurred under ambient conditions, indicating sensitization of naturally hypoxic cells. The enhancement observed (SER') varied with the radiation dose, as would be predicted for a mixed oxic/hypoxic cell population. The maximum SER' in these 13 tumours did not depend on histology or regrowth rate. The 14th tumour, a slow-growing sarcoma, was not sensitized under ambient conditions, but showed marked sensitization when clamped to produce acutely hypoxic cells. This is consistent with no hypoxic cells occurring naturally in a sarcoma with a slow rate of growth. Faster-growing variants of this tumour showed radiosensitization under ambient conditions. The slow-growing carcinoma, RH, however, appears to contain hypoxic cells and did show sensitization. The cytotoxic action of MISO was compared with the radiosensitization by administering it after irradiation in 8 of the tumour lines. In 2 tumours no cytotoxicity was observed. In the rest cytotoxicity was significant, but much smaller than the sensitization observed when MISO was administered before irradiation. These regrowth-delay data have been used to calculate hypoxic fractions in 3 ways. Estimates of hypoxic fraction ranged from less than 0.1% in the slow sarcoma to greater than or equal to 30% in several tumours. There is considerable variation in the estimate, according to the technique used.