Cytotoxic effect of misonidazole and cyclophosphamide on aerobic and hypoxic cells in a C3H mammary carcinoma in vivo

Abstract

Summary The chemosensitising effect of the nitroaromatic radiosensitiser misonidazole (MISO) on the alkylating agent cyclophosphamide (CTX) has been investigated in a C3H mammary carcinoma in CDF, mice.The selective cytotoxicity against aerobic and hypoxic cells was measured indirectly, using a local tumour control (TCD50) assay. The hypoxic fraction was calculated from the dose difference between the TCD50s for tumours irradiated either in air or under clamped conditions. The relative survival of tumour cells after drug therapy was expressed as a surviving fraction (SF). CTX (100 mg kg-) was found to be considerably more toxic towards hypoxic than aerobic cells (SF 4% versus 52%). MISO (1000mg kg-) was almost exclusively toxic to hypoxic cells (SF 22%). When MISO and CTX were administered simultaneously a decrease in the surviving fraction was observed. The effect on aerated cells was found to be 10-fold more than expected from addition of toxicities, suggesting a chemosensitising effect on these cells by MISO when used in combination with CTX. No synergistic effect was found on radiobiologically hypoxic cells. The exact role of hypoxia for the development of chemosensitisation seems to be complex and requires additional research in the future.The ability of nitroimidazoles to enhance the tumour response of anti-cancer drugs has been shown by several investigators in different animal models (see review by Siemann, 1982) and is currently being investigated in phase II clinical trials. However, the mechanisms underlying the observed chemosensitisation in vivo is still not settled, although several suggestions have been made. These include the preferential killing of hypoxic cells by the nitroimidazole, changes in the pharmacokinetics and metabolism of the cancer chemotherapeutic drug, interference with the repair of potentially lethal damage and a manifestation of the in vitro pre-incubation effect observed under hypoxic conditions (Brown, 1982;Siemann, 1982. In vitro, hypoxia has been found to be a prerequisite for chemopotentiation to occur. No effect has been observed if cells were exposed to the sensitiser under aerobic conditions, unless extremely high doses were used (Smith et al., 1982). Some studies in vivo have also suggested that hypoxia plays a role in the development of chemosensitisation. The observation is primarily based on a relationship between the amount of sensitisation observed and the degree of tumour hypoxia Wheeler et al., 1984). However, the observed sensitisation generally exceeds what would be expected if the interaction between the sensitiser and the drug was restricted to the radiobiologically hypoxic tumour cell population (Brown & Hirst, 1982;Hinchliffe et al., 1983;Horsman et al., 1984). Thus the importance of hypoxic conditions for chemosensitisation in vivo is still unsettled.The aim of the present study was to investigate the selective effect of MISO and the alkylating agent cyclophosphamide (CTX) on aerobic and hypoxic cells in situ, using a clamped local tumour control assay ap...

Early and late changes in the normal mouse bladder reservoir function due to irradiation and cis-DDP

“…This DEF was defined as the ratio between the RDm values for radiation alone and when radiation was combined with cis-DDP. A comparison between the estimated RD50 was performed as described recently (Grau et al, 1990).…”

The effect of radiosensitizers on radio‐immunotherapy, using 131I‐Labelled anti‐cea antibodies in a human colonic xenograft model

“…Perhaps the situation most similar to this is where misonidazole was combined with continuous (up to 18.5 hr) low-dose-rate irradiation in a mouse model, producing a radiosensitizing effect (Fu et al, 1980). It also resembles the use of misonidazole as a chemosensitizer, selectively sensitizing tumour cells to some cytotoxic drugs via both hypoxia-mediated mechanisms (inhibition of DNA repair or intracellular thiol depletion) and inhibition of hepatic drugmetabolizing enzymes (Twentyman and Workman, 1983;Grau et al, 1990). High tumour levels of misonidazole, obtained at the time of cytotoxic drug administration, continue throughout the active life of the drug.…”

Phase I/pharmacokinetic/biochemical study of the nitroimadazole hypoxic cell sensitiser SR2508 (etanidazole) in combination with cyclophosphamide

“…An obvious interpretation is that SR2508 (not a known myelotoxic agent) may potentiate the toxicity of cyclophosphamide to bone marrow stem cells. This may occur as a result of GSH depletion in oxic cells as occurred in peripheral mononuclear cells; enhanced toxicity of cyclophosphamide (Grau et al, 1990) and other alkylators (Durand & Chaplin, 1987;Horsmann et al, 1989) to oxic cells is welldescribed in vivo and in spheroids. Allalunis et al have shown in addition that a proportion of normal marrow cells exist in a hypoxic environment (Allalunis et al, 1983).…”