Chemosensitization by misonidazole in CCNU-treated spheroids and tumours

Abstract

Summary Misonidazole has been demonstrated to enhance the cytotoxicity of several common antineoplastic drugs in vitro and in vivo, and its mechanism of action as a chemosensitizer, though still unknown, is thought to be dependent upon hypoxia. We have used fluorescence-activated cell sorting to evaluate chemopotentiation by misonidazole as a function of cell position in V79 spheroids and KHT tumours. CCNU toxicity was enhanced in all cell subpopulations of both tumours and spheroids, with greater consistency than might be predicted on the basis of the known variations in oxygen tension. Further, both misonidazole and CCNU as single agents were preferentially toxic in the less well oxygenated regions of each system, arguing that differential toxicity canno t be implicated in the chemopotentiation observed. In fact, increased treatment toxicity did not necessarily lead to increased chemopotentiation, nor was potentiation directly related to the metabolism/binding of the misonidazole. Chemopotentiation in multicell systems thus appears to be a complex, multi-factorial process.Hypoxic cells develop spontaneously during the growth of many solid tumours, and are known to be resistant to ionizing radiation and implicated in resistance to several antineoplastic drugs. Clearly, their presence in human tumours may limit the effectiveness of cancer therapy. Development of hypoxic cell radiosensitizers thus has a clear rationale; the observations that many of these sensitizers were preferentially toxic to hypoxic cells (Olive & McCalla 1975; Hall & Roizin-Towle 1975;Moore et al., 1976) suggested that they may be complementary to many conventional cancer chemotherapeutic agents, and could thus be used effectively in combination treatments. Indeed, in a number of instances an interaction between the sensitizer and the chemotherapeutic agent was observed (Kelly et al., 1979;Clement et al., 1980;Rose et al., 1980, and reviewed by Siemann, 1984).Many factors have been implicated in the phenomenon of chemosensitization (see Brown 1982, and Siemann 1984 for reviews), including differential toxicity (Kelly et al., 1979), direct interactions between the sensitizer (or its toxic metabolites, which are generally produced under hypoxic conditions) and the chemotherapeutic agent itself (Taylor et al., 1982), alterations of drug pharmacology or delivery (Urtasun et al., 1982;Workman et al., 1983;Lee & Workman 1986), inhibition of repair processes by chemosensitizer treatment (Taylor et al., 1982;Mulcahy 1986), and alterations of cellular drug sensitivity, by, for example, the sensitizer selecting for surviving cells more vulnerable to the chemotherapeutic agent due to cell cycle status or other factors . Perhaps the most consistent observation, however, has been the apparent requirement of hypoxia both in vitro (Brown, 1982;Mulcahy, 1984), and in vivo Wheeler et al., 1984).In this report, we present data evaluating the cytotoxic effects of misonidazole, MISO, and N-(2-chloro-ethyl)-N'-cyclohexyl-N-nitrosourea, CCNU, singly and in combi...

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

“…This has been demonstrated by several investigators. Durand and Chaplin (1987) used the fluorochrome Hoechst 33342 to separate tumour cells in fractions as a function of distance from the blood vessels. They found that the chemosensitisation by MISO in KHT tumours treated wth CCNU was almost constant throughout the tumour, irrespective of the oxygenation status of the cells.…”

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).…”