Potentiation of chemotherapy by hypoxic cell radiation sensitizers—A review

There recently has been considerable interest in combining conventional anti-tumor agents and nitroimidazole radiation sensitizers as a new approach in the treatment of tumors. This interest stems at least in part from the observations that oxygen-deficient or hypoxic tumor cells may be preferentially spared by certain chemotherapeutic agents because (i) these cells may be relatively inaccessible to the drug because they are located in poorly vascularized areas of the tumor, (ii), hypoxia per se may reduce the efficacy of the chemotherapeutic agent, or (iii) under hypoxic conditions a large proportion of cells may be in a noncycling state which protects the cells from cell cycle specific agents.33 On the other hand nitroimidazoles such as misonidazole (MISO) have been shown to possess, besides their radiation sensitizing ability, a direct cytotoxic effect against hypoxic cells.‘,’ ‘.25,29.44.47,50.54 Consequently, it was suggested33,47 that if hypoxia resulted in a chemo-protective effect, then the combination of two agents, one aimed against the oxic tumor cell population (chemotherapeutic agent), the other against the hypoxic tumor cells (nitroimidazole sensitizer), might enhance the treatment efficacy. That hypoxic conditions may modify the response to certain chemotherapeutic agents has been well-documented in cell culture systems’7.‘9.2’,33 and multicellular spheroids.7.4X.49.55 Less information is available for in vivo chemotherapy, although some reports of the occurrence of this phenomenon in mouse tumors treated with conventional anti-tumor agents such as BCNU* and nitrogen mustard have been described.‘*,” Initial demonstrations of enhanced cell killing resulting from combinations of chemotherapeutic agents and radiation sensitizers came from in vitro investigations. Previously it had been shown that hypoxic cells, exposed to radiation sensitizers such as MIS0 under conditions of prolonged incubation prior to irradiation, were more readily killed than would have been predicted solely on the basis of the radiation sensitization mechanism.‘0.62.h3 Subsequently it was shown that pre-treating cells under hypoxic conditions with MIS0 prior to the administration of anti-cancer agents such as bleomycin,33,‘4 adriamycin,4R melphalan, mustine and cisplatinum45 also markedly reduced cell survival. These “pre-incubation experiments” usually consisted of exposing cells under hypoxic conditions to fairly high doses of MIS0 (3-5 mM) for a few hours (spheroids were exposed for up to 24 hours), washing out the sensitizer, and then treating the cells under aerobic conditions either with various doses of the chemotherapeutic agent for a fixed time period or with a fixed drug dose for a variable time period. Compared to treatments with the chemotherapeutic agents alone, such sensitizer pre-treatments led to steeper slopes and/or smaller shoulders on the cell survival curves, and when evaluated by isobologram analysis to supra-additive cell kill in spheroids. Following these initial observations, chemo-potentiation in vitro has been reported by many investigators for various chemotherapeutic agents and radiation sensitizers (for review see Int. J. Radiat. Oncol. Biol. Phys. 8, No. 314, 1982). The first in vivo demonstrations of enhanced tumoricidal effects resulting from the combined treatments of radiosensitizers and conventional chemotherapy were reported in 1980 by Clement et ~1.~ and Rose et ~1.~~ Using in vivo to in vitro excision or in situ tumor regrowth assays, these authors showed that combining MIS0 with anti-tumor agents such as melphalan or cyclophosphamide (CYC) could result in enhancement ratios (ERs) of up to 2.5. Of special interest in these studies was the finding of a smaller enhancement of these agents by MIS0 in bone marrow stem cell toxicity (ER = 1.3-l .5) than in the tumor. Since these initial observations a variety ef combinations of chemotherapeutic agents and sensitizers have been tested in a number of different mouse tumor models (for review see Int. J. Rudiut. Oncol.