Drug resistance of hypoxic tumour cells in vitro

Abstract

In 1959 Thomlinson and Gray postulated that hypoxic cells might exist in the larger solid tumours ( 13), and some evidence to support this has been obtained in trials of hyperbaric oxygen (3). Hypoxic cells in solid tumours, however, appear to be not only relatively resistant to the cytotoxic effect of ionizing radiation, but also to some chemotherapeutic agents (12). Cytotoxic drugs which may be at least as active under hypoxic as under euoxic conditions are therefore of considerable clinical interest. Lin et al. (8) postulated that bioreductive drugs like Mitomycin C should have a particular toxicity for hypoxic tumours. The decreased oxygen tension in hypoxic tumour cells should create conditions conducive to reductive processes thought to be necessary for activation of Mitomycin C. The reduction of the benzoquinone ring of the Mitomycin molecule to dihydrobenzoquinone has been proposed as an essential step for the biological activity of this drug (8), and it has therefore been classed as a “bioreductive alkylating agent”. Using the murine sarcoma cell lines EMT6 and S180, Kennedy et al. (7) have demonstrated that Mitomycin C is preferentially activated and metabolized by hypoxic murine tumour cells. We have studied the cytotoxic effect ofMitomycin C on human carcinomas under euoxic and hypoxic conditions in the tumour clonogenic assay (HTCA) of Hamburger et al. (6). We also measured the rate of metabolism of Mitomycin C by EMT6 cells and the human colon tumour cell line WIDR, under both euoxic and hypoxic conditions. Additionally we have studied the cytotoxic effect of Bisantrene, a new anthracene bishydrazone derivative, on human carcinomas under euoxic and hypoxic conditions in the HTCA. This drug has shown antitumour activity in phase I trials (1, 10, 14) and in the HTCA (1). In contrast to Mitomycin C or Adriamycin, the drug has no quinone ring and in cardiac studies carried out in beagle dogs, it showed no cardiotoxicity (11). In addition to the clonogenic experiments, the induction of protein associated DNA breaks by Bisantrene has also been investigated under euoxic and hypoxic conditions in the murine L12 10 cell line, using the alkaline elution technique (2). For our experiments, cells were rendered hypoxic by gassing the suspension containing the medium and 10% serum in completely gas tight tubes, designed to obtain and maintain