Interactions between antitumor drugs and radiation in mammalian tumor cell lines: differential drug responses and mechanisms of resistance following fractionated X-irradiation or continuous drug exposure in vitro.

Drug-resistant mammalian tumor cell lines have been derived by either fractionated x-irradiation treatment or exposure to vincristine or etoposide (VP-16-213) in vitro. Analyses of the patterns of responses expressed by these differently derived, resistant cell lines have shown variations in responses to a range of antitumor drugs depending upon the agent used to induce resistance. However, all treated cell lines express resistance to vincristine and, with one exception, to VP-16-213. Preliminary evidence has indicated that resistance to vincristine in drug-treated cells, but not x-irradiation-treated cells, is associated with impaired vincristine uptake; resistance to VP-16-213 in both differently derived, resistant sublines is associated with a reduction of VP-16-213-induced DNA single-strand breakage; and collateral sensitivity to cisplatin in x-irradiation-treated cells is associated with enhanced drug-induced DNA cross-linking. These data indicate that patterns of responses to antitumor drugs and the mechanisms associated with these altered responses differ depending upon the agent used to induce resistance.