TOXICITY OF CAMPTOTHECIN TO CHINESE-HAMSTER CELLS CONTAINING 5-HYDROXYMETHYL-2'-DEOXYURIDINE IN THEIR DNA

5-Hydroxymethyl-2'-deoxyuridine (hmdUrd) is incorporated into the DNA of V79 Chinese hamster cells as an analogue of thymidine. Incorporated residues are then recognized and excised by hmUra-DNA glycosylase (hm UDG). The removal of large numbers of hmUra residues and subsequent st rand breakage is cytotoxic, as has been demonstrated by our finding th at a mutant cell line, which is deficient in this enzyme, is resistant to hmdUrd (Boorstein et al., 1992a). In order to determine whether to poisomerase I plays a role in hmUDG initiated base excision repair, V7 9 cells and repair deficient V79mut1 cells were exposed to combination s of hmdUrd and the topoisomerase I inhibitors camptothecin (CPT), CPT -11, and beta-lapachone. Treatment of V79 cells with hmdUrd followed b y non-toxic concentrations of camptothecin or CPT-11 showed significan t enhancement of the baseline cytotoxicity of the hmdUrd alone. In con trast, camptothecin and CPT-11 had no effect in combination with hmdUr d in the V79mut1 cells. Non-toxic concentrations of beta-lapachone, wh ich inhibits topoisomerase I by a different mechanism than camptotheci n and CPT-11, produced no synergistic toxicity in V79 cells. Neither c amptothecin nor CPT-11 inhibited removal of hmdUrd from hmdUrd treated cells, nor did they affect hmdUrd-induced poly(ADP-ribose) synthesis. Camptothecin did not alter the cell cycle distribution of either hmdU rd treated cells or untreated cells at concentrations sufficient to ca use synergistic toxicity with hmdUrd. Results from our study indicate that the utility of topoisomerase I inhibitors may be enhanced by sens itizing cells with hmdUrd initiated repair activity which arrests cell s in S-phase and produces DNA lesions that are further converted into lethal damage by camptothecin.