Evaluation of cell survival, DNA double strand breaks, and DNA synthesis during concurrent camptothecin and X-radiation treatments

We evaluated cell survival, DNA double strand breaks (dsbs), and DNA synthe sis following camptothecin (CPT) alone or concurrent CPT and X-radiation tr eatments in exponential-phase cultures of a radioresistant human melanoma c ell line. Cell survival was measured by a clonogenic assay. DNA dsbs were m easured by pulsed-field gel electrophoresis. DNA synthesis was measured by incorporation of H-3-thymidine. We found that (i) concurrent CPT and X-radi ation interacted additively, unlike previous results with plateau-phase cul tures of these cells, which showed synergistic interaction; (ii) there were strong negative correlations (correlation coefficients of at least 0.82) b etween clonogenic surviving fractions and DNA dsbs following CPT alone or c oncurrent CPT and radiation treatments; and (iii) concurrent CPT and radiat ion (10 Gy) treatment did not completely inhibit DNA synthesis, even though addition of radiation to CPT did further decrease DNA synthesis (relative to CPT alone) at CPT concentrations below 20 muM. Our results suggest that during concurrent CPT and radiation treatment residual DNA dsb levels were good indicators of cell killing and that the absence of complete inhibition of DNA synthesis could at least in part explain the additive interaction b etween CPT and radiation. (C) 2001 Wiley-Liss, Inc.