Differential sensitivity to etoposide (VP-16)-induced S phase delay in a panel of small-cell lung carcinoma cell lines with G1/S phase checkpoint dysfunction

Purpose: The highly schedule-dependent cytotoxic agent etoposide (VP-16) is initially effective in the treatment of small-cell lung cancer (SCLC), par ticularly in multidrug combination chemotherapy. Heterogeneity in cellular sensitivity to cell cycle arrest may underpin the inadequacy of low-dose ex tended-cycle single-agent regimes in tumours with partially dysfunctional a poptotic signalling pathways. We have studied the longevity and dose depend ency of cell cycle and to a limited extent the apoptotic responses of a pan el of seven unselected SCLC cell lines, screened for TP53 status. Methods: Cells were analysed using flow cytometry for the cell cycle responses and f ield inversion gel electrophoresis for apoptotic patterns. The mitotic inhi bitor nocodazole was used to assess and correct cell line response data for differences in cell cycle traverse per se. Results: An overall lack of G1/ S arrest and muted DNA fragmentation were consistent with the presence of T P53 gene abnormalities. Maximal G2 arrest but with clear recovery potential occurred at an exposure dose (ED, concentration of drug x time) value of a pproximately 24 muM .h. Higher doses (ED values > 48 muM .h) revealed a wid e variation in S phase delay that was independent of population doubling ti me and could not be compensated for by drug concentration changes alone. Co nclusion: The results suggest that heterogeneity in the in vitro sensitivit y of unselected SCLC cell lines to S phase arrest is demonstrable at ED Val ues projected to be critical for clinical activity. Such variation in S pha se responsiveness may reflect differences in checkpoint activation and offe r a functional target for the design of more-effective combination therapy.