The importance of drug scheduling and recovery phases in determining drug activity. Improving etoposide efficacy in BCR-ABL-positive CML cells.

K562 leukaemic cells are known to be less sensitive to etoposide than other cell lines, despite having similar topo II mRNA levels and cleavable complex formation. We have investigated the effect of etoposide schedule on cell cycle distribution, apoptosis and p21(waf1) and cdk1(p34) status in two bcr-abl-positive chronic myeloid leukaemia (CML) cell lines (K562 and KU812) and two small cell lung cancer (SCLC) cell lines (H69 and GLC4). During a continuous 5-day exposure, the SCLC cell lines showed a time and concentration-dependent loss of cell viability, with an initial block in the G2/M phase of the cell cycle followed by apoptosis. In contrast, the two CML cell lines showed no significant apoptosis or loss of viability after a similar block in G2/M. However, when K562 or KU812 cells were placed in drug-free medium following a 3-day drug exposure there was marked, concentration-dependent apoptosis (% apoptosis after release at 1 microM etoposide in K562, 10% at 24 h, 30% at 48 h). Our data also show that p21(waf1) does not increase after etoposide treatment in either H69 or GLC4 (both with mutated-p53). Although K562 and KU812 cells are null-p53, the arrest in G2/M during drug exposure was associated with increased p21(waf1) and a decrease in cdk1 (both P<0.001 compared with controls). Upon release of these cells from drug-medium, p21(waf1) gradually returned to control levels, which was associated with an easing of the block at G2/M and an induction of apoptosis. This study highlights the importance of cell cycle regulatory proteins in drug sensitivity and resistance, and suggests that in cells such as K562 and KU812, a pulsed schedule may be more active than a single prolonged exposure.