A–C. Apoptotic rates in Tu212, Tu177, and SqCC/Y1 cells upon sequential treatment. Z→ DP consistently shows a lower apoptotic rate than the DP combination. D. Immunoblotting for apoptosis-related markers in sequentially treated Tu212 cell line. Z→DP shows less cleavage of PARP and caspases than DP, DPZ or DP→Z drug treatments (black arrows, pre-cleaved; white arrows, cleaved bands). E. Representative graph of SqCC/Y1 cells treated for 24 hours with DP, Z, DPZ, DP→ Z, or Z→DP. DP treatment induced S and G2/M arrest while Z induced G0/G1 arrest. The cell cycle distributions by DP, DPZ and DP→Z were similar. When Z was given first, there was an increase in G1 arrest (gray line: the control).