Figure 6. Nutlin-3 protects only p53 wild-type cells from S phase-specific drug combinations. (A) Cells were incubated with 0–10 ng/ml SN38 alone or in combination with 10 µM KU55933 (ATM inhibitor) for 24 h. (B) Cells were incubated with 0–10 mM hydroxyurea alone or in combination with 1 µM MK-8776 (Chk1 inhibitor) for 24 h. Cells were also pretreated with 10 µM Nutlin-3 for 24 h, and this was left on during incubation with the other drugs. Drugs were removed 24 h later and cells were collected 5–7 d later when the untreated or Nutlin-3 only treated cells reached 70–80% confluency. The error bars represent SE of at least two independent experiments.