CASP9 and neuroblastoma: We then investigated whether Rk1 induces caspase-dependent cell death in neuroblastoma cells by western blot and treatment with caspase inhibitors Z-DEVD (caspase-3 inhibitor), Z-IETD (caspase-8 inhibitor), Z-LEHD (caspase-9 inhibitor), and Z-VAD (pan-caspase inhibitor) for 3 h prior to 15 μM Rk1 exposure (Figure 4E).