Fig. 9. Combination of vorinostat or panobinostat and gambogic acid is more effective against the neuroblastoma cells tested. (A) IMR-32 and SK-N-DZ cells both have relatively high levels of MYCN. Vorinostat or panobinostat and GA were added to these cells. (A) Incubation time for vorinostat or panobinostat was 24 h and that for GA was 6 h. For combination treatment, cells were pretreated with vorinostat or panobinostat for 18 h before the GA administration and were then incubated with GA for 6 h more. (B-C) The relative percentages of viability with proliferation were calculated by the following formula: % viability, proliferation = (optical density (OD) of the drug-treated sample/OD of the control sample) × 100. Values were plotted as a histogram by using GraphPad Prism 5. Combination index (CI) was calculated by using the Chou-Talalay method with CompuSyn software. The calculated CI was calculated and plotted using GraphPad Prism 5.