The cytotoxic activity of the compounds, vorinostat and quisinostat were evaluated by an improved MTT assay as previously described. Cal27, HepG2 and MRC-5 cell lines were seeded at a density of 2500, 6000 or 5000 cells/well (c/w) in 96 well plates (Corning, Incorporated, New York City, New York). The cell line A2780 was seeded at 8000 cells/well and U87 at 4000 cells/well. After 24 h cells were exposed to increasing concentrations of test compounds. Incubation was ended after 72 h and cell survival was determined by addition of MTT (Serva, Heidelberg, Germany) solution (5 mg/mL in phosphate buffered saline). After approximatly 20 min (Cal27, A2780 and U87), 30 min (HepG2) or 120min (MRC-5) incubation at 37 °C in a humidified incubator resulting formazan precipitate was dissolved in DMSO (VWR, Darmstadt, Germany) and absorbance measured at 540 nm and 690 nm using a ThermoFisher Multiskan FC Microplate Photometer (Thermo Scientific, Waltham, Massachusetts).