In vitro activity: MS-275 shows inhibitory to HDACs by 2'-amino group. MS-275 induces accumulation of p21WAF1/CIP1 and gelsolin in K562 cell. MS-275 could reduce S-phase cells and induce G1-phase cells in A2780 cell. MS-275 inhibits the proliferation of human tumor cell lines including A2780, Calu-3, HL-60, K562, St-4, HT-29, KB-3-1, Capan-1, 4-1St and HCT-15 with IC50 from 41.5 nM to 4.71 μM, which due to HAD-inhibition. MS-275 is not sensitive to other HDACs (4, 6, 8 and 10) with IC50 about/above 100 μM. MS-275 shows great inhibition to human leukemia and lymphoma cells, including U937, HL-60, K562, and Jurkat. MS-275 also decreases expression of cyclin D1 and the antiapoptotic proteins Mcl-1 and XIAP.

Kinase Assay: Rat liver enzyme is diluted 1:6 with HDAC buffer. Recombinant human HDACs are diluted 1:4 in HDAC buffer. For standard HDAC assays, 60 μL of HDAC buffer is mixed with 10 μL of diluted enzyme solution at 30 °C. The HDAC reaction is started by adding 30 μL substrate solution in HDAC buffer followed by 30 min of incubation at 30 °C. The reaction is stopped by adding 100 μL trypsin solutions (10 mg/ml trypsin in 50 mM Tris-HCl [pH 8.0], 100 mM NaCl, 2 μM TSA). After a 20 min incubation period at 30 °C, the release of AMC is monitored by measuring the fluorescence at 460 nm (λex = 390 nm). Fluorescence intensity is calibrated using free AMC. For standard time course experiments, 20 pmol of substrate is used in the initial 100 μL HDAC reaction. Km and Vmax values are determined by measuring the fluorescence AMC generated by enzymatic cleavage of 2–50 pmol of substrate. The experimental data are analyzed using a Hanes plot. The AMC signals are recorded against a blank with buffer and substrate but without the enzyme.

Cell Assay: Cancer cells (A2780, Calu-3, HL-60, K562, St-4, HT-29, KB-3-1, Capan-1, 4-1St and HCT-15 cells, 5 × 103 ) are seeded into each well of 96-well plates and cultured with graded concentrations of MS-275 for 3 days. The cells are stained with 0.1 mg/mL neutral red for 1 hour in a CO2-incubator, and, after aspiration of the medium, OD540 of the neutral red solubilized with 50 μL of ethanol and 150 μL of 0.1 M Na2HPO4 is measured. The IC50 value is determined by plotting growth inhibition of the cells against the logarithm of the drug concentration.

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