Belinostat (PXD101)

Subconfluent cultures are harvested and washed twice in ice cold PBS and pelleted by centrifugation at 200 × g for 5 min. The cell pellet is resuspended in two volumes of lysis buffer [60 mM Tris buffer (pH 7.4) containing 30% glycerol and 450 mM NaCl] and lysed by three freeze (dry ice) thaw (30 °C water bath) cycles. Cell debris is removed by centrifugation at 1.2 × 10⁴ g for 5 min, and the supernatant is stored at −80 °C. Histone H4 peptide (sequence SGRGKGGKGLGKGGAKRHRK corresponding to the 20 NH2-terminal residues) is acetylated by a recombinant protein containing the hypoxanthine-aminopterin-thymidine domain of p300, using [³H]acetyl CoA as a source of acetate. H4 peptide (100 μg) is mixed with hypoxanthine-aminopterin-thymidine buffer (50 mM Tris HCl pH 8.0, 5% glycerol, 50 mM KCl, and 0.1 mM EDTA), 1 mM DTT, 1 mM 4-(2-aminoethyl) benzenesulfonylfluoride, 1 × complete protease inhibitors, 50 μL of purified p300, and 1.85 m [³H]acetyl CoA (4.50Ci/mmol) in a final volume of 300 μL and incubated at 30 °C for 45 min. The p300 protein is removed by incubation with 20 μL of 50% Ni-agaroase beads for 1 hour at 4 °C and centrifugation. The supernatant is applied to a 2 mL Sephadex G15 column, and the flow through is collected. One milliliter of distilled H₂O is gently applied, and three drop fractions are collected; this is repeated until 4–5 mL of distilled H₂O has been added, and ∼40 fractions are collected. Three microliters of each fraction are diluted in 2 mL of scintillation fluid and counted in a scintillation counter to identify the fractions containing the labeled peptide. These fractions are pooled, and 1 μL of the combined sample is measured to assess the radioactivity in every peptide batch (3-7×10³ cpm/μL). For activity assays, the reaction is carried out in a total volume of 150 μL of buffer [60 mM Tris (pH 7.4) containing 30% glycerol] containing 2 μL of cell extract and, where used, 2 μL of belinostat. The reaction is started by the addition of 2 μL of [³H] labeled substrate (acetylated histone H4 peptide corresponding to the 20 NH₂-terminal residues). Samples are incubated at 37 °C for 45 min, and the reaction stopped by the addition of HCl and acetic acid (0.72 and 0.12 M final concentrations, respectively). Released [³H]acetate is extracted into 750 μL of ethyl acetate, and samples are centrifuged at 1.2× 10⁴ g for 5 min. The upper phase (600 μL) is transferred to 3 mL of scintillation fluid and counted.