In vitro activity: Resminostat [HCl] is acting as a potent inhibitor of recombinant HDAC 1, 3 and 6 isoenzymes with a substrate competitive binding mode. It can induce hyperacetylation of histone H4 in MM cells. Low micromolar concentrations of resminostat abrogates cell growth and strongly induces apoptosis in MM cell lines (OPM-2, NCI-H929, U266 ) as well as primary MM cells. At 1 μM, resminostat inhibits proliferation and induces G0/G1 cell cycle arrest in OPM-2, NCI-H929, U266 MM cell lines accompanied with decreased levels of cyclin D1, cdc25a, Cdk4 and pRb as well as upregulation of p21. Resminostat decreases phosphorylation of 4E-BP1 and p70S6k indicating an interference with Akt pathway signalling. Treatment with resminostat results in increased protein levels of Bim and Bax and decreases levels of Bcl-xL. Caspases 3, 8 and 9 are activated by resminostat. Furthermore, synergistic effects are observed for combinations of resminostat with melphalan and the proteasome inhibitors bortezomib and S-2209.

Kinase Assay: Forty microliter enzyme buffer (15 mM Tris HCl pH 8.1, 0.25 mM EDTA, 250 mM NaCl, 10% v:v glycerol) containing HDAC1, 3, 6 or 8 activity, 29 μL enzyme buffer and 1 μL resminostat [HCl] at different concentrations are added to a 96-well microtitre plate and the reaction started by the addition of 30μL substrate peptide Ac-NH-GGK(Ac)-AMC (HDAC1, 3 and 6 assays, final concentrations 6 μM for HDAC1, 10μM for HDAC6 and 25μM for HDAC3/DAD) or Ac-RHK(Ac)K(Ac)-AMC (HDAC8 assay, final concentration 50 μM). After incubation for 180 min (HDAC1, HDAC6, HDAC8) or 120 min (HDAC3) at 30°C, the reaction is terminated by the addition of 25 μL stop solution (50 mM Tris HCl pH 8, 100 mM NaCl, 0.5 mg/ml trypsin and 2 μM trichostatin A [TSA]). After incubation at room temperature for further 40 min, fluorescence is measured using a multilabel counter (extinction 355 nm, emission 460 nm) for quantification of AMC generated by tryptic cleavage of the deacetylated peptide. For the calculation of the 50% inhibitory concentration (IC50) values the fluorescence in wells without test compound (1% DMSO, negative control) is set as 100% enzymatic activity and the fluorescence in wells with 2 μM TSA (positive control) are set at 0% enzymatic activity (background fluorescence substracted).

Cell Assay: WST-1 assay using OPM-2, NCI-H929, RPMI-8226 and U266 cells.

Br J Haematol. 2010 May;149(4):518-28.