EDO-S101 (Tinostamustine) is a pan HDAC inhibitor with IC50 values of 9, 9 and 25 nM for HDAC1, HDAC2 and HDAC3 , respectively.

EDOS101 inhibits HDAC activity in rat peripheral blood mononuclear cells (PBMCs) in a cellular assay by approximately 90% one hour after dosing with 10 mg/kg i.v. HDAC inhibition in PBMCs could not be increased with higher doses up to 50 mg/kg. EDO-S101 triggers apoptosis and shows strong antitumor activity in HL60 and Daudi cells. Initial in vitro experiments in HL60 cells shows an activation of the intrinsic pathway of apoptosis with cleavage of caspases 3, 9 and PARP and a marked reduction of anti-apoptotic proteins XIAP and Mcl-1.

Intracellular HDAC inhibition of EDO-S101, which occurs rapidly after dosing is at maximum activity already at the lowest dose of 10 mg/kg and lasts for about 12-16 hours. Exposure to EDO-S101 causes a strong DNA repair response evidenced by activation of pH2AX and p53 in tumors taken from mice bearing subcutaneous human Burkitt's lymphoma. Tumors of BL rapidly shrink or are completely eradicated after i.v. administration of EDO-S101.

EDO-S101 is dissolved in DMSO and added to the assay buffer solution. EDO-S101 dilutions of 5 μL of each dilution is added to 50 μL of the reaction mixture including the Fluor de Lys substrate and all of the enzymatic reactions are conducted in duplicate at 37oC for 30 minutes. After enzymatic reactions, 50 μL of 2xHDAC developer is added to each well and fluorescence intensity is measured[1].