Histone Deacetylase Inhibitors: The Abbott Experience

Histone deacetylase inhibitors have generated significant interest as anti-cancer agents due to their ability to cause growth arrest, terminal differentiation and / or apoptosis in carcinoma cells. Abbott entered this area after the serendipitous discovery of the biaryl hydroxamate A-161906 in a TGFβ mimetic screen and the subsequent identification of this compound as an inhibitor of selected HDACs. The complex biology of these enzymes became evident when cloning and expression of the HDACs demonstrated that they were present as multiprotein and, in some cases, multi-HDAC containing complexes in their active forms. This discovery suggested that any selectivity determinations would have to be considered in the context of these multi-protein / HDAC complexes. However, siRNA gene knockdown studies did demonstrate that reduction of the Class I HDACs resulted in a phenotype similar to that observed with small molecule HDAC inhibitors. Evaluation of the Abbott small molecule HDAC inhibitors utilized a Class I HDAC (HDAC 1 / 2) preparation and antiproliferation assays using HT1080 fibrosarcoma and MDA435 breast carcinoma cells. Characterization of several series of hydroxamic acids indicated that while many of these analogs possessed potent enzymatic and cellular activity, in general these compounds had unacceptable pharmacokinetic profiles and marginal antitumor effects. Replacement of the potentially labile hydroxamic acid moiety with a trifluoromethyl ketone or a ketooxazole gave measurable HDAC potency but only modest cellular and in vivo activity. However, hydroxamate replacement with an α-ketoamide moiety provided potent HDAC inhibitors (IC50 values as low as 3 nM) with excellent cellular activity (IC50 values < 0.2 μM) and measurable anti-tumor activity in a flank tumor growth model.