3-(4-Aroyl-1-methyl-1H-2-pyrrolyl)-N-hydroxy-2-alkylamides as a New Class of Synthetic Histone Deacetylase Inhibitors. 1. Design, Synthesis, Biological Evaluation, and Binding Mode Studies Performed through Three Different Docking Procedures

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Abstract
Recently we reported a novel series of hydroxamates, called 3-(4-aroyl-1H-2-pyrrolyl)-N-hydroxy-2-propenamides (APHAs), acting as HDAC inhibitors (Massa, S.; et al. J. Med. Chem. 2001, 44, 2069−2072). Among them, 3-(4-benzoyl-1-methyl-1H-2-pyrrolyl)-N-hydroxy-2-propenamide 1 was chosen as lead compound, and its binding mode into the modeled HDAC1 catalytic core together with its histone hyperacetylation, antiproliferative, and cytodifferentiating properties in cell-based assays were investigated (Mai, A.; et al. J. Med. Chem. 2002, 45, 1778−1784). Here we report the results of some chemical manipulations performed on (i) the aroyl portion at the C4-pyrrole position, (ii) the N1-pyrrole substituent, and (iii) the hydroxamate moiety of 1 to determine structure−activity relationships and to improve enzyme inhibitory activity of APHAs. In the 1 structure, pyrrole N1-substitution with groups larger than methyl gave a reduction in HDAC inhibiting activity, and replacement of hydroxamate function with various non-hydroxamate, metal ion-complexing groups yielded poorly active or totally inactive compounds. On the contrary, proper substitution at the C4-position favorably affected enzyme inhibiting potency, leading to 8 (IC50 = 0.1 μM) and 9 (IC50 = 1.0 μM) which were 38- and 3.8-fold more potent than 1 in in vitro anti-HD2 assay. Against mouse HDAC1, 8 showed an IC50 = 0.5 μM (IC50 of 1 = 4.9 μM), and also in cell-based assay, 8 was endowed with higher histone hyperacetylating activity than 1, although it was less potent than TSA and SAHA. Such enhancement of inhibitory activity can be explained by the higher flexibility of the pyrrole C4-substituent of 8 which accounts for a considerably better fitting into the HDAC1 pocket and a more favorable enthalpy ligand receptor energy compared to 1. The enhanced fit allows a closer positioning of 8 hydroxamate moiety to the zinc ion. These findings were supported by extensive docking studies (SAD, DOCK, and Autodock) performed on both APHAs and reference drugs (TSA and SAHA).