2-Trifluoroacetylthiophene oxadiazoles as potent and selective class II human histone deacetylase inhibitors Ester Muraglia * , Sergio Altamura, Danila Branca, Ottavia Cecchetti, Federica Ferrigno, Maria Vittoria Orsale, Maria Cecilia Palumbi, Michael Rowley, Rita Scarpelli, Christian Steinkühler, Philip Jones IRBM—Merck Research Laboratories Rome, Via Pontina km 30,600—Pomezia, 00040 Rome, Italy article info Article history: Received 29 August 2008 Revised 17 September 2008 Accepted 18 September 2008 Available online 24 September 2008 Keywords: HDAC Inhibitors Trifluoroacetyl Oxadiazoles abstract Trifluoroacetylthiophene carboxamides have recently been reported to be class II HDAC inhibitors, with moderate selectivity. Exploration of replacements for the carboxamide with bioisosteric pentatomic het- eroaromatic like 1,3,4-oxadiazoles, 1,2,4-oxadiazoles and 1,3-thiazoles, led to the discovery that 2-triflu- oroacetylthiophene 1,3,4-oxadiazole derivatives are very potent low nanomolar HDAC4 inhibitors, highly selective over class I HDACs (HDAC 1 and 3), and moderately stable in HCT116 cell culture. Ó 2008 Elsevier Ltd. All rights reserved. Histone deacetylase (HDAC) and histone acetyltransferase (HAT) enzymes regulate the acetyl removal/addition to the N-ter- minal Lys residues on histones. The acetylation status of histones is directly related to gene expression, with hyperacetylation being associated with a less condensed chromatin state, a conformation that induces activation of gene transcription. HDACs are also involved in the deacetylation of other non-his- tone proteins (HSP90, a-tubuline), and in interactions with tran- scription factors and nuclear receptors (p53, Era). 1 The involvement of HDACs in regulating cell proliferation, to- gether with the experimental evidence that inhibition of HDACs induces growth arrest or apoptosis in various tumor cell lines, 2 has highlighted HDACs as an appealing target for cancer therapy. The 18 known human HDACs can be divided into four classes: class I (HDAC1, 2, 3 and 8), class II, which can be further divided into the two subclasses HDAC IIa (HDAC 4, 5, 7, 9) and IIb (HDAC 6 and 10) and class IV (HDAC11) are Zn-dependent HDACs, while class III, or Sirtuins (SIRT1-7), are structurally unrelated NAD- dependent deacetylases. Class I, II and IV HDACs are sensitive to the classical HDAC inhibitor trichostatin A (TSA), whereas those of class III are insen- sitive to this inhibitor. 1b The cellular function of each HDAC class is not fully under- stood and also the specific role in the antitumor activity of the single isoforms is still not completely deciphered. Notwith- standing, the urgency for alternative therapies for cancer has allowed a rapid development of broad spectrum HDAC inhibi- tors. Indeed, most of the HDAC inhibitors that are known or in clinical trials are unselective, or are just partially selective for one class of HDACs 2,3 (e.g. Vorinostat, formerly SAHA, ap- proved by the FDA in 2006, is an inhibitor of HDAC1, 2, 3 and 6). 4 Elucidation of the function of each HDAC subtype would potentially address toxicity issues and clinical adverse effects of HDAC inhibitors. Consequently, there remains a widely recognized need to identify selective HDAC inhibitors. 5 Selective HDAC inhibitors would also provide more focused cancer therapies and new clinical opportunities. For instance, it has been demonstrated for HDAC inhibitors to be additive or synergistic with conventional anticancer chemotherapeutics and to produce cellular sensitization to ionizing radiation. 6 The former seems to be correlated to the inhibition of class I 7 HDACs, while studies on modulation of cellular response to radiation provide evidence of class II HDAC involvement, implying a role for the HDAC4 isoform. 8 In our company a research program was initiated aiming at developing a portfolio of different chemotypes, each with a dif- ferent HDAC selectivity profile. This effort led to the discovery of potent and selective class I HDAC inhibitors, belonging to various chemical classes, including methyl ketone A (Fig. 1) 9 and aminobenzamides B 10 and C, 11 which have been demon- strated to cause tumor growth inhibition in a HCT-116 xeno- graft model. 0960-894X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.bmcl.2008.09.076 * Corresponding author. Tel.: +39 06 91093276; fax: +39 06 91093654. E-mail address: ester_muraglia@merck.com (E. Muraglia). Bioorganic & Medicinal Chemistry Letters 18 (2008) 6083–6087 Contents lists available at ScienceDirect Bioorganic & Medicinal Chemistry Letters journal homepage: www.elsevier.com/locate/bmcl