Aiming to Miss a Moving Target: Bromo and Extra Terminal Domain (BET) Selectivity in Constrained ATAD2 Inhibitors Paul Bamborough,* ,† Chun-wa Chung, † Rebecca C. Furze, ‡ Paola Grandi, ∥ Anne-Marie Michon, ∥ Robert J. Watson, ‡ Darren J. Mitchell, ‡ Heather Barnett, § Rab K. Prinjha, ‡ Christina Rau, ∥ Robert J. Sheppard, ‡,⊥ Thilo Werner, ∥ and Emmanuel H. Demont* ,‡ † Molecular Discovery Research, ‡ Epigenetics Discovery Performance Unit, and § Flexible Discovery Unit, GlaxoSmithKline Medicines Research Centre, Gunnels Wood Road, Stevenage, Hertfordshire SG1 2NY, U.K. ∥ Molecular Discovery Research, Cellzome GmbH, GlaxoSmithKline, Meyerhofstrasse 1, 69117 Heidelberg, Germany *S Supporting Information ABSTRACT: ATAD2 is a cancer-associated protein whose bromodomain has been described as among the least druggable of its class. In our recent disclosure of the first chemical probe against this bromodomain, GSK8814 (6), we described the use of a conformationally constrained methoxy piperidine to gain selectivity over the BET bromodomains. Here we describe an orthogonal conformational restriction strategy of the piperidine ring to give potent and selective tropane inhibitors and show structural insights into why this was more challenging than expected. Greater understanding of why different rational approaches succeeded or failed should help in the future design of selectivity in the bromodomain family. ■ INTRODUCTION ATAD2, also known as ATAD2A or ANCCA (AAA nuclear coregulator cancer-associated protein), is a nuclear protein normally expressed in germ cells and embryonic stem cells with a key role in chromatin remodelling. 1 Since its identification as a target gene in breast cancer, 2,3 ATAD2 has been shown to be highly expressed in a wide variety of unrelated cancers with increased expression levels correlating with disease recurrence and poor clinical prognosis. 4 Potential oncology indications include lung adenocarcinoma, 5 hepatocellular carcinoma, 6,7 colorectal, 8 ovarian, 9 and endometrial cancer 10 as well as osteosarcoma 11 and gastric cancer. 12 As an oncology target, ATAD2 is supported by down- regulation of protein expression using siRNA technology. This has implicated the role of ATAD2 in pathways including apoptosis, cell survival, proliferation, and migration. 2,4,13−16 Mechanistically, ATAD2 has been shown to function as a coactivator for a variety of transcription factors including MYC, estrogen/androgen receptors, and E2F family members, directing transcription of a number of genes involved in cell proliferation and survival, including cyclins, Cdk2, and kinesins. 17−21 Furthermore, it has been demonstrated that ATAD2 is recruited to newly synthesized histones during DNA replication via a direct interaction with diacetylation modifications at K5 and K12 on histone H4. 22,23 ATAD2 possesses a bromodomain and an ATPase associated with diverse cellular activities (AAA+) domain. So far, drug discovery efforts for oncology have focused on the ATAD2 bromodomain. 24,25 The discovery that molecules identified through phenotypic screens act by inhibition of the bromodomains of the BET family demonstrated the ligandability of bromodomains and highlighted their potential in a wide range of clinical indications, including oncology and inflammation. 26−28 A number of drug-like BET bromodomain inhibitors have now been reported, and over 20 clinical trials are underway studying their effects in oncology and inflammatory diseases. 29 A growing number of tool molecules have been reported to inhibit the bromodomains of non-BET proteins, including but not limited to CBP/EP300, BRD7/9, BRPF1, and SMARCA2/4. 30−32 Because of the potent pharmacology of even quite weak BET inhibitors, 33 cellular chemical probes of other bromodomains require exceptional selectivity. We have typically tried to reach 100−1000-fold selectivity. However, because of the conserved nature of bromodomain KAc sites, this is not always easily achieved. Greater understanding of which rational attempts to engineer greater selectivity succeed, and why they can unexpectedly fail, provides insights to inform future efforts. We 34,35 and others 36−38 have identified small molecule inhibitors of the ATAD2/ATAD2B bromodomain to probe its functional role in disease and to assess if its inhibition was Received: May 30, 2018 Article pubs.acs.org/jmc Cite This: J. Med. Chem. XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acs.jmedchem.8b00862 J. Med. Chem. XXXX, XXX, XXX−XXX Downloaded via UNIV OF SUNDERLAND on September 18, 2018 at 18:15:56 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.