Structure–activity relationship studies on a series of piperazinebenzylalcohols and their ketone and amine analogs as melanocortin-4 receptor ligands Dragan Marinkovic a , Fabio C. Tucci a , Joe A. Tran a , Beth A. Fleck b , Jenny Wen c , Chen Chen a, * a Department of Medicinal Chemistry, Neurocrine Biosciences, Inc., 12790 El Camino Real, San Diego, CA 92121, USA b Department of Pharmacology, Neurocrine Biosciences, Inc., San Diego, CA 92121, USA c Department of Preclinical Development, Neurocrine Biosciences, Inc., San Diego, CA 92121, USA article info Article history: Received 1 July 2008 Revised 18 July 2008 Accepted 22 July 2008 Available online 25 July 2008 Keywords: Melanocortin Receptor Ligand Structure–activity relationship Benzylalcohol Ketone Amine Synthesis abstract A series of piperazinebenzylalcohols were prepared and studied to compare with their ketone and amine analogs as MC4R antagonists. Several benzylalcohols such as 14a and 14g displayed low nanomolar bind- ing affinities (K i < 10 nM), and high selectivities over other melanocortin receptor subtypes. Ó 2008 Elsevier Ltd. All rights reserved. The melanocortin receptors, which consist of five subtypes MC1-5R and belong to the class A GPCR superfamily, 1 have been identified and cloned. 2 Among them, MC4R controls feeding behav- ior. 3 Therefore, MC4R agonists have been sought for the possible treatment of obesity. 4 Moreover, recent studies have also demon- strated that MC4R-selective antagonists might be useful in cachexia. 5 The melanocyte-stimulating hormones (a-MSH, b-MSH and c-MSH) and adrenocorticotropin (ACTH) are the endogenous ago- nists for the melanocortin receptors. All these peptides possess a His-Phe-Arg-Trp motif which is crucial for receptor binding and activation. 6 In addition, two endogenous antagonists, agouti-pro- tein and agouti-related protein (AgRP) contain an Arg-Phe-Phe moiety located in a loop known to interact with the melanocor- tin receptors, 7 and are believed to be important for functional antagonism. 8 Mutagenesis and modeling studies have provided some information for understanding how a peptide ligand inter- acts with the receptor. 9 The requirement of a basic and an aro- matic group for a MC4R ligand is also evidenced by many known non-peptide MC4R agonists and antagonists from several chemical classes. 10 For example, studies have shown that the dipeptide D-Tic-D-(4-Cl)Phe attaching to a piperazinebenzene that bears a polar functionality at the ortho-position is a potent MC4R agonist with high binding affinity. 11–13 Thus, Dyck et al. reported a K i value of 270 nM for the weakly basic triazole 1a (Fig. 1). In comparison to 1a, the basic benzylamine 1b possessed a similar K i value of 380 nM, while its 2-thienylethyl derivative 1c (K i = 11 nM) displayed a substantially increased binding affinity, indicating that a basic nitrogen in combination with a small lipo- philic group significantly contributes to receptor binding. 14 Com- pounds 1a–c are full agonists, suggesting an important role of the Tic (1,2,3,4-tetrahydro-isoquinolinecarbonyl) group in the receptor binding and activation. In contrast, the b-Ala-D-(2,4- Cl)Phe analog of 2 (K i = 1.8 nM) is a highly potent MC4R antagonist. 14 Based on receptor modeling studies, the basic amine of the Tic group of 1 possibly interacts with an aspartic residue (Asp-126) at the top of the transmembrane domain-3 (TM-3) of the human MC4 receptor (hMC4R), while another acidic Asp-122 may contact with the polar triazole of 1a via a hydrogen-bond, or the amine of 1b and 1c via a charge–charge interaction. 15–17 We have shown that the binding affinity of benzylamine 3 (K i = 490 nM) is improved when a small isobutyl group is incorpo- rated (4, K i = 74 nM). 18 In addition, b-Ala-D-(2,4-Cl)Phe-piperazines bearing a cyclohexyl-carboxylate such as 5 are potent MC4R antag- onists, 19 indicating a basic amine in 4 might not be crucial for receptor binding. To further explore the structure–activity rela- 0960-894X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.bmcl.2008.07.076 * Corresponding author. Tel.: +1 858 617 7600; fax: +1 858 617 7967. E-mail address: cchen@neurocrine.com (C. Chen). Bioorganic & Medicinal Chemistry Letters 18 (2008) 4817–4822 Contents lists available at ScienceDirect Bioorganic & Medicinal Chemistry Letters journal homepage: www.elsevier.com/locate/bmcl