Synthesis and evaluation of a series of 2,4-diaminopyridine derivatives as potential positron emission tomography tracers for neuropeptide Y Y1 receptors Minoru Kameda a, * , Makoto Ando a , Chisato Nakama a , Kensuke Kobayashi a , Hiroshi Kawamoto a , Sayaka Ito b , Tomoki Suzuki b , Takeshi Tani c , Satoshi Ozaki d , Shigeru Tokita b , Nagaaki Sato a a Department of Medicinal Chemistry, Tsukuba Research Institute, Merck Research Laboratories, Banyu Pharmaceutical Co., Ltd, Okubo 3, Tsukuba, Ibaraki 300-2611, Japan b Department of Metabolic Disorder, Tsukuba Research Institute, Merck Research Laboratories, Banyu Pharmaceutical Co., Ltd, Okubo 3, Tsukuba, Ibaraki 300-2611, Japan c Department of Drug Metabolism, Tsukuba Research Institute, Merck Research Laboratories, Banyu Pharmaceutical Co., Ltd, Okubo 3, Tsukuba, Ibaraki 300-2611, Japan d Department of Pharmacology, Tsukuba Research Institute, Merck Research Laboratories, Banyu Pharmaceutical Co., Ltd, Okubo 3, Tsukuba, Ibaraki 300-2611, Japan article info Article history: Received 12 May 2009 Revised 27 June 2009 Accepted 2 July 2009 Available online 10 July 2009 Keywords: Neuropeptide Y Neuropeptide Y Y1 Positron emission tomography abstract A series of 2,4-diaminopyridine derivatives was synthesized and evaluated as potential candidates for neuropeptide Y (NPY) Y1 receptor positron emission tomography (PET) tracers. Derivatives bearing sub- stitutions allowing reliable access to radiolabeling were designed, focusing on Y1 binding affinity and lipophilicity. The advanced derivatives 2n and 2o were identified as promising PET tracer candidates. Ó 2009 Elsevier Ltd. All rights reserved. Neuropeptide Y (NPY) is a 36-amino acid peptide abundantly distributed in the central nervous systems. 1–3 NPY is implicated in the regulation of a variety of physiological functions including feeding behavior, energy homeostasis, 4,5 cardiovascular function, 6 hormone secretion, 7 and pain. 8 The biological effects of NPY are mediated by a family of G-protein-coupled receptors consisting of five distinct receptor subtypes of which Y1, Y2, Y4, Y5, and Y6 have been characterized. 9 NPY is one of the most potent orexigenic substances when directly administrated into the brain, and a num- ber of studies have suggested that Y1 and Y5 receptors play a role in NPY-induced food intake and development of obesity. 9,10 Over the past decade, many pharmaceutical companies have de- voted significant efforts towards discovering potent and selective NPY Y1 antagonists to probe the physiological roles of the Y1 recep- tor. 11 To better understand NPY Y1 biology in vivo, a suitable NPY Y1 positron emission tomography (PET) tracer would be a powerful tool allowing non-invasive Y1 receptor imaging and determination of receptor occupancy. In addition, PET tracers would be valuable tools for designing and testing promising drug candidates. In theory, a successful PET tracer targeting a receptor in the central nervous sys- tem needs to have high affinity for the target receptor, ideally with a B max /K d > 10. Reasonable lipophilicity with log P or log D =1À3.5 is also necessary for appropriate brain penetrability and achievement of an optimal specific/non-specific binding ratio. 12,13 Another criti- cal criterion for the selection of a brain-targeting tracer candidate is lack of susceptibility to P-glycoprotein (P-gp), which is an efflux transporter expressed at the blood–brain barrier. 14 To our knowl- edge, there are no reports of PET tracers available for in vivo studies of the NPY Y1 receptor. 15 Here we report the synthesis and optimiza- tion of 2,4-diaminopyridine derivatives as potential candidates for NPY Y1 PET tracers. Previously, we reported the discovery of the potent and selec- tive 2,4-diaminopyridine-based NPY Y1 antagonist 1 (Fig. 1). 16a Compound 1 showed excellent selectivity over other NPY receptor subtypes (Y2, Y4, Y5 > 10 lM) and demonstrated food intake inhi- bition in rodents. During the course of structure–activity relation- ship (SAR) studies of the 2,4-diaminopyridine class, we identified the 2-fluoropyridine derivative 2a as a potential lead for PET tracer development. Compound 2a has appropriate lipophilicity and is amenable to radiolabeling with 18 F, 17 although its Y1 binding affin- ity is moderate. Accordingly, we directed our efforts towards mod- ifying compound 2a by specifically focusing on improving the Y1 binding affinity and lipophilicity of 2a. Preparation of 2,4-diaminopyridine derivatives 2a–l, 3a, 3b, 4a, and 4b is illustrated in Scheme 1. Esterification of chelidamic acid (5) followed by protection of the 4-hydroxy group as its benzyl ether produced 6. The two symmetrical ester groups were differen- tiated by half-reduction using sodium borohydride in the presence of calcium chloride to give 7. The hydroxyl group of 7 was pro- tected as its tetrahydropyranyl (THP) ether. The ester of 8 was hydrolyzed to the corresponding carboxylic acid, which was trea- 0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.bmcl.2009.07.030 * Corresponding author. Tel.: +81 29 877 2004; fax: +81 29 877 2029. E-mail address: minorukame@yahoo.co.jp (M. Kameda). Bioorganic & Medicinal Chemistry Letters 19 (2009) 5124–5127 Contents lists available at ScienceDirect Bioorganic & Medicinal Chemistry Letters journal homepage: www.elsevier.com/locate/bmcl