pubs.acs.org/jmc Published on Web 07/28/2010 r 2010 American Chemical Society 6228 J. Med. Chem. 2010, 53, 6228–6239 DOI: 10.1021/jm100639f Design, Synthesis, and Structure-Affinity Relationships of Regioisomeric N-Benzyl Alkyl Ether Piperazine Derivatives as σ-1 Receptor Ligands Iman A. Moussa, † Samuel D. Banister, † Corinne Beinat, † Nicolas Giboureau, ‡,§ Aaron J. Reynolds, † and Michael Kassiou* ,†,‡,§ † School of Chemistry, and ‡ Discipline of Medical Radiation Sciences, University of Sydney, NSW 2006 Australia, and § Brain and Mind Research Institute, Sydney, NSW 2050 Australia Received May 26, 2010 A series of N-(benzofuran-2-ylmethyl)-N 0 -benzylpiperazines bearing alkyl or fluoroalkyl aryl ethers were synthesized and evaluated at various central nervous system receptors. Examination of in vitro σ 1 {[ 3 H](þ)-pentazocine} and σ 2 ([ 3 H]DTG) receptor binding profiles of piperazines 11-13 and 25-36 revealed several highly potent and σ 1 selective ligands, notably, N-(benzofuran-2-ylmethyl)-N 0 - (4 0 -methoxybenzyl)piperazine (13, K i = 2.7 nM, σ 2 /σ 1 = 38) and N-(benzofuran-2-ylmethyl)-N 0 -(4 0 - (2 00 -fluoroethoxy)benzyl)piperazine (30, K i = 2.6 nM, σ 2 /σ 1 = 187). Structural features for optimal σ 1 receptor affinity and selectivity over the σ 2 receptor were identified. On the basis of its favorable log D value, 13 was selected as a candidate for the development of a σ 1 receptor positron emission tomography radiotracer. [ 11 C]13 showed high uptake in the brain and other σ receptor-rich organs of a Papio hamadryas baboon. The in vivo evaluation of [ 11 C]13 indicates that this radiotracer is a suitable candidate for imaging the σ 1 receptor in neurodegenerative processes. Introduction σ receptors were mistakenly identified as an opioid receptor subtype over 30 years ago. 1 The investigation of σ receptors with nonselective ligands led to a decade of confusion as σ receptors were erroneously reclassified as the phencyclidine/ N-methyl-D-aspartate receptor complex. 2 Currently, σ recep- tors are recognized as a distinct class of mammalian protein, widely distributed in the central nervous system (CNS) a and peripheral organs, comprised of two pharmacologically well- defined subtypes: σ 1 and σ 2 . 3 The σ 1 receptor has been cloned from numerous sources, including human brain tissue, and shows no similarity to any other mammalian protein. 4,5 Very recently, the σ 2 receptor was identified as a member of the histone protein family, but it has not been cloned. 6,7 Crystal structure data do not exist for either σ receptor subtype. An endogenous ligand for σ receptors has not been definitively identified; however, various neurosteroids, particularly proges- terone, as well as the hallucinogenic trace amine, N,N-dimethyl- tryptamine (DMT), have been proposed to fulfill this role. 8-11 It is believed that σ 1 receptors reside primarily at the interface between the endoplasmic reticulum (ER) and the mitochon- drion, the mitochondria-associated ER membrane (MAM), 12 and modulate Ca 2þ efflux from the ER by acting as mole- cular chaperones of inositol (1,4,5)-triphosphate receptors. 13,14 However, σ 1 receptors are also known to translocate to the plasma membrane where they regulate voltage-dependent Ca 2þ channels and K þ channels. 15-18 Much less is known about the nature of σ 2 signaling. Like σ 1 receptors, 19 the primary role of σ 2 receptors is thought to involve the main- tenance of Ca 2þ homeostasis. 20 Although σ receptors are linked to a variety of signaling pathways, the precise trans- duction mechanisms are complex and have not been fully elucidated. In the CNS, σ receptors are known to modulate several classical neurotransmitter systems, including those of acetylcholine, 21-23 dopamine, 24-27 glutamate, 28,29 5-hydroxy- tryptamine, 30,31 and norepinephrine, 32-34 accounting for their implication in a diverse spectrum of CNS disorders. 35-37 Initial interest in σ receptors as therapeutic targets was largely motivated by their high affinity for a range of clinically used typical antipsychotics, such as haloperidol (1, Figure 1). 38 The antipsychotic effect of functional antagonists of σ recep- tors is well-known, and several σ ligands have entered clinical trials for the treatment of schizophrenia. 39 Similarly, the finding that a number of structurally and pharmacologically diverse antidepressants, including fluvoxamine (2) and imi- pramine (3), also interact with σ receptors led to the investiga- tion of σ receptors as a novel therapeutic target for the treatment of affective disorders. 40,41 Implication of σ receptors in anxiety and depression is widely accepted and has been comprehensively reviewed. 42-44 Widely abused psychostimu- lants, like cocaine (4) and methamphetamine (5), also interact with σ receptors at physiologically relevant concentrations, and it is believed that certain behavioral effects, addictive properties, and the toxicity of these drugs involve their activity at σ receptors. 45-49 Dozens of functional σ receptor antago- nists have been shown to attenuate 4-induced locomotor stimulation, convulsion, and lethality, thereby identifying σ receptors as an important target for the development of novel anti-4 therapies. 46,47,49 Numerous human tumor cell lines overexpress σ recep- tors, 50-52 and many σ ligands possess antiproliferative acti- vity. 53-55 Indeed, σ 2 agonists have been shown to activate *To whom correspondence should be addressed. Tel: þ61-2-9351- 0849. Fax: þ61-2-9351-9146. E-mail: michael.kassiou@sydney.edu.au. a Abbreviations: CNS, central nervous system; DMF, N,N-dimethyl- formamide; EDC 3 HCl, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride; ER, endoplasmic reticulum; HOBt, N-hydroxybenzotria- zole; NMM, N-methylmorpholine; PET, positron emission tomography; RT, room temperature; TBAH, tetra-n-butylammonium hydroxide.