Exploration of Synthetic Approaches and Pharmacological Evaluation of PNU-69176E and Its Stereoisomer as 5-HT2C Receptor Allosteric Modulators

The serotonin (5-HT) 2C receptor (5-HT2CR) is implicated in a diversity of physiological functions, such as nociception, motor behavior, endocrine secretion, thermoregulation, appetite modulation, and the control of exchanges between the central nervous system (CNS) and the cerebrospinal fluid.1 This receptor has also been implicated in numerous psychiatric pathologies, and the modulation of 5-HT2CR function holds a tremendous amount of therapeutic promise for the treatment of diseases of significant unmet medical need, including addiction, anxiety, depression, obesity/eating disorders, Parkinson’s disease, and schizophrenia.1b,1d,2 Successful development of 5-HT2CR ligands requires selectivity versus the highly homologous 5-HT2AR and 5-HT2BR, as 5-HT2A/2BR agonists can result in significant CNS (5-HT2AR) and cardiovascular (5-HT2BR) adverse effects.3 Allosteric modulators of 5-HT2CR present a novel and attractive drug design strategy to augment the response to endogenous 5-HT and to achieve high receptor subtype selectivity and specificity with ligand binding to an allosteric site rather than to the orthosteric binding site that binds the endogenous agonist.4 PNU-69176E [(2S,4R)-N-((1S,2S)-2-chloro-1-((2R,3R,4S,5R,6R)-3,4,5-trihydroxy-6-(methylthio)tetrahydro-2H-pyran-2-yl)propyl)-4-undecylpiperidine-2-carboxamide (1); Figure ​Figure1)]1)] is the only reported 5-HT2CR selective positive allosteric modulator, which was identified via screening of a chemical library of Pharmacia (now Pfizer).5 Structurally, 1 consists of two moieties, a piperidinyl ring with a long alkyl chain (undecyl) and a polar moiety including the α-d-galactopyranoside, and can be viewed as an analogue of pirlimycin with a long alkyl chain (Figure ​(Figure1).1). Although the preparation of pirlimycin and its analogues is available in the literature,6 to our knowledge neither the synthesis nor the chemical characterization of 1 and its diastereomer 2 have been reported. Figure 1 Structures of PNU-69176E (1) and its diastereomer (2). The goal of our drug discovery efforts is to identify novel, positive allosteric modulators of the 5-HT2CR with high potency, specificity, and druglike properties based on 1. Thus, it is imperative to establish a practical method to readily access 1, a fairly complex molecule, on a large scale as a reference compound as well as to evaluate pharmacological and biological properties of the parent molecule and its derivatives. In the current report, we investigated two synthetic routes and established a feasible method to readily access 1 and its diastereomer 2 for the first time. Meanwhile, we have evaluated and compared the allosteric modulation of 1 and its diastereomer 2 employing an intracellular calcium (Cai2+) release assay in two cell lines stably transfected with either the 5-HT2CR or 5-HT2AR. The exploration of the synthetic approaches elucidates an efficient synthesis of key building blocks which can be used to develop novel and simplified 5-HT2CR positive allosteric modulators.