A Decade’s Progress in the Development of Molecular Imaging Agents Targeting the Growth Hormone Secretagogue Receptor

Abstract

The growth hormone secretagogue receptor 1a (GHSR), also called the ghrelin receptor, is a G protein-coupled receptor known to play an important metabolic role in the regulation of various physiological processes, including energy expenditure, growth hormone secretion, and cell proliferation. This receptor has been implicated in numerous health issues including obesity, gastrointestinal disorders, type II diabetes, and regulation of body weight in patients with Prader-Willi syndrome, and there has been growing interest in studying its mechanism of behavior to unlock further applications of GHSR-targeted therapeutics. In addition, the GHSR is expressed in various types of cancer including prostate, breast, and testicular cancers, while aberrant expression has been reported in cardiac disease. Targeted molecular imaging of the GHSR could provide insights into its role in biological processes related to these disease states. Over the past decade, imaging probes targeting this receptor have been discovered for the imaging modalities PET, SPECT, and optical imaging. High-affinity analogues of ghrelin, the endogenous ligand for the GHSR, as well as small molecule inhibitors have been developed and evaluated both in vitro and in pre-clinical models. This review provides a comprehensive overview of the molecular imaging agents targeting the GHSR reported to the end of 2019.

A survey of stapling methods to increase affinity, activity, and stability of ghrelin analogues

“…To determine the effect of different stapling techniques on the bioactivity and stability of [Dpr 3 ]ghrelin (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20), five ghrelin analogues bearing helix-inducing staples were synthesized. For their synthesis, the natural ghrelin sequence had substitutions at positions 12 and 16 with residues bearing the functional groups required for the diverse stapling techniques with an i, i + 4 spacing (Scheme 1).…”

“…Thus, it has become a target of interest in the field of molecular imaging. [18][19][20] Like many natural peptides, ghrelin's linear structure makes it susceptible to enzymatic degradation by proteases. As a result, ghrelin analogues that were developed as imaging agents have been noted as having poor in vitro stability in human serum, with structural modifications required in order to improve stability.…”

“…4 However, studies have demonstrated that when ghrelin binds to its receptor, part of its peptide structure folds into an α-helical conformation approximately from Pro 7 to Gln 14 . [23][24][25] This suggests that a section of ghrelin's sequence has helical character in certain environments and could be stabilized using a helix-inducing chemical staple, although in contrast, a NMR spectroscopy analysis describes considerable fluidity in the Glu 8 to Ser 18 region for truncated ghrelin (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18). 26 Chemical staples have become recognized as an effective molecular tool for modulating the pharmacodynamic and pharmacokinetic properties of peptides.…”

“…In prior work, a lactam staple (Glu-Lys) was introduced into the peptide structure of [Dpr 3 (n-octanoyl)]ghrelin(1-20)-NH 2 in what was referred to as a "staple scan" to discover the optimal location that yields a cyclic peptide with the highest affinity towards the ghrelin receptor. In the resulting publication, Lalonde and coworkers described how an i, i + 4 staple between the 12 and 16 position of [Dpr 3 ]ghrelin (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) resulted in the highest affinity towards the ghrelin receptor with a reported IC 50 of 7.9 nM. 37 The resulting compound was then appended with a Cy5 fluorescent dye and was successful at imaging the ghrelin receptor in ovarian cancer cell lines using confocal microscopy.…”

“…Since the optimal position for an i, i + 4 lactam (Glu-Lys) staple in ghrelin (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) analogues was determined to be positions 12 and 16, the question arose whether different stapling techniques at this same location would yield compounds with greater affinity towards the ghrelin receptor, and if these techniques would have varied influence on the metabolic stability of ghrelin analogues. As a result, we present an investigation of five different helix inducing staples at positions 12 and 16 in truncated ghrelin (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) analogues. The five staples investigated are the triazole, lactam (Lys-Asp), hydrocarbon, xylene-thioether, and Glaser staples (Fig.…”