The development of a GPR44 targeting radioligand [11C]AZ12204657 for in vivo assessment of beta cell mass

Abstract

BackgroundThe G-protein-coupled receptor 44 (GPR44) is a beta cell-restricted target that may serve as a marker for beta cell mass (BCM) given the development of a suitable PET ligand.MethodsThe binding characteristics of the selected candidate, AZ12204657, at human GPR44 were determined using in vitro ligand binding assays. AZ12204657 was radiolabeled using 11C- or 3H-labeled methyl iodide ([11C/3H]CH3I) in one step, and the conversion of [11C/3H]CH3I to the radiolabeled product [11C/3H]AZ12204657 was quantitative. The specificity of radioligand binding to GPR44 and the selectivity for beta cells were evaluated by in vitro binding studies on pancreatic sections from human and non-human primates as well as on homogenates from endocrine and exocrine pancreatic compartments.ResultsThe radiochemical purity of the resulting radioligand [11C]AZ12204657 was > 98%, with high molar activity (MA), 1351 ± 575 GBq/μmol (n = 18). The radiochemical purity of [3H]AZ12204657 was > 99% with MA of 2 GBq/μmol. Pancreatic binding of [11C/3H]AZ12204657 was co-localized with insulin-positive islets of Langerhans in non-diabetic individuals and individuals with type 2 diabetes (T2D). The binding of [11C]AZ12204657 to GPR44 was > 10 times higher in islet homogenates compared to exocrine homogenates. In human islets of Langerhans GPR44 was co-expressed with insulin, but not glucagon as assessed by co-staining and confocal microscopy.ConclusionWe radiolabeled [11C]AZ12204657, a potential PET radioligand for the beta cell-restricted protein GPR44. In vitro evaluation demonstrated that [3H]AZ12204657 and [11C]AZ12204657 selectively target pancreatic beta cells. [11C]AZ12204657 has promising properties as a marker for human BCM.

GPR44 as a Target for Imaging Pancreatic Beta-Cell Mass

“…The screen used the Human Protein Atlas to identify proteins with strong immunostaining in the islets of Langerhans, combined with no evidence of presence in the exocrine pancreas. Further evaluation of GPR44 localization as assessed by confocal microscopy in individual human islets demonstrated co-localization between the GPR44 antibody with insulin, but not glucagon or somatostatin, indicating that the expression was restricted to beta cells [29•, 30••].…”

“…Using AZD3825 as starting point, a demethylated precursor for AZ12204657 (Fig. 1a) was developed [30••].…”

“…AZ12204657 was shown to displace tritiated PGD 2 , as well as tritiated AZ12204657 itself, dose-dependently from membranes of HEK293 cells overexpressing human GPR44 [30••]. AZ12204657 also acts as an antagonist on GPR44 as it could inhibit the PGD 2 signaling in human immortalized EndoC-BH1 beta cells [30••]. Demethylated AZ12204657 was then radiolabeled with Carbon-11 by addition of 11 C-CH 3 I as described in detail previously (Fig.…”

“…Demethylated AZ12204657 was then radiolabeled with Carbon-11 by addition of 11 C-CH 3 I as described in detail previously (Fig. 1b) [30••].…”

Beta Cell Imaging—From Pre-Clinical Validation to First in Man Testing

“…Furthermore, availability is limited to local production facilities as the short half-life restricts the ability to distribute these tracers. Several tracers developed as surrogate markers of beta-cell mass have utilized 11 C for use with PET including [ 11 C]DTBZ [ 49 ], [ 11 C]5-HTP [ 50 ], [ 11 C]AZ12204657 [ 134 ], and [ 11 C]MK7246 [ 135 ].…”