PET imaging of GLP-1 receptor
GLP-1 and GLP-1R
Incretin peptides, including neuropeptides GLP-1 and GIP (glucose-dependent insulinotropic polypeptide), function as gut hormones which increase the secretion of insulin after a meal. Intravenously administered glucose does not cause the release of incretin hormones, and therefore leads to lower insulin secretion than orally given glucose (“incretin effect”).
The glucagon-like peptide 1 (GLP-1) is a 30 amino acid hormone that is secreted by L-cells of small intestine and proximal colon following food ingestion. The proglucagon gene sequence contains coding regions for glucagon (expressed in pancreatic α-cells), GLP-1, and GLP-2. GLP-1 inhibits glucagon secretion, while GIP and GLP-2 can stimulate it. GLP-1 is agonist to G-protein -coupled glucagon-like peptide 1 receptor (GLP-1R). When binding to its ligand GLP-1R internalizes, like other G-protein coupled receptors, and is partially recycled back to the plasma membrane. The activated GLP-1R/GLP-1-complex stimulates the cAMP-dependent pathway, increasing release of insulin and decreasing release of glucagon in the pancreas. GLP-1 stimulates β-cell proliferation and inhibits β-cell apoptosis. Endogenous GLP-1 is degraded within minutes in vivo by dipeptidyl peptidase 4 (DPP-4) via the cleavage of two N-terminal residues.
GLP-1 and GIP secretion is highly variable between subjects. In type 2 diabetes, both the secretion of GLP-1 and the affinity of GLP-1 to GLP-1R are impaired. GIP levels are increased in obesity. Exendin-4 (Exenatide, Byetta, Amylin) is a subcutaneously administered GLP-1R agonist peptide that is used in the treatment of type 2 diabetes. Another GLP-1R agonist, liraglutide, can be used in treatment of obesity. DDP-4 inhibitors can be used to increase GLP-1 levels.
GLP-1R is highly expressed in pancreatic β-cells, with minimal or no expression in other pancreatic islet cell types, and relatively low expression in pancreatic ductal cells. Due to the relatively low β-cell mass, the GLP-1Rs in other pancreatic tissue still affects the overall uptake of GLP-1R tracers (Eriksson et al., 2017). GLP-1R is overexpressed in certain types of cancers derived from endocrine and neuroendocrine origins, including insulinoma. GLP-1R is also expressed in the brain, liver, heart, and in lung and renal vasculature. GLP-1Rs in the hypothalamus are responsible for the GLP-1 induced satiety and reduced appetite.
There are some species differences in the GLP-1R expression. For example, GLP-1R expression in the thyroid gland is substantial in rodents, but virtually absent in humans. Receptor density is also higher in the lungs of rats and mice than in humans. In the hearts of rats and mice, GLP-1Rs are expressed mainly in the atria, while in humans the expression is similar in all four chambers (Baggio et al., 2018).
GLP-1R is expressed in injured myocardium, where GLP-1 limits apoptosis and reduces fibrosis, and therefore has become a therapy target in cardiovascular disease (Gao et al., 2012). In type 2 diabetic patients with cardiomyopathy, GLP-1R agonist Exenatide did not improve CFR or myocardial oxygen consumption (Chen et al., 2017). In healthy men, GLP-1 increases myocardial glucose uptake in subjects that have low baseline glucose uptake, and decreases glucose uptake in those subjects that have high baseline glucose uptake in the myocardium (Gejl et al., 2014).
Neuropeptide CART is expressed in the small intestine, and increases glucose-stimulated GLP-1 and GIP secretion (Shcherbina et al., 2018).
GLP-1 peptide is not a suitable tracer for imaging because of its short biological halflife. Therefore several GLP-1 analogues with much longer in vivo halflife have been developed, including exendin-3, exendin-4, and exendin(9-39); these have been labelled with radionuclides such as 111In, 99mTc, 68Ga, 18F, and 64Cu, for SPECT and PET. Some of these tracers have shown promise for imaging neuroendocrine tumours, transplanted islets, insulinomas, and myocardial infarction.
GLP-1R agonist exendin-4 is a 39 amino acid peptide, originally derived from the saliva of the Gila monster. Exendin(9-39) is a truncated form of exendin-4, but functions as GLP-1R antagonist. Exendin-3, derived from related species, has almost similar structure, but is less specific to GLP-1R.
When GLP-1R binds GLP-1, the receptor-ligan complex is internalized rapidly with half-life of 2-3 min, and recycle half-life of 15 min (Widman et al., 1995). GLP-1R recycling is faster with GLP-1 than with exendin-4 or liraglutide (Roed et al., 2014).
Internalization of GLP-1R tracers, is different between tracers, even just exendin-4 tracers (Jodal et al., 2014), and possibly also in different organs. Internalization, although relatively low, may correlate with the tumour uptake (Wild et al., 2010). Optimal scan time and analysis method must therefore be studied for each tracer and target organ separately. Irreversible uptake must not be assumed without validation. For example, 68Ga labelled exendin-4 has been analyzed using reversible one-tissue compartment model, with the volume of distribution as the reported parameter (Selvaraju et al., 2013; Nalin et a., 2014), suggesting very slow internalization. 125I-labelled exendin-4 has shown fast internalization, while antagonist exendin-(9-39) did not internalize (Läppchen et al., 2017).
GLP-1 receptor density in normal tissues (excluding some GLP-1R expressing tumours) is relatively low. Therefore the cold mass of injected tracer must be kept as low as possible (high specific activity) to avoid pharmacological and receptor blocking effects (Wild et al., 2006; Mikkola et al., 2014 and 2016; Rydén et al., 2016). In labelling with 68Ga, the requirement for low mass may lead to formation of 68Ga colloids which, if not properly removed, can be seen as uptake in the spleen and liver (Brom et al., 2016).
Quantification of GLP-1R using PET
[18F]FBEM-Cys40-exendin-4 was used to study GLP-1R expression in vivo in rat model of myocardial infarction and reperfusion; a static 10 min scan was performed 1 h after tracer injection, and regional %ID/g values were calculated (Gao et al., 2012).
Ex vivo autoradiography has shown that [68Ga]NODAGA-exendin-4 uptake is localized in macrophage-rich, GLP-1R-positive atherosclerotic lesion areas in the aorta of atherosclerotic and diabetic mice (Ståhle et al., 2017).
Radiolabelled peptides, including exendin-4, are excreted mainly to urine via the kidneys. Kidneys express GLP-1R, but specific binding is negligible compared to the non-specific build-up of intact and metabolized radioligand. Part of the peptides are reabsorbed by scavenger receptors like megalin into proximal tubular cells, where peptides are then degraded. High kidney uptake and concentration in urine hampers GLP-1R imaging in organs that are close to the kidneys and bladder. Albumin-derived peptides can be used to inhibit renal uptake of radiolabelled peptides.
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Created at: 2014-12-05
Updated at: 2018-10-26
Written by: Vesa Oikonen, Anne Roivainen