Imaging of the Glucose-Dependent Insulinotropic Polypeptide Receptor Using a Novel Radiolabeled Peptide Rationally Designed Based on Endogenous GIP and Synthetic Exendin-4 Sequences

Introduction of a fatty acid chain modification to prolong circulatory half-life of a radioligand towards glucose-dependent insulinotropic polypeptide receptor

BACKGROUND

The beneficial role of glucose-dependent insulinotropic polypeptide receptor (GIPR) in weight control and maintaining glucose levels has led to the development of several multi-agonistic peptide drug candidates, targeting GIPR and glucagon like peptide 1 receptor (GLP1R) and/or the glucagon receptor (GCGR). The in vivo quantification of target occupancy by these drugs would accelerate the development of new drug candidates. The aim of this study was to evaluate a novel peptide (GIP1234), based on previously reported ligand DOTA-GIP-C803, modified with a fatty acid moiety to prolong its blood circulation. It would allow higher target tissue exposure and consequently improved peptide uptake as well as in vivo PET imaging and quantification of GIPR occupancy by novel drugs of interest.

METHOD

A 40 amino acid residue peptide (GIP1234) was synthesized based on DOTA-GIP-C803, in turn based on the sequences of endogenous GIP and Exendin-4 with specific amino acid modifications to obtain GIPR selectivity. A palmitoyl fatty acid chain was furthermore added at Lys14 via a glutamic acid linker to prolong its blood circulation time by the interaction with albumin. GIP1234 was conjugated with a DOTA chelator at the C-terminal cysteine residue to achieve 68Ga radiolabeling. The resulting PET probe, [68Ga]Ga-DOTA-GIP1234 was evaluated for receptor binding specificity and selectivity using HEK293 cells transfected with human GIPR, GLP1R, or GCGR. Blocking experiments with tirzepatide (2 μM) were conducted using huGIPR HEK293 cells to investigate binding specificity. Ex vivo and in vivo organ distribution of [68Ga]Ga-DOTA-GIP1234 was studied in rats and a pig in comparison to [68Ga]Ga-DOTA-C803-GIP. Binding of [68Ga]Ga-DOTA-GIP1234 to albumin was assessed in situ using polyacrylamide gel electrophoresis (PAGE). The stability was tested in formulation buffer and rat blood plasma.

RESULTS

[68Ga]Ga-DOTA-GIP1234 was synthesized with non-decay corrected radiochemical yield of 88 ± 3.7 % and radiochemical purity of 97.8 ± 0.8 %. The molar activity for the radiotracer was 8.1 ± 1.1 MBq/nmol. [68Ga]Ga-DOTA-GIP1234 was stable and maintained affinity to huGIPR HEK293 cells (dissociation constant (Kd) = 40 ± 12.5 nM). The binding of [68Ga]Ga-DOTA-GIP1234 to huGCGR and huGLP1R cells was insignificant. Pre-incubation of huGIPR HEK293 cell sections with tirzepatide resulted in the decrease of [68Ga]Ga-DOTA-GIP1234 binding by close to 90 %. [68Ga]Ga-DOTA-GIP1234 displayed slow blood clearance in pigs with SUV = 3.5 after 60 min. Blood retention of the tracer in rat was 2-fold higher than that of [68Ga]Ga-DOTA-C803-GIP. [68Ga]Ga-DOTA-GIP1234 also demonstrated strong liver uptake in both pig and rat combined with decreased renal excretion. The concentration dependent binding of [68Ga]Ga-DOTA-GIP1234 to albumin was confirmed in situ by PAGE.

CONCLUSION

[68Ga]Ga-DOTA-GIP1234 demonstrated nanomolar affinity and selectivity for huGIPR in vitro. Addition of a fatty acid moiety prolonged blood circulation time and tissue exposure in both rat and pig in vivo. However, the liver uptake was also increased which may make PET imaging of abdominal tissues such as pancreas challenging. The investigation of the influence of fatty acid moiety on the biological performance of the peptide ligand paved the way for further rational design of GIPR ligand analogues with improved characteristics.

The historical progression of positron emission tomography research in neuroendocrinology

The rapid and continual development of a number of radiopharmaceuticals targeting different receptor, enzyme and small molecule systems has fostered Positron Emission Tomography (PET) imaging of endocrine system actions in vivo in the human brain for several decades. PET radioligands have been developed to measure changes that are regulated by hormone action (e.g., glucose metabolism, cerebral blood flow, dopamine receptors) and actions within endocrine organs or glands such as steroids (e.g., glucocorticoids receptors), hormones (e.g., estrogen, insulin), and enzymes (e.g., aromatase). This systematic review is targeted to the neuroendocrinology community that may be interested in learning about positron emission tomography (PET) imaging for use in their research. Covering neuroendocrine PET research over the past half century, researchers and clinicians will be able to answer the question of where future research may benefit from the strengths of PET imaging.