Characterization of 111In-labeled Glucose-Dependent Insulinotropic Polypeptide as a Radiotracer for Neuroendocrine Tumors

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

Imaging and radiotherapy targeting the glucose-dependent insulinotropic polypeptide receptor (GIPR) could potentially benefit the management of neuroendocrine neoplasms (NENs), complementing clinically established radiopharmaceuticals. The aim of this study was to evaluate a GIPR-targeting positron emission tomography (PET) radioligand with receptor-specific binding, fast blood clearance, and low liver background uptake. The peptide DOTA-bioconjugate, C803-GIP, was developed based on the sequence of the endogenous GIP(1-30) and synthetic exendin-4 peptides with selective amino acid mutations to combine their specificity for the GIPR and in vivo stability, respectively. The ⁶⁸Ga-labeled bioconjugate was evaluated in vitro in terms of binding affinity, specificity, and internalization in HEK293 cells transfected with the human GIPR, GLP1, or GCG receptors and in sections of human insulinoma and NENs. In vivo binding specificity, biodistribution, and tissue background were investigated in mice bearing huGIPR-HEK293 xenografts and in a pig. Ex vivo organ distribution, pharmacokinetics, and dosimetry were studied in normal rats. [⁶⁸Ga]Ga-C803-GIP was stable and demonstrated a high affinity to the huGIPR-HEK293 cells. Binding specificity was demonstrated in vitro in frozen sections of NENs and huGIPR-HEK293 cells. No specific uptake was observed in the negative controls of huGLP1R and huGCGR cells. A novel rationally designed PET radioligand, [⁶⁸Ga]Ga-C803-GIP, demonstrated promising binding characteristics and specificity towards the GIPR.

Molecular imaging phenotyping for selecting and monitoring radioligand therapy of neuroendocrine neoplasms

Neuroendocrine neoplasia (NEN) is an umbrella term that includes a widely heterogeneous disease group including well-differentiated neuroendocrine tumours (NETs), and aggressive neuroendocrine carcinomas (NECs). The site of origin of the NENs is linked to the intrinsic tumour biology and is predictive of the disease course. It is understood that NENs demonstrate significant biologic heterogeneity which ultimately translates to widely varying clinical presentations, disease course and prognosis. Thus, significant emphasis is laid on the pre-therapy evaluation of markers that can help predict tumour behavior and dynamically monitors the response during and after treatment. Most well-differentiated NENs express somatostatin receptors (SSTRs) which make them appropriate for peptide receptor radionuclide therapy (PRRT). However, the treatment outcomes of PRRT depend heavily on the adequacy of patient selection by molecular imaging phenotyping not only utilizing pre-treatment SSTR PET but ¹⁸F-Fluorodeoxyglucose (¹⁸F-FDG) PET to provide insights into the intra- or inter-tumoural heterogeneity of the metastatic disease. Molecular imaging phenotyping may go beyond patient selection and provide useful information during and post-treatment for monitoring of temporal heterogeneity of the disease and dynamically risk-stratify patients. In addition, advances in the understanding of genomic-phenotypic classifications of pheochromocytomas and paragangliomas led to an archetypical example in precision medicine by utilizing molecular imaging phenotyping to guide radioligand therapy. Novel non-SSTR based peptide receptors have also been explored diagnostically and therapeutically to overcome the tumour heterogeneity. In this paper, we review the current molecular imaging modalities that are being utilized for the characterization of the NENs with special emphasis on their role in patient selection for radioligand therapy.

Drug Occupancy Assessment at the Glucose-Dependent Insulinotropic Polypeptide Receptor by Positron Emission Tomography

Targeting of the glucose-dependent insulinotropic polypeptide receptor (GIPR) is an emerging strategy in antidiabetic drug development. The aim of this study was to develop a positron emission tomography (PET) radioligand for the GIPR to enable the assessment of target distribution and drug target engagement in vivo. The GIPR-selective peptide S02-GIP was radiolabeled with ⁶⁸Ga. The resulting PET tracer [⁶⁸Ga]S02-GIP-T4 was evaluated for affinity and specificity to human GIPR (huGIPR). The in vivo GIPR binding of [⁶⁸Ga]S02-GIP-T4 as well as the occupancy of a drug candidate with GIPR activity were assessed in nonhuman primates (NHPs) by PET. [⁶⁸Ga]S02-GIP-T4 bound with nanomolar affinity and high selectivity to huGIPR in overexpressing cells. In vivo, pancreatic binding in NHPs could be dose-dependently inhibited by coinjection of unlabeled S02-GIP-T4. Finally, subcutaneous pretreatment with a high dose of a drug candidate with GIPR activity led to a decreased pancreatic binding of [⁶⁸Ga]S02-GIP-T4, corresponding to a GIPR drug occupancy of almost 90%. [⁶⁸Ga]S02-GIP-T4 demonstrated a safe dosimetric profile, allowing for repeated studies in humans. In conclusion, [⁶⁸Ga]S02-GIP-T4 is a novel PET biomarker for safe, noninvasive, and quantitative assessment of GIPR target distribution and drug occupancy.

Cancer Biology and Prevention in Diabetes

The available evidence suggests a complex relationship between diabetes and cancer. Epidemiological data suggest a positive correlation, however, in certain types of cancer, a more complex picture emerges, such as in some site-specific cancers being specific to type I diabetes but not to type II diabetes. Reports share common and differential mechanisms which affect the relationship between diabetes and cancer. We discuss the use of antidiabetic drugs in a wide range of cancer therapy and cancer therapeutics in the development of hyperglycemia, especially antineoplastic drugs which often induce hyperglycemia by targeting insulin/IGF-1 signaling. Similarly, dipeptidyl peptidase 4 (DPP-4), a well-known target in type II diabetes mellitus, has differential effects on cancer types. Past studies suggest a protective role of DPP-4 inhibitors, but recent studies show that DPP-4 inhibition induces cancer metastasis. Moreover, molecular pathological mechanisms of cancer in diabetes are currently largely unclear. The cancer-causing mechanisms in diabetes have been shown to be complex, including excessive ROS-formation, destruction of essential biomolecules, chronic inflammation, and impaired healing phenomena, collectively leading to carcinogenesis in diabetic conditions. Diabetes-associated epithelial-to-mesenchymal transition (EMT) and endothelial-to-mesenchymal transition (EndMT) contribute to cancer-associated fibroblast (CAF) formation in tumors, allowing the epithelium and endothelium to enable tumor cell extravasation. In this review, we discuss the risk of cancer associated with anti-diabetic therapies, including DPP-4 inhibitors and SGLT2 inhibitors, and the role of catechol-o-methyltransferase (COMT), AMPK, and cell-specific glucocorticoid receptors in cancer biology. We explore possible mechanistic links between diabetes and cancer biology and discuss new therapeutic approaches.

News ways of understanding the complex biology of diabetes using PET

The understanding of metabolic disease and diabetes on a molecular level has increased significantly due to the recent advances in molecular biology and biotechnology. However, in vitro studies and animal models do not always translate to the human disease, perhaps illustrated by the failure of many drug candidates in the clinical phase. Non-invasive biomedical imaging techniques such as Positron Emission Tomography (PET) offer tools for direct visualization and quantification of molecular processes in humans. Developments in this area potentially enable longitudinal in vivo studies of receptors and processes involved in diabetes guiding drug development and diagnosis in the near future. This mini-review focuses on describing the overall perspective of how PET can be used to increase our understanding and improve treatment of diabetes. The methodological aspects and future developments and challenges are highlighted.

The pathogenic role of the GIP/GIPR axis in human endocrine tumors: emerging clinical mechanisms beyond diabetes

The glucose-dependent insulinotropic polypeptide (GIP) is an incretin hormone produced in the gastrointestinal tract in response to nutrients. GIP has a variety of effects on different systems, including the potentiation of insulin secretion from pancreatic β-cells after food intake (i.e. incretin effect), which is probably the most important. GIP effects are mediated by the GIP receptor (GIPR), a G protein-coupled receptor expressed in several tissues, including islet β-cells, adipocytes, bone cells, and brain. As well as its involvement in metabolic disorders (e.g. it contributes to the impaired postprandial insulin secretion in type 2 diabetes (T2DM), and to the pathogenesis of obesity and associated insulin resistance), an inappropriate GIP/GIPR axis activation of potential diagnostic and prognostic value has been reported in several endocrine tumors in recent years. The ectopic GIPR expression seen in patients with overt Cushing syndrome and primary bilateral macronodular adrenal hyperplasia or unilateral cortisol-producing adenoma has been associated with an inverse rhythm of cortisol secretion, with low fasting morning plasma levels that increase after eating. On the other hand, most acromegalic patients with an unusual GH response to oral glucose suppression have GIPR-positive somatotropinomas, and a milder phenotype, and are more responsive to medical treatment. Neuroendocrine tumors are characterized by a strong GIPR expression that may correlate positively or inversely with the proliferative index MIB-1, and that seems an attractive target for developing novel radioligands. The main purpose of this review is to summarize the role of the GIP/GIPR axis in endocrine neoplasia, in the experimental and the clinical settings.