In vivo imaging of insulin receptors in monkeys using 18F-labeled insulin and positron emission tomography

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.

Metal-ion coordinated self-assembly of human insulin directs kinetics of insulin release as determined by preclinical SPECT/CT imaging

Human insulin (HI) has fascinating metal-facilitated self-assembly properties that are essential for its biological function. HI has a natural Zn²⁺ binding site and we have previously shown that covalently attached abiotic ligands (e.g., bipyridine, terpyridine) can lead to the formation of nanosized oligomeric structures through the coordination of metal ions. Here we studied the hypothesis that metal ions can be used to directly control the pharmacokinetics of insulin after covalent attachment of an abiotic ligand that binds metal ions. We evaluated the pharmacokinetics (PK) and biodistribution of HI self-assemblies directed by metal ion coordination (i.e., Fe²⁺/Zn²⁺, Eu³⁺/Zn²⁺, Fe²⁺/Co³⁺) using preclinical SPECT/CT imaging and ex vivo gamma counting. HI was site-specifically modified with terpyridine (Tpy) at the Phe^B1 or Lys^B29 position to create conjugates that bind either Fe²⁺ or Eu³⁺, while its natural binding site (e.g., His^B10) preferentially coordinates with either Zn²⁺ or Co³⁺. HI was also functionalized with trans-cyclooctene (TCO) opposite to Tpy at Phe^B1 or Lys^B29, respectively, to allow for tetrazine-TCO coupling via a tetrazine-modified DTPA followed by ¹¹¹In-radiolabeling for SPECT/CT imaging. When the ¹¹¹In-B29Tpy-HI conjugate was coordinated with Fe²⁺/Zn²⁺, its retention at the injection site 6 h after injection was ~8-fold higher than the control without the metal ions, while its kidney accumulation was lower. ¹¹¹In-B1Tpy-HI showed comparable retention at the injection site 6 h after injection and slightly increased retention at 24 h. However, higher kidney accumulation and residence time of degraded ¹¹¹In-B1Tpy-HI was observed compared to that of ¹¹¹In-B29Tpy-HI. Quantitative PK analysis based on SPECT/CT images confirmed slower distribution from the injection site of the HI-metal ion assemblies compared to control HI conjugates. Our results show that the Tpy-binding site (i.e., Phe^B1 or Lys^B29) on HI and its coordination with the added metal ions (i.e., Fe²⁺/Zn²⁺ or Fe²⁺/Co³⁺) directed the distribution half-life of HI significantly. This clearly indicates that the PK of insulin can be controlled by complexation with different metal ions.

Brain insulin action in schizophrenia: Something borrowed and something new

Insulin signaling in the central nervous system is at the intersection of brain and body interactions, and represents a fundamental link between metabolic and cognitive disorders. Abnormalities in brain insulin action could underlie the development of comorbid schizophrenia and type 2 diabetes. Among its functions, central nervous system insulin is involved in regulation of striatal dopamine levels, peripheral glucose homeostasis, and feeding regulation. In this review, we discuss the role and importance of central nervous system insulin in schizophrenia and diabetes pathogenesis from a historical and mechanistic perspective. We describe central nervous system insulin sites and pathways of action, with special emphasis on glucose metabolism, cognitive functioning, inflammation, and food preferences. Finally, we suggest possible mechanisms that may explain the actions of central nervous system insulin in relation to schizophrenia and diabetes, focusing on glutamate and dopamine signaling, intracellular signal transduction pathways, and brain energetics. Understanding the interplay between central nervous system insulin and schizophrenia is essential to disentangling this comorbid relationship and may provide novel treatment approaches for both neuropsychiatric and metabolic dysfunction. This article is part of the issue entitled 'Special Issue on Antipsychotics'.

Probing insulin sensitivity in diabetic kidney disease: is there a stronger role for functional imaging?

Clinical and experimental evidence support a cause-effect relationship between altered insulin signaling and development of kidney disease of metabolic and non-metabolic origin. However, the current criteria to measure and/or estimate the insulin resistance (IR) are available as research tool but are very difficult to implement in the clinical practice. Therefore, a better understanding of the key players contributing to IR may lead to the development of new non-invasive tools to assess organ-specific insulin sensitivity (IS). We will therefore first introduce the concept that IR and kidney disease may be causally linked as suggested by clinical and experimental studies. We will then, expand on the potential mechanisms leading to altered renal insulin signaling. After reviewing the limitation of currently available strategies to determine IR, this review article will focus on imaging techniques that could be utilized to determine renal IR and that could be tested to predict kidney disease development and progression.

Evaluation of Fluorine-18-Labeled α1(I)-N-Telopeptide Analogs as Substrate-Based Radiotracers for PET Imaging of Melanoma-Associated Lysyl Oxidase

Accumulating evidence suggests an unequivocal role of lysyl oxidases as key players of tumor progression and metastasis, which renders this enzyme family highly attractive for targeted non-invasive functional imaging of tumors. Considering their function in matrix remodeling, malignant melanoma appears as particularly interesting neoplasia in this respect. For the development of radiotracers that enable PET imaging of the melanoma-associated lysyl oxidase activity, substrates derived from the type I collagen α1 N-telopeptide were labeled with fluorine-18 using N-succinimidyl 4-[¹⁸F]fluorobenzoate ([¹⁸F]SFB) as prosthetic reagent. With regards to potential crosslinking to tumor-associated collagen in vivo, their interaction with triple-helical type I collagen was studied by SPR. A mouse model of human melanoma was established on the basis of the A375 cell line, for which the expression of the oncologically relevant lysyl oxidase isoforms LOX and LOXL2 was demonstrated in Western blot and immunohistochemical experiments. The radiopharmacological profiles of the peptidic radiotracers were evaluated in normal rats and A375 melanoma-bearing mice by ex vivo metabolite analysis, whole-body biodistribution studies and dynamic PET imaging. Out of three ¹⁸F-labeled telopeptide analogs, the one with the most favorable substrate properties has shown favorable tumor uptake and tumor-to-muscle ratio. Lysyl oxidase-mediated tumor uptake was proven by pharmacological inhibition using β-aminopropionitrile and by employing negative-control analogs of impeded or abolished targeting capability. The latter were obtained by substituting the lysine residue by ornithine and norleucine, respectively. Comparing the tumor uptake of the lysine-containing peptide with that of the non-functional analogs indicate the feasibility of lysyl oxidase imaging in melanoma using substrate-based radiotracers.

Synthesis and radiopharmacological characterisation of a fluorine-18-labelled azadipeptide nitrile as a potential PET tracer for in vivo imaging of cysteine cathepsins

A fluorinated cathepsin inhibitor based on the azadipeptide nitrile chemotype was prepared and selected for positron emission tomography (PET) tracer development owing to its high affinity for the oncologically relevant cathepsins L, S, K and B. Labelling with fluorine-18 was accomplished in an efficient and reliable two-step, one-pot radiosynthesis by using 2-[(18) F]fluoroethylnosylate as a prosthetic agent. The pharmacokinetic properties of the resulting radiotracer compound were studied in vitro, ex vivo and in vivo in normal rats by radiometabolite analysis and small-animal positron emission tomography. These investigations revealed rapid conjugate formation of the tracer with glutathione in the blood, which is associated with slow blood clearance. The potential of the developed (18) F-labelled probe to image tumour-associated cathepsin activity was investigated by dynamic small-animal PET imaging in nude mice bearing tumours derived from the human NCI-H292 lung carcinoma cell line. Computational analysis of the obtained image data indicated the time-dependent accumulation of the radiotracer in the tumours. The expression of the target enzymes in the tumours was confirmed by immunohistochemistry with specific antibodies. This indicates that azadipeptide nitriles have the potential to target thiol-dependent cathepsins in vivo despite their disadvantageous pharmacokinetics.

Synthesis and in Vitro Evaluation of 18F- and 19F-Labeled Insulin: A New Radiotracer for PET-based Molecular Imaging Studies

A new and regioselective strategy was developed for the preparation of fluorine-18-labeled insulin as a novel positron emission tomography (PET) tracer. [18F]-4-Fluorobenzoic acid (4-18FBA), which was produced in 83 +/- 8% yield (n = 10), through the use of succinimidyl [18F]-4-fluorobenzoate (4-(18)FSB), was conjugated through a short spacer (6-aminohexanoic acid, AHx) to the PheB1 residue of a protected form of insulin. 18FB-AHx-insulin (8b) was repeatedly prepared in practical quantities (10-20 mCi, 370-740 MBq) in good radiochemical yield (9 +/- 5%, n = 9) and in a specific activity of 7.8 mCi/micromol. The final product was characterized by comparing the radioHPLC and radioTLC of 8b with that of the 19F-analogue (19FB-AHx-insulin, 8a) and by analyzing a carrier-added synthesis by mass spectrometry. Dithiothreitol and endoproteinase Glu-C digestion experiments on 8a confirmed that the prosthetic group was in fact conjugated to the PheB1 residue. An insulin receptor (IR) phosphorylation assay using CHO-hIR cells overexpressing recombinant human insulin receptors indicated no statistical difference in the extent of autophosphorylation stimulated by 8a as compared to that for human insulin (EC50 values of 0.82 nM and 1.0 nM, respectively). The stimulation of 2-deoxyglucose uptake in 3T3-L1 mouse adipocytes utilizing 8a versus unmodified human insulin gave similar EC50 values of 0.68 nM and 0.41 nM, respectively. The IC50 values for 8a versus native insulin for the displacement of 125I-insulin from HEK-293 cells were also the same within experimental error (2.6 nM for 8a versus 2.4 nM for unmodified human insulin). These results support the use of the 18F-insulin analogue as a PET tracer for imaging the distribution of insulin in vivo.