Systematic screening of imaging biomarkers for the Islets of Langerhans, among clinically available positron emission tomography tracers

Evaluation of PET Brain Radioligands for Imaging Pancreatic β-Cell Mass: Potential Utility of 11C-(+)-PHNO

Type 1 diabetes mellitus (T1DM) is characterized by a loss of β-cells in the islets of Langerhans of the pancreas and subsequent deficient insulin secretion in response to hyperglycemia. Development of an in vivo test to measure β-cell mass (BCM) would greatly enhance the ability to track diabetes therapies. β-cells and neurologic tissues have common cellular receptors and transporters, therefore, we screened brain radioligands for their ability to identify β-cells. Methods: We examined a β-cell gene atlas for endocrine pancreas receptor targets and cross-referenced these targets with brain radioligands that were available at our institution. Twelve healthy control subjects and 2 T1DM subjects underwent dynamic PET/CT scans with 6 tracers. Results: The D₂/D₃ receptor agonist radioligand ¹¹C-(+)-4-propyl-9-hydroxynaphthoxazine (PHNO) was the only radioligand to demonstrate sustained uptake in the pancreas with high contrast versus abdominal organs such as the kidneys, liver, and spleen, based on the first 30 min of data. Mean SUV from 20 to 30 min demonstrated high uptake of ¹¹C-(+)-PHNO in healthy controls (SUV, 13.8) with a 71% reduction in a T1DM subject with undetectable levels of C-peptide (SUV, 4.0) and a 20% reduction in a T1DM subject with fasting C-peptide level of 0.38 ng/mL (SUV, 11.0). SUV in abdominal organs outside the pancreas did not show measurable differences between the control and T1DM subjects, suggesting that the changes in SUV of ¹¹C-(+)-PHNO may be specific to changes in the pancreas between healthy controls and T1DM subjects. When D₃ and D₂ antagonists were used in nonhuman primates, specific pancreatic binding (SUVR-1) of ¹¹C-PHNO was reduced by 57% and 38%, respectively. Conclusion:¹¹C-(+)-PHNO is a potential marker of BCM, with 2:1 binding of D₃ receptors over D₂ receptors. Further in vitro and in vivo studies to establish D₂/D₃ receptor specificity to β-cells is warranted to characterize ¹¹C-(+)-PHNO as a candidate for clinical measurement of BCM in healthy control and diabetic subjects.

Imaging pancreatic islet cells by positron emission tomography

It was estimated that every year more than 30000 persons in the United States - approximately 80 people per day - are diagnosed with type 1 diabetes (T1D). T1D is caused by autoimmune destruction of the pancreatic islet (β cells) cells. Islet transplantation has become a promising therapy option for T1D patients, while the lack of suitable tools is difficult to directly evaluate of the viability of the grafted islet over time. Positron emission tomography (PET) as an important non-invasive methodology providing high sensitivity and good resolution, is able to accurate detection of the disturbed biochemical processes and physiological abnormality in living organism. The successful PET imaging of islets would be able to localize the specific site where transplanted islets engraft in the liver, and to quantify the level of islets remain alive and functional over time. This information would be vital to establishing and evaluating the efficiency of pancreatic islet transplantation. Many novel imaging agents have been developed to improve the sensitivity and specificity of PET islet imaging. In this article, we summarize the latest developments in carbon-11, fluorine-18, copper-64, and gallium-68 labeled radioligands for the PET imaging of pancreatic islet cells.

In vivo imaging of beta cells with radiotracers: state of the art, prospects and recommendations for development and use

Radiotracer imaging is characterised by high in vivo sensitivity, with a detection limit in the lower picomolar range. Therefore, radiotracers represent a valuable tool for imaging pancreatic beta cells. High demands are made of radiotracers for in vivo imaging of beta cells. Beta cells represent only a small fraction of the volume of the pancreas (usually 1-3%) and are scattered in the tiny islets of Langerhans throughout the organ. In order to be able to measure a beta cell-specific signal, one has to rely on highly specific tracer molecules because current in vivo imaging technologies do not allow the resolution of single islets in humans non-invasively. Currently, a considerable amount of preclinical data are available for several radiotracers and three are under clinical evaluation. We summarise the current status of the evaluation of these tracer molecules and put forward recommendations for their further evaluation.

GPR44 is a pancreatic protein restricted to the human beta cell

AIMS

To address questions regarding onset and progression of types 1 and 2 diabetes (T1D/T2D), surrogate imaging biomarkers for beta cell function and mass are needed. Here, we assess the potential of GPR44 as a surrogate marker for beta cells, in a direct comparison with clinically used biomarker VMAT2.

METHODS

GPR44 surface availability was assessed by flow cytometry of human beta cells. RNA transcription levels in different pancreas compartments were evaluated. The density of GPR44 receptor in endocrine and exocrine tissues was assessed by the radiolabeled GPR44 ligand [(3)H]AZD 3825. A direct comparison with the established beta cell marker VMAT2 was performed by radiolabeled [(3)H]DTBZ.

RESULTS

GPR44 was available on the cell surface, and pancreatic RNA levels were restricted to the islets of Langerhans. [(3)H]AZD 3825 had nanomolar affinity for GPR44 in human islets and EndoC-βH1 beta cells, and the specific binding to human beta cells was close to 50 times higher than in exocrine preparations. The endocrine-to-exocrine binding ratio was approximately 10 times higher for [(3)H]AZD 3825 than for [(3)H]DTBZ.

CONCLUSION

GPR44 is a highly beta cell-specific target, which potentially offers improved imaging contrast between the human beta cell and the exocrine pancreas.