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 beta-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 beta-cell mass (BCM) would greatly enhance the ability to track diabetes therapies. beta-cells and neurologic tissues have common cellular receptors and transporters, therefore, we screened brain radioligands for their ability to identify beta-cells. Methods: We examined a beta-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 C-11-(1)-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 C-11-(1)-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 C-11-(1)-PHNO may be specific to changes in the pancreas between healthy controls and T1DM subjects. When D-3 and D-2 antagonists were used in nonhuman primates, specific pancreatic binding (SUVR-1) of C-11-PHNO was reduced by 57% and 38%, respectively. Conclusion: C-11-(1)-PHNO is a potential marker of BCM, with 2: 1 binding of D-3 receptors over D-2 receptors. Further in vitro and in vivo studies to establish D-2/D-3 receptor specificity to beta-cells is warranted to characterize C-11-(1)-PHNO as a candidate for clinical measurement of BCM in healthy control and diabetic subjects.