PET imaging of GABAA receptors in pancreatic islets by [11C]flumazenil.

Abstract

Background: Type 1 diabetes (T1D) is an autoimmune disease characterized by a progressive β-cell destruction. There are no clinically established methods for quantifying endocrine cells of the pancreas and current knowledge is almost exclusively based on autopsy material and functional measurements. Based on the expression of the γ-aminobutyric acid A receptors (GABA_ARs) in pancreatic islets and the fact that GABA_AR agonists are being explored as treatment for T1D, we hypothesized that the positron emission tomography (PET) tracer [¹¹C]flumazenil ([¹¹C]FMZ) could serve as a marker of the endocrine mass of the pancreas. The in vivo uptake of [¹¹C]FMZ in pig pancreas was evaluated by PET/CT, either tracer alone or after blockade of GABA_AR by unlabeled flumazenil. The pancreatic binding of [¹¹C]FMZ was investigated in vitro with frozen sections of pig pancreas as well as human organ donors, in addition to isolated mouse and human islets and exocrine preparations. The expression of GABA_AR subunits in pig, human and mouse pancreas was explored by immunohistochemistry.

Results: Strong specific in vivo uptake of [¹¹C]FMZ was observed in the pig brain as expected, but in the pancreas the signal was moderate and only partially reduced by blockade. In vitro experiments revealed a positive but weak and variable binding of [¹¹C]FMZ in islets compared to exocrine tissue in the mouse, pig and human pancreas. In pig and mouse pancreatic islets we identified the GABA_AR subunits β2 and γ2 but not α2. In the human pancreas from non-diabetic donors, we have identified the α2, β2 (although weak) and γ2 subunits, whereas from a T2D donor the α2 subunit was missing.

Conclusions: Our findings suggest that [¹¹C]FMZ bind to GABA_ARs in the islets, but not with a sufficient contrast or magnitude to be implemented as an in vivo PET marker for the endocrine mass of the pancreas. However, GABA_ARs with different subunits are widely expressed in the endocrine cells within the pancreas in pig, human and mouse. Hence, the GABA_AR could still be a potential imaging target for the endocrine cells of the pancreas but would require tracers with higher affinity and selectivity for individual GABA_AR subunits.