Synthesis and evaluation of F-18-exendin as a potential biomarker to measure pancreatic beta-cell mass

Objectives Glucagon-like peptide 1 (GLP-1) is released in response to food intake and plays an important role in maintaining blood glucose homeostasis. Exendin (EXEN, 9-39), a potent GLP-1R antagonist, has been labeled with In-111 for SPECT imaging. We conducted in vitro binding studies and carried out the first radiosynthesis of F-18 EXEN and an evaluation of its potential as a biomarker for in vivo PET imaging of pancreatic β-cell mass (BCM) in diabetic rat models.

Methods In vitro selectivity was assessed by comparing the uptake of [I-125]EXEN by INS-1 vs. PANC-1 cell lines. F-18 label was introduced by conjugation of [18F]4-fluorobenzaldehyde with an EXEN derivative having a 6-hydrazinonicotinyl group on the ϵ-amine of lys-27 through the formation of a hydrazone. PET imaging was carried out in Sprague-Dawley rats (5 control, 5 streptozotocin [STZ] treated) and BioBreeding-Diabetes Prone rats (3 at 7wks, 3 at 12 wks) using HRRT following 0.187±0.084 mCi EXEN. Time activity curves (TACs) were obtained from pancreas, liver and kidney. Animals were then sacrificed and the pancreases were assayed for insulin content.

Results In vitro assay showed the selectivity of [I-125]EXEN to the islet β-cell. Site-specifically F-18 labeled EXEN was purified on either a G15 open column or a Superdex peptide column with radiochemical and chemical purities > 98% based on size exclusion HPLC. PET imaging showed pancreatic SUV peaked at 10 min, and plateaued by 50 min until the end of scan (240 min). No correlations of pancreatic SUV with post-mortem measures of insulin content were seen.

Conclusions F-18 EXEN was successfully prepared and used for PET imaging for the first time to measure the pancreatic β-cell mass. The results from F-18 EXEN suggest that derivatization of the lys 27 residue might lead to a change in binding affinity, as seen by the absence of specific binding. Preparation of EXEN analogs with the F-18 label at other sites may allow us to understand the location of the active site that is required for binding.

Research Support JDR