Detection of pancreatic beta cell mass in vivo in humans: studies in individuals with long-standing type 1 diabetes and in individuals with obesity

BackgroundPET-CT scans of radioactive exendin-4, a ligand of the GLP-1 receptor, are claimed to provide a biomarker of pancreatic beta cell mass (BCM), although the GLP-1 receptor is expressed also in the exocrine pancreas (PX). Parotid glands may be a reference tissue for GLP-1 receptor expression in exocrine cells of the GI system. Our aims were 1. To assess biomarker(s) of BCM derived from 68Ga-NODAGA-exendin-4 PET-CT scans in participants with long-standing type 1 diabetes (T1DM) or in subjects with obesity (OBESE); 2. To investigate the relationship between biomarker(s) of BCM and a biomarker of beta cell functional mass (BCFxM) in OBESE. MethodsT1DM (n=8, Age: 50.4{+/-}3.8 yrs; T1DM duration: 34.2{+/-}3.0 yrs; BMI: 26.6{+/-}1.1 kg/m2; HbA1c: 7.5{+/-}0.36%) and OBESE (n=9; Age:48.2{+/-}2.2 yrs; BMI: 37.4{+/-}1.1 kg/m2; HbA1c: 5.4{+/-}0.17%) underwent two studies: 1) 68Ga-NODAGA-exendin-4 PET-CT scan of both PX and parotid glands 45-60 after i.v. injection and with CT-assessment of PX volume to compute biomarkers of BCM based on SUV (BCMSUV) or clearance (CLEAR; BCMCLEAR); 2) Mixed meal test (MMT), with measurement of plasma glucose, C-peptide, GLP-1 and GIP curves to assess BCFxM with state-of-art mathematical modeling. ResultsThe C-peptide response to the MMT in T1DM participants was absent or negligible, whereas the OBESE displayed a robust BCFxM. The PX volume was smaller in T1DM than in OBESE (51.7{+/-}6.6 vs 92.9{+/-}10.9 cc; p=0.007). The biomarkers of BCM, as assessed by 68Ga-NODAGA-exendin-4 SUV or CLEAR, were 6.6-fold (p=0.003) and 5.0-fold (p=0.002) lower, respectively, in T1DM than in OBESE. BCFxM was correlated in OBESE to both biomarkers of BCM (r=0.91 p<0.001, and r=0.82 p=0.006, respectively). Conclusion/interpretation68Ga-NODAGA-exendin-4 derived biomarkers of BCM can discriminate T1DM from OBESE. In OBESE 68Ga-NODAGA-exendin-4 derived BCM appears to be a pivotal determinant of the beta cell response to MMT and may be valuable to compare and monitor BCM both in research and in clinical settings. Research in contextO_ST_ABSWhat is already known about this subject?C_ST_ABSO_LIChanges in pancreatic beta cell functional mass are at the heart of alterations in glucose regulation, including diabetes mellitus. Beta cell functional mass can be assessed by mathematical modeling of the in vivo beta cell response to intravenous or oral challenges. C_LIO_LIBeta cell functional mass is the product of beta cell mass times beta cell function per mass unit, i.e. the result of two distinct entities, mass and function. No in vivo methods can dissect out beta cell mass and function. C_LIO_LIPancreatic 68Ga-exendin-4 uptake, as measured by PET-CT, has been proposed as a non-invasive biomarker of beta cell mass. However, the ratio of 3.6:1 between endocrine and exocrine pancreas 68Ga-exendin 4 uptake suggests that there is room for improvement. C_LI What are the key questions?O_LIDoes an improved 68Ga-exendin4 method provide a better separation between participants with type 1 diabetes and expected zero/nil beta cell mass vs people with nondiabetic obesity? C_LIO_LIWhat is the role of beta cell mass in determining beta cell functional mass in people living with obesity? C_LI What are the new findings?O_LIThe improvement in the quantitation of beta cell 68Ga-exendin-4 binding to beta cells resulted in a clearcut separation of participants with type 1 diabetes and expected zero/nil beta cell mass from people living with obesity C_LIO_LIIn people living with obesity, beta cell mass, as assessed by 68Ga-exendin-4 PET-CT scan, is a pivotal determinant of beta cell functional mass, as assessed by mathematical modeling of a frequently sampled mixed meal test C_LI How might this impact on clinical practice in the foreseeable future?O_LIThis method has the potential to track changes in beta cell mass both between-subjects and within-subjects over time C_LIO_LINatural history of glucose (in)tolerance and the impact of disease modifier candidates in diabetes mellitus can be assessed with the present method C_LI