In vivo variation in same-day estimates of metabotropic glutamate receptor subtype 5 binding using [ 11 C]ABP688 and [ 18 F]FPEB.

Positron emission tomography tracers [ ¹¹ C]ABP688 and [ ¹⁸ F]FPEB target the metabotropic glutamate receptor subtype 5 providing quantification of the brain glutamatergic system in vivo. Previous [ ¹¹ C]ABP688 positron emission tomography human test-retest studies indicate that, when performed on the same day, significant binding increases are observed; however, little deviation is reported when scans are >7 days apart. Due to the small cohorts examined previously (eight and five males, respectively), we aimed to replicate the same-day test-retest studies in a larger cohort including both males and females. Results confirmed large within-subject binding differences (ranging from -23% to 108%), suggesting that measurements are greatly affected by study design. We further investigated whether this phenomenon was specific to [ ¹¹ C]ABP688. Using [ ¹⁸ F]FPEB and methodology that accounts for residual radioactivity from the test scan, four subjects were scanned twice on the same day. In these subjects, binding estimates increased between 5% and 39% between scans. Consistent with [ ¹¹ C]ABP688, mean absolute test-retest variability was previously reported as <12% when scans were >21 days apart. This replication study and pilot extension to [ ¹⁸ F]FPEB suggest that observed within-day binding variation may be due to characteristics of mGluR5; for example, diurnal variation in mGluR5 may affect measurement of this receptor.