Test-retest reproducibility of quantitative binding measures of [11C]Ro15-4513, a PET ligand for GABAA receptors containing alpha5 subunits

Introduction: Alteration of &ggr;‐aminobutyric acid “A” (GABAA) receptor‐mediated neurotransmission has been associated with various neurological and psychiatric disorders. [11C]Ro15‐4513 is a PET ligand with high affinity for &agr;5‐subunit‐containing GABAA receptors, which are highly expressed in limbic regions of the human brain (Sur et al., 1998). We quantified the test‐retest reproducibility of measures of [11C]Ro15‐4513 binding derived from six different quantification methods (12 variants). Methods: Five healthy males (median age 40 years, range 38–49 years) had a 90‐min PET scan on two occasions (median interval 12 days, range 11–30 days), after injection of a median dose of 441 MegaBequerels of [11C]Ro15‐4513. Metabolite‐corrected arterial plasma input functions (parent plasma input functions, ppIFs) were generated for all scans. We quantified regional binding using six methods (12 variants), some of which were region‐based (applied to the average time‐activity curve within a region) and others were voxel‐based: 1) Models requiring arterial ppIFs – regional reversible compartmental models with one and two tissue compartments (2kbv and 4kbv); 2) Regional and voxelwise Logan’s graphical analyses (Logan et al., 1990), which required arterial ppIFs; 3) Model‐free regional and voxelwise (exponential) spectral analyses (SA; (Cunningham and Jones, 1993)), which also required arterial ppIFs; 4) methods not requiring arterial ppIFs – voxelwise standardised uptake values (Kenney et al., 1941), and regional and voxelwise simplified reference tissue models (SRTM/SRTM2) using brainstem or alternatively cerebellum as pseudo‐reference regions (Lammertsma and Hume, 1996; Gunn et al., 1997). To compare the variants, we sampled the mean values of the outcome parameters within six bilateral, non‐reference grey matter regions‐of‐interest. Reliability was quantified in terms of median absolute percentage test‐retest differences (MA‐TDs; preferentially low) and between‐subject coefficient of variation (BS‐CV, preferentially high), both compounded by the intraclass correlation coefficient (ICC). These measures were compared between variants, with particular interest in the hippocampus. Results: Two of the six methods (5/12 variants) yielded reproducible data (i.e. MA‐TD <10%): regional SRTMs and voxelwise SRTM2s, both using either the brainstem or the cerebellum; and voxelwise SA. However, the SRTMs using the brainstem yielded a lower median BS‐CV (7% for regional, 7% voxelwise) than the other variants (8–11%), resulting in lower ICCs. The median ICCs across six regions were 0.89 (interquartile range 0.75–0.90) for voxelwise SA, 0.71 (0.64–0.84) for regional SRTM‐cerebellum and 0.83 (0.70–0.86) for voxelwise SRTM‐cerebellum. The ICCs for the hippocampus were 0.89 for voxelwise SA, 0.95 for regional SRTM‐cerebellum and 0.93 for voxelwise SRTM‐cerebellum. Conclusion: Quantification of [11C]Ro15‐4513 binding shows very good to excellent reproducibility with SRTM and with voxelwise SA which, however, requires an arterial ppIF. Quantification in the &agr;5 subunit‐rich hippocampus is particularly reliable. The very low expression of the &agr;5 in the cerebellum (Fritschy and Mohler, 1995; Veronese et al., 2016) and the substantial &agr;1 subunit density in this region may hamper the application of reference tissue methods.