PET imaging of neurokinin-1 receptors with [(18)F]SPA-RQ in human subjects: assessment of reference tissue models and their test-retest reproducibility.

[(18)F]SPA-RQ (substance P antagonist receptor quantifier) labels the substance P-preferring (NK(1)) receptor in human brain. A prior study showed that [(18)F]SPA-RQ brain uptake can be quantified with a reference tissue method and thereby avoid invasive blood sampling. The purposes of this study were to compare three different reference tissue methods and to assess test-retest reproducibility. Eight healthy subjects underwent two [(18)F]SPA-RQ scans. We calculated the binding potential (BP), which is proportional to receptor density, from both regional volume of interest and voxel-wise data. We compared three reference tissue methods: simplified reference tissue model, multilinear reference tissue model (MRTM), and its two-parameter version (MRTM2). The three methods generated equivalent values of regional BP, but MRTM2 was the most resistant to noise. Temporally stable values of BP were obtained with 240 min of imaging data. MRTM2 had excellent test-retest reproducibility, with high reliability (intraclass correlation > 0.9) and low variability (< 10%). In addition to regional volume of interest analysis, we also created parametric images of BP, variability, and reliability based on voxel-wise time-activity data. The reproducibility of parametric BP was also good, with variability < 20% and reliability > 0.7 in gray matter regions. In conclusion, a two-parameter reference tissue method (MRTM2) provided reproducible and reliable measurements of [(18)F]SPA-RQ brain uptake using 240 min of both regional and voxel-wise data.

Specific in vitro binding of (S,S)-[3H]MeNER to norepinephrine transporters.

The aim of this study was to determine the selectivity of (S,S)-2-(alpha-(2-methoxyphenoxy)benzyl)morpholine (MeNER) binding to norepinephrine transporters (NET). Quantitative autoradiography studies of NET binding were performed in brains of wildtype mice and those of mutant mice lacking one or two alleles of the NET gene. [3H]MeNER binding in the wildtype mouse brains was consistent with previously reported distributions of NET. Highest levels were found in the locus coeruleus, thalamus, hypothalamus, and bed nucleus of stria terminalis. Specific binding in these regions was approximately 50% in the heterozygous NET mice and negligible in the NET knockout mice. Binding in the wildtype mouse brains was displaced by the NET ligand, nisoxetine, but not by the serotonin or dopamine transporter blockers, citalopram or GBR 12935. [3H]MeNER displayed much higher affinity for NET than for SERT or DAT in homogenate binding studies. Each of these features supports the binding specificity of this candidate in vivo NET ligand.