A graphical method to compare the in vivo binding potential of PET radioligands in the absence of a reference region: application to [(11)C]PBR28 and [(18)F]PBR111 for TSPO imaging

Positron emission tomography (PET) radioligands for a reversible central nervous system (CNS) demand a high specific to nonspecific signal characterized by the binding potential (BPND). The quantification of BPND requires the determination of the nondisplaceable binding usually derived from a reference region devoid of the target of interest. However, for many CNS targets, there is no valid reference region available. In such cases, the total volume of distribution (VT) is often used as the outcome measure, which includes both the specific and nonspecific binding signals. Here we present a graphical method that allows for direct comparison of the binding potential of ligands using the regional VT data alone via linear regression. The method was first validated using literature data for five serotonin transporter ligands, for which a reference region exists, and then applied to two second generation 18 kDa translocator protein radioligands, namely [(11)C]PBR28 and [(18)F]PBR111. The analysis determined that [(11)C]PBR28 had a higher BPND than [(18)F]PBR111.