Mixed Affinity Binding in Humans with 18 kDa Translocator Protein (TSPO) Ligands

[ 11 C]PBR28-PET can detect the 18kDa Translocator Protein (TSPO) expressed within macrophages. However, quantitative evaluation of the signal in brain tissue from donors with multiple sclerosis (MS) shows that PBR28 binds the TSPO with high ( K i ~4nM), low ( K i ~200nM) or mixed affinity (two sites with K i ~4nM and ~300nM). Our study tested whether similar binding behaviour could be detected 1) in brain tissue from donors with no history of neurological disease, 2) with TSPO-binding PET ligands other than [ 11 C]PBR28, 3) for TSPO present in peripheral blood and 4) with human brain PET data acquired in vivo with [ 11 C]PBR28. Methods The affinity of TSPO ligands was measured in human brain post mortem from donors with a history of MS (n=13), donors without any history of neurological disease (n=20) and in platelets from healthy volunteers (n=13). Binding potential estimates from 35 [ 11 C]PBR28 PET scans from an independent sample of healthy volunteers were analysed using a Gaussian mixture model. Results Three binding affinity patterns were found in brains from subjects without neurological disease in similar proportions to those reported previously from studies of MS brains. TSPO ligands showed substantial differences in affinity between subjects classified as high (HAB) and low affinity binders (LAB): differences in affinity between HABs and LABs are ~50-fold with PBR28, ~17-fold with PBR06, and ~4-fold with DAA1106, DPA713 and PBR111. Where differences in affinity between HABs and LABs were low (~4 fold), distinct affinities were not resolvable in binding curves for mixed affinity binders (MABs), who appeared to express one class of sites with an affinity approximately equal to the mean of those for HABs and LABs. Mixed affinity binding was detected in platelets from an independent sample (HAB 69%, MAB 31%), although LABs were not detected. Analysis of [ 11 C]PBR28 PET data was not inconsistent with the existence of distinct sub-populations of HABs, MABs and LABs. application recognise high, low and mixed-affinity binders in brain tissue in vitro. Knowledge of subjects’ binding patterns will be required to accurately quantify TSPO expression in vivo using PET.