An 18-kDa translocator protein (TSPO) polymorphism explains differences in binding affinity of the PET radioligand PBR28

Abstract

[(11)C]PBR28 binds the 18-kDa Translocator Protein (TSPO) and is used in positron emission tomography (PET) to detect microglial activation. However, quantitative interpretations of signal are confounded by large interindividual variability in binding affinity, which displays a trimodal distribution compatible with a codominant genetic trait. Here, we tested directly for an underlying genetic mechanism to explain this. Binding affinity of PBR28 was measured in platelets isolated from 41 human subjects and tested for association with polymorphisms in TSPO and genes encoding other proteins in the TSPO complex. Complete agreement was observed between the TSPO Ala147Thr genotype and PBR28 binding affinity phenotype (P value=3.1 × 10(-13)). The TSPO Ala147Thr polymorphism predicts PBR28 binding affinity in human platelets. As all second-generation TSPO PET radioligands tested hitherto display a trimodal distribution in binding affinity analogous to PBR28, testing for this polymorphism may allow quantitative interpretation of TSPO PET studies with these radioligands.

Figure 1

(A) Competition binding assay using unlabelled PBR28 to displace [³H]PK11195 in human platelets isolated from whole blood (n=41). The dashed vertical line indicates the concentration of PBR28 used to generate panel B. The fractional binding is described by the following equations, One site model Two site model where B, binding signal; NS, nonspecific binding; K_i, binding affinity; and f_H, the fraction of high-affinity binding sites. (B) Box-whisker plot of the residual [³H]PK11195 binding in the presence of 100 nmol/L unlabelled PBR28 (expressed as a percentage of the total [³H]PK11195 binding in the absence of PBR28) stratified on rs6971 genotype. Percentage residual of total binding is plotted as blue diamonds for each individual. HAB, high affinity binder; LAB, low affinity binder; MAB, mixed affinity binder.