PET radioligands tend to be quite lipophilic (log P > 3). Relatedly, the radiometabolites of lipophilic drugs—like those typically used for PET imaging of brain—tend to be less lipophilic than their parent compounds and, thus, less likely to enter brain. However, if the radiometabolites of a PET radioligand are lipophilic enough to enter the brain, their contribution to the PET signal will preclude accurate quantitation using only parent radioligand as the input function. In addition, the concentration of parent radioligand typically decreases over time, while that of radiometabolites tends to increase. Thus, if radiometabolites enter the brain, they tend to contribute to an increasing percentage of total brain radioactivity over time. For this reason, one typical pattern that occurs for radiometabolites that enter the brain is that the kinetically determined value of brain binding (i.e., V
T) increases with PET scan duration and never reaches a stable value within the relatively short scan times of 1–2 h. Some radioligands may produce radiometabolite(s) that binds to the target receptor. For these radioligands, accurate quantitation requires more than one input function, i.e., parent and radiometabolite(s). Quantitation using only parent as the single input would cause greater errors at later time points because of increase in radiometabolites.
The effect of radiometabolites accumulating in brain can be observed with the greatest sensitivity when they represent a high percentage of total radioactivity in brain—that is, when parent radioligand represents a small percentage of brain radioactivity. Recent studies examining the four TSPO radioligands of interest presented two scenarios when radiometabolites were a relatively high percentage of brain radioactivity: (1) after administration of XBD173, which blocked uptake of parent radioligand; and (2) in LABs, where low uptake of parent radioligand was caused by the low affinity of TSPO in this genotype. In both of these conditions, we found that V
T increased with scan duration for three of the four ligands, but not for ER176 (Table 1 and Fig. 1). Combined with a moderate BP
ND of 1.4 in LABs, an important implication is that LABs do not need to be excluded from studies using ER176. It should be noted here that ER176 is still sensitive to genotype, and that V
T values must be corrected post hoc; however, LABs need not be excluded a priori.
Notably, radiometabolites are particularly problematic only for LABs, who have a small percentage of parent radioligand in brain. Radiometabolites are much less problematic for HABs and MABs. For example, the specific binding of PBR28—and particularly of DPA-713—is so high (Table 1) that radiometabolites do not preclude their accurate quantitation in HABs and MABs.
Time stability of V
T, i.e., stable V
T values with longer length of data, is an indirect method to assess accumulation of radiometabolites in brain. A direct method is sampling brain and performing ex vivo analysis in animals. However, even between human and non-human primate, the concentration of radiometabolites is usually markedly different, and TSPO density in brain is also about 20 times different [4, 6] although the structure of TSPO is expected to be similar across species because of high homology of the TSPO gene. Therefore, ex vivo experiments in animals including non-human primate are unlikely to provide good guidance to interpret human data. As a caveat, it should be noted that good time stability of V
T does not necessarily mean that radiometabolites are not present in brain. For example, if radiometabolites remain a constant percentage of brain radioactivity over time, V
T will be stable, though it will still be contaminated by these radiometabolites.