Alzheimer disease (AD) is associated with an increase in the brain of the 18-kDa translocator protein (TSPO), which is overexpressed in activated microglia and reactive astrocytes. Measuring the density of TSPO with PET typically requires absolute quantitation with arterial blood sampling, because a reference region devoid of TSPO does not exist in the brain. We sought to determine whether a simple ratio method could substitute for absolute quantitation of binding with ¹¹C-PBR28, a second-generation radioligand for TSPO. Methods: ¹¹C-PBR28 PET imaging was performed in 21 healthy controls, 11 individuals with mild cognitive impairment, and 25 AD patients. Group differences in ¹¹C-PBR28 binding were compared using 2 methods. The first was the gold standard method of calculating total distribution volume (V_T), using the 2-tissue-compartment model with the arterial input function, corrected for plasma-free fraction of radiotracer (f_P). The second method used a ratio of brain uptake in target regions to that in cerebellum—that is, standardized uptake value ratio (SUVR). Results: Using absolute quantitation, we confirmed that TSPO binding (V_T/f_P) was greater in AD patients than in healthy controls in expected temporoparietal regions and was not significantly different among the 3 groups in the cerebellum. When the cerebellum was used as a pseudo-reference region, the SUVR method detected greater binding in AD patients than controls in the same regions as absolute quantification and in 1 additional region, suggesting SUVR may have greater sensitivity. Coefficients of variation of SUVR measurements were about two-thirds lower than those of absolute quantification, and the resulting statistical significance was much higher for SUVR when comparing AD and healthy controls (e.g., P < 0.0005 for SUVR vs. P = 0.023 for V_T/f_P in combined middle and inferior temporal cortex). Conclusion: To measure TSPO density in AD patients and control subjects, a simple ratio method SUVR can substitute for, and may even be more sensitive than, absolute quantitation. The SUVR method is expected to improve subject tolerability by allowing shorter scanning time and not requiring arterial catheterization. In addition, this ratio method allows smaller sample sizes for comparable statistical significance because of the relatively low variability of the ratio values.

For PET imaging of 18-kDa translocator protein (TSPO), a biomarker of neuroinflammation, most second-generation radioligands are sensitive to the single nucleotide polymorphism rs6971; however, this is probably not the case for the prototypical agent ¹¹C-PK11195 (¹¹C-labeled N-butan-2-yl-1-(2-chlorophenyl)-N-methylisoquinoline-3-carboxamide), which has a relatively lower signal-to-noise ratio. We recently found that ¹¹C-ER176 (¹¹C-(R)-N-sec-butyl-4-(2-chlorophenyl)-N-methylquinazoline-2-carboxamide), a new analog of ¹¹C-(R)-PK11195, showed little sensitivity to rs6971 when tested in vitro and had high specific binding in monkey brain. This study sought, first, to determine whether the sensitivity of ¹¹C-ER176 in humans is similar to the low sensitivity measured in vitro and, second, to measure the nondisplaceable binding potential (BP_ND, or the ratio of specific-to-nondisplaceable uptake) of ¹¹C-ER176 in human brain. Methods: Nine healthy volunteers—3 high-affinity binders (HABs), 3 mixed-affinity binders (MABs), and 3 low-affinity binders (LABs)—were studied with whole-body ¹¹C-ER176 PET imaging. SUVs from 60 to 120 min after injection derived from each organ were compared between genotypes. Eight separate healthy volunteers—3 HABs, 3 MABs, and 2 LABs—underwent brain PET imaging. The 3 HABs underwent a repeated brain scan after TSPO blockade with XBD173 (N-benzyl-N-ethyl-2-(7-methyl-8-oxo-2-phenylpurin-9-yl)acetamide) to determine nondisplaceable distribution volume (V_ND) via Lassen occupancy plotting and thereby estimate BP_ND in brain. Results: Regional SUV averaged from 60 to 120 min after injection in brain and peripheral organs with high TSPO densities such as lung and spleen were greater in HABs than in LABs. On the basis of V_ND determined via the occupancy plot, the whole-brain BP_ND for LABs was estimated to be 1.4 ± 0.8, which was much lower than that for HABs (4.2 ± 1.3) but about the same as that for HABs with ¹¹C-PBR28 ([methyl-¹¹C]N-acetyl-N-(2-methoxybenzyl)-2-phenoxy-5-pyridinamine)) (∼1.2). Conclusion: Obvious in vivo sensitivity to rs6971 was observed in ¹¹C-ER176 that had not been expected from in vitro studies, suggesting that the future development of any improved radioligand for TSPO should consider the possibility that in vitro properties will not be reflected in vivo. We also found that ¹¹C-ER176 has adequately high BP_ND for all rs6971 genotypes. Thus, the new radioligand would likely have greater sensitivity in detecting abnormalities in patients.

Objective: To investigate cerebral oxidative stress based on an over-reductive state caused by mitochondrial dysfunction and its relationship to disease severity in patients with amyotrophic lateral sclerosis (ALS) using PET with [⁶²Cu]diacetyl-bis(N⁴-methylthiosemicarbazone) (⁶²Cu-ATSM). Methods: Twelve patients with ALS and 9 age-matched healthy controls underwent a 20-minute dynamic brain PET scan after ⁶²Cu-ATSM injection. The standardized uptake value (SUV) images obtained from the last 10 minutes of frames were normalized by the global mean (nSUV). Regional ⁶²Cu-ATSM retention in the nSUV images was compared between groups using statistical parametric mapping (SPM) and region of interest (ROI) analysis. Secondary analyses evaluated the correlations between regional nSUVs and the clinical characteristics of the participants. Results: In SPM mapping, patients with ALS showed a significantly greater accumulation of ⁶²Cu-ATSM compared to controls in the bilateral cortices around the central sulcus, including the motor cortex, and the right superior parietal lobule. ROI analysis also revealed significantly greater nSUVs in patients than controls in these regions. Increases in nSUV for these regions were associated with decreases in the revised ALS Functional Rating Scale score, suggesting a good correlation with the severity of ALS. In controls, age was correlated with nSUV for the bilateral cortices around the central sulcus, although this correlation was not observed in patients with ALS. Conclusions: ⁶²Cu-ATSM PET imaging demonstrated increased oxidative stress based on an over-reductive state, primarily in the motor cortex, in patients with ALS. The magnitude of oxidative stress correlated well with clinical severity, indicating that it may be associated with neurodegenerative changes in ALS.

Neurofibrillary tangles, which consist of highly phosphorylated tau protein, and senile plaques (SPs) are pathological hallmarks of Alzheimer’s disease (AD). In swollen axons, many autophagic vacuoles are observed around SP in the AD brain. This suggests that autophagy function is disturbed in AD. We used a neuronal cellular model of tauopathy (M1C cells), which harbors wild type tau (4R0N), to assess the effects of the lysosomotrophic agent NH4Cl, and autophagy inhibitors chloroquine and 3 methyladenine (3MA). It was found that chloroquine, NH4Cl and 3MA markedly increased tau accumulation. Thus, autophagy lysosomal system disturbances disturbed the degradation mechanisms of tau protein. Other studies also revealed that tau protein, including aggregated tau, is degraded via the autophagy lysosome system. Phosphorylated and C terminal truncated tau were also reported to disturb autophagy function. As a therapeutic strategy, autophagy upregulation was suggested. Thus far, as autophagy modulators, rapamycin, mTOCR1 inhibitor and its analogues, lithium, metformin, clonidine, curcumin, nicotinamide, bexaroten, and torehalose have been proposed. As a therapeutic strategy, autophagic modulation may be the next target of AD therapeutics.

Translocator protein (TSPO) is a biomarker for detecting neuroinflammation by PET. ¹¹C-(R)-PK11195 has been used to image TSPO since the 1980s. Here, we compared the utility of four ¹¹C-labeled ligands—(R)-PK11195, PBR28, DPA-713, and ER176—to quantify TSPO in healthy humans. For all of these ligands, BP _ND (specific-to-non-displaceable ratio of distribution volumes) was measured by partially blocking specific binding with XNBD173 administration. In high-affinity binders, DPA-713 showed the highest BP _ND of 7.3 followed by ER176 (4.2), PBR28 (1.2), and PK11195 (0.8). Only ER176 allows the inclusion of low-affinity binders because of little influence of radiometabolites and high BP _ND. If inclusion of all three genotypes is important for study logistics, ER176 is the best of these four radioligands for studying neuroinflammation.