Examples of progress made during the prior year are summarized below. 1. Building a database for brain 18kDa translocator protein (TSPO) imaged using 11CPBR28 in healthy subjects Translocator protein 18 kDa (TSPO) has been widely imaged as a marker of neuroinflammation using several radioligands, including 11CPBR28. In order to study the effects of potential contributors to neuroinflammation (i.e., age, sex, obesity) and to determine whether 11CPBR28 binding can be accurately measured with fewer radio high-performance liquid chromatography (radioHPLC) measurements of arterial samples, we created an 11CPBR28 database of healthy subjects. 11CPBR28 scans from 48 healthy subjects (23 high-affinity binders (HABs) and 25 mixed-affinity binders (MABs), 20F/28M, age: 40.616.8 years) were analyzed using Logan plot with metabolite-corrected arterial input obtained from 23 arterial samples. Total distribution volume (VT) was found to be 1.2-fold higher in HABs across all brain regions. The polymorphism plot estimated nondisplaceable uptake (VND) as 1.40 mL cm-3, which generated a specific-to-nondisplaceable ratio (BPND) of 1.60.6 in HABs and 1.10.6 in MABs. VT increased significantly with age in nearly all regions, but did not correlate with body mass index and was not affected by sex. VT was well estimated with radioHPLC measurements from six arterial samples. These results underscore which patient characteristics should be accounted for during 11CPBR28 studies and suggest ways to perform such studies more easily and with fewer blood samples. This work was performed under clinical protocols NCT01547780, NCT00526916, NCT01322555, NCT01851356, NCT02233868 and NCT00613119. 2. Replication of the finding that PET radioligand binding to TSPO, a putative biomarker of inflammation, is increased during major depressive episodes Translocator protein 18 kDa (TSPO), a potential biomarker of neuroinflammation, can be accurately quantified using positron emission tomography (PET) and 11CPBR28, a TSPO tracer. A recent study found increased TSPO binding in multiple brain regions in unmedicated subjects with major depressive disorder (MDD) currently experiencing a major depressive episode (MDE). Building on this work, we sought to: 1) replicate these findings using a different patient population and a different tracer; 2) investigate the effects of antidepressants on TSPO binding in these subjects; and 3) determine the relationship of peripheral and central inflammatory markers to TSPO binding. Towards this end, unmedicated MDD subjects (n = 12), medicated MDD subjects (n = 16), and healthy controls (n = 20) underwent PET imaging using 11CPBR28. Total distribution volume (VT) was measured using arterial input function and corrected for TSPO genotype. The subgenual prefrontal cortex and anterior cingulate cortex were chosen as regions of interest. Peripheral blood samples and cerebrospinal fluid were analyzed to investigate the relationship between peripheral and central inflammatory markers and TSPO binding. We found that TSPO binding was significantly higher in MDD versus healthy control subjects in the subgenual prefrontal cortex as well as the anterior cingulate cortex. Unmedicated MDD subjects had the highest level of TSPO binding, followed by medicated MDD subjects and healthy controls. TSPO binding correlated with interleukin-5 (IL-5) in cerebrospinal fluid, but with no other peripheral or central inflammatory markers. Taken together, our results replicated previous findings showing increased TSPO binding in the brain of MDD subjects experiencing an MDE, suggesting that inflammation is a key target for developing novel therapeutics to treat MDD. The findings are particularly important because they may help further elucidate pathways involved in the development of MDD as well as identify potential novel treatments and pharmacological targets. This work was performed under clinical protocol NCT01851356. 3. Cellular localization of the inflammatory biomarkers COX-1, COX-2, and TSPO in brain tissue samples from epileptic subjects. It is presently unclear what type of role neuroinflammation plays in mesial temporal lobe epilepsy. However, animal models of epilepsy have shown increases in the inflammatory enzymes cyclooxygenase (COX)-1, and -2. COX-1 and -2 work by converting arachidonic acid into prostanoids. Which prostanoids are produced depend on the type of cell in which this pathway is expressed. Therefore, knowing the cellular expression of COX-1 and COX-2 is important for understanding their function. Only two studies have investigated the expression of COX-2 in epilepsy, but there are no reports on COX-1. To investigate this issue, we sought to measure the cellular expression of COX-1, COX-2, and 18kDa translocator protein (TSPO) in brain tissue samples from 33 people with drug-resistant mesial temporal lobe epilepsy. TSPO, a biomarker of inflammation, was included as a positive control. We found that COX-1 is expressed in microglia, and COX-2 and TSPO are expressed in microglia and neurons. In-situ hybridization confirmed a higher expression of all three proteins in microglia than in astrocytes. The brain tissue could be classified into two groups based on histopathological assessment: sclerotic and non-sclerotic. No differences were found in the expression of any protein between the two groups. To the best of our knowledge, ours is the first study to measure the cellular expression of COX-1, COX-2, and TSPO amongst microglia, astrocytes, and neurons in surgical brain tissue samples drug-resistant mesial temporal lobe epilepsy. Further research is needed to determine the effects of the COX inflammatory pathway in epilepsy. 4. 11C-ER176, a radioligand for TSPO, can image all three afnity genotypes in human brain 18-kDa translocator protein (TSPO) is a key biomarker of neuroinammation. However, PET imaging of TSPO is complicated by the fact that most second-generation radioligands are sensitive to the single nucleotide polymorphism rs6971, though this is probably not the case for the prototypical agent 11C-PK11195. We recently found that 11C-ER176, a new analog of 11C-(R)-PK11195, showed little sensitivity to rs6971 when tested in vitro and had high specic binding in monkey brain. Building on this work, we sought to 1) determine whether the sensitivity of 11C-ER176 in humans was similar to the low sensitivity measured in vitro, and 2) measure the nondisplaceable binding potential (BPND, or the ratio of specic-to-nondisplaceable uptake) of 11C-ER176 in human brain. Nine healthy volunteers3 high-afnity binders (HABs), 3 mixed-afnity binders (MABs), and 3 low-afnity binders (LABs)were studied with whole-body 11C-ER176 PET imaging. Eight separate healthy volunteers3 HABs, 3 MABs, and 2 LABsunderwent brain PET imaging. The 3 HABs underwent a repeat brain scan after TSPO blockade with XBD173. We found that regional standardized uptake values (SUVs) averaged from 60 to 120 minutes after injection in brain and peripheral organs with high TSPO densities such as lung and spleen were greater in HABs than in LABs. The whole-brain BPND for LABs was much lower than that for HABs but about the same as that for HABs with 11C-PBR28. Taken together, the results show that 11C-ER176 has obvious in vivo sensitivity to rs6971 that had not been expected from the in vitro studies with this agent. This suggests that the future development of any improved radioligand for TSPO should consider the possibility that in vitro properties will not be reected in vivo. We also found that 11C-ER176 has adequately high BPND for all rs6971 genotypes. Thus, this new radioligand is likely to have greater sensitivity in detecting abnormalities in patients. This work was performed under clinical protocols NCT02147392 and NCT02181582.
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