The Section on PET Neuroimaging Sciences develops and uses positron emission tomographic (PET) radioligands to study pathophysiology in several neuropsychiatric disorders. We use in vivo imaging to evaluate novel PET radioligands, first in animals, then in healthy human subjects, and finally in patients. Our current research focuses on two broad areas of study: neuroinflammation and cyclic adenosine monophosphate (cAMP) signaling.
We first sought to develop a radioligand capable of imaging translocator protein (TSPO), a biomarker of neuroinflammation. Among numerous analogs synthesized and evaluated in vitro for TSPO, [11C]PBR28 was selected to image the neuroinflammatory response, first in rat brain then in monkey brain. Imaging in humans showed the widespread but low density of this target in healthy subjects. Having confirmed the sensitivity of [11C]PBR28, we used it to explore the inflammatory pathology associated with a number of neuropsychiatric disorders, including Alzheimer’s disease (AD) and major depressive disorder (MDD).
With regard to AD, studies from our laboratory found that this marker of neuroinflammation was increased in patients with AD compared to subjects with mild cognitive impairment (MCI, a precursor syndrome) and to control subjects (Kreisl et al., 2013). Furthermore—and unlike amyloid—the amount of inflammation, indirectly measured as TSPO binding, was correlated with the severity of cognitive impairment. The results of this between-group comparison were confirmed in a larger group of subjects and, more importantly, were extended with repeated PET scans in patients over a two- to three-year follow-up period (Kreisl et al., 2016). Our laboratory further found that TSPO binding increased with progression of AD but not in healthy aging. The increased TSPO binding was significantly correlated with increased cognitive impairment during the follow-up period. Taken together, these results in AD suggest that neuroinflammation is a biomarker of disease severity (based on the cross-sectional study) as well as a biomarker of disease progression (based on the longitudinal study).
We also recently used this TSPO tracer, [11C]PBR28, to establish that TSPO binding is increased in multiple brain regions in unmedicated subjects with MDD currently experiencing a major depressive episode. We subsequently replicated this finding in a different population and also investigated the effects of antidepressants on TSPO binding in MDD subjects. 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 suggested 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.
Building on this work, we recently established that 11C-ER176, a radioligand for TSPO, can image all three afﬁnity genotypes in human brain (Ikawa et al., 2017). PET imaging of TSPO, though promising, has been complicated by the fact that most second-generation radioligands are sensitive to the single nucleotide polymorphism rs6971 (Kreisl et al., 2013), 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 speciﬁc binding in monkey brain. We extended this work to determine whether the sensitivity of 11C-ER176 in humans was similar to the low sensitivity measured in vitro as well as to measure the nondisplaceable binding potential (BPND, or the ratio of speciﬁc-to-nondisplaceable uptake) of 11C-ER176 in human brain. We found that 11C-ER176 had obvious in vivo sensitivity to rs6971 that had not been expected from the in vitro studies with this agent. This suggested 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 had adequately high BPND for all rs6971 genotypes. Thus, this new radioligand is likely to have greater sensitivity for detecting abnormalities in patients.
Relatedly, the cyclooxygenase (COX) system is implicated in the pathophysiology of brain diseases, including AD and MDD, and is another potential biomarker for neuroinflammation. The COX system comprises two isoforms, COX-1 and COX-2, which are key enzymes in neuroinflammation. We recently developed two PET radioligands: 11C-PS13 for COX-1 and 11C-MC1 for COX-2. Using these new radioligands, a recent study from our laboratory explored the in vivo expression of COX-1 and COX-2 in rhesus monkeys. We found that COX-1 was constitutively expressed in major organs while COX-2 was only induced by inflammation. Different patterns of expression between the two COX isoforms were successfully demonstrated by 11C-PS13 and 11C-MC1, suggesting that these two radioligands are promising biomarkers for measuring inflammation in various disorders, as well as target engagement of therapeutic drugs.
Phosphodiesterase-4 (PDE4) is the predominant enzyme in brain that metabolizes the second messenger cAMP. As a result, it has been a target of drug development, especially for MDD and cognitive improvement. The cAMP theory of depression posits that depression is caused by low cAMP signaling, and the corresponding theory regarding the relevant mechanism of treatment is that chronic, but not acute, administration of antidepressants upregulates cAMP signaling. According to this theory, depression might be treated with a PDE4 inhibitor, a strategy that has been tried with limited success because of side effects of nausea and vomiting. The identification of four subtypes of PDE4 (4A, 4B, 4C, and 4D), and especially the discovery of subtype selective inhibitors, has invigorated the field, both to examine distinct roles for these subtypes and the therapeutic potential of subtype-selective inhibitors. We first confirmed in animals that increased binding of [11C](R)-rolipram, a reversible PDE4 inhibitor, reflects the phosphorylated/active state of PDE4. Using PET imaging with [11C](R)-rolipram, studies from our laboratory found that cAMP signaling was decreased in patients experiencing a major depressive episode (Fujita et al., 2012). A follow-up study confirmed that PDE4 levels were decreased in unmedicated individuals with MDD in vivo, but increased after two months of treatment with a selective serotonin reuptake inhibitor (SSRI), consistent with the cAMP theories of depression and of antidepressant action (Fujita et al., 2017). Rolipram binding did not correlate with severity of baseline symptoms, and increased rolipram binding during treatment did not correlate with symptom improvement. These results suggest that PDE4 inhibitors, which increase cAMP cascade activity, may have antidepressant effects.
We also investigated the links between PDE4 and the protein Disrupted in Schizophrenia 1 (DISC1). In PET studies, [11C]-(R)-rolipram was used to visualize the activation of PDE4 by protein kinase A (PKA). Because DISC1 is hypothesized to inhibit PDE4 activation, the absence of DISC1 would be expected to increase PDE4 activity. A recent study from our group used a Disc1 locus impairment (LI) mouse model to investigate whether [11C]-(R)-rolipram PET imaging could quantify the interaction between PDE4 and DISC1. We observed significant group differences in PDE4 enzyme activity; the activity levels of LI mice were greater than those of heterozygotes which, in turn, were greater than the activity levels of wild-type mice. Our findings demonstrate a clear increase in PDE4 activity in the absence of most critical Disc1 protein isoforms both in vivo and ex vivo. In addition, the study demonstrated that [11C]-(R)-rolipram PET imaging was sensitive enough to assess differences in PDE4 activity caused by the interaction between PDE4 and DISC1, creating avenues for future research in this area.