Current Research
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 participants, and finally in patients. Our current research focuses on two broad areas of study: neuroinflammation and cyclic adenosine monophosphate (cAMP) signaling.
Recent Publications
Summary
Overview
PET has unsurpassed sensitivity to localize and quantify specific proteins in the living brain that are hypothesized to underlie the pathophysiology of mental illnesses. Although PET has promising performance characteristics for research, the global results to date have had limited impact on our knowledge of the pathophysiology of mental disorders, and almost no impact on their treatment. Reasons for this limited impact include a relative paucity of radioligands compared to the large number of potential protein targets in brain, as well as a dearth of research trying to link PET to early treatment studies of psychiatric illnesses. Thus, three urgent needs exist as regards such research: 1) to develop new PET radioligands that can be used to determine molecular abnormalities underlying a variety of mental disorders; 2) to assess the practical utility of these radioligands in patient populations; and 3) to meaningfully use these radioligands in experimental medicine studies—e.g., proof-of-concept and proof-of-mechanism. In general, my laboratory has focused on developing radioligands and applying these radioligands in patient populations. In recent years, however, we have also sought to link newly developed radioligands to early therapeutic trials of experimental medicines. Towards this end, the two major targets currently being investigated in my laboratory are neuroinflammation and cyclic adenosine monophosphate (cAMP) signaling, indirectly assessed by imaging phosphodiesterase-4 (PDE4).
Project 1: Neuroinflammation and TSPO
Translocator protein (TSPO) is a mitochondrial protein that is highly expressed in activated microglia and reactive astrocytes. As such, TSPO is a putative biomarker of neuroinflammation. We developed a second generation TSPO radioligand, [11C]PBR28, which is now used worldwide at several PET centers to explore neuroinflammation. Using this radioligand, we found elevated TSPO density in unmedicated patients with major depressive disorder (MDD) and in patients with Alzheimer’s disease (AD). These findings have broad applications for precision medicine in neuropsychiatric disorders. For example, six of seven recent studies of MDD found statistically significant increases of PET TSPO binding in brain (Meyer et al, 2020 ), suggesting that at least a subset of patients with MDD could be treated with anti-inflammatory medication. Furthermore, in healthy volunteers, we are using our recently developed third-generation TSPO radioligand, [11C]ER176, synaptic density using [11C]SV2A, and amyloid accumulation using [18F]florbetaben to study the effects of week-long sleep restriction on neuroinflammation.
Project 2: Neuroinflammation and COX
Building on our work with TSPO, and to make the connection to experimental medicine, we developed radioligands for cyclooxygenase-1 (COX-1) and COX-2, which are themselves targets of non-steroidal anti-inflammatory drugs (NSAIDs). These radioligands—[11C]PS13 for COX-1 and [11C]MC1 for COX-2—are potent and selective for each isoform and have shown promising results in monkeys and humans. We found that COX-1 is expressed constitutively (i.e., under healthy conditions) in several organs, including brain, spleen, kidney, and GI tract. The binding of the COX-1 radioligand was selective for its targeted isozyme, as it could be blocked by COX-1-selective inhibitors in both monkeys and humans, but not by COX-2-selective inhibitors. To evaluate [11C]MC1, we first had to upregulate COX-2 in monkey brain using intracerebral injection of the inflammogen lipopolysaccharide (LPS). Under baseline/healthy conditions, [11C]MC1 detected no COX-2 in brain. However, after LPS injection, [11C]MC1 clearly visualized areas of elevated COX-2 and showed appropriate subtype selectivity. Our interpretation was that [11C]MC1 lacked the sensitivity to measure the low baseline concentrations of COX-2, but could detect the enzyme after upregulation, which can be a 10- to 20-fold effect. To determine whether [11C]MC1 can detect COX-2 in human participants, we studied individuals with rheumatoid arthritis (RA), as COX-2 is upregulated in the affected joints. In the four participants studied to date, symptomatic joints had increased uptake of [11C]MC1, which was blocked by celecoxib. These preliminary results confirmed that [11C]MC1 is capable of imaging COX-2 in humans if COX-2 concentrations are elevated by peripheral inflammation. Moving forward, it appears that COX-1 may be a biomarker of microgliosis (i.e., increased density of microglia), and COX-2 may be a dynamic biomarker of inflammation and of the effects of anti-inflammatory drugs. We are now examining these biomarkers in several mental disorders, including MDD and AD, as well as in multiple sclerosis (MS) and RA.
Project 3: Phosphodiesterase-4 (PDE4)
PDE4 metabolizes and thereby terminates the actions of cAMP. Rolipram is a reversible PDE4 inhibitor, and binding of [11C](R)-rolipram provides a measure of the activity of this enzyme in brain. Consistent with the ‘cAMP theory of depression’, we found that [11C](R)-rolipram binding is decreased in unmedicated patients with major depressive disorder (MDD) and increased (normalized) after two months of treatment with a selective serotonin reuptake inhibitor (SSRI). Rolipram was developed by Pfizer in the 1990s as a potential antidepressant and was shown to have some preliminary antidepressant effects, but studies were terminated because of nausea and vomiting. It was subsequently learned that PDE4 has four subtypes (PDE4B and PDE4D are most prevalent in brain), and that rolipram inhibits all four. Pharmaceutical companies sought to develop a subtype-selective PDE4 inhibitor, hoping that it would have antidepressant efficacy but lack side effects. We successfully collaborated with Mark Gurney (Tetra Discovery Partners) to develop a radioligand selective for PDE4D and with Pfizer/Cerevel to develop the first PET radioligand selective for PDE4B. We recently began human studies to image individuals with MDD experiencing a major depressive episode with our recently developed radioligand for PDE4B to determine whether that subtype is decreased; this builds on prior work from our laboratory that used [11C](R)-rolipram to demonstrate decreased binding to all PDE4 subtypes. Future studies will examine whether MDD participants with low PDE4B binding preferentially respond to a PDE4B inhibitor. We hope that PDE4B PET imaging may be able to identify those MDD patients most likely to respond to future treatment with a PDE4B inhibitor.
Project 4: PET Reporter Probe Systems
In collaboration with Barry Richmond (NIMH) and Michael Michaelides (NIDA), we are developing radioligands to localize and quantify transfected genes activated by exogenously administered drugs in two systems: Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) and Pharmacologically Selective Actuator Modules (PSAMs). Monitoring the survival and expression of the transfected gene is critically important for current studies in animals and in any future application of this gene therapy in individuals with mental disorders
Project 5: OpenNeuroPET
Dr. Innis is the PI on a BRAIN grant (2021–2026) to establish an archive for primary PET data so they can be meaningfully shared and combined. The proposal has three specific aims: 1) to establish the OpenNeuroPET archive; 2) to support the use and adoption of the OpenNeuroPET archive by reaching out to the community, maintaining a helpdesk, and ensuring compliance with international regulations; and 3) to establish and provide human brain atlases of molecular imaging targets that provide their mean three-dimensional distributions in brain in aggregated samples of people. To accomplish these goals, we have established a collaboration between four institutions: the NIMH Intramural Research Program, Copenhagen University Hospital, Massachusetts General Hospital (MGH), and Stanford University. NIMH and Copenhagen University will work closely together on the first two specific aims; MGH will develop the molecular imaging brain atlases; and Stanford will provide expert guidance for software development.
Significance
The significance of this work can be assessed relative to its intrinsic scientific impact as well as its overall impact on the field of molecular imaging. With regard to intrinsic scientific impact, our research is widely respected as some of the most rigorous and valuable investigation into new areas of general importance to the field. As an example, imaging of TSPO with the prototypical radioligand [11C]PK11195 was flawed by poorly reproducible results due to a radioligand that had low specific signal. Our discovery and development of the second-generation radioligand [11C]PBR28, which has a much higher specific signal (Kobayashi et al., 2018 ), helped reinvigorate the imaging of neuroinflammation; the radioligand is now used at many PET centers throughout the world. Building on that work, our laboratory also developed [11C]-ER176, which is potentially even more useful to the field because of its even higher percentage-specific signal and its ability to measure TSPO in homozygous low-affinity binders (Ikawa et al., 2017 ).
Beyond its intrinsic scientific merit, this research has advanced the field of molecular imaging in several important ways.
- Developing radioligands. The crux of our work is to develop novel and truly useful radioligands with broad scientific utility. Towards this end, we have developed novel radioligands for more than 15 targets, and these are now used at centers throughout the world; these novel ligands are either measurably better than ligands previously in use or, often, are the first to successfully image a particular target. These targets include: TSPO, PDE4D, PDE4B, COX-1, COX-2, metabotropic glutamate receptor 1 (mGluR1), mGluR5, cannabinoid receptor type 1 (CB1), serotonin 1A receptor (5-HT1A), the nociceptin-orphanin peptide (NOP) receptor, OGlcNAcase (an enzyme involved in tau protein clearance from brain), and NR2B, among others.
- Sharing expertise and information. The Molecular Imaging Branch has provided guidance to numerous PET centers and makes critical information regarding the synthesis of radioligands publicly available. By the time the first human results are published, Drs. Innis and Pike post the complete Investigational New Drug application, which includes detailed information on synthesis and quality control; the animal toxicology data; the human protocol; the FDA review; our response to the review; and detailed instructions for synthesis and quality control (Chemistry, Manufacturing, and Controls).
- Senior advisor and leader. Dr. Innis is a highly regarded leader in the international field of brain PET imaging and is often consulted by researchers throughout the world for advice on radioligands, experimental and analytical methods, and regulatory issues. Perhaps his most important contributions in this area during the past four years have been establishing consensus on the format of content for PET data and spearheading the proposed OpenNeuroPET archive. His efforts in this area continue to give significant structure to this growing field, vastly improving the sharing, reproducibility, and overall usefulness of results worldwide.
- Training. Dr. Innis has trained more than 60 graduate students and postdoctoral fellows. This talented crop of future leaders in the field have taken important positions in academia and industry.