Examples of progress made during the prior year are summarized below. 1) We have been studying phosphodiesterase type IV (PDE4), an important component of the cyclic adenosine monophosphate (cAMP) cascade, for several years. Here, we sought to investigate the interactions between PDE4 and DISC1 which limits PDE4 activityin live animals. We found increased rolipram binding to PDE4 in Disc1 KO mice,consistent with DISC1 inhibition of PDE4 activity by binding to its active site. Our work is the first to demonstrate that PDE4 is regulated by DISC1 in live animals. PDE4 selectively metabolizes cAMP in the brain to the inactive monophosphate. PDE4 is regulated by protein kinase A (PKA) and by disrupted-in-schizophrenia 1 (DISC1). DISC1 inhibits PDE4 activity, and DISC1 mutations are involved in both schizophrenia and mood disorders. Rolipram, a selective PDE4 inhibitor, has been labeled with carbon-11 (11C(R)-rolipram) for PET imaging of PDE4 and PKA interaction in vivo. Prior work from our laboratory confirmed in animals that increased 11C(R)-rolipram binding reflects the phosphorylated / active state of PDE4. To investigate the interactions between PDE4 and DISC1 in live animals, we compared 11C(R)-rolipram binding in the brains of Disc1 gene locus impaired C57BL/6 model mice lacking a large genomic region between exons 1 and 3 (Disc1 KO, n=11) and wild type mice (WT, n=9). The average change in brain radioactivity per hour after the 50-minute scan time was only 5 to 6% for both mouse groups, indicating that equilibrium was achieved within the scan time. Disc1 KO mice showed a 41% significant increase in VT (P = 0.04) compared to WT mice. The VT/fP, which more accurately reflects 11C(R)-rolipram binding than VT, showed a 73% significant increase (P = 0.004) in Disc1 KO compared to WT mice. The increased rolipram binding to PDE4 observed in Disc1 KO mice is consistent with DISC1 inhibition of PDE4 activity by binding to its active site. This is the first study to demonstrate that PDE4 is regulated by DISC1 in live animals. The results also suggest that PET imaging of PDE4 activity could facilitate the development of drugs such as PDE4 inhibitors, or could be used to monitor treatment in individuals affected with DISC1 gene variants. 2) We sought to develop a PET radioligand for cyclooxygenase-1 (COX-1) and COX-2; the expression of both enzymes is increased by neuroinflammation. We found that, in monkey, the PET radioligands 11CPS13 and 11CMC1 showed excellent imaging properties for selectively measuring COX-1 or COX-2, respectively. PET imaging of neuroinflammation has largely been restricted to studies of translocator protein (TSPO), which transports cholesterol across the mitochondrial membrane and is highly expressed in activated microglia and reactive astrocytes. However, use of TSPO as a biomarker of neuroinflammation is limited because TSPO is highly expressed both in microglia (the major source of inflammatory mediators) and astrocytes (which can form scars and merely mark sites of former damage and inflammation). For this reason, we sought to develop a PET radioligand for COX-1, which is almost exclusively localized in microglia and whose expression is increased by neuroinflammation. In parallel with this effort, we sought to develop a radioligand selective for COX-2, which is expressed in both neurons and glia. Although COX-2 is known to play a much larger role than COX-1 in peripheral inflammation, the relative role of these two enzymes in brain is controversial. We synthesized and screened over 50 compounds for inhibitory potency using in vitro enzymatic assays in whole blood from monkey and human. In vitro enzymatic assays in monkey and human blood showed that 11CPS13 was potent and selective for COX-1 compared to COX-2 and, conversely, 11CMC1 was potent and selective for COX-2 compared to COX-1. Both ligands showed good uptake in monkey brain and washed out relatively quickly (demonstrating that the binding was reversible, as expected). In addition, VT values were stable after about 70 minutes, suggesting that brain uptake was not contaminated by radiometabolites. To determine the percentage of brain uptake specifically bound to each enzyme, we intravenously injected pharmacological doses of drugs selective for COX-1 and COX-2. From these studies, we estimated that specific binding as a percentage of total uptake was 80% for 11CPS13 and 40% for 11CMC1. Our results indicate that these two PET radioligands showed excellent imaging properties for selectively measuring COX-1 or COX-2 in monkey. The percentage of total brain uptake specifically bound to each target (40 to 80%) was moderate to good at baseline but would be expected to markedly increase in inflammatory conditions. The COX-1 radioligand, 11CPS13, should provide a selective measure of microglial activation and, thus, of active inflammation. The COX-2 radioligand, 11CMC1, will help identify the relative roles of both COX enzymes in neuroinflammation. Together, these two radioligands can be used to assess the relative in vivo selectivity and brain entry of non-steroidal anti-inflammatory drugs, two areas for which relatively little information is available. Finally, both COX-1 and COX-2 are obvious therapeutic targets. Thus, in addition to being used as biomarkers of disease, these two PET radioligands can also measure target engagement of anti-inflammatory drugs and potentially monitor therapeutic response.
Showing the most recent 10 out of 101 publications
