First-in-Human Evaluation of 18F-PF-06445974, a PET Radioligand That Preferentially Labels Phosphodiesterase-4B.

Phosphodiesterase-4 (PDE4), which metabolizes the second messenger cyclic adenosine monophosphate (cAMP), has 4 isozymes: PDE4A, PDE4B, PDE4C, and PDE4D. PDE4B and PDE4D have the highest expression in the brain and may play a role in the pathophysiology and treatment of depression and dementia. This study evaluated the properties of the newly developed PDE4B-selective radioligand 18F-PF-06445974 in the brains of rodents, monkeys, and humans. Methods: Three monkeys and 5 healthy human volunteers underwent PET scans after intravenous injection of 18F-PF-06445974. Brain uptake was quantified as total distribution volume (V T) using the standard 2-tissue-compartment model and serial concentrations of parent radioligand in arterial plasma. Results: 18F-PF-06445974 readily distributed throughout monkey and human brain and had the highest binding in the thalamus. The value of V T was well identified by a 2-tissue-compartment model but increased by 10% during the terminal portions (40 and 60 min) of the monkey and human scans, respectively, consistent with radiometabolite accumulation in the brain. The average human V T values for the whole brain were 9.5 ± 2.4 mL ⋅ cm-3 Radiochromatographic analyses in knockout mice showed that 2 efflux transporters-permeability glycoprotein (P-gp) and breast cancer resistance protein (BCRP)-completely cleared the problematic radiometabolite but also partially cleared the parent radioligand from the brain. In vitro studies with the human transporters suggest that the parent radioligand was a partial substrate for BCRP and, to a lesser extent, for P-gp. Conclusion: 18F-PF-06445974 quantified PDE4B in the human brain with reasonable, but not complete, success. The gold standard compartmental method of analyzing brain and plasma data successfully identified the regional densities of PDE4B, which were widespread and highest in the thalamus, as expected. Because the radiometabolite-induced error was only about 10%, the radioligand is, in the opinion of the authors, suitable to extend to clinical studies.

PDE4B as a microglia target to reduce neuroinflammation.

The importance of microglia in immune homeostasis within the brain is undisputed. Their role in a diversity of neurological and psychiatric diseases as well as CNS injury is the subject of much investigation. Cyclic adenosine monophosphate (AMP) is a critical regulator of microglia homeostasis; as the predominant negative modulator of cyclic AMP signaling within microglia, phosphodiesterase 4 (PDE4) represents a promising target for modulating immune function. PDE4 expression is regulated by inflammation, and in turn, PDE4 inhibition can alter microglia reactivity. As the prototypic PDE4 inhibitor, rolipram, was tested clinically in the 1980s, drug discovery and clinical development of PDE4 inhibitors have been severely hampered by tolerability issues involving nausea and emesis. The two PDE4 inhibitors approved for peripheral inflammatory disorders (roflumilast and apremilast) lack brain penetration and are dose-limited by side effects making them unsuitable for modulating microglial function. Subtype selective inhibitors targeting PDE4B are of high interest given the critical role PDE4B plays in immune function versus the association of PDE4D with nausea and emesis. The challenges and requirements for successful development of a novel brain-penetrant PDE4B inhibitor are discussed in the context of early clinical development strategies. Furthermore, the challenges of monitoring the state of microglia in vivo are highlighted, including a description of the currently available tools and their limitations. Continued drug discovery efforts to identify safe and well-tolerated, brain-penetrant PDE4 inhibitors are a reflection of the confidence in the rationale for modulation of this target to produce meaningful therapeutic benefit in a wide range of neurological conditions and injury. GLIA 2016;64:1698-1709.

Estrogen receptor neurobiology and its potential for translation into broad spectrum therapeutics for CNS disorders.

Estrogens are hormones that modulate a diverse array of effects during development and adulthood. The effects of estrogen are mediated by two estrogen receptor (ER) isotypes, ERalpha and ERbeta, which classically function as transcription factors to modulate specific target gene expression and in addition regulate a growing list of intracellular signaling cascades. These receptors share protein sequence homology and protein-motif organization but have distinct differences in their tissue distribution and binding affinities for their ligands. In the nervous system estrogen has been implicated to play a role in a number of processes which regulate synaptic plasticity including synaptogenesis and neurogenesis. The role for estrogen in a range of neurological and neuropsychiatric diseases is also becoming very apparent. Estrogen is able to regulate processes and behaviours relevant for both Alzheimer's disease and schizophrenia and to modulate neuroendocrine and inflammatory processes important in neuroinflammation, anxiety and depressive disorders as well as chronic pain. We will consider the rationale for estrogen-based therapies for diseases of the nervous system. In particular we will highlight the molecular mechanisms and signal transduction pathways most likely underlying the effects of estrogen in the CNS.

WAY-200070, a selective agonist of estrogen receptor beta as a potential novel anxiolytic/antidepressant agent.

Recent studies have reported that estrogen has antidepressant-like effects in animal models. In this study we used the highly selective ER beta agonist, WAY-200070, to examine the role of ER beta activation on brain neurochemistry and activity in antidepressant and anxiolytic models in male mice. Within 15 min of administration, WAY-200070 (30 mg/kg s.c.) caused the nuclear translocation of striatal ER beta receptors from the cytosol. WAY-200070 also increased c-fos activation 4h, but not 15 min after administration. Both nuclear translocation and c-fos induction effects of WAY-200070 demonstrate that WAY-200070 has bound to estrogen receptors and triggered downstream events. The absence of these effects in the ER beta KO mice confirms that WAY-200070 was targeting ER beta. Administration of WAY-200070 (30 mg/kg s.c.) produced a delayed approximately 50% increase in dopamine in the striatum of wild type mice. The effect was significant and maintained from 90 to 240 min. This increase was absent in ER beta KO mice. In wild type mice, WAY-200070 (30 mg/kg s.c.) also produced a delayed and transient approximately 100% increase in 5-HT. To further investigate the role of ER beta receptors on serotonergic function, 5-HTP accumulation was measured. ER beta KO mice were found to have reduced frontal cortex levels of 5-HTP, indicating reduced tryptophan hydroxylase activity. WAY-200070 (3-30 mg/kg s.c.) was also tested in behavioural models. WAY-200070 (30 mg/kg s.c.) reduced immobility time in the mouse tail suspension test indicating an antidepressant-like effect. WAY-200070 (30 mg/kg) showed anxiolytic-like effects in the four-plate test (increased punished crossings) and stress-induced hyperthermia (attenuation of hyperthermic response). The effects of the selective ER beta agonist, WAY-200070, on dopamine and serotonin, the anxiolytic-like and antidepressant-like effects as well as the genotype specific effects on neurochemistry support that positive modulation of ER beta function may provide a novel treatment for affective disorders.

Use of Arc expression as a molecular marker of increased postsynaptic 5-HT function after SSRI/5-HT1A receptor antagonist co-administration.

An increase in central postsynaptic 5-hydroxytryptamine (5-HT) function activates expression of activity-related cytoskeletal protein (Arc). Here, Arc expression was used to test whether, in rats, co-administration of a 5-HT re-uptake inhibitor (paroxetine) and a 5-HT1A receptor antagonist (WAY 100635) increases postsynaptic 5-HT function. After pre-treatment with WAY 100635 (0.3 mg/kg s.c.), paroxetine (5 mg/kg s.c.) caused a threefold increase in 5-HT in prefrontal cortex microdialysates. In situ hybridization studies found that neither paroxetine (5 mg/kg s.c.) nor WAY 1000635 (0.3 mg/kg s.c.) altered Arc mRNA abundance in any region examined. In contrast, paroxetine (5 mg/kg s.c.) increased Arc mRNA after pre-treatment with WAY 100635 (0.3 mg/kg s.c.). This increase was apparent in cortical regions (frontal, parietal and cingulate) and caudate nucleus but was absent in hippocampus (CA1). Increases in Arc mRNA were accompanied by an increase in c-fos mRNA. The increase in Arc expression induced by paroxetine/WAY 100635 was abolished by the 5-HT synthesis inhibitor, p-chlorophenylalanine (300 mg/kg i.p., daily for two days). In conclusion, paroxetine and WAY 100635 injected in combination (but not alone) caused a region-specific, 5-HT-mediated increase in Arc expression. These data provide molecular evidence that co-administration of a 5-HT re-uptake inhibitor and 5-HT1A receptor antagonist increases 5-HT function at the postsynaptic level.