[11C]Mirtazapine binding in depressed antidepressant nonresponders studied by PET neuroimaging

Antidepressant efficacy is correlated with plasma levels: mega-analysis and further evidence

The debate around optimal target dose for first-line antidepressants (ADs) is still ongoing. Along this line, therapeutic drug monitoring (TDM) represents one of the most promising tools to improve clinical outcome. Nevertheless, a few data exist regarding the concentration-effect relationship of first-line ADs which limits TDM implementation in routine clinical practice. We conducted the first patient-level concentration-response mega-analysis including data acquired by us previously and explored the concentration dependency of first-line AD (206 subjects). Further, new data on mirtazapine are reported (18 subjects). Hamilton Depression Rating Scale-21 administered at baseline, at month 1 and month 3 was used as the measure of efficacy to assess antidepressant response (AR). When pooling all four first-line ADs together, normalized plasma levels and AR significantly fit a bell-shaped quadratic function with a progressive increase of AR up to around the upper normalized limit of the therapeutic reference range with a decrease of AR at higher serum levels. Our results complement the available evidence on the issue and the recent insights gained from dose-response studies. A concentration-dependent clinical efficacy, such as previously demonstrated for tricyclic compounds, also emerge for first-line ADs. Our study supports a role for TDM as a tool to optimize AD treatment to obtain maximum benefit.

PET Radiotracers for CNS-Adrenergic Receptors: Developments and Perspectives

Epinephrine (E) and norepinephrine (NE) play diverse roles in our body’s physiology. In addition to their role in the peripheral nervous system (PNS), E/NE systems including their receptors are critical to the central nervous system (CNS) and to mental health. Various antipsychotics, antidepressants, and psychostimulants exert their influence partially through different subtypes of adrenergic receptors (ARs). Despite the potential of pharmacological applications and long history of research related to E/NE systems, research efforts to identify the roles of ARs in the human brain taking advantage of imaging have been limited by the lack of subtype specific ligands for ARs and brain penetrability issues. This review provides an overview of the development of positron emission tomography (PET) radiotracers for in vivo imaging of AR system in the brain.

Application of cross-species PET imaging to assess neurotransmitter release in brain

RATIONALE

This review attempts to summarize the current status in relation to the use of positron emission tomography (PET) imaging in the assessment of synaptic concentrations of endogenous mediators in the living brain.

OBJECTIVES

Although PET radioligands are now available for more than 40 CNS targets, at the initiation of the Innovative Medicines Initiative (IMI) "Novel Methods leading to New Medications in Depression and Schizophrenia" (NEWMEDS) in 2009, PET radioligands sensitive to an endogenous neurotransmitter were only validated for dopamine. NEWMEDS work-package 5, "Cross-species and neurochemical imaging (PET) methods for drug discovery", commenced with a focus on developing methods enabling assessment of changes in extracellular concentrations of serotonin and noradrenaline in the brain.

RESULTS

Sharing the workload across institutions, we utilized in vitro techniques with cells and tissues, in vivo receptor binding and microdialysis techniques in rodents, and in vivo PET imaging in non-human primates and humans. Here, we discuss these efforts and review other recently published reports on the use of radioligands to assess changes in endogenous levels of dopamine, serotonin, noradrenaline, γ-aminobutyric acid, glutamate, acetylcholine, and opioid peptides. The emphasis is on assessment of the availability of appropriate translational tools (PET radioligands, pharmacological challenge agents) and on studies in non-human primates and human subjects, as well as current challenges and future directions.

CONCLUSIONS

PET imaging directed at investigating changes in endogenous neurochemicals, including the work done in NEWMEDS, have highlighted an opportunity to further extend the capability and application of this technology in drug development.

Exploratory meta-analysis on deep brain stimulation in treatment-resistant depression

OBJECTIVE

Deep brain stimulation is currently an experimental treatment for major depressive disorder. Information is lacking, however, on how sham responding may affect efficacy. This article applies exploratory meta-analysis to address that topic.

METHODS

Data on benefits of deep brain electrical stimulation come from a recent review. Stimulated brain regions included subgenual cingulate, capsular interna, nucleus accumbens, and medial forebrain bundle. Expert opinion plus random number software was used to generate hypothetical values for sham responding.

RESULTS

An effect size of 1.71 (95% CI: 1.47-1.96) was obtained for deep brain stimulation versus sham treatment in patients suffering from long-term treatment-resistant depression.

CONCLUSION

Preliminary findings on deep brain electrical stimulation suggest that the procedure may be 71% more effective than sham treatment. Expressing these findings as patients-needed-to-treat, deep brain electrical stimulation is required by 2.9 patients with long-term treatment-resistant depression in order for one of them to benefit.

Molecular Neurobiology of Depression: PET Findings on the Elusive Correlation with Symptom Severity

Molecular mechanisms in the brain are assumed to cause the symptoms and severity of neuropsychiatric disorders. This review concerns the elusive nature of relationships between the severity of depressive disorders and neuromolecular processes studied by positron emission tomography (PET). Recent PET studies of human depression have focused on serotonergic, dopaminergic, muscarinic, nicotinic, and GABAergic receptors, as well as central processes dependent on monoamine oxidase, phosphodiesterase type 4, amyloid plaques, neurofibrillar tangles, and P-glycoprotein. We find that reliable causal links between neuromolecular mechanisms and relief from depressive disorders have yet to be convincingly demonstrated. This situation may contribute to the currently limited use of PET for exploring the neuropathways that are currently viewed as being responsible for beneficial effects of antidepressant treatment regimes.

Quest for biomarkers of treatment-resistant depression: shifting the paradigm toward risk

The search for potential biomarkers of psychiatric disorders is a central topic in biological psychiatry. This review concerns published studies on potential biomarkers of treatment-resistant depression (TRD). The search for biomarkers of TRD in the bloodstream has focused on cytokines and steroids as well as brain-derived neurotropic factor. Additional approaches to identifying biomarkers of TRD have dealt with cerebrospinal fluid analysis, magnetic resonance imaging, and positron emission tomography. Some studies have also investigated potential genetic and epigenetic factors in TRD. Most studies have, however, used a post hoc experimental design that failed to determine the association between biomarkers and the initial risk of TRD. Particular attention in future studies should be on shifting the experimental paradigm toward procedures that can determine the risk for developing treatment resistance in untreated depressed individuals.

Key players in major and bipolar depression--a retrospective analysis of in vivo imaging studies

In the present study, we evaluated the contribution of the individual synaptic constituents of all assessed neurotransmitter systems by subjecting all available in vivo imaging studies on patients with unipolar major depressive disorder (MDD) and bipolar depression (BD) to a retrospective analysis. In acute MDD, findings revealed significant increases of prefrontal and frontal DA synthesis, decreases of thalamic and midbrain SERT, increases of insular SERT, decreases of midbrain 5-HT(1A) receptors and decreases of prefrontal, frontal, occipital and cingulate 5-HT(2A) receptors, whereas, in remission, decreases of striatal D₂ receptors, midbrain SERT, frontal, parietal, temporal, occipital and cingulate 5-HT(1A) receptors and parietal 5-HT(2A) receptors were observed. In BD, findings indicated a trend towards increased striatal D₂ receptors in depression and mania, decreased striatal DA synthesis in remission and decreased frontal D₁ receptors in all three conditions. Additionally, there is some evidence that ventrostriatal and hippocampal SERT may be decreased in depression, whereas in remission and mania elevations of thalamic and midbrain SERT, respectively, were observed. Moreover, in depression, limbic 5-HT(1A) receptors were elevated, whereas in mania a decrease of both cortical and limbic 5-HT(2A) receptor binding was observed. Furthermore, in depression, prefrontal, frontal, occipital and cingulate M2 receptor binding was found to be reduced. From this, a complex pattern of dysregulations within and between neurotransmitter systems may be derived, which is likely to be causally linked not only with the subtype and duration of disease but also with the predominance of individual symptoms and with the kind and duration of pharmacological treatment(s).

PET kinetics of radiolabeled antidepressant, [N-methyl-11C]mirtazapine, in the human brain

Background

We compared six kinetic models with and without the requirement of arterial cannulation for estimating the binding potential of [N-methyl-¹¹C]mirtazapine in the living human brain.

Methods

Distribution volumes of [N-methyl-¹¹C]mirtazapine in brain regions were estimated using single- and two-tissue compartment models as well as a graphical plasma input model. The two-tissue compartment model provided a direct estimate of the binding potentials of [N-methyl-¹¹C]mirtazapine in brain regions, while binding potentials of the single-tissue compartment model and the graphical plasma input model were estimated indirectly from ratios of distribution volumes in brain regions. We obtained also direct estimates of binding potentials using a graphical reference tissue model and two nonlinear reference tissue models.

Results

The two-tissue compartment model required several fits with different initial guesses for avoiding negative values of parameters. Despite the extra fits, estimates of distribution volumes and binding potentials of [N-methyl-¹¹C]mirtazapine obtained by the two-tissue compartment model were far more variable than those produced by the other methods. The graphical plasma input method and the graphical reference tissue method provided estimates of the binding potential that correlated closely, but differed in magnitude. The single-tissue compartment model provided relatively low estimates of binding potentials with curves that failed to fit the data as well as the three other methods that used the entire series of positron emission tomography data. The reference tissue method and the simplified reference tissue method provided similar, consistent estimates of binding potentials. However, certain assumptions of the simplified reference tissue method may not be fulfilled by the radioligand.

Conclusion

The reference tissue method is appropriate for estimating the binding potential of [N-methyl-¹¹C]mirtazapine in regions of the human brain so that the binding potential of [N-methyl-¹¹C]mirtazapine can be estimated without arterial cannulation.

Molecular imaging and the neuropathologies of Parkinson's disease

The main motor symptoms of Parkinson's disease (PD) are linked to degeneration of the nigrostriatal dopamine (DA) fibers, especially those innervating the putamen. This degeneration can be assessed in molecular imaging studies with presynaptic tracers such as [(18)F]-fluoro-L-DOPA (FDOPA) and ligands for DA transporter ligands. However, the pathologies of PD are by no means limited to nigrostriatal loss. Results of post mortem and molecular imaging studies reveal parallel degenerations of cortical noradrenaline (NA) and serotonin (5-HT) innervations, which may contribute to affective and cognitive changes of PD. Especially in advanced PD, cognitive impairment can come to resemble that seen in Alzheimer's dementia, as can the degeneration of acetylcholine innervations arising in the basal forebrain. The density of striatal DA D(2) receptors increases in early untreated PD, consistent with denervation upregulation, but there is an accelerated rate of DA receptor loss as the disease advances. Animal studies and post mortem investigations reveal changes in brain opioid peptide systems, but these are poorly documented in imaging studies of PD. Relatively minor changes in the binding sites for GABA are reported in cortex and striatum of PD patients. There remains some controversy about the expression of the 18 kDa translocator protein (TSPO) in activated microglia as an indicator of an active inflammatory component of neurodegeneration in PD. A wide variety of autonomic disturbances contribute to the clinical syndrome of PD; the degeneration of myocardial sympathetic innervation can be revealed in SPECT studies of PD patients with autonomic failure. Considerable emphasis has been placed on investigations of cerebral blood flow and energy metabolism in PD. Due to the high variance of these physiological estimates, researchers have often employed normalization procedures for the sensitive detection of perturbations in relatively small patient groups. However, a widely used normalization to the global mean must be used with caution, as it can result in spurious findings of relative hypermetabolic changes in subcortical structures. A meta-analysis of the quantitative studies to date shows that there is in fact widespread hypometabolism and cerebral blood flow in the cerebral cortex, especially in frontal cortex and parietal association areas. These changes can bias the use of global mean normalization, and probably represent the pathophysiological basis of the cognitive impairment of PD.