Occupancy of Norepinephrine Transporter by Duloxetine in Human Brains Measured by Positron Emission Tomography with (S,S)-[18F]FMeNER-D2

The utility of PET imaging in depression

This educational review article aims to discuss growing evidence from PET studies in the diagnosis and treatment of depression. PET has been used in depression to explore the neurotransmitters involved, the alterations in neuroreceptors, non-neuroreceptor targets (e.g., microglia and astrocytes), the severity and duration of the disease, the pharmacodynamics of various antidepressants, and neurobiological mechanisms of non-pharmacological therapies like psychotherapy, electroconvulsive therapy, and deep brain stimulation therapy, by showing changes in brain metabolism and receptor and non-receptor targets. Studies have revealed alterations in neurotransmitter systems such as serotonin, dopamine, GABA, and glutamate, which are linked to the pathophysiology of depression. Overall, PET imaging has furthered the neurobiological understanding of depression. Despite these advancements, PET findings have not yet led to significant changes in evidence-based practices. Addressing the reasons behind inconsistencies in PET imaging results, conducting large sample size studies with a more standardized methodological approach, and investigating further the genetic and neurobiological aspects of depression may better leverage PET imaging in future studies.

Update Lessons from PET Imaging Part II: A Systematic Critical Review on Therapeutic Plasma Concentrations of Antidepressants

BACKGROUND

Compared with antipsychotics, the relationship between antidepressant blood (plasma or serum) concentrations and target engagement is less well-established.

METHODS

We have discussed the literature on the relationship between plasma concentrations of antidepressant drugs and their target occupancy. Antidepressants reviewed in this work are citalopram, escitalopram, fluoxetine, fluvoxamine, paroxetine, sertraline, venlafaxine, duloxetine, milnacipran, tricyclic antidepressants (amitriptyline, nortriptyline, and clomipramine), bupropion, tranylcypromine, moclobemide, and vortioxetine. Four electronic databases were systematically searched.

RESULTS

We included 32 articles published 1996-2022. A strong relationship between serotonin transporter (SERT) occupancy and drug concentration is well established for selective serotonin reuptake inhibitors. Lower limits of recommended therapeutic reference ranges largely corroborate with the findings from positron emission tomography studies (80% SERT occupancy). Only a few novel studies have investigated alternative targets, that is, norepinephrine transporters (NETs), dopamine transporters (DATs), or monoamine oxidase A (MAO-A). For certain classes of drugs, positron emission tomography study data are inconclusive. Low DAT occupancy after bupropion treatment speculates its discussed mechanism of action. For MAO inhibitors, a correlation between drug concentration and MAO-A occupancy could not be established.

CONCLUSIONS

Neuroimaging studies are critical in TDM-guided therapy for certain antidepressants, whereas for bupropion and MAO inhibitors, the available evidence offers no further insight. Evidence for selective serotonin reuptake inhibitors is strong and justifies a titration toward suggested ranges. For SNRIs, duloxetine, and venlafaxine, NETs are sufficiently occupied, well above the SERT efficacy threshold. For these drugs, a titration toward higher concentrations (within the recommended range) should be considered in case of no response at lower concentrations.

Beyond monoamines: I. Novel targets and radiotracers for Positron emission tomography imaging in psychiatric disorders

With the emergence of positron emission tomography (PET) in the late 1970s, psychiatry had access to a tool capable of non-invasive assessment of human brain function. Early applications in psychiatry focused on identifying characteristic brain blood flow and metabolic derangements using radiotracers such as [¹⁵ O]H₂ O and [¹⁸ F]FDG. Despite the success of these techniques, it became apparent that more specific probes were needed to understand the neurochemical bases of psychiatric disorders. The first neurochemical PET imaging probes targeted sites of action of neuroleptic (dopamine D₂ receptors) and psychoactive (serotonin receptors) drugs. Based on the centrality of monoamine dysfunction in psychiatric disorders and the measured success of monoamine-enhancing drugs in treating them, the next 30 years witnessed the development of an armamentarium of PET radiopharmaceuticals and imaging methodologies for studying monoamines. Continued development of monoamine-enhancing drugs over this time however was less successful, realizing only modest gains in efficacy and tolerability. As patent protection for many widely prescribed and profitable psychiatric drugs lapsed, drug development pipelines shifted away from monoamines in search of novel targets with the promises of improved efficacy, or abandoned altogether. Over this period, PET radiopharmaceutical development activities closely paralleled drug development priorities resulting in the development of new PET imaging agents for non-monoamine targets. Part one of this review will briefly survey novel PET imaging targets with relevance to the field of psychiatry, which include the metabotropic glutamate receptor type 5 (mGluR5), purinergic P₂ X₇ receptor, type 1 cannabinoid receptor (CB₁ ), phosphodiesterase 10A (PDE10A), and describe radiotracers developed for these and other targets that have matured to human subject investigations. Current limitations of the targets and techniques will also be discussed.

Adaptation of Lipid Profiling in Depression Disease and Treatment: A Critical Review

Major depressive disorder (MDD), also called depression, is a serious disease that impairs the quality of life of patients and has a high incidence, affecting approximately 3.8% of the world population. Its diagnosis is very subjective and is not supported by measurable biomarkers mainly due to the lack of biochemical markers. Recently, disturbance of lipid profiling has been recognized in MDD, in animal models of MDD or in depressed patients, which may contribute to unravel the etiology of the disease and find putative new biomarkers, for a diagnosis or for monitoring the disease and therapeutics outcomes. In this review, we provide an overview of current knowledge of lipidomics analysis, both in animal models of MDD (at the brain and plasma level) and in humans (in plasma and serum). Furthermore, studies of lipidomics analyses after antidepressant treatment in rodents (in brain, plasma, and serum), in primates (in the brain) and in humans (in plasma) were reviewed and give evidence that antidepressants seem to counteract the modification seen in lipids in MDD, giving some evidence that certain altered lipid profiles could be useful MDD biomarkers for future precision medicine.

The relationship between dose and serotonin transporter occupancy of antidepressants-a systematic review

Brain imaging techniques enable the visualization of serotonin transporter (SERT) occupancy as a measure of the proportion of SERT blocked by an antidepressant at a given dose. We aimed to systematically review the evidence on the relationship between antidepressant dose and SERT occupancy. We searched PubMed and Embase (last search 20 May 2021) for human in vivo, within-subject PET, or SPECT studies measuring SERT occupancy at any dose of any antidepressant with highly selective radioligands ([¹¹C]-DASB, [¹²³I]-ADAM, and [¹¹C]-MADAM). We summarized and visualized the dose-occupancy relationship for antidepressants across studies, overlaying the plots with a curve based on predicted values of a standard 2-parameter Michaelis-Menten model fitted using the observed data. We included seventeen studies of 10 different SSRIs, SNRIs, and serotonin modulators comprising a total of 294 participants, involving 309 unique occupancy measures. Overall, following the Michaelis-Menten equation, SERT occupancy increased with a higher dose in a hyperbolic relationship, with occupancy increasing rapidly at lower doses and reaching a plateau at approximately 80% at the usual minimum recommended dose. All the studies were small, only a few investigated the same antidepressant, dose, and brain region, and few reported information on factors that may influence SERT occupancy. The hyperbolic dose-occupancy relationship may provide mechanistic insight of relevance to the limited clinical benefit of dose-escalation in antidepressant treatment and the potential emergence of withdrawal symptoms. The evidence is limited by non-transparent reporting, lack of standardized methods, small sample sizes, and short treatment duration. Future studies should standardize the imaging and reporting procedures, measure occupancy at lower antidepressant doses, and investigate the moderators of the dose-occupancy relationship.

Tools for optimising pharmacotherapy in psychiatry (therapeutic drug monitoring, molecular brain imaging and pharmacogenetic tests): focus on antidepressants

Objectives: More than 40 drugs are available to treat affective disorders. Individual selection of the optimal drug and dose is required to attain the highest possible efficacy and acceptable tolerability for every patient.Methods: This review, which includes more than 500 articles selected by 30 experts, combines relevant knowledge on studies investigating the pharmacokinetics, pharmacodynamics and pharmacogenetics of 33 antidepressant drugs and of 4 drugs approved for augmentation in cases of insufficient response to antidepressant monotherapy. Such studies typically measure drug concentrations in blood (i.e. therapeutic drug monitoring) and genotype relevant genetic polymorphisms of enzymes, transporters or receptors involved in drug metabolism or mechanism of action. Imaging studies, primarily positron emission tomography that relates drug concentrations in blood and radioligand binding, are considered to quantify target structure occupancy by the antidepressant drugs in vivo. Results: Evidence is given that in vivo imaging, therapeutic drug monitoring and genotyping and/or phenotyping of drug metabolising enzymes should be an integral part in the development of any new antidepressant drug.Conclusions: To guide antidepressant drug therapy in everyday practice, there are multiple indications such as uncertain adherence, polypharmacy, nonresponse and/or adverse reactions under therapeutically recommended doses, where therapeutic drug monitoring and cytochrome P450 genotyping and/or phenotyping should be applied as valid tools of precision medicine.

PET technology for drug development in psychiatry

Positron emission tomography (PET) is a non‐invasive imaging method to measure the molecule in vivo. PET imaging can evaluate the central nervous system drugs as target engagement in the human brain. For antipsychotic drugs, adequate dopamine D2 receptor occupancy (“therapeutic window”) is reported to be from 65%‐70% to 80% to achieve the antipsychotic effect without extrapyramidal symptoms. For antidepressants, the clinical threshold of serotonin transporter (5‐HTT) occupancy is reported to be 70%‐80% although the relation between the side effect and 5‐HTT occupancy has not yet been established. Evaluation of norepinephrine transporter (NET) occupancy for antidepressant is ongoing as adequate PET radioligands for NET were developed recently. Measurement of the target occupancy has been a key element to evaluate the in vivo target engagement of the drugs. In order to evaluate new drug targets for disease conditions such as negative symptoms/cognitive impairment of schizophrenia and treatment‐resistant depression, new PET radioligands need to be developed concurrently with the drug development.

PET imaging can evaluate the central nervous system drugs as target engagement in the human brain. The uptake of [¹¹C]raclopride for dopamine D2 receptors decreased from (A) baseline to (B) antipsychotic administration conditions.

Recent advances in radiotracers targeting norepinephrine transporter: structural development and radiolabeling improvements

The norepinephrine transporter (NET) is a major target for the evaluation of the cardiac sympathetic nerve system in patients with heart failure and Parkinson's disease. It is also used in the therapeutic applications against certain types of neuroendocrine tumors, as exemplified by the clinically used ^123/131I-MIBG as theranostic single-photon emission computed tomography (SPECT) agent. With the development of more advanced positron emission tomography (PET) technology, more radiotracers targeting NET have been reported, with superior temporal and spatial resolutions, along with the possibility of functional and kinetic analysis. More recently, fluorine-18-labelled NET tracers have drawn increasing attentions from researchers, due to their longer radiological half-life relative to carbon-11 (110 min vs. 20 min), reduced dependence on on-site cyclotrons, and flexibility in the design of novel tracer structures. In the heart, certain NET tracers provide integral diagnostic information on sympathetic innervation and the nerve status. In the central nervous system, such radiotracers can reveal NET distribution and density in pathological conditions. Most radiotracers targeting cardiac NET-function for the cardiac application consistent of derivatives of either norepinephrine or MIBG with its benzylguanidine core structure, e.g. ¹¹C-HED and ¹⁸F-LMI1195. In contrast, all NET tracers used in central nervous system applications are derived from clinically used antidepressants. Lastly, possible applications of NET as selective tracers over organic cation transporters (OCTs) in the kidneys and other organs controlled by sympathetic nervous system will also be discussed.

Item-based analysis of the effects of duloxetine in depression: a patient-level post hoc study

Oft-cited trial-level meta-analyses casting doubt on the usefulness of antidepressants have been based on re-analyses of to what extent the active drug has outperformed placebo in reducing the sum score of the Hamilton Depression Rating Scale (HDRS-17-sum) in clinical trials. Recent studies, however, suggest patient-level analyses of individual HDRS items to be more informative when assessing the efficacy of an antidepressant. To shed further light on both symptom-reducing and symptom-aggravating effects of a serotonin and noradrenaline reuptake inhibitor, duloxetine, when used for major depression in adults, we hence applied this approach to re-analyse data from 13 placebo-controlled trials. In addition, using patient-level data from 28 placebo-controlled trials of selective serotonin reuptake inhibitors (SSRIs), the response profile of duloxetine was compared to that of these drugs. Duloxetine induced a robust reduction in depressed mood that was not dependent on baseline severity and not caused by side-effects breaking the blind. A beneficial effect on depressed mood was at hand already after one week; when outcome was assessed using HDRS-17-sum as effect parameter, this early response was however masked by a concomitant deterioration with respect to adverse event-related items. No support for a suicide-provoking effect of duloxetine was obtained. The response profile of duloxetine was strikingly similar to that of the SSRIs. We conclude that the use of HDRS-17-sum as effect parameter underestimates the true efficacy and masks an early effect of duloxetine on core symptoms of depression. No support for major differences between duloxetine and SSRIs in clinical profile were obtained.

Duloxetine plasma level and antidepressant response

BACKGROUND

Major Depressive Disorder (MDD) is associated with a high rate of inadequate treatment response, which is mainly due to the large inter-individual genetic variability in pharmacokinetic and pharmacodynamic targets of antidepressant drugs. Little is still known about the exact association between plasma level of first-line antidepressants and clinical response. This is particularly true for duloxetine, a dual serotonin and norepinephrine reuptake inhibitor recommended as first-line treatment for MDD. The aim of this study was to investigate the association between serum concentration of duloxetine (SCD) and antidepressant response (AR).

METHODS

66 MDD patients treated with duloxetine 60 mg/day monotherapy were recruited in an outpatient setting and followed for three months. Hamilton Depression Rating Scale - 21 (HAMD-21) was administrated at baseline, at month 1, and at month 3 to assess AR. SCD was measured at steady state. Linear regression analysis and nonlinear least-squares regression were used to estimate association between SCD and AR.

RESULTS

SCD showed a high inter-individual variability in our sample, despite the duloxetine fixed oral dosage. We found a strong association between SCD and AR following a bell-shaped function at month 1 and at month 3. Nonetheless, within the recommended SCD range of 30-120 ng/mL a more linear correlation between SCD and AR was observed.

DISCUSSION

Our results suggest that for duloxetine the association between SCD and AR likely follows a bell-shaped quadratic function with poor AR at subtherapeutic SCD and progressive decrease of AR at higher SCD. The maximum antidepressant efficacy seems to require SCD values next to the highest recommended SCD (30-120 ng/mL), probably because of the optimal saturation of both serotonin and norepinephrine transporters. Thus, taking into account the observed high interindividual variability of SCD, our findings suggest that for MDD patients treated with duloxetine, SCD could be a useful tool to guide the treatment by optimizing the oral dosage in order to increase the AR rate.

Distinguishing analgesic drugs from non-analgesic drugs based on brain activation in macaques with oxaliplatin-induced neuropathic pain

The antineoplastic agent oxaliplatin is a first-line treatment for colorectal cancer. However, neuropathic pain, characterized by hypersensitivity to cold, emerges soon after treatment. In severe instances, dose reduction or curtailing treatment may be necessary. While a number of potential treatments for oxaliplatin-induced neuropathic pain have been proposed based on preclinical findings, few have demonstrated efficacy in randomized, placebo-controlled clinical studies. This failure could be related, in part, to the use of rodents as the primary preclinical species, as there are a number of distinctions in pain-related mechanisms between rodents and humans. Also, an indicator of preclinical pharmacological efficacy less subjective than behavioral endpoints that is translatable to clinical usage is lacking. Three days after oxaliplatin treatment in Macaca fascicularis, a significantly reduced response latency to cold (10 °C) water was observed, indicating cold hypersensitivity. Cold-evoked bilateral activation of the secondary somatosensory (SII) and insular (Ins) cortex was observed with functional magnetic resonance imaging. Duloxetine alleviated cold hypersensitivity and significantly attenuated activation in both SII and Ins. By contrast, neither clinically used analgesics pregabalin nor tramadol affected cold hypersensitivity and cold-evoked activation of SII and Ins. The current findings suggest that suppressing SII and Ins activation leads to antinociception, and, therefore, could be used as a non-behavioral indicator of analgesic efficacy in patients with oxaliplatin-induced neuropathic pain.

Venlafaxine ER Blocks the Norepinephrine Transporter in the Brain of Patients with Major Depressive Disorder: a PET Study Using [18F]FMeNER-D2

BACKGROUND

The in vivo binding of clinical dose of venlafaxine on norepinephrine transporter has been questioned because venlafaxine has higher in vitro affinity to serotonin transporter than that to norepinephrine transporter. Although serotonin transporter occupancy of clinically relevant doses of venlafaxine has been reported, there has been no report of norepinephrine transporter occupancy in the human brain.

METHODS

This was an open-label, single center, exploratory positron emission tomography study. Twelve major depressive disorder patients who had responded to venlafaxine extended-release and 9 control subjects were recruited. Each subject participated in one positron emission tomography measurement with [18F]FMeNER-D2. Binding potential in brain was quantified by the area under the curve ratio method with thalamus as target and white matter as reference regions. The difference of binding potential values between control and patient groups divided to 2 dose ranges were evaluated. Norepinephrine transporter occupancy (%) for all the major depressive disorder patients was calculated using mean binding potential of control subjects as baseline. The relationships between dose or plasma concentration of total active moiety and occupancies of norepinephrine transporter were also estimated.

RESULTS

The binding potential of the patient group with 150 to 300 mg/d was significantly lower than that in the control subjects group (P = .0004 < .05/2). The norepinephrine transporter occupancy (8-61%) increased in a dose-dependent manner although a clear difference beyond 150 mg/d was not observed.

CONCLUSIONS

This study demonstrates that clinically relevant doses of venlafaxine extended-release block the norepinephrine transporter of the major depressive disorder patient's brain. The data support the notion that the antidepressant effect of venlafaxine involves a combination of serotonin transporter and norepinephrine transporter blockades.

Understanding the pathophysiology of depression: From monoamines to the neurogenesis hypothesis model - are we there yet?

A number of factors (biogenic amine deficiency, genetic, environmental, immunologic, endocrine factors and neurogenesis) have been identified as mechanisms which provide unitary explanations for the pathophysiology of depression. Rather than a unitary construct, the combination and linkage of these factors have been implicated in the pathogenesis of depression. That is, environmental stressors and heritable genetic factors acting through immunologic and endocrine responses initiate structural and functional changes in many brain regions, resulting in dysfunctional neurogenesis and neurotransmission which then manifest as a constellation of symptoms which present as depression.