Using Fenfluramine to Test the Serotonin Deficiency Theory of Depression (FenDep)

Clinical depression is a common and disabling condition characterised by persistent low mood and loss of interest that interferes with daily functioning. Serotonin is a key brain neurotransmitter involved in mood regulation, and a leading theory proposes that depression is associated with impaired serotonin function (the serotonin deficiency hypothesis), which underpins the use of selective serotonin reuptake inhibitors (SSRIs) as first-line antidepressant treatments. However, the strength and specificity of the link between serotonin dysfunction and depressive symptoms in humans remains uncertain and requires direct evidence in living human brains.

Positron Emission Tomography (PET) allows in vivo quantification of neurotransmitter receptor systems using a radioactive tracer that binds to specific brain targets. The serotonin 2A receptor (5-HT2A) agonist tracer [11C]Cimbi-36 enables measurement of the active-state 5-HT2A receptor, which is highly expressed in cortical regions implicated in mood regulation. When combined with a pharmacological challenge that acutely increases serotonin levels, changes in [11C]Cimbi-36 binding can be used to estimate serotonin release capacity across different brain regions.

Previous work using an amphetamine challenge with [11C]Cimbi-36 has shown reduced serotonin release capacity in the frontal cortex of patients with depression compared with healthy controls, providing preliminary support for the serotonin deficiency hypothesis. However, amphetamine releases multiple neurotransmitters in addition to serotonin, limiting the ability to attribute these effects specifically to serotonergic dysfunction. Dl-fenfluramine is a more selective serotonin-releasing agent and therefore offers a targeted approach to probe serotonin release in the human brain.

This case-control observational study will compare serotonin release capacity between unmedicated adults with Major Depressive Disorder (MDD) and healthy control participants using dl-fenfluramine challenge combined with [11C]Cimbi-36 PET imaging. The primary objective is to test whether individuals with MDD show reduced fenfluramine-induced serotonin release, indexed by changes in [11C]Cimbi-36 binding, relative to healthy controls. Secondary objectives include exploring how multimodal imaging, blood biomarkers, and behavioural measures relate to serotonin release capacity and depressive symptom severity.

Following completion of imaging, participants with MDD who will start SSRI treatment as part of their usual clinical care will be followed for 8 weeks with remote assessments. The study will examine whether baseline measures of serotonin release capacity predict subsequent clinical response to SSRIs, defined primarily by change in clinician-rated depression scores over the treatment period. Together, these data aim to provide a more precise test of the serotonin deficiency hypothesis of depression and to identify potential biomarkers of SSRI treatment response in MDD.

Study Overview

• Status: Not yet recruiting
• Conditions: Depression - Major Depressive Disorder
• Intervention / Treatment: Drug: Fenfluramine Hydrochloride; Radiation: [11C]Cimbi-36; Drug: Placebo

Detailed Description

Major depressive disorder (MDD) is a leading cause of disability worldwide and is associated with substantial personal, social, and economic burden. Despite the availability of antidepressant medications, fewer than half of patients achieve an adequate response to first-line treatments, and many fail to reach full remission, highlighting the need for a clearer understanding of the underlying neurobiology to guide more effective and personalised interventions. The most established biological theory of depression is the serotonin deficiency hypothesis, which proposes that in a proportion of patients, depression arises from impaired brain serotonin neurotransmission, particularly within cortical and limbic regions involved in mood regulation. While the efficacy of selective serotonin reuptake inhibitors (SSRIs) and other serotonergic agents is consistent with this hypothesis, most supporting data have been indirect, and prior imaging and peripheral biomarker studies have not provided a definitive, mechanistic test in living humans.

Recent advances in positron emission tomography (PET) have made it possible to measure endogenous serotonin release capacity in the human brain. The agonist radiotracer [11C]Cimbi-36 binds to the active state of the serotonin 2A receptor (5-HT2A), which is densely expressed in frontal and other cortical regions implicated in mood and affect regulation. Because agonist tracers are more sensitive than antagonist tracers to competition from endogenous neurotransmitters, increases in synaptic serotonin can be inferred from reductions in [11C]Cimbi-36 binding following a pharmacological challenge that releases serotonin. Using this methodology with a d-amphetamine challenge, prior work in healthy volunteers established that acute monoamine release reduces [11C]Cimbi-36 binding in the frontal cortex and other cortical areas. In a subsequent case-control PET study, patients undergoing a major depressive episode were found to have significantly reduced serotonin release capacity, defined as the percentage reduction in [11C]Cimbi-36 binding after d-amphetamine compared to placebo, providing the first direct in vivo evidence of impaired serotonin release in depression.

However, d-amphetamine releases dopamine and noradrenaline in addition to serotonin, so it remains unclear whether the observed deficit in release capacity is specific to the serotonin system or reflects broader monoaminergic dysfunction. Dl-fenfluramine is a more selective serotonin-releasing agent that has previously been used as a single-dose challenge in neuroendocrine and neurobiological studies of mood and anxiety disorders. Fenfluramine increases presynaptic serotonin release and leads to downstream effects such as prolactin elevation and changes in thermoregulation, which can be blocked by 5-HT2A receptor antagonists, indicating that its functional effects depend on serotonin acting at postsynaptic 5-HT receptors rather than direct agonist activity at 5-HT2A itself. Preclinical PET and microdialysis work has shown that fenfluramine produces robust serotonin release and larger reductions in [11C]Cimbi-36 binding than d-amphetamine, suggesting that a fenfluramine challenge may offer a more sensitive and specific assay of cortical serotonin release capacity.

The FenDep study is a case-control, observational brain imaging study designed to use dl-fenfluramine and [11C]Cimbi-36 PET to test the serotonin deficiency hypothesis of depression and to explore whether baseline serotonin release capacity predicts subsequent SSRI treatment response. The study will enrol approximately 26 adults with moderate to severe MDD and 20 healthy control (HC) participants.

Following an initial pre-screening telephone contact, potentially eligible individuals will be screened for mental and physical health. Screening also includes completion of a battery of psychometric instruments assessing depression, anxiety, stress, rumination, anhedonia, well-being, personality traits, and connectedness.

Eligible participants then attend two imaging visits, separated by approximately 14 days. The study uses a single-blind, balanced-order design in which each participant receives dl-fenfluramine (60 mg oral liquid) on one imaging day and a placebo on the other, with the order counterbalanced across participants. Dosing occurs approximately 3.5 hours before the PET scan to coincide with peak plasma levels of dl-fenfluramine. PET imaging is performed with intravenous administration of up to 300 MBq of [11C]Cimbi-36, followed by 90 minutes of dynamic data acquisition and arterial blood sampling via a radial arterial cannula to allow kinetic modelling of radiotracer uptake and metabolism. A venous cannula is used for tracer injection and additional venous sampling. The primary region of interest is the frontal cortex, with the cerebellum serving as a reference region for calculation of non-displaceable binding potential (BPND) using the multilinear analysis-1 (MA1) model. Serotonin release capacity (SRC) is operationalised as the relative reduction in [11C]Cimbi-36 BPND after dl-fenfluramine compared with placebo.

Alongside PET, a comprehensive multimodal imaging, including Magnetic Resonance Imaging (MRI) and electroencephalography (EEG), and a behavioural task battery is collected during the imaging visits. Visit procedures also include repeated measurements of blood pressure and heart rate, pharmacokinetic assessment of dl-fenfluramine and prolactin, and subjective drug effect ratings.

After completing both imaging visits, MDD participants return to their usual clinical care. In case participants initiate SSRI treatment as part of standard practice, the study team conducts three remote follow-up assessments by video call over an 8-week observational period. At these contacts, clinician-rated and self-report depression scales and other relevant questionnaires are administered. The primary clinical outcome is change in the Montgomery-Åsberg Depression Rating Scale (MADRS) score after 8 weeks of SSRI treatment, with response defined as a 50% or greater reduction from baseline, enabling investigation of whether baseline serotonin release capacity and related imaging/biomarker measures predict antidepressant response.

The primary objective of FenDep is to compare fenfluramine-induced serotonin release capacity in cortical regions between unmedicated adults with MDD and healthy controls using [11C]Cimbi-36 PET. By combining a selective serotonin releaser with a validated PET methodology for measuring serotonin release, multimodal imaging, behavioural tasks, blood-based biomarkers, and prospective SSRI treatment follow-up, FenDep aims to deliver a rigorous test of the serotonin deficiency hypothesis in MDD and to identify mechanistically informed biomarkers that could support future stratified treatment approaches.

• Study Type: Interventional
• Enrollment (Estimated): 46
• Phase: Not Applicable

Contacts and Locations

• Study Contact: Name: David Erritzoe, MD., PhD.; Phone Number: +447503289090; Email: d.erritzoe@imperial.ac.uk
• Study Contact Backup: Name: Claudio Agnorelli, PhD; Phone Number: 07341723258; Email: c.agnorelli21@imperial.ac.uk

Study Locations

• United Kingdom
  • London, United Kingdom, W120NN
    • entre for Psychedelics Research, Division of Psychiatry Imperial College London, Level 2, Commonwealth Building, Hammersmith Campus, Du Cane Road, London

Participation Criteria

Eligibility Criteria

• Ages Eligible for Study: Adult; Older Adult
• Accepts Healthy Volunteers: Yes

Description

Inclusion Criteria for All Participants:

• Aged 21 years and over.
• Able to lie comfortably on their back for scanning.
• Participants must agree to use one of the contraception methods listed in Appendix 1.
• Capable of providing written informed consent and willing to comply with the requirements and restrictions listed in the consent form.
• Able to read, comprehend, and record information written in English.
• Able to access the internet through their own electronic device.

Inclusion Criteria for Participants with Major Depressive Disorder (MDD):

• Major depressive episode diagnosed according to the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) as moderate to severe.
• Scoring above 20 on the Montgomery-Åsberg Depression Rating Scale (MADRS).
• Have never taken antidepressants, or are currently unmedicated for at least 8 weeks before signing the informed consent form.
• Not classified as treatment-resistant.
• Ongoing relationship with a general practitioner (GP) or other healthcare professional.

Inclusion Criteria for Healthy Controls:

- Healthy as determined by a responsible physician, based on a medical evaluation including medical history, physical examination, and laboratory tests.

Exclusion Criteria for All Participants:

• Presence of a general medical or psychiatric condition (excluding MDD in the relevant population), as revealed by a physical and psychiatric examination, which, in the opinion of the Principal Investigator (PI), would impair the safety of the participant or the scientific integrity of the study.
• Ongoing treatment with medication that, in the opinion of the investigator, would compromise participant safety or the scientific integrity of the study, including but not limited to compounds known to interact with the serotonin 2A (5-HT2A) receptor or to have a significant effect on the synthesis and/or release of serotonin (5-HT).
• Use of illicit compounds in the 3 months before consent, including but not limited to classic psychedelics, stimulants, 3,4-methylenedioxymethamphetamine (MDMA), and cannabis.
• Unwillingness or inability to follow the procedures outlined in the protocol.
• The participant is mentally or legally incapacitated.
• Contraindications to blood sampling and/or arterial cannulation, including but not limited to peripheral vascular disease or Raynaud's phenomenon.
• Contraindications to the administration of dl-fenfluramine include medications that have a significant effect on the synthesis and/or release of 5-HT.
• Abnormal Allen's test and/or prolonged Prothrombin Time (PT), due to the arterial cannulation required for the positron emission tomography (PET) scans.
• Participation in another research study involving ionising radiation exceeding 10 mSv within the last year.
• Contraindications to magnetic resonance imaging (MRI) scans include, but are not limited to, pacemakers, recent metallic implants, foreign bodies in the eye, or other contraindications identified by a standard pre-MRI questionnaire.
• Claustrophobia or any other condition that would render the participant incapable of undergoing MRI/PET scanning.
• Pregnancy or breastfeeding.

Exclusion Criterion for MDD Participants:

- History of suicide attempts requiring hospitalisation.

Exclusion Criteria for Healthy Controls:

• History of an Axis I psychiatric diagnosis.
• History of a neurological or general medical illness that, in the opinion of the investigator, would compromise participant safety or the scientific integrity of the study.

Study Plan

How is the study designed?

Design Details

• Primary Purpose: Basic Science
• Allocation: Randomized
• Interventional Model: Crossover Assignment
• Masking: Double

Number of Arms

2

Arms and Interventions

Participant Group / Arm	Intervention / Treatment
Experimental: Major Depressive Disorder (MDD) Arm; This arm includes adults with a current moderate to severe Major Depressive Disorder episode, diagnosed according to the Diagnostic and Statistical Manual of Mental Disorders 5th Edition, and currently unmedicated for at least 8 weeks. Participants attend one screening visit and two imaging visits (approx. 14 days apart). At each imaging visit, they undergo [11C]Cimbi-36 brain scans, pharmacokinetic, behavioural, and psychometric assessment. On one imaging day, they receive a single oral dose of dl-fenfluramine 60 mg, and on the other they receive placebo, with order balanced and allocation randomised across participants. Dosing occurs approximately 3.5 hours before [11C]Cimbi-36 to capture peak pharmacodynamic effects. After completion of both imaging visits, participants in this arm commencing Selective Serotonin Reuptake Inhibitor treatment under their treating clinician are monitored over an 8-week observational period with remote clinical assessments of depressive symptomatology.	Drug: Fenfluramine Hydrochloride; Single oral dose of 60 mg dl-fenfluramine oral solution, administered once on one of the two imaging days, approximately 3.5 hours before [11C]Cimbi-36 brain scan to coincide with peak pharmacodynamic effects. Dl-fenfluramine is a selective serotonin-releasing agent (licensed in the United Kingdom for Dravet syndrome) used here as an acute pharmacological challenge to probe cortical serotonin release capacity, with associated venous sampling for plasma levels and prolactin as pharmacodynamic markers.; Radiation: [11C]Cimbi-36; [11C]Cimbi-36 is a carbon-11-labelled serotonin 2A receptor (5-HT2A) agonist Positron emission tomography (PET) radiotracer that preferentially binds to the active state of cortical 5-HT2A receptors and is sensitive to displacement by endogenous serotonin. In this study, up to 300 MBq is administered intravenously on each of two PET sessions (one after dl-fenfluramine and one after placebo).; Drug: Placebo; Placebo consists of an oral solution of orange juice matched in volume, appearance, and taste to the dl-fenfluramine preparation but containing no active drug. It is administered once on one of the two imaging days, approximately 3.5 hours before the [11C]Cimbi-36 brain scan, with allocation randomised and order balanced across participants to maintain single-blind conditions.
Active Comparator: Healthy Control (HC) Arm; This arm includes adults who are physically and psychiatrically healthy, as determined by medical history, physical examination, and screening investigations, and who have no current or past Axis I psychiatric disorder. Participants attend one screening visit and two imaging visits (approx.14 days apart). At each imaging visit, they undergo [11C]Cimbi-36 brain scan, pharmacokinetic blood sampling, behavioural tasks, and psychometric assessment. On one imaging day, they receive a single oral dose of dl-fenfluramine 60 mg, and on the other, they receive a placebo, with orders balanced and allocation randomised across participants. Dosing occurs approximately 3.5 hours before [11C]Cimbi-36 to capture peak pharmacodynamic effects. Unlike the patients' arm, healthy control participants do not receive antidepressant treatment or longitudinal clinical follow-up, but provide the reference comparison group for dl-fenfluramine-induced changes in [11C]Cimbi-36 scan.	Drug: Fenfluramine Hydrochloride; Single oral dose of 60 mg dl-fenfluramine oral solution, administered once on one of the two imaging days, approximately 3.5 hours before [11C]Cimbi-36 brain scan to coincide with peak pharmacodynamic effects. Dl-fenfluramine is a selective serotonin-releasing agent (licensed in the United Kingdom for Dravet syndrome) used here as an acute pharmacological challenge to probe cortical serotonin release capacity, with associated venous sampling for plasma levels and prolactin as pharmacodynamic markers.; Radiation: [11C]Cimbi-36; [11C]Cimbi-36 is a carbon-11-labelled serotonin 2A receptor (5-HT2A) agonist Positron emission tomography (PET) radiotracer that preferentially binds to the active state of cortical 5-HT2A receptors and is sensitive to displacement by endogenous serotonin. In this study, up to 300 MBq is administered intravenously on each of two PET sessions (one after dl-fenfluramine and one after placebo).; Drug: Placebo; Placebo consists of an oral solution of orange juice matched in volume, appearance, and taste to the dl-fenfluramine preparation but containing no active drug. It is administered once on one of the two imaging days, approximately 3.5 hours before the [11C]Cimbi-36 brain scan, with allocation randomised and order balanced across participants to maintain single-blind conditions.

What is the study measuring?

Primary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Change in cortical [11C]Cimbi-36 non-displaceable binding potential (BPND) after dl-fenfluramine versus placebo (serotonin release capacity)	Serotonin release capacity (SRC) will be quantified as the percentage change in [11C]Cimbi-36 non-displaceable binding potential (BPND) in cortical regions (frontal, occipital, parietal, temporal, limbic) between the placebo Positron emission tomography (PET) scan and the dl-fenfluramine PET scan within each participant. The primary region of interest is the frontal cortex. BPND will be derived from 90-minute dynamic [11C]Cimbi-36 PET data using arterial input and multilinear analysis-1 (MA1), with cerebellum as the reference region. SRC will then be compared between the Major Depressive Disorder (MDD) and healthy control groups as the primary mechanistic endpoint.	Approximately 3.5 hours after dl-fenfluramine or placebo dosing on each of the two imaging visits (within 14 days)

Secondary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Dl-fenfluramine plasma concentration after single 60 mg oral dose	Plasma concentrations of dl-fenfluramine will be measured from venous blood samples obtained on the dl-fenfluramine imaging day at approximately 0, 3.5, and 5 hours after dosing. For each visit, dl-fenfluramine plasma concentration will be summarised (e.g. as average or change from baseline across post-dose time points) and compared between dl-fenfluramine and placebo within subjects, and between Major Depressive Disorder (MDD) and healthy control groups, and related to Positron emission tomography (PET) scan-derived serotonin release capacity.	Baseline, 3.5 hours, and 5 hours after dosing on each of the two imaging visits (dl-fenfluramine and placebo)
Prolactin levels after dl-fenfluramine and placebo in healthy controls and patients with Major Depressive Disorder (MDD)	Plasma prolactin concentrations will be measured from venous blood samples taken on both imaging days (dl-fenfluramine and placebo) approximately 0, 3.5, and 5 hours after dosing. For each visit, prolactin response will be summarised (e.g. as average or change from baseline across post-dose time points) and compared between dl-fenfluramine and placebo within subjects, and between Major Depressive Disorder (MDD) and healthy control groups, and related to Positron emission tomography (PET) scan-derived serotonin release capacity.	Baseline, 3.5 hours, and 5 hours after dosing on each of the two imaging visits (dl-fenfluramine and placebo)
Depression scores in Major Depressive Disorder (MDD) patients	In patients with Major Depressive Disorder (MDD), depressive symptom severity will be assessed with the clinician-rated Montgomery-Åsberg Depression Rating Scale (MADRS), a 10-item scale with total scores ranging from 0 to 60, where higher scores indicate more severe depression. MADRS will be administered at each study visit in MDD participants. For those patients initiating Selective Serotonin Reuptake Inhibitor (SSRI) treatment after the imaging visits as part of their standard clinical care, the MADRS will also be assessed at treatment initiation, 2 weeks, and 8 weeks after treatment. Change in MADRS scores (absolute change and percentage reduction from baseline) will be used to define clinical response and remission, and these outcomes will be related to baseline positron emission tomography (PET)-derived serotonin release capacity and other biomarkers.	Baseline, Scan Visit 1, Scan Visit 2, SSRI treatment start, 2 weeks after SSRI treatment, and 8 weeks after SSRI treatment
Functional magnetic resonance imaging (fMRI) measures of emotional face reactivity after dl-fenfluramine and placebo in healthy controls and patients with Major Depressive Disorder (MDD)	On both imaging days (dl-fenfluramine and placebo), participants complete a Functional magnetic resonance imaging (fMRI) emotional face-reactivity task during which brain responses to emotional facial expressions (e.g. neutral, positive, and negative valenced) are measured. Task-evoked Blood-Oxygen-Level-Dependent (BOLD) signal, particularly in regions such as the amygdala and frontal cortex, will be compared between dl-fenfluramine and placebo within subjects, between Major Depressive Disorder (MDD) and healthy control groups, and examined for associations with Positron emission tomography (PET) scan-derived serotonin release capacity and clinical measures.	During the MRI session on each of the two imaging visits (approximately 5 hours after dosing)
Arterial spin labelling (ASL) cerebral blood flow after dl-fenfluramine and placebo in healthy controls and patients with Major Depressive Disorder (MDD)	On both imaging days (dl-fenfluramine and placebo), participants undergo arterial spin labelling (ASL) to quantify regional cerebral blood flow at rest. ASL-derived cerebral blood flow values in predefined regions of interest (e.g. frontal cortex and limbic areas) will be compared between dl-fenfluramine and placebo within subjects, between Major Depressive Disorder (MDD) and healthy control groups, and examined for relationships with Positron emission tomography (PET) scan-derived serotonin release capacity and clinical/behavioural measures.	During the MRI session on each of the two imaging visits (approximately 5 hours after dosing)
Neuromelanin-sensitive Magnetic Resonance Imaging (MRI) measures of striatal dopamine signal after dl-fenfluramine and placebo in healthy controls and patients with Major Depressive Disorder (MDD)	On both imaging days (dl-fenfluramine and placebo), participants undergo a neuromelanin-sensitive Magnetic Resonance Imaging (MRI) sequence to quantify signal intensity in striatal regions as an indirect marker related to dopaminergic innervation. Neuromelanin-weighted signal in predefined striatal regions of interest will be compared between dl-fenfluramine and placebo within subjects, between Major Depressive Disorder (MDD) and healthy control groups, and explored in relation to Positron emission tomography (PET) scan-derived serotonin release capacity and clinical/behavioural measures.	During the MRI session on each of the two imaging visits (approximately 5 hours after dosing)
Magnetic resonance spectroscopy (MRS) measures of glutamate and Gamma-Aminobutyric acid (GABA) in anterior cingulate cortex after dl-fenfluramine and placebo in healthy controls and patients with Major Depressive Disorder (MDD)	On both imaging days (dl-fenfluramine and placebo), single-voxel proton Magnetic resonance spectroscopy (MRS) is acquired in the anterior cingulate cortex to quantify metabolites including glutamate and Gamma-Aminobutyric acid (GABA). Metabolite concentrations (e.g. glutamate, GABA and related ratios) will be compared between dl-fenfluramine and placebo within participants, between Major Depressive Disorder (MDD)and healthy control groups, and examined for associations with Positron emission tomography (PET) scan-derived serotonin release capacity and clinical/behavioural measures.	During the MRI session on each of the two imaging visits (approximately 5 hours after dosing)
Electroencephalography (EEG) measures after dl-fenfluramine and placebo in healthy controls and patients with Major Depressive Disorder (MDD)	On both imaging days (dl-fenfluramine and placebo), Electroencephalography (EEG) is recorded to characterise resting-state and task-related electrophysiological activity (including linear and non-linear measures) in healthy controls and Major Depressive Disorder (MDD) participants. EEG metrics will be compared between dl-fenfluramine and placebo within subjects, between MDD and healthy control groups, and explored in relation to Positron emission tomography (PET) scan-derived serotonin release capacity and other imaging, behavioural, and clinical measures.	During EEG sessions conducted on each of the two imaging visits (approximately 2 hours after dosing)
Behavioural measures of reinforcement learning after dl-fenfluramine and placebo in healthy controls and patients with Major Depressive Disorder (MDD)	On both imaging days (dl-fenfluramine and placebo), participants will complete the Probabilistic Instrumental Learning Task (PILT), a computerised reinforcement-learning task. The outcome measure will be the proportion of optimal choices on loss trials (choosing the symbol associated with the lower probability of monetary loss), expressed as a percentage from 0 to 100, where higher values indicate better performance (more optimal choices). This measure will be compared between dl-fenfluramine and placebo within subjects, between Major Depressive Disorder (MDD) and healthy control groups, and examined in relation to positron emission tomography (PET)-derived serotonin release capacity and clinical measures.	During behavioural testing on each of the two imaging visits (approximately 6 hours after dosing)
Behavioural measures of affective interference after dl-fenfluramine and placebo in healthy controls and patients with Major Depressive Disorder (MDD)	On both imaging days (dl-fenfluramine and placebo), participants will complete the Affective Interference Go/No-Go task, which measures response inhibition during exposure to emotional face distractors. The outcome measure will be the proportion of correct no-go responses (trials on which participants correctly withhold a response when a no-go signal is presented), expressed as a percentage from 0 to 100, where higher values indicate better inhibitory control. This measure will be compared between dl-fenfluramine and placebo within subjects, between participants with Major Depressive Disorder (MDD) and healthy controls, and examined in relation to positron emission tomography (PET)-derived serotonin release capacity and clinical measures.	During behavioural testing on each of the two imaging visits (approximately 6 hours after dosing)
Subjective depression scores in Major Depressive Disorder (MDD) patients	In patients with Major Depressive Disorder (MDD), depressive symptom severity will be assessed with the the Beck Depression Inventory-II (BDI-II), a 21-item self-report scale assessing the presence and severity of depressive symptoms over the past two weeks. Total scores range from 0 to 63, where higher scores indicate more severe depression. BDI-II will be administered at each study visit in MDD participants. For those patients initiating Selective Serotonin Reuptake Inhibitor (SSRI) treatment after the imaging visits as part of their standard clinical care, the BDI-II will also be assessed at treatment initiation, 2 weeks, and 8 weeks after treatment. Change in BDI-II scores (absolute change and percentage reduction from baseline) will be used to define clinical response and remission, and these outcomes will be related to baseline positron emission tomography (PET)-derived serotonin release capacity and other biomarkers.	Baseline, Scan Visit 1, Scan Visit 2, SSRI treatment start, 2 weeks after SSRI treatment, and 8 weeks after SSRI treatment
Subjective drug effect - Visual Analogue Scale (VAS): Any drug effect	On both imaging days (dl-fenfluramine and placebo), participants rated their subjective experience using a Visual Analogue Scale (VAS), a 100 mm line on which the distance from the left endpoint determines the score. For this item, scores range from 0 ("not at all") to 100 ("extremely"), where higher scores indicate a stronger perceived drug effect. Ratings are obtained at approximately 3 hours after dosing and will be compared between dl-fenfluramine and placebo within participants, between Major Depressive Disorder (MDD) and healthy control groups, and examined in relation to dl-fenfluramine plasma levels, prolactin response, and positron emission tomography (PET)-derived serotonin release capacity.	Approximately 3 hours after dosing on each of the two imaging visits
Subjective drug effect - Visual Analogue Scale (VAS): Good drug effect	n both imaging days (dl-fenfluramine and placebo), participants rate their subjective experience using a Visual Analogue Scale (VAS). For this item, scores range from 0 ("not at all") to 100 ("extremely"), where higher scores indicate a stronger perceived good/positive drug effect. Ratings are obtained at approximately 3 hours after dosing and will be compared between dl-fenfluramine and placebo within participants, between Major Depressive Disorder (MDD) and healthy control groups, and examined in relation to dl-fenfluramine plasma levels, prolactin response, and positron emission tomography (PET)-derived serotonin release capacity.	Approximately 3 hours after dosing on each of the two imaging visits
Subjective drug effect - Visual Analogue Scale (VAS): Bad drug effect	On both imaging days (dl-fenfluramine and placebo), participants rate their subjective experience using a Visual Analogue Scale (VAS). For this item, scores range from 0 ("not at all") to 100 ("extremely"), where higher scores indicate a stronger perceived bad/negative drug effect. Ratings are obtained at approximately 3 hours after dosing and will be compared between dl-fenfluramine and placebo within participants, between Major Depressive Disorder (MDD) and healthy control groups, and examined in relation to dl-fenfluramine plasma levels, prolactin response, and positron emission tomography (PET)-derived serotonin release capacity.	Approximately 3 hours after dosing on each of the two imaging visits
Subjective drug effect - Visual Analogue Scale (VAS): Drug liking	On both imaging days (dl-fenfluramine and placebo), participants rate their subjective experience using a Visual Analogue Scale (VAS). For this item, scores range from 0 ("not at all") to 100 ("extremely"), where higher scores indicate greater liking of the drug effect. Ratings are obtained at approximately 3 hours after dosing and will be compared between dl-fenfluramine and placebo within participants, between Major Depressive Disorder (MDD) and healthy control groups, and examined in relation to dl-fenfluramine plasma levels, prolactin response, and positron emission tomography (PET)-derived serotonin release capacity.	Approximately 3 hours after dosing on each of the two imaging visits
Subjective drug effect - Visual Analogue Scale (VAS): Drug high	On both imaging days (dl-fenfluramine and placebo), participants rate their subjective experience using a Visual Analogue Scale (VAS). For this item, scores range from 0 ("not at all") to 100 ("extremely"), where higher scores indicate a greater sense of drug-induced high. Ratings are obtained at approximately 3 hours after dosing and will be compared between dl-fenfluramine and placebo within participants, between Major Depressive Disorder (MDD) and healthy control groups, and examined in relation to dl-fenfluramine plasma levels, prolactin response, and positron emission tomography (PET)-derived serotonin release capacity	Approximately 3 hours after dosing on each of the two imaging visits
Subjective drug effect - Visual Analogue Scale (VAS): Stimulated	On both imaging days (dl-fenfluramine and placebo), participants rate their subjective experience using a Visual Analogue Scale (VAS). For this item, scores range from 0 ("not at all") to 100 ("extremely"), where higher scores indicate greater perceived stimulation. Ratings are obtained at approximately 3 hours after dosing and will be compared between dl-fenfluramine and placebo within participants, between Major Depressive Disorder (MDD) and healthy control groups, and examined in relation to dl-fenfluramine plasma levels, prolactin response, and positron emission tomography (PET)-derived serotonin release capacity.	Approximately 3 hours after dosing on each of the two imaging visits
Subjective drug effect - Visual Analogue Scale (VAS): Talkative	On both imaging days (dl-fenfluramine and placebo), participants rate their subjective experience using a Visual Analogue Scale (VAS). For this item, scores range from -50 ("not at all") to +50 ("extremely"), with 0 representing the participant's normal state, where higher scores indicate greater than normal talkativeness and lower scores indicate less than normal talkativeness. Ratings are obtained at approximately 3 hours after dosing and will be compared between dl-fenfluramine and placebo within participants, between Major Depressive Disorder (MDD) and healthy control groups, and examined in relation to dl-fenfluramine plasma levels, prolactin response, and positron emission tomography (PET)-derived serotonin release capacity.	Approximately 3 hours after dosing on each of the two imaging visits
Subjective drug effect - Visual Analogue Scale (VAS): Open	On both imaging days (dl-fenfluramine and placebo), participants rate their subjective experience using a Visual Analogue Scale (VAS). For this item, scores range from -50 ("not at all") to +50 ("extremely"), with 0 representing the participant's normal state, where higher scores indicate greater than normal feelings of openness and lower scores indicate less than normal openness. Ratings are obtained at approximately 3 hours after dosing and will be compared between dl-fenfluramine and placebo within participants, between Major Depressive Disorder (MDD) and healthy control groups, and examined in relation to dl-fenfluramine plasma levels, prolactin response, and positron emission tomography (PET)-derived serotonin release capacity.	Approximately 3 hours after dosing on each of the two imaging visits
Subjective drug effect - Visual Analogue Scale (VAS): Concentration	On both imaging days (dl-fenfluramine and placebo), participants rate their subjective experience using a Visual Analogue Scale (VAS). For this item, scores range from -50 ("not at all") to +50 ("extremely"), with 0 representing the participant's normal state, where higher scores indicate greater than normal concentration and lower scores indicate reduced concentration relative to normal. Ratings are obtained at approximately 3 hours after dosing and will be compared between dl-fenfluramine and placebo within participants, between Major Depressive Disorder (MDD) and healthy control groups, and examined in relation to dl-fenfluramine plasma levels, prolactin response, and positron emission tomography (PET)-derived serotonin release capacity.	Approximately 3 hours after dosing on each of the two imaging visits
Subjective drug effect - Visual Analogue Scale (VAS): Sense of time	On both imaging days (dl-fenfluramine and placebo), participants rate their subjective experience using a Visual Analogue Scale (VAS). For this item, scores range from -50 ("not at all") to +50 ("extremely"), with 0 representing the participant's normal state, where higher scores indicate a greater than normal sense of time passing and lower scores indicate a reduced sense of time relative to normal. Ratings are obtained at approximately 3 hours after dosing and will be compared between dl-fenfluramine and placebo within participants, between Major Depressive Disorder (MDD) and healthy control groups, and examined in relation to dl-fenfluramine plasma levels, prolactin response, and positron emission tomography (PET)-derived serotonin release capacity.	Approximately 3 hours after dosing on each of the two imaging visits
Subjective drug effect - Visual Analogue Scale (VAS): Speed of thinking	On both imaging days (dl-fenfluramine and placebo), participants rate their subjective experience using a Visual Analogue Scale (VAS). For this item, scores range from -50 ("slowed") to +50 ("racing"), with 0 representing the participant's normal state, where positive scores indicate faster than normal thinking and negative scores indicate slower than normal thinking. Ratings are obtained at approximately 3 hours after dosing and will be compared between dl-fenfluramine and placebo within participants, between Major Depressive Disorder (MDD) and healthy control groups, and examined in relation to dl-fenfluramine plasma levels, prolactin response, and positron emission tomography (PET)-derived serotonin release capacity.	Approximately 3 hours after dosing on each of the two imaging visits

Other Outcome Measures

Outcome Measure	Measure Description	Time Frame
Baseline trait anxiety - Spielberger State-Trait Anxiety Inventory, Trait subscale (STAI-T) total score	At the screening/baseline visit, all participants complete the Spielberger State-Trait Anxiety Inventory - Trait subscale (STAI Form Y-2), a 20-item self-report measure of stable, dispositional anxiety. Total scores range from 20 to 80, where higher scores indicate greater trait anxiety. This baseline score will be used to characterise the psychological profile of each group and examined in relation to PET-derived serotonin release capacity, other biological measures, and subsequent SSRI treatment response in the MDD group.	Screening/baseline visit prior to any imaging or pharmacological intervention
Baseline anhedonia - Snaith-Hamilton Pleasure Scale (SHAPS) total score	At the screening/baseline visit, all participants complete the Snaith-Hamilton Pleasure Scale (SHAPS), a 14-item self-report measure of hedonic tone (the capacity to experience pleasure). Each item is rated on a 4-point Likert scale (0 = strongly disagree to 3 = strongly agree), with positively worded items reverse-coded, giving total scores ranging from 0 to 42, where higher scores indicate greater anhedonia (reduced ability to experience pleasure). This baseline score will be used to characterise the psychological profile of each group and examined in relation to Positron emission tomography (PET) scan-derived serotonin release capacity, other biological measures, and subsequent Selective Serotonin Reuptake Inhibitors (SSRIs) treatment response in the Major Depressive Disorder (MDD) group.	Screening/baseline visit prior to any imaging or pharmacological intervention
Baseline depressive rumination - Ruminative Responses Scale (RRS) total score	At the screening/baseline visit, all participants complete the Ruminative Responses Scale (RRS), a 22-item self-report measure of the tendency to ruminate in response to low mood. Total scores range from 22 to 88, where higher scores indicate greater ruminative responding. This baseline score will be used to characterise the psychological profile of each group and examined in relation to Positron emission tomography (PET) scan-derived serotonin release capacity, other biological measures, and subsequent Selective Serotonin Reuptake Inhibitors treatment response in the Major Depressive Disorder (MDD) group.	Screening/baseline visit prior to any imaging or pharmacological intervention
Baseline mental wellbeing - Warwick-Edinburgh Mental Wellbeing Scale (WEMWBS) total score	At the screening/baseline visit, all participants complete the Warwick-Edinburgh Mental Wellbeing Scale (WEMWBS), a 14-item self-report measure of mental wellbeing covering positive affect and functioning over the past two weeks. Total scores range from 14 to 70, where higher scores indicate greater mental well-being. This baseline score will be used to characterise the psychological profile of each group and examined in relation to Positron emission tomography (PET) scan-derived serotonin release capacity, other biological measures, and subsequent Selective Serotonin Reuptake Inhibitors treatment response in the Major Depressive Disorder (MDD) group.	Screening/baseline visit prior to any imaging or pharmacological intervention
Baseline personality traits - Big Five Inventory (BFI-44) Extraversion	At the screening/baseline visit, all participants complete the 44-item Big Five Inventory (BFI-44). The Extraversion subscale comprises 8 items (items 1, 6R, 11, 16, 21R, 26, 31R, 36), each rated on a 5-point scale (1 = disagree strongly to 5 = agree strongly). Subscale scores range from 8 to 40, where higher scores indicate greater extraversion. This baseline score will be used to characterise the personality profile of each group and examined in relation to Positron emission tomography (PET) scan-derived serotonin release capacity, other biological measures, and subsequent Selective Serotonin Reuptake Inhibitors treatment response in the Major Depressive Disorder (MDD) group.	Screening/baseline visit prior to any imaging or pharmacological intervention
Baseline personality trait - Big Five Inventory (BFI-44) Agreeableness subscale score	At the screening/baseline visit, all participants complete the 44-item Big Five Inventory (BFI-44). The Agreeableness subscale comprises 9 items (items 2R, 7, 12R, 17, 22, 27R, 32, 37R, 42), each rated on a 5-point scale (1 = disagree strongly to 5 = agree strongly). Subscale scores range from 9 to 45, where higher scores indicate greater agreeableness. This baseline score will be used to characterise the personality profile of each group and examined in relation to positron emission tomography (PET)-derived serotonin release capacity, other biological measures, and subsequent selective serotonin reuptake inhibitor (SSRI) treatment response in the Major Depressive Disorder (MDD) group.	Screening/baseline visit prior to any imaging or pharmacological intervention
Baseline personality trait - Big Five Inventory (BFI-44) Conscientiousness subscale score	At the screening/baseline visit, all participants complete the 44-item Big Five Inventory (BFI-44). The Conscientiousness subscale comprises 9 items (items 3, 8R, 13, 18R, 23R, 28, 33, 38, 43R), each rated on a 5-point scale (1 = disagree strongly to 5 = agree strongly). Subscale scores range from 9 to 45, where higher scores indicate greater conscientiousness. This baseline score will be used to characterise the personality profile of each group and examined in relation to positron emission tomography (PET)-derived serotonin release capacity, other biological measures, and subsequent selective serotonin reuptake inhibitor (SSRI) treatment response in the Major Depressive Disorder (MDD) group.	Screening/baseline visit prior to any imaging or pharmacological intervention
Baseline personality trait - Big Five Inventory (BFI-44) Neuroticism subscale score	At the screening/baseline visit, all participants complete the 44-item Big Five Inventory (BFI-44). The Neuroticism subscale comprises 8 items (items 4, 9R, 14, 19, 24R, 29, 34R, 39), each rated on a 5-point scale (1 = disagree strongly to 5 = agree strongly). Subscale scores range from 8 to 40, where higher scores indicate greater neuroticism (emotional instability). This baseline score will be used to characterise the personality profile of each group and examined in relation to positron emission tomography (PET)-derived serotonin release capacity, other biological measures, and subsequent selective serotonin reuptake inhibitor (SSRI) treatment response in the Major Depressive Disorder (MDD) group.	Screening/baseline visit prior to any imaging or pharmacological intervention
Baseline personality trait - Big Five Inventory (BFI-44) Openness subscale score	At the screening/baseline visit, all participants complete the 44-item Big Five Inventory (BFI-44). The Openness to Experience subscale comprises 10 items (items 5, 10, 15, 20, 25, 30, 35R, 40, 41R, 44), each rated on a 5-point scale (1 = disagree strongly to 5 = agree strongly). Subscale scores range from 10 to 50, where higher scores indicate greater openness to experience. This baseline score will be used to characterise the personality profile of each group and examined in relation to positron emission tomography (PET)-derived serotonin release capacity, other biological measures, and subsequent selective serotonin reuptake inhibitor (SSRI) treatment response in the Major Depressive Disorder (MDD) group.	Screening/baseline visit prior to any imaging or pharmacological intervention
Baseline sense of connectedness - Watts Connectedness Scale (WCS) total score	At the screening/baseline visit, all participants complete the Watts Connectedness Scale (WCS), a 23-item self-report measure of felt connectedness to self, others, and world/universe, with each item rated on a visual analogue scale from 0 to 100. The overall General Connectedness score is the mean of the three subscale means and ranges from 0 to 100, where higher scores indicate a greater sense of connectedness. This baseline score will be used to characterise the psychological profile of each group and examined in relation to PET-derived serotonin release capacity, other biological measures, and subsequent SSRI treatment response in the MDD group.	Screening/baseline visit prior to any imaging or pharmacological intervention

Collaborators and Investigators

• Sponsor: Imperial College Healthcare NHS Trust
• Collaborators: University of Oxford
• Investigators: Principal Investigator: David Nutt, Professor, Centre for Psychedelic Research, Division of Psychiatry, Department of Brain Sciences, Imperial College London, London, UK; Principal Investigator: Phillip Cowen, Professor, Department of Psychiatry, University of Oxford

Publications and helpful links

General Publications

• Colwell MJ, Tagomori H, Shang F, Cheng HI, Wigg CE, Browning M, Cowen PJ, Murphy SE, Harmer CJ. Direct serotonin release in humans shapes aversive learning and inhibition. Nat Commun. 2024 Aug 9;15(1):6617. doi: 10.1038/s41467-024-50394-x.
• Agnorelli C, Garling HD, Paterson LM, Erritzoe D, Knudsen GM. Recent advances in PET measures of brain 5-HT release. J Cereb Blood Flow Metab. 2026 Mar 19:271678X261427944. doi: 10.1177/0271678X261427944. Online ahead of print.
• Erritzoe D, Godlewska BR, Rizzo G, Searle GE, Agnorelli C, Lewis Y, Ashok AH, Colasanti A, Boura I, Farrell C, Parfitt H, Howes O, Passchier J, Gunn RN, Politis M, Nutt DJ, Cowen PJ, Knudsen GM, Rabiner EA. Brain Serotonin Release Is Reduced in Patients With Depression: A [11C]Cimbi-36 Positron Emission Tomography Study With a d-Amphetamine Challenge. Biol Psychiatry. 2023 Jun 15;93(12):1089-1098. doi: 10.1016/j.biopsych.2022.10.012. Epub 2022 Oct 29.

Helpful Links

• Related Info

Study record dates

Study Major Dates

• Study Start (Estimated): July 1, 2026
• Primary Completion (Estimated): July 1, 2028
• Study Completion (Estimated): December 1, 2028

Study Registration Dates

• First Submitted: June 2, 2026
• First Submitted That Met QC Criteria: June 12, 2026
• First Posted (Actual): June 16, 2026

Study Record Updates

• Last Update Posted (Actual): June 16, 2026
• Last Update Submitted That Met QC Criteria: June 12, 2026
• Last Verified: June 1, 2026

More Information

Terms related to this study

• Keywords: PET; Serotonin; Major Depressive Disorder; SSRI; Fenfluramine; [11C]Cimbi-36; Serotonin Deficiency Theory of Depression
• Additional Relevant MeSH Terms: Mental Disorders; Mood Disorders; Depressive Disorder; Depressive Disorder, Major; Organic Chemicals; Amines; Phenethylamines; Ethylamines; Fenfluramine; Cimbi-36
• Other Study ID Numbers: 354525; APP28983 (Other Grant/Funding Number: Medical Research Council (MRC))

Plan for Individual participant data (IPD)

• Plan to Share Individual Participant Data (IPD)?: YES
• IPD Plan Description: The main study outcomes (primary and secondary) will be reported in peer-reviewed, open-access publications after study completion and once the main analyses have been concluded. Results will be presented in aggregate and, where appropriate, in pseudo-anonymised form that does not allow identification of individual participants. No personally identifiable information (e.g. names, dates of birth, contact details, National Health System numbers) will be shared with other researchers or collaborators outside the sponsor sites. At present, there is no confirmed plan to share de-identified individual participant-level datasets beyond the immediate study team. Any future IPD sharing would require additional approvals and agreements.
• IPD Sharing Time Frame: IPD will be available following publication of primary results (anticipated within 12 months of study completion) and will remain accessible for a minimum of 10 years after the end of data collection
• IPD Sharing Access Criteria: Pseudonymised IPD will be available to researchers for non-commercial research purposes upon submission of a data sharing request to the Chief Investigator. Access is granted under a Data Sharing Agreement requiring review by the sponsor.
• IPD Sharing Supporting Information Type: STUDY_PROTOCOL; SAP; ICF; ANALYTIC_CODE; CSR

Drug and device information, study documents

• Studies a U.S. FDA-regulated drug product: No
• Studies a U.S. FDA-regulated device product: No