Using Fenfluramine to Test the Serotonin Deficiency Theory of Depression

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.