Early effects of a novel 5-HT₄R agonist (PF-04995274) and the SSRI citalopram on emotional cognition in unmedicated...

Selective serotonin reuptake inhibitors (SSRIs) are limited by inadequate response in a significant proportion of patients, slow onset, minimal cognitive benefit and side-effects. Preclinical studies suggest selective serotonin 4 receptor (5-HT₄R) agonists may produce faster antidepressant effects via distinct mechanisms; however, there has been no experimental research in clinical populations to date.

To test whether the novel 5-HT₄R partial agonist PF-04995274 produces early behavioural and neural changes in emotional cognition similar to SSRIs in patients with unmedicated major depressive disorder (MDD).

In a double-blind, placebo-controlled trial, 90 participants with MDD were randomised to 7 days of PF-04995274 (15 mg), citalopram (20 mg) or placebo. Emotional processing was assessed using a behavioural facial expression recognition task and functional magnetic resonance imaging (fMRI) of implicit emotional face processing (days 6-9). Observer- and self-reported symptoms of depression were also measured at baseline and study end.

As anticipated, citalopram reduced relative accuracy and increased relative reaction time to identify negative faces, with corresponding changes in neural activity (reduced left amygdala activation to emotional faces and valence-specific shifts in cortical regions). In contrast, PF-04995274 produced no change in behavioural negative bias or amygdala activity but increased medial-frontal cortex activation across valences. While this was not a clinical trial, both active treatments demonstrated an early treatment response with reduced observer-rated depression severity relative to placebo; PF-04995274 also reduced self-reported depression, state anxiety and negative affect.

PF-04995274 did not show the typical antidepressant profile of negative bias reductions observed with citalopram. Instead, it was associated with distinct increased medial-frontal activation during an emotional faces task, coupled with preliminary evidence of early clinical improvement, suggesting a potential alternative pathway for antidepressant effects. Findings support further clinical trials of 5-HT₄R agonists and investigation of pro-cognitive and mood effects.

NCT03516604.