Updated Viloxazine Pharmacology: Experiments Establish Norepinephrine Transporter Occupancy and Serotonin 5-HT_2C,...

Viloxazine, which has been used to treat depression and attention-deficit/hyperactivity disorder (ADHD), has been termed a moderate-affinity, selective norepinephrine reuptake inhibitor based on high selectivity for norepinephrine relative to serotonin and dopamine transporters. However, accumulated research suggests a more complex mechanism of action, based on studies showing activity at serotonin 5-HT_2C, 5-HT_2B, and 5-HT₇ receptors, as well as findings that viloxazine increases extracellular serotonin (along with norepinephrine and dopamine) in the rat prefrontal cortex. This in vitro pharmacology study aimed to replicate and expand prior experiments to better characterize viloxazine's affinity for and activity at the norepinephrine transporter (NET) and individual serotonin receptors and to clarify how these effects contribute to the mechanism of action.

Using in vitro binding competition and functional assays and ex vivo receptor occupancy studies in rats, we assessed viloxazine activity at human NET isoforms and 5-HT_2C, 5-HT_2B, and 5-HT₇ receptors relative to clinically relevant unbound viloxazine plasma concentrations (0.4-3.6 μM).

Viloxazine showed moderate binding affinity for NET (inhibition constant [K_i] = 0.13 µM) and 5-HT_2C (K_i = 0.66 µM), 5-HT_2B (K_i = 0.83 µM), and 5-HT₇ (K_i = 1.90 µM) receptors. In vitro functional studies showed viloxazine acted as a NET inhibitor, 5-HT_2C partial agonist, and 5-HT_2B and 5-HT₇ antagonist. At clinically relevant concentrations, viloxazine could potentially occupy nearly 95% of NET, more than 80% of 5-HT_2C and 5-HT_2B, and 65% of 5-HT₇ receptors. Subsequent ex vivo studies in rats confirmed high NET occupancy (67-94%) at clinically relevant concentrations.

These results validate previous experiments showing that viloxazine, in addition to displaying high NET occupancy, acts as a partial agonist at 5-HT_2C receptors and an antagonist at 5-HT_2B and 5-HT₇ receptors at clinically relevant concentrations for ADHD treatment. Therefore, both NET inhibition and serotonin receptor activity may contribute to viloxazine's clinical efficacy. These findings are contributing to a renewed understanding of viloxazine's pharmacodynamic profile and likely multimodal mechanism of action.