Viloxazine Acts Beyond Norepinephrine: Serotonin Receptor Binding Confirmed at Therapeutic Doses

Viloxazine, the active ingredient in the nonstimulant ADHD medication Qelbree, has long been labeled a selective norepinephrine reuptake inhibitor. A new open-access study published in Drugs in R&D argues that this description is incomplete. In a series of in vitro binding competition assays, functional cellular experiments, and ex vivo receptor occupancy measurements in rats, researchers established that viloxazine binds not only the norepinephrine transporter (NET) but also three serotonin receptors—the 5-HT2C, 5-HT2B, and 5-HT7 subtypes—at concentrations that patients taking the drug actually reach in their blood. The work, led by Jennie Garcia-Olivares, Brittney Yegla, Jennifer Koch, Chungping Yu, and Jonathan Rubin, replicates and extends earlier findings using contemporary methodology, and it points toward a multimodal mechanism of action rather than a single-transporter story.

The motivation for the study came from a persistent puzzle. Viloxazine was synthesized in the 1970s during the search for second-generation antidepressants and was approved in Europe to treat depression, but early pharmacology showed it had poor affinity for beta-adrenergic receptors despite clear antidepressant effects. Multiple early studies indicated that viloxazine affects serotonin neurotransmission, yet these effects were often omitted from descriptions of the drug, which came to be characterized solely by its ability to inhibit NET with little inhibition of the serotonin and dopamine transporters. When the extended-release formulation was approved in the United States for ADHD in children and adults, an unanswered question remained: viloxazine’s potency at NET is far lower than that of atomoxetine, the only other selective NET inhibitor approved for ADHD, with inhibition constants of roughly 2300 nanomolar versus 3.4 nanomolar. Could NET inhibition alone explain viloxazine’s clinical effects?

To address this, the team ran binding competition assays against human isoforms of NET, the serotonin transporter (SERT), and the 5-HT2A, 5-HT2B, 5-HT2C, and 5-HT7 receptors, all expressed in cloned cell systems. The calculated inhibition constants confirmed moderate affinity for NET (Ki = 0.13 micromolar) and very weak binding to SERT (Ki = 14.00 micromolar), a 100-fold selectivity gap. Among the serotonin receptors, viloxazine showed moderate affinity for the agonist site of the nonedited 5-HT2C receptor (Ki = 0.66 micromolar), the antagonist site of 5-HT2B (Ki = 0.83 micromolar), and the agonist site of 5-HT7 (Ki = 1.90 micromolar). Binding to 5-HT2A, included as a homology control, was more than 50-fold weaker than binding to NET. The resulting affinity ranking was NET greater than 5-HT2C approximately equal to 5-HT2B greater than 5-HT7 far greater than 5-HT2A and far greater still than SERT.

Functional assays then revealed what viloxazine actually does at these targets. In a radiolabeled norepinephrine uptake assay using cells expressing human NET, viloxazine inhibited uptake with an IC50 of 0.370 micromolar, consistent with a previously reported value of 0.26 micromolar in rat hypothalamic synaptosomes. At the 5-HT2C receptor, monitored through calcium mobilization, viloxazine acted as an agonist with an EC50 of 1.6 micromolar but displayed submaximal efficacy, with a maximal response of 89.2 percent of the reference—behavior characteristic of a partial agonist. Notably, viloxazine did not recruit beta-arrestin or promote receptor internalization even at concentrations above 150 micromolar, indicating functional selectivity toward the canonical G-protein signaling pathway at 5-HT2C. At 5-HT2B and 5-HT7, viloxazine acted as an antagonist, with weak potencies of IC50 = 36.4 micromolar in the calcium release assay and IC50 = 27.2 micromolar in a cyclic AMP assay, respectively.

The critical question was whether any of this matters at real-world doses. The researchers anchored their analysis to clinically relevant unbound viloxazine plasma concentrations estimated from population pharmacokinetic models: a range of 0.4 to 3.6 micromolar, spanning the lower confidence interval of the average concentration for the 100-milligram-per-day dose to the upper confidence interval for the 400-milligram-per-day dose in children. Plasma concentrations at approved effective doses for adolescents and adults also fall within this range. Using the experimentally measured Ki values in a standard occupancy equation, the team predicted that viloxazine could occupy nearly 95 percent of NET at the high end of the therapeutic range, more than 80 percent of 5-HT2C and 5-HT2B receptors, and up to about 65 percent of 5-HT7 receptors. Potential occupancy at 5-HT2A remained below 40 percent even at the highest concentration, and SERT occupancy was negligible.

To test whether the predicted NET engagement holds in living brain tissue, the group performed an ex vivo occupancy study in rats using the radioligand [11C]methyl reboxetine, developed and conducted at Imperial College London. Twenty rats were divided into five groups receiving viloxazine at 1, 3, 10, or 30 milligrams per kilogram intraperitoneally, or saline vehicle, before radioligand administration; the doses were chosen because they produce unbound plasma concentrations in rats that span the human therapeutic range. Tissue gamma counting across cortex, thalamus, striatum, brain stem, and cerebellum yielded a shared occupancy EC50 of 0.19 micromolar in unbound plasma concentration terms. From this, the authors calculated that viloxazine occupies between 67 and 94 percent of NET at concentrations corresponding to approved therapeutic doses—confirming the in vitro predictions and placing viloxazine in the same occupancy territory as established ADHD medications.

Those comparisons are striking. Positron emission tomography studies with [11C]methyl reboxetine in rhesus monkeys have estimated that atomoxetine occupies more than 90 percent of NET at clinical concentrations, and a study in human patients estimated that methylphenidate occupies 70 to 80 percent of NET at average recommended maintenance doses. Viloxazine’s rat-brain occupancy of 67 to 94 percent falls squarely within that band, supporting the idea that NET inhibition plays a central role in its effects on ADHD symptoms, likely by raising norepinephrine and dopamine levels in the prefrontal cortex. The authors note that the greater NET inhibition at higher doses could be consistent with the somewhat higher response rates observed with doses above 100 milligrams in short-term pediatric studies.

The serotonergic component adds a genuinely new dimension. Prior in vivo microdialysis work by the same group showed that clinically relevant doses of viloxazine increase extracellular serotonin, along with norepinephrine and dopamine, in the rat prefrontal cortex—an effect that cannot be attributed to SERT inhibition, since viloxazine’s potency there is clinically irrelevant and the metabolite profile did not fit a reuptake-blocking mechanism. The new receptor data suggest candidate explanations: 5-HT2B antagonism has been reported to augment dopamine and serotonin prefrontally in animal studies, and modulation at 5-HT2C and 5-HT7 could also shape monoamine signaling. The therapeutic implications remain speculative but intriguing. Research suggests 5-HT2C partial agonism could contribute to reductions in impulsivity, and a recent post hoc analysis of the adult phase III trial suggested viloxazine may contribute to weight loss in adults with ADHD, with greater effects among participants with high baseline body mass index. Potential roles for 5-HT7 antagonists in depression, anxiety, sleep, and cognition, and for 5-HT2B antagonists in substance use disorders and schizophrenia, have been suggested in the literature—conditions that overlap with ADHD.

The authors are careful about the limits of their work. The in vitro and ex vivo methods do not exactly replicate viloxazine’s action at NET and serotonin receptors in the human brain, and extrapolating rat plasma concentrations to human brain concentrations is inexact, so conclusions drawn from comparisons to clinically relevant human plasma ranges are approximate. The contribution of the serotonergic component—specifically the effects of 5-HT2C partial agonism on attention and impulsivity—is being evaluated in a parallel study to be reported separately, and future PET studies in humans could directly measure receptor occupancy at therapeutic doses. Viloxazine metabolites were not evaluated because human studies have shown no metabolites meaningfully contribute to its pharmacological effects.

Even with those caveats, the study delivers a clear message: at plasma concentrations clinically relevant to ADHD treatment, viloxazine has the potential to act simultaneously as a moderate NET inhibitor, a 5-HT2C partial agonist, and a 5-HT2B and 5-HT7 antagonist. That profile distinguishes it from stimulants, which target the dopamine transporter in combination with NET and/or SERT, and from atomoxetine, which is not known to affect serotonin receptors in this way. The authors propose that viloxazine may be best described as a moderate norepinephrine reuptake inhibitor with direct serotonin receptor activity—a renewed, multimodal understanding of a drug that has been in clinical use for half a century, and one that raises new questions about how serotonin modulation might be harnessed in ADHD and other psychiatric conditions.

Subject of Research: Pharmacological characterization of viloxazine's activity at the norepinephrine transporter and serotonin receptors at clinically relevant concentrations

Article Title: Updated Viloxazine Pharmacology: Experiments Establish Norepinephrine Transporter Occupancy and Serotonin 5-HT2C, 5-HT2B, and 5-HT7 Receptor Binding at Therapeutically Relevant Concentrations

Article References: Garcia-Olivares, J., Yegla, B., Koch, J., Yu, C., & Rubin, J. (2026). Updated Viloxazine Pharmacology: Experiments Establish Norepinephrine Transporter Occupancy and Serotonin 5-HT2C, 5-HT2B, and 5-HT7 Receptor Binding at Therapeutically Relevant Concentrations. Drugs in R&D, 26(1), 129-139. https://doi.org/10.1007/s40268-026-00543-y

Image Credits: AI Generated

DOI: 10.1007/s40268-026-00543-y

Keywords: viloxazine, ADHD, norepinephrine transporter, serotonin receptors, 5-HT2C, 5-HT2B, 5-HT7, receptor occupancy, pharmacology, nonstimulant medication, in vitro assays, rat ex vivo study