2 results
Characterization of Viloxazine Effects on Cortical Serotonin Neurotransmission at Doses Relevant for ADHD Treatment
- Jennie Garcia-Olivares, Brittney Yegla, Jami Earnest, Vladimir Maletic, ChungPing Yu
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- Journal:
- CNS Spectrums / Volume 28 / Issue 2 / April 2023
- Published online by Cambridge University Press:
- 14 April 2023, p. 235
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Introduction
Most ADHD treatments are thought to be effective due to augmentation of dopamine (DA) and norepinephrine (NE). Our prior preclinical studies found that the ADHD treatment, viloxazine, may augment serotonin (5-HT) in addition to NE and DA; however, it was unclear if these effects occurred at clinically relevant concentrations. To further understand these potential 5-HT effects, we conducted a series of experiments with two objectives: 1) Can we confirm and better elucidate the previously observed serotonergic effects of viloxazine and determine if they occur at clinically relevant concentrations? 2) Are these effects observed in species with close physiology to humans?
MethodsObjective 1: The affinity of viloxazine for human isoforms of 5-HT2A, 5-HT2B, 5-HT2C, and 5-HT7 receptors was assessed via cell-based binding assays. Viloxazine agonism of 5-HT2C and antagonism at 5-HT7 was elucidated with IP1, Ca2+, β-arrestin, internalization, and cAMP assays in cells expressing human receptor isoforms. A microdialysis study was conducted in rats to determine the relationship between viloxazine concentrations in the interstitial fluid (ISF) and changes in NE, DA, 5-HT, and their metabolite concentrations in the prefrontal cortex (PFC). Objective 2: A PET imaging study using a 5-HT2A/2C radioligand agonist, [11C]CIMBI-36, is being conducted in non-human primates (NHPs) to evaluate if viloxazine binds these receptors and/or increases 5-HT release.
Animal research was approved by animal care and use committees. Animals were cared for according to international standards.
ResultsObjective 1: Cell-based assays to measure viloxazine affinity for NET, 5-HT2B, 5-HT2C, and 5-HT7 found Ki values of 0.14, 0.65, 0.84, 1.90 μM respectively. These values were lower than therapeutically relevant rat ISF concentrations (3.5 ± 1.6 μM) approximating pediatric ADHD patients unbound plasma concentrations (2.1-3.3 μM), indicating receptor recruitment. Binding affinity and functional activity assays found viloxazine had negligible activity for 5-HT2A and SERT at therapeutic concentrations. Viloxazine 5-HT2C agonism activated Gq-protein signaling (EC50=1.6 μM, Ca2+ assay), but not β-arrestin or internalization pathways (EC50 values >150 μM). Viloxazine 5-HT7 antagonism decreased Gs-protein signaling (IC50 =6.7 μM). The microdialysis study found that at therapeutically relevant ISF concentrations, 5-HT levels were significantly increased over baseline; no changes were seen in the 5-HIAA metabolite, indicating 5-HT increase is not due to 5-HT reuptake inhibition. Objective 2: PET imaging studies are ongoing.
ConclusionsTo date, our experiments to further elucidate the potential 5-HT effects of viloxazine have shown that the previously observed effects of viloxazine on 5-HT receptors and its augmentation of 5-HT in rat PFC occur at clinically relevant concentrations. Further exploration is needed to ascertain if these effects occur in NHPs and are relevant to ADHD.
FundingSupernus Pharmaceuticals, Inc.
Viloxazine Increases Cortical Serotonin Without Inhibiting Serotonin Reuptake at Doses Used to Treat ADHD
- Jennie Garcia-Olivares, Brittney Yegla, David Zweibaum, Jennifer Koch, Frank Bymaster, ChungPing Yu
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- Journal:
- CNS Spectrums / Volume 28 / Issue 2 / April 2023
- Published online by Cambridge University Press:
- 14 April 2023, p. 237
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- Article
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- You have access Access
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Background
Most FDA-approved ADHD treatments increase norepinephrine (NE) and dopamine (DA); however, our prior preclinical studies of the non-stimulant ADHD treatment viloxazine ER (Qelbree®) demonstrated that viloxazine also increases serotonin (5-HT). A prior microdialysis study showed increases in NE, DA, and 5-HT in the rat prefrontal cortex (PFC); however, the 50 mg/kg dose resulted in supratherapeutic plasma concentrations. Viloxazine is a moderate affinity selective NE reuptake inhibitor, structurally different than traditional SSRI antidepressants. Viloxazine has negligible activity at the serotonin reuptake transporter (SERT), suggesting viloxazine has a different mechanism of 5-HT PFC elevation than SSRIs. The current microdialysis study was undertaken to further characterize if viloxazine affects 5-HT and its 5-HIAA metabolite at therapeutically relevant plasma concentrations. Results are compared to similar microdialysis studies of SSRIs.
MethodsRats were implanted with I-shaped microdialysis probes connected to a microperfusion pump, delivering artificial cerebrospinal fluid, in the PFC. After a 2-hour baseline period, viloxazine (1, 3, 10, or 30 mg/kg) was administered (ip). Dialysate samples were collected from the interstitial fluid (ISF) of the PFC before and after dosing. LC-MS/MS was used to determine the dialysate concentrations of viloxazine and viloxazine-induced changes in NE, 5-HT, and their respective metabolites, DHPG and 5-HIAA. Viloxazine plasma concentrations were also measured.
Animal research was approved by the Institutional Animal Care and Use Committee and conducted in accordance with the National Research Council’s Guide for the Care and Use of Laboratory Animals.
ResultsViloxazine administration resulted in significant dose-dependent increases in ISF NE levels and corresponding decreases in DHPG (NE metabolite) at all doses tested, reflecting viloxazine’s activity as a NET inhibitor. Viloxazine treatment also resulted in a dose-dependent elevation of ISF 5-HT levels in the PFC. Of the doses tested, 30 mg/kg was found to be clinically relevant as it induced ISF concentrations approximating unbound plasma concentrations in pediatric ADHD patients. At this dose, 5-HT levels were significantly increased over baseline and higher than vehicle levels. Coincident changes in 5-HIAA concentrations were not observed, reaffirming viloxazine’s lack of activity as a SERT inhibitor.
ConclusionViloxazine induced dose-dependent increases in NE and 5-HT in the PFC, a critical target region for ADHD therapies. At clinically relevant viloxazine plasma concentrations, 5-HT was increased in the PFC. Unlike SSRIs, which correspondingly decrease the 5-HT metabolite in the PFC (indicating serotonin reuptake inhibition), viloxazine did not affect 5-HIAA levels. Thus, viloxazine increases cortical 5-HT levels by a different mechanism than SSRIs. Whether 5-HT effects aid in viloxazine therapeutic efficacy in ADHD is yet unknown.
FundingSupernus Pharmaceuticals, Inc.