Fenfluramine: a plethora of mechanisms?

Abstract

Developmental and epileptic encephalopathies are rare, treatment-resistant epilepsies with high seizure burden and non-seizure comorbidities. The antiseizure medication (ASM) fenfluramine is an effective treatment for reducing seizure frequency, ameliorating comorbidities, and potentially reducing risk of sudden unexpected death in epilepsy (SUDEP) in patients with Dravet syndrome and Lennox-Gastaut syndrome, among other rare epilepsies. Fenfluramine has a unique mechanism of action (MOA) among ASMs. Its primary MOA is currently described as dual-action sigma-1 receptor and serotonergic activity; however, other mechanisms may be involved. Here, we conduct an extensive review of the literature to identify all previously described mechanisms for fenfluramine. We also consider how these mechanisms may play a role in the reports of clinical benefit in non-seizure outcomes, including SUDEP and everyday executive function. Our review highlights the importance of serotonin and sigma-1 receptor mechanisms in maintaining a balance between excitatory (glutamatergic) and inhibitory (γ-aminobutyric acid [GABA]-ergic) neural networks, and suggests that these mechanisms may represent primary pharmacological MOAs in seizures, non-seizure comorbidities, and SUDEP. We also describe ancillary roles for GABA neurotransmission, noradrenergic neurotransmission, and the endocrine system (especially such progesterone derivatives as neuroactive steroids). Dopaminergic activity underlies appetite reduction, a common side effect with fenfluramine treatment, but any involvement in seizure reduction remains speculative. Further research is underway to evaluate promising new biological pathways for fenfluramine. A better understanding of the pharmacological mechanisms for fenfluramine in reducing seizure burden and non-seizure comorbidities may allow for rational drug design and/or improved clinical decision-making when prescribing multi-ASM regimens.

Full text

Fenfluramine: a plethora of
mechanisms?
Jo Sourbron and Lieven Lagae*
Department of Development and Regeneration, Section Pediatric Neurology, University Hospital KU
Leuven, Leuven, Belgium
Developmental and epileptic encephalopathies are rare, treatment-resistant
epilepsies with high seizure burden and non-seizure comorbidities. The
antiseizure medication (ASM) fenfluramine is an effective treatment for
reducing seizure frequency, ameliorating comorbidities, and potentially
reducing risk of sudden unexpected death in epilepsy (SUDEP) in patients with
Dravet syndrome and Lennox-Gastaut syndrome, among other rare epilepsies.
Fenfluramine has a unique mechanism of action (MOA) among ASMs. Its primary
MOA is currently described as dual-action sigma-1 receptor and serotonergic
activity; however, other mechanisms may be involved. Here, we conduct an
extensive review of the literature to identify all previously described
mechanisms for fenfluramine. We also consider how these mechanisms may
play a role in the reports of clinical benefit in non-seizure outcomes, including
SUDEP and everyday executive function. Our review highlights the importance of
serotonin and sigma-1 receptor mechanisms in maintaining a balance between
excitatory (glutamatergic) and inhibitory (γ-aminobutyric acid [GABA]-ergic)
neural networks, and suggests that these mechanisms may represent primary
pharmacological MOAs in seizures, non-seizure comorbidities, and SUDEP. We
also describe ancillary roles for GABA neurotransmission, noradrenergic
neurotransmission, and the endocrine system (especially such progesterone
derivatives as neuroactive steroids). Dopaminergic activity underlies appetite
reduction, a common side effect with fenfluramine treatment, but any
involvement in seizure reduction remains speculative. Further research is
underway to evaluate promising new biological pathways for fenfluramine. A
better understanding of the pharmacological mechanisms for fenfluramine in
reducing seizure burden and non-seizure comorbidities may allow for rational
drug design and/or improved clinical decision-making when prescribing multi-
ASM regimens.
KEYWORDS
fintepla, pathways, serotonin, sigma, disease modification, epilepsy
1 Introduction
Epilepsy, a neurological disorder characterized by seizures, affects up to 70 million
people worldwide (Singh and Trevick, 2016). The mainstay of treatment remains controlling
seizures by antiseizure medications (ASMs). Since epilepsy is a heterogeneous condition,
there is no perfect ASM for all epilepsy patients. The optimal treatment strategy is dependent
on etiology, patient-specific factors (e.g., seizure type, sex, age, comorbidities, family history)
and ASM characteristics (drug interaction profile, adverse effects, costs) (www.nice.org.uk/
guidance/CG137). ASMs can act through different pathways and subsequently increase
neuronal inhibition and/or decrease neuronal excitation. A primary mechanism of many
ASMs is by sodium channel blockade and/or enhancement of neurotransmission by γ-
OPEN ACCESS
EDITED BY
Philippe De Deurwaerdere,
Université de Bordeaux, France
REVIEWED BY
Kinga Aurelia Gawel,
Medical University of Lublin, Poland
*CORRESPONDENCE
Lieven Lagae,
lieven.lagae@uzleuven.be
RECEIVED 22 March 2023
ACCEPTED 10 April 2023
PUBLISHED 12 May 2023
CITATION
Sourbron J and Lagae L (2023),
Fenfluramine: a plethora of mechanisms?
Front. Pharmacol. 14:1192022.
doi: 10.3389/fphar.2023.1192022
COPYRIGHT
© 2023 Sourbron and Lagae. This is an
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Attribution License (CC BY). The use,
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which does not comply with these terms.
Frontiers in Pharmacology frontiersin.org01
TYPE Mini Review
PUBLISHED 12 May 2023
DOI 10.3389/fphar.2023.1192022

aminobutyric acid (GABA) (Löscher and Klein, 2021;Strzelczyk and
Schubert-Bast, 2022). The development of ASMs increased
tremendously in the past 30 years. Second- and third-generation
ASMs have various novel molecular targets (e.g., voltage-gated
cation channels, glutamate [GLUT], GABA turnover, synaptic
vesicle protein 2A) (Loscher et al., 2013;Löscher, 2021).
Nonetheless, about one-third of patients with epilepsy are
unable to achieve seizure control on their current ASM regimens
(Loscher et al., 2013) (i.e., patients with developmental and epileptic
encephalopathies [DEE]). Patients with DEE experience severe,
drug-resistant seizures and developmental delay due to both
epileptiform activity and the underlying pathology of their
condition (Specchio et al., 2022). DEE can cause developmental,
social, emotional, and physical dysfunctions secondary to seizures or
as a direct result of either the underlying pathology or the induced
neurochemical alterations (Nabbout et al., 2013;Gataullina and
Dulac, 2017). To reduce seizure frequency and, ideally, also alleviate
comorbidities, ASMs should have novel, preferably multimodal,
mechanisms of action.
In this short review, we will focus on fenfluramine (FFA), an
ASM with mechanisms of action unique among ASMs (Reeder et al.,
2021a;Martin et al., 2021), which is now approved in the US,
Europe, the UK, and Japan as add-on therapy in patients with
Dravet syndrome, as well as in the US for treating patients with
Lennox-Gastaut syndrome (LGS) (Zogenix, 2022). Clinical trials are
currently underway to evaluate FFA’s potential as an ASM for other
DEEs when added to a patient’s current standard-of-care regimen
(https://clinicaltrials.gov/ct2/show/NCT05232630)(Devinsky et al.,
2021;Aledo-Serrano ÁCabal-Paz et al., 2022).
FFA’s mechanisms of action have been studied extensively.
High-dose FFA (60–120 mg/day) was originally marketed as an
anti-obesity drug that reduced food intake through serotonergic
activation of hypothalamic energy homeostasis circuits. With the
discovery of its potent antiseizure properties (Schoonjans et al.,
2015), low-dose FFA (0.2–0.7 mg/kg/day; maximum 26 mg/day)
was re-developed to an ASM (Schoonjans et al., 2015;
Johannessen Landmark et al., 2021). The pharmacological
mechanisms underlying the antiseizure effects of FFA have been
the subject of extensive research in recent years. According to the
current hypothetical model, FFA enhances GABAergic signaling via
activity at serotonin (5-hydroxytryptophan, 5-HT) receptors and
inhibits excitatory signaling through sigma-1 (σ1)-mediated
mechanisms, thereby restoring the balance between inhibition
and excitation (Sourbron et al., 2017;Martin et al., 2020;
Sourbron and Lagae, 2022). Nonetheless, other mechanisms are
likely to be involved. Recent data suggest that FFA confers clinical
benefit beyond seizure reduction alone (Jensen et al., 2022;Jensen
et al., 2023), including improvements in everyday executive function
(defined as self-regulation of emotions, behavior, and cognition or
working memory operations) (Bishop et al., 2021a;Bishop et al.,
2022a) and reduction in sudden unexpected death in epilepsy
(SUDEP) (Cross et al., 2021).
Since there is currently no clear, comprehensive overview
regarding FFA’s pharmacological mechanisms in the literature,
our aim was to concisely summarize all the known mechanisms
of FFA on seizures and ancillary mechanisms that may be related to
seizure control, as well as consider additional mechanisms of its
observed clinical benefit in non-seizure comorbidities and survival.
Our PubMed search (July 2022) retrieved 622 articles, of which
79 contained relevant information regarding the mechanisms
of FFA.
The proposed mechanisms of fenfluramine at the synaptic level
of neurotransmission in the context of DEEs are presented in
Figure 1. At a synaptic and cellular level, FFA modulates
serotonergic and σ1-related pathways, respectively (Figure 1, left
and right, respectively). In Figure 2, we provide an overview of the
A) mechanisms and B) clinical efficacy data of FFA.
2 Primary mechanisms of fenfluramine
antiseizure activity
2.1 Serotonergic neurotransmission
Fenfluramine (FFA, 3-trifluoromethyl-N-ethylamphetamine) is
a racemic mixture of levo-FFA and dextro-FFA (Balagura et al.,
2020;Odi et al., 2021). Both enantiomers are rapidly metabolized to
norfenfluramine, which is also pharmacologically active via multiple
mechanisms (Marchant et al., 1992;Bever and Perry, 1997). Dextro-
FFA (dexfenfluramine) promotes serotonergic neurotransmission
by inhibition of serotonin (5-hydroxytryptophan, 5-HT) reuptake
and stimulation of 5-HT release (Kannengiesser et al., 1976;
Garattini and Samanin, 1978;Baumann et al., 2014).
Subsequently, different 5-HT subtype receptors can be activated,
of which several have been associated with the anticonvulsant effects
of FFA in the last 5 years (Supplemental Table S1). In addition, FFA
has agonist activity at distinct 5-HT receptors (see section on 5-HT
receptors below). Furthermore, 5-HT itself plays a crucial role in
normal brain physiology, and distinct 5-HT receptors are involved
in seizure-reducing effects (as well as non-seizure outcomes). Hence,
it is not surprising that defective serotonergic neurotransmission
could be related to epilepsy (Di Giovanni, 2013;Guiard and
Giovanni, 2015;Svob Strac et al., 2016;Zarcone and Corbetta,
2017;Deidda et al., 2021).
2.2 Serotonin receptors
Of the 14 known 5-HT receptors, six subtypes have confirmed
FFA activity (Supplemental Table S1), including agonist activity at
5-HT1D, 5-HT2A, 5-HT2B, 5-HT2C, and 5-HT4, and antagonist
activity at 5-HT1A (Rothman et al, 2003;Sourbron et al., 2017;
Rodríguez-Muñoz et al., 2018;Tupal and Faingold, 2019;Martin
et al., 2020;Reeder et al., 2021b;Tupal and Faingold, 2021). Activity
at 5-HT7 has more recently been described (Faingold and Tupal,
2019). Early binding studies showed high affinity of norfenfluramine
for 5-HT2A, 5-HT2B, and 5-HT2C, while fenfluramine was a weak
agonist with low affinity for any 5-HT2 receptor (Porter et al., 1999;
Rothman et al., 2003). Binding assays confirmed low affinity for 5-
HT1A, with antagonist activity in functional assays in vitro (Martin
et al., 2020). Studies in a zebrafish model of Dravet syndrome
demonstrated that treatment with FFA in the presence of
antagonists to 5-HT1D, 5-HT2A, and/or 5-HT2C receptors no
longer inhibited spontaneous seizures, suggesting that agonist
activity at these receptor subtypes may be responsible for
reducing seizure frequency (Sourbron et al., 2017). Consistent
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with this report, subsequent studies showed that other 5-HT1D
agonists were effective in several zebrafish seizure models (Sourbron
et al., 2016;Sourbron et al., 2017;Gooshe et al., 2018;Sourbron et al.,
2019) and two rodent seizure models (Gooshe et al., 2018;Hatini
and Commons, 2020). More recent preclinical studies showed that
seizure reduction and/or reduction of SUDEP by FFA also may be
associated, at least partially, with stimulation of the 5-HT4 (Tupal
and Faingold, 2021).
FFA was reported to induce valvular heart disease and
pulmonary arterial hypertension, potentially due to 5-HT2B
stimulation (Rothman et al., 2000); however, these effects were
most likely related to high dosages (up to 160 mg/day),
combination treatment with other 5-HT2B agonists (such as
phentermine), and/or other cardiovascular risk factors (older age/
female sex/hypertension) (Rothman et al., 2000). The 5-HT2B
receptor subtype is expressed in low abundance in the brain
(Rothman et al., 2000;Launay et al., 2002;Elangbam, 2010;
Sourbron and Lagae, 2022) and does not appear to play a role in
FFA’s antiseizure effects (Sourbron et al., 2017;Odi et al., 2021).
Current long-term clinical data support the cardiovascular safety of
FFA in treating patients with epilepsy at much lower dosages
(Schoonjans and Ceulemans, 2022). In a comprehensive long-
term open label study conducted in patients with Dravet
syndrome, no cases of valvular heart disease or pulmonary
arterial hypertension were reported in 327 patients treated with
FFA for a median treatment duration of 23.9 months at a median
FFA dose of 0.44 mg/kg/day (Agarwal et al., 2022). Regular follow-
up echocardiography is advised before initiating FFA and during
treatment (Schoonjans et al., 2017).
As summarized in a prior review (Sourbron and Lagae, 2022),
the serotonergic mechanisms of FFA include: 1) increase of
GABAergic dendritic arborization via serotonergic and
GABAergic activity (see below); 2) decrease of 5-HT reuptake by
inhibition of 5-HT transporters (SERT); 3) increase of release and
fusion of synaptic vesicles (filled with 5-HT); 4) 5-HT increase in the
synaptic cleft—via (2) and (3)—and subsequently stimulation of
FIGURE 1
Schematic mechanisms of fenfluramine at a synaptic level (5-HT; left) and cellular level (σ1; right).
FIGURE 2
Proposed mechanisms of fenfluramine and its efficacy (A) Previously reported pathways are in black; mechanisms that are not yet confirmed in
animal models of epilepsy are presented in orange (B) Efficacies confirmed by clinical data before the start of fenfluramine in clinical trials of epilepsy
patients are presented in black; activities that were recently confirmed by clinical trials of epilepsy patients are presented in orange. For the corresponding
references, please refer to the text. 5-HT, serotonin; ACTH, adrenocorticotropic hormone; ADHD, attention deficit hyperactivity disorder; GABA, γ-
aminobutyric acid; GLUT, glutamine; OCD, obsessive compulsive disorder; σ1, sigma-1; SERT, serotonin transporter; SUDEP, sudden unexpected death in
epilepsy.
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different 5-HT receptor subtypes and 5) direct stimulation of at least
four.
5-HT receptor subtypes (5-HT1D, 2A, 2C, and 4), which
increases GABA inhibitory input and decreases glutaminergic
excitatory output.
2.3 Sigma-1 pathway
FFA has high (sub-micromolar) affinity for the σ1 receptor
(Martin et al., 2020). Contradictory findings have been reported
about the action of FFA on σ1 receptors (Rodríguez-Muñoz et al.,
2018;Sourbron and Lagae, 2022). However, a growing body of
corroborating evidence supports that FFA acts as a positive
modulator of σ1 receptors. In a mouse model of dizocilpine-
induced learning deficits, FFA acted as a positive modulator of
σ1 receptors (Martin et al., 2020). Further, dextro-FFA reduced
dizocilpine-induced deficits in spatial memory by positive
modulation of 5-HT receptors by the σ1 receptor (Martin et al.,
2022). Further in vitro and in vivo studies underlined these positive
modulatory effects of FFA, which were related to their antiseizure
activities (Vavers et al., 2019;Martin et al., 2021) and potentially also
contribute to the prevention of SUDEP (Ning et al., 2021). The
reason for both agonist and antagonist activity reported at the σ1
receptor with FFA treatment is unclear, but may be due to the
biphasic dose response of σ1 receptor modulation (Maurice, 2021).
As outlined concisely in a prior review (Martin et al., 2021), FFA
restores the loss of GABAergic tone via mediating the σ1interaction
with the NMDA receptor that leads to a dampening of calcium influx
and decreasing seizure activities at glutaminergic synapses. Modulation
of these calcium fluxes (in the endoplasmic reticulum via aGq/
inositolphosphate3-receptor mediated mechanism) is also under the
control of serotonergic neurotransmission. σ1 receptor-client protein
interactions initiate a host of signal transduction cascades, including
other ion channels besides NMDA (e.g., potassium, sodium, and
voltage-regulated chloride channels), as well as interaction with
trophic factor receptors and kinases. Finally, the interaction with
Rac-GTPases promotes dendritic spine formation and affects
neuronal redox processes, which likely contributes to its antiseizure
(Vavers et al., 2017) and potentially antidepressant effects (Voronin
et al., 2020). The effects of FFA on these downstream second messenger
systems remain to be elucidated, but FFA interaction with the
σ1 receptor could potentially mediateanyofthesedownstream
effects to produce antiseizure effects. Further studies are needed to
determine which second messenger systems contribute to antiseizure
effects of FFA in response to σ1 receptor activation.
2.4 GABA neurotransmission
The loss of GABAergic neurotransmission is a major
contributor to epileptogenesis in numerous preclinical models of
epilepsy (de Lanerolle et al., 1989;Sundstrom et al., 2001;Swartz
et al., 2006;Oakley et al., 2011;Houser, 2014). FFA enhances
GABAergic neurotransmission by 5-HT release at GABAergic
synapses and stimulating 5-HT2A and 5-HT2C receptors (Shen
and Andrade, 1998;Higgins et al., 2014;Martin et al., 2014;Guiard
and Giovanni, 2015). Further, FFA has been shown to restore
dendritic arborization of GABAergic neurons in a Dravet
syndrome zebrafish model of Dravet syndrome (Tiraboschi et al.,
2020). Taken together, FFA may restore inhibitory synaptic inputs
by a combined effect of preserving the GABAergic dendritic
architecture and enhancing GABA neurotransmission in Dravet
syndrome and other DEEs.
3 Ancillary mechanisms of fenfluramine
3.1 Dopaminergic neurotransmission
Levo-FFA, lacking serotonergic activity in contrast to dextro-
FFA, can modulate dopaminergic transmission (Invernizzi et al.,
1989;Baumann et al., 2000;Wurtman and Fenfluramine, 2018).
Some studies reported that the increase of extracellular dopamine by
FFA is mediated by its primary effect on 5-HT (Balcioglu and
Wurtman, 1998;Ledonne et al., 2009). However, these effects on
dopaminergic transmission are rather small compared to 5-HT
modulation (Crespi et al., 1997;Rothman et al., 2008) and
appear to be high-dose related (Balcioglu and Wurtman, 1998).
Furthermore, FFA does not seem to bind directly to dopaminergic
receptors (Invernizzi et al., 1989;Martin et al., 2020). There is only
one case report that links the assumed dopaminergic-enhancing
effects of FFA to seizure control (Clemens, 1988), and moreover this
study did not involve experiments to prove that the beneficial effects
of FFA on self-induced seizures were related to dopamine. In
contrast, other studies suggest a decrease of dopamine or
dopaminergic neurotransmission by FFA (Garattini and Samanin,
1978;Invernizzi et al., 1989;Sourbron et al., 2017). The impact of
FFA on dopaminergic modulation appears to be more relevant to
reduced appetite (a known side effect of FFA) than to seizure control
by affecting the pleasurable aspects of feeding behavior (Rothman
et al., 2008;Ledonne et al., 2009).
3.2 Noradrenergic neurotransmission
FFA modulates noradrenergic neurotransmission, a mechanism
that may contribute to the clinical benefit associated with
amelioration of concentration problems, learning difficulties, and
attention deficit hyperactivity disorder (ADHD) (Donnelly et al.,
1989;Aman et al., 1993;Reeder et al., 2021a;Jensen et al., 2021;
Jensen et al., 2022). However, in epilepsy patients, FFA-associated
improvements in cognitive domains, including self-regulation and
everyday executive function, can also be related to FFA-induced
seizure reduction (Besag and Vasey, 2021) in addition to a direct
effect not mediated by seizure reduction (Martin et al., 2022).
FFA has direct effects on adrenergic receptors and their target
receptors. At high, supratherapeutic concentrations in vitro
(>10 µM), dextro-FFA can stimulate alpha 1-adrenergic
receptors, resulting in a metabolic shift from glucose production
(gluconeogenesis) to glycose degradation (glycolysis), which is
mediated by a change in glucose 6-phosphate (Comte et al.,
1997). One could speculate that the increase of glycolysis could
be related to a decrease in epileptic activity since decreased glycolysis
impairs neuronal function, and glycolysis sustains normal synaptic
function (Li et al., 2000). However, most studies indicate that
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inhibition, rather than stimulation, of glycolysis is associated with
antiseizure activities (Fei et al., 2020). Without additional data with
FFA at physiologically relevant concentrations, it is difficult to
conclude what effects FFA has on alpha-adrenergic receptors,
and whether these effects contribute to FFA’s antiseizure effects
observed in patients with DEEs.
Inhibiting beta-adrenergic receptors attenuated maximal
electroshock-induced seizures in mice and audiogenic seizures in
DBA/2 mice (Lints and Nyquist-Battie, 1985;Luchowska et al.,
2001), suggesting a role for beta-adrenergic receptors in
epileptogenesis. FFA and norfenfluramine bind to beta 2-
adrenergic receptors with micromolar affinity (1.26 x 10
−5
and
8.77 x 10
−6
, respectively (Martin et al., 2016)). Selective
antagonism of beta 2-adrenergic receptors in a zebrafish model
of Dravet syndrome had no effect on spontaneous epileptiform
activity (Sourbron et al., 2017), arguing against a direct effect on this
receptor subtype. However, we cannot exclude an indirect effect of
FFA on the adrenergic receptors and their pharmacologic targets, as
FFA can decrease the noradrenaline content in the brains of
zebrafish larvae (Sourbron et al., 2017) and rats (Calderini et al.,
1975). This decrease is likely the result of FFA’s effects on 5-HT
(Astorne Figari et al., 2014). Of interest, elevated noradrenaline
transmission has been related to some cases of epilepsy (Fitzgerald,
2010) and even though there are contradictory data (Svob Strac
et al., 2016), there clearly is evidence for the use of noradrenaline-
decreasing drugs for treating neurological diseases, including
epilepsy (Fitzgerald, 2015). Taken together, the data suggest that
any antiseizure activity of FFA on noradrenergic neurotransmission
is likely to be an indirect result decreased levels of noradrenaline in
the brain.
3.3 Endocrine system
FFA targets several neuropeptides, even though the exact role of
these neuro-endocrinological activities remains elusive. First, FFA
increases prolactin in humans (Kavoussi et al., 1999) and primates
(Bethea et al., 2013). Although epileptiform activity in the
hypothalamic pituitary axis (HPA) putatively causes prolactin
secretion (Lusićet al., 1999), there is currently no evidence to
support an antiseizure effect of prolactin secretion.
Second, 5-HT release by FFA stimulates 5-HT2C receptors in
proopiomelanocortin (POMC) neurons of the hypothalamic
melanocortin system that regulate energy homeostasis and
feeding (Smith et al., 2010;He et al., 2021). The melanocortin
peptide adrenocorticotropic hormone (ACTH) is another POMC
derivative that is elevated after FFA treatment (Schürmeyer et al.,
1996). These effects of FFA on the HPA axis have typically been
interpreted in relation to the anorectic properties of FFA as a former
anti-obesity drug. However, ACTH also has antiseizure activity and
is commonly used as an ASM in treating DEEs such as LGS, and
Ohtahara and West syndromes (Strzelczyk and Schubert-Bast,
2022). Further evidence is needed to determine whether activity
of FFA on the HPA after 5-HT2C-induced ACTH release from
POMC neurons contributes to its antiseizure activity.
Third, dextro-FFA specifically activates oxytocinergic and
vasopressinergic neurons in the rat brain (Mikkelsen et al., 1999).
The balance between oxytocin and vasopressin regulates emotions
and behaviors such as anxiety and social behavior. Oxytocin also
reduces epileptic seizures in preclinical studies (Erfanparast et al.,
2017), and vasopressin is related to the pathogenesis of some
epilepsies (Gulec and Noyan, 2002). Additional studies are
needed to determine whether FFA affects the balance between
oxytocin and vasopressin in a way that is clinically meaningful to
its antiseizure effects.
Fourth, Martin et al. (2022) demonstrated that positive
modulation of FFA and the dextro-FFA enantiomer (but not the
levo-enantiomer) on σ1 receptors reversed dizocilpine-induced
amnesia in rodent models, while norfenfluramine (both dextro-
and levo-isomers) acted as an antagonist at σ1 receptors (Martin
et al., 2022). Furthermore, FFA and dextro-FFA activity interacted
synergistically with the neuroactive steroids pregnenolone sulfate or
dehydroepiandrosterone sulfate (both σ1 receptor agonists), and
progesterone (a σ1 receptor antagonist) blocked the anti-amnesic
effect of FFA. These data suggested that the anti-amnesic effects of
FFA may be mediated by amplification of endogenous σ1 receptor
agonists such as neuroactive steroids. Antagonists to 5-HT1A and 5-
HT2A inhibited the effects of FFA, suggesting that the interaction
between σ1 receptors and neuroactive steroids may involve these
receptor subtypes. Clinical studies with the neuroactive steroid
ganaxolone suggest neuroactive steroids may have antiseizure
efficacy in patients with DEEs by acting as non-competitive
antagonists of GABA-A receptors (Knight et al., 2022), but
further studies are needed to determine whether neuroactive
steroids play a role in FFA’s effects on seizures.
In summary, data to date suggest that the effects of FFA on
hormones associated with the HPA (e.g., ACTH, prolactin) or
oxytocin/vasopressin are most likely to affect food intake, with
only weak or speculative evidence for involvement in antiseizure
properties. Neuroactive steroids (e.g., progesterone derivatives) are
attractive candidates for further investigation.
4Efficacy of fenfluramine, beyond
seizures
Clinical and preclinical data support that FFA treatment may
positively impact non-seizure comorbidities in addition to
improving seizure control in patients with DEEs (Figure 2). First,
FFA promoted survival in clinical data and preclinical models
(Reeder et al., 2021b;Cross et al., 2021;Ning et al., 2021;Tupal
and Faingold, 2021). FFA reduced SUDEP mortality rates compared
to pre-treatment rates (1.7 deaths per patient-years after FFA
compared to 11.7 deaths per patient-years pre-FFA treatment)
and historical controls without FFA treatment (9.3 deaths per
1,000 person-years) (Cross et al., 2021). The mechanisms of these
effects are under investigation, but some evidence supports a role for
5-HT4 and σ1 receptors (Ning et al., 2021;Tupal and Faingold,
2021). Preclinical data demonstrated that FFA reduced seizure-
induced respiratory arrest in a mouse model of SUDEP by acting
at 5-HT4 receptors (Tupal and Faingold, 2021). Additional
preliminary data in a mouse model of Dravet syndrome showed
that FFA reduced mortality of FFA-treated animals (Reeder et al.,
2021b). This report showed that FFA may also reduce
neuroinflammation, demyelination, and apoptosis in the
hippocampus, corpus callosum, and/or parietal cortex,
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contributing to survival (Reeder et al., 2021b), but further studies are
needed to confirm these preliminary results and definitively link
these observations to SUDEP or survival.
Second, FFA improved everyday executive functioning,
including regulation of emotions and behavior, in some patients
with Dravet syndrome (Bishop et al., 2021a;Bishop et al., 2022a) and
LGS (Bishop et al., 2021c;Bishop et al., 2022b;Bishop et al., 2021b).
These effects of FFA appeared to be, at least in part, independent of
seizure control (Bishop et al., 2022a). The mechanism of these effects
remains to be established, but data in a dizocilpine-induced amnesia
model suggests that FFA improves spatial learning and memory
(i.e., aspects of cognition) by positively modulating σ1 receptors
(Martin et al., 2020). Further, the activity of FFA at 5-HT4 receptors
may positively affect cognition, as evidence suggests that 5-HT4
receptor agonism enhances learning and memory in clinical studies
(Murphy et al., 2020). Survey data of caregivers of patients with
Dravet syndrome suggest additional clinical benefit beyond seizure
control after FFA treatment, including improved cognitive function,
alertness, education-related outcomes, and focus (Jensen et al., 2022;
Jensen et al., 2023). Additional clinical and preclinical data suggest
improvement of autistic-like behavior, obsessive-compulsive
behavior, everyday executive functioning (i.e., self-regulation of
emotions, cognition, and behavior), alertness, cognition, and QoL
with FFA treatment (Gastaut, 1984;Aicardi and Gastaut, 1985;
Donnelly et al., 1989;Hollander et al., 1992;Aman et al., 1993;
Higgins and Fletcher, 2015;Schoonjans et al., 2017;Bishop et al.,
2021a;Bishop et al., 2021c;Reeder et al., 2021b;Jensen et al., 2021;
Bishop et al., 2022a;Bishop et al., 2022b;Jensen et al., 2022;Bishop
et al., 2021b). Additional studies are needed to determine the
mechanisms underlying these observations in relation to the
clinical effects observed after FFA treatment in patients with DEEs.
5 Conclusion
Current therapeutic approaches to treating severe DEEs support
targeting multiple mechanisms to optimize clinical efficacy. Rationally
designed ASMs have targeted a single receptor or pathway (Roth et al.,
2004).Morerecently,ASMsorcombinationsofASMswithmultimodal
mechanisms of action have been developed to improve clinical efficacy
in treating seizures and non-seizure comorbidities (Cardamone et al.,
2013). FFA is an ASM with multimodal mechanisms of action. We
consider dual-action 5-HT and σ1 receptor activity to be the primary
pharmacological mechanisms of action for FFA’s antiseizure effects.
Those mechanisms, which are well-supported by preclinical data on
antiseizure effects, include balancing inhibitory (GABAergic) and
excitatory (glutamatergic) inputs by: 1) serotonergic neurotransmission
and 5-HT receptor activation, 2) enhancing GABAergic neurotransmission
and preserving GABA neuron dendritic arborization, and 3) activity at
the σ1 receptor. We also reviewed additional pharmacological
mechanisms demonstrated in the literature for FFA and evaluated
the strength of the evidence mediating antiseizure activity and
comorbidities. Of the pathways described, some evidence exists for
neuroactive (progesterone-derivative) steroids, with weaker or
speculative evidence for ACTH, noradrenergic, or dopaminergic
endocrine systems. Interesting additional speculative pharmacological
pathways for further research include myelination and
neuroinflammation. It is important to note that FFA’s multimodal
mechanisms of action will not be mutually exclusive, but rather will act
cooperatively in antiseizure and non-seizure effects. Overall, we
delineate possible specific pathways relevant to FFA that may
inform future studies and contribute to greater understanding of the
pharmacological mechanisms of action of FFA in treating epilepsy and
other conditions.
Author contributions
JS: conceptualization, methodology, validation, formal analysis,
investigation, resources, data curation, writing (Original draft,
review and editing), Visualization. LL: conceptualization,
investigation, writing (review and editing), supervision.
Funding
Medical writing and editing support were provided by Danielle
L. Ippolito, PhD, CMPP, MWC, and Barbara Schwedel, MS, ELS, of
PharmaWrite LLC (Princeton, NJ), and was funded by UCB
Pharma, Inc. In addition, PharmaWrite LLC was involved in the
article processing (open access) charge and provided support with
the submission process, funded by UCB.
Acknowledgments
The authors thank Christian Wolff, PhD, Amélie Lothe, PhD,
and Shikha Polega, PhD (UCB), for critically reviewing the
manuscript.
Conflict of interest
LL received grants, and is a consultant and/or speaker for Zogenix,
LivaNova, UCB, Shire, Eisai, Novartis, Takeda/Ovid, NEL, and
Epihunter.
The remaining author declares that the research was conducted
in the absence of any commercial or financial relationships that
could be construed as a potential conflict of interest.
Publisher’s note
All claims expressed in this article are solely those of the authors
and do not necessarily represent those of their affiliated
organizations, or those of the publisher, the editors and the
reviewers. Any product that may be evaluated in this article, or
claim that may be made by its manufacturer, is not guaranteed or
endorsed by the publisher.
Supplementary material
The Supplementary Material for this article can be found online
at: https://www.frontiersin.org/articles/10.3389/fphar.2023.1192022/
full#supplementary-material
Frontiers in Pharmacology frontiersin.org06
Sourbron and Lagae 10.3389/fphar.2023.1192022

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