Etifoxine Hydrochloride: Uses, Properties, and Safety Precautions Explained

Most anxiolytics people have heard of belong to one of two families: benzodiazepines, which act on GABA-A receptors at the benzodiazepine site, and serotonergic agents such as buspirone and the SSRIs. Etifoxine hydrochloride belongs to neither. It is a benzoxazine, a small heterocyclic molecule that has been prescribed for decades in parts of Europe and elsewhere for the psychosomatic manifestations of anxiety, and that modulates inhibitory neurotransmission through a different route from benzodiazepines. It also engages a second target, the translocator protein (TSPO), which connects it to neurosteroid biosynthesis and has driven a surge of preclinical research into neuroprotection and nerve regeneration.

For the pharmaceutical scientist, etifoxine hydrochloride is also an instructive molecule in its own right: a cyclic isourea built on a chlorinated benzo-fused ring, carrying a stereogenic quaternary carbon, and supplied as a hydrochloride salt. Its regulatory story is equally instructive, since a benefit–risk re-evaluation in Europe ended with the drug staying on the market but with firmer contraindications for rare, serious skin and liver reactions. This guide covers the chemical structure and structural formula of etifoxine hydrochloride, its physical and chemical properties, mechanism and uses, the impurity and stability considerations that matter to quality laboratories, and the safety precautions that apply to patients and to anyone handling the material.

What Is Etifoxine Hydrochloride?

Etifoxine hydrochloride is the hydrochloride salt of etifoxine, a non-benzodiazepine anxiolytic of the 4H-3,1-benzoxazine class; anticonvulsant activity has also been described in animal models. It is also known in the research literature by the code HOE 36-801, and the finished product is best known under the brand name Stresam.

It answers one specific question: What is this anxiolytic, and how does its structure differ from the benzodiazepines it is often compared with?

Etifoxine is indicated, where authorized, for psychosomatic manifestations of anxiety, typically when psychotherapeutic approaches alone are not sufficient. Its pharmacology rests on two complementary actions: positive allosteric modulation of GABA-A receptors containing β2 or β3 subunits, and binding to TSPO, which promotes synthesis of neuroactive steroids.

Quick identification reference:

Property	Value
Chemical name (IUPAC)	6-Chloro-N-ethyl-4-methyl-4-phenyl-4H-3,1-benzoxazin-2-amine hydrochloride
Common names / codes	Etifoxine hydrochloride, etifoxine HCl, HOE 36-801 hydrochloride; brand: Stresam
CAS number (hydrochloride)	56776-32-0
CAS number (free base)	21715-46-8
Molecular formula (free base)	C₁₇H₁₇ClN₂O
Molecular formula (hydrochloride)	C₁₇H₁₇ClN₂O·HCl (C₁₇H₁₈Cl₂N₂O)
Molar mass	300.78 g/mol (base); 337.24 g/mol (hydrochloride)
SMILES (salt)	CCNC1=NC2=C(C=C(C=C2)Cl)C(O1)(C)C3=CC=CC=C3.Cl
InChIKey (salt)	SCBJXEBIMVRTJE-UHFFFAOYSA-N
Drug class	Non-benzodiazepine anxiolytic; GABA-A positive allosteric modulator; TSPO ligand

Etifoxine Hydrochloride Structure: The Chemical Structure and Structural Formula Explained

The etifoxine hydrochloride chemical structure is where its pharmacology, its analytical behavior, and its impurity profile all originate, so it is worth reading the structure carefully rather than treating it as a drawing to memorize.

Reading the structural formula

The free base, etifoxine, is built on a 4H-3,1-benzoxazine core: a benzene ring fused to a six-membered ring that contains one nitrogen and one oxygen atom. Working around that heterocyclic ring from the benzo-fused nitrogen:

• Position 1 is a ring nitrogen attached directly to the aromatic ring and double-bonded to C2 (an imine-type nitrogen).
• Position 2 is a carbon that carries an ethylamino group (–NH–CH₂CH₃) and is bonded to both the ring nitrogen and the ring oxygen.
• Position 3 is the ring oxygen.
• Position 4 is a quaternary carbon bearing a methyl group and a phenyl group, and bonded to the ring oxygen and to the benzo-fused carbon.
• Position 6, on the benzo ring, carries a chlorine atom.

The structural formula for etifoxine hydrochloride is therefore the free base C₁₇H₁₇ClN₂O combined with one equivalent of hydrogen chloride, giving the salt C₁₇H₁₇ClN₂O·HCl (also written C₁₇H₁₈Cl₂N₂O). In the salt, protonation occurs at the basic nitrogen of the amino-substituted C=N system, which is why the hydrochloride is the preferred form for handling and formulation.

Key structural features:

• Cyclic isourea (2-amino-benzoxazine) motif — the carbon at position 2 is bonded to a ring oxygen, a ring nitrogen, and an exocyclic amino nitrogen. This arrangement makes the system basic enough to form a stable hydrochloride salt, and it is also the structural feature most likely to be sensitive to hydrolytic stress, which stability studies should therefore probe deliberately
• Stereogenic quaternary carbon at C4 — with four different substituents (methyl, phenyl, ring oxygen, and the benzo-fused carbon), C4 is a stereocenter, so etifoxine exists as a pair of enantiomers. The drug substance is generally described as a racemate; always confirm the stereochemical status against the specific monograph or certificate of analysis
• Chlorinated aromatic ring — the chlorine at position 6 contributes lipophilicity, shapes receptor interactions, and gives a distinctive isotopic pattern in mass spectrometry (a roughly 3:1 M/M+2 ratio) that is useful for identifying related impurities
• Two aromatic rings (benzo-fused and phenyl) — these supply strong UV absorbance, which is why HPLC-UV is the natural workhorse for assay and related substances
• Ethylamino side group — a secondary amine-type N–H that is relevant both to hydrogen bonding and to risk assessments for nitrosamine formation, as discussed below

Why the structural distinction matters

The structure explains why etifoxine is not a benzodiazepine despite acting on the GABA system. Benzodiazepines contain a fused benzene–diazepine ring and bind at the interface of α and γ subunits of the GABA-A receptor. Etifoxine's benzoxazine skeleton is a different ring system, and it acts through the β2 and β3 subunits at a different site. The same structure explains why etifoxine behaves as a basic, salt-forming compound, why it is analytically straightforward to detect by UV, and why its stereocenter and isourea linkage need attention in quality control.

Reading the structure correctly is the first step toward understanding the pharmacology, the stability profile, and the analytical strategy for this compound.

Physical and Chemical Properties of Etifoxine Hydrochloride

Physical properties

Property	Value / Description
Appearance	Typically, a white to off-white crystalline solid (confirm against supplier or monograph description)
Salt form	Hydrochloride, the form used in the marketed product; it improves crystallinity and handling relative to the free base, although aqueous solubility remains limited
Molar mass	337.24 g/mol (salt); 300.78 g/mol (free base)
Purity (research-grade material)	Commonly offered at ≥98% by HPLC
Recommended storage (research material)	Supplier data commonly specify refrigerated storage at about +4°C, in a closed container
Chirality	One stereocenter (C4); generally supplied as the racemate
UV behavior	Absorbs in the UV range owing to two aromatic chromophores

Specific values such as melting point and solubility in particular solvents should be taken from the certificate of analysis or the current monograph for the lot in hand, because they vary with salt form, hydration state, and crystalline form.

Chemical properties and reactivity

Property/Behavior	Description
Basicity	The amino-substituted C=N system is basic; the compound forms a stable hydrochloride salt
Hydrolytic sensitivity	The cyclic isourea linkage is the most plausible site of hydrolytic degradation (plausible products include the corresponding 3,1-benzoxazin-2-one, formed with loss of ethylamine, and ring-opened urea/amino-alcohol species); acid, base, and humidity stress should be evaluated in forced degradation studies
Oxidative behavior	The exocyclic N–H, the N-ethyl group and the aromatic rings warrant evaluation under oxidative (e.g., peroxide) stress; C4 carries no hydrogen, so classical benzylic C–H oxidation at that position is not expected
Photostability	Aromatic, chlorinated systems should be assessed under ICH Q1B photostability conditions
Counter-ion	The chloride content is a quality attribute and can be assayed by titration as a complement to chromatographic purity
Stereochemical stability	C4 carries no hydrogen, so racemization by deprotonation is not possible. However, C4 is a tertiary, doubly benzylic carbon bonded to oxygen, so acid-promoted C–O ionization could in principle cause racemization or ring opening/elimination; include this in acid stress studies and verify the chiral purity of any single-enantiomer material

Mechanism of Action: Two Targets, One Molecule

Etifoxine's pharmacology is unusual because it combines a direct and an indirect route to enhanced GABAergic inhibition.

Direct pathway: GABA-A receptor modulation. Etifoxine acts as a positive allosteric modulator of GABA-A receptors, with preferential activity at receptors containing β2 or β3 subunits. This site differs from the benzodiazepine binding site, which helps explain why etifoxine produces anxiolytic effects in preclinical and clinical settings with a reportedly lower burden of sedation, muscle relaxation, and memory impairment than classical benzodiazepines.

Indirect pathway: TSPO and neurosteroid synthesis. Etifoxine also binds the 18 kDa translocator protein (TSPO), formerly called the peripheral-type benzodiazepine receptor. TSPO is involved in moving cholesterol from the outer to the inner mitochondrial membrane, the rate-limiting step in steroidogenesis. By stimulating this process, etifoxine increases the synthesis of neurosteroids such as allopregnanolone, which are themselves potent positive modulators of GABA-A receptors. Preclinical work also links the TSPO action to neuroprotective and nerve-regenerative effects.

Pharmacological comparison with benzodiazepines

Feature	Etifoxine Typical benzodiazepine (e.g., lorazepam)
Chemical class	Benzoxazine Benzodiazepine
GABA-A binding site	β2/β3 subunits (distinct from benzodiazepine site) α/γ subunit interface (benzodiazepine site)
Second target	TSPO (neurosteroid synthesis) None of comparable significance
Sedation and psychomotor effects	Reported to be lower in available studies, though drowsiness can occur Prominent, dose-dependent
Rebound on discontinuation	Studies reviewed by regulators did not show rebound anxiety on stopping Recognized risk
Distinctive serious risks	Rare severe skin reactions and cytolytic hepatitis Dependence, withdrawal, cognitive and psychomotor impairment

These comparisons are drawn from the published and regulatory literature and are intended as a scientific overview, not as clinical guidance.

Uses of Etifoxine Hydrochloride

Clinical use

• Psychosomatic manifestations of anxiety — the authorized indication in the countries where etifoxine is marketed, typically 150–200 mg per day, given as 50 mg capsules in two or three divided doses, for a few days to a few weeks and generally for no longer than 12 weeks, according to the product information
• Adjustment disorder with anxiety — the setting of the AMETIS study, which compared etifoxine with lorazepam and placebo; neither active treatment showed superiority over placebo on the primary endpoint, and this result contributed to the EU-level benefit–risk review

Etifoxine is marketed in around 50 countries, including France, Bulgaria, Luxembourg, Malta and Romania within the EU, but it is not approved in the United States, so local regulatory status should always be verified.

Research and development use

• Neuropharmacology tool compound — used to study GABA-A β-subunit pharmacology and TSPO biology
• Neuroprotection and nerve regeneration research — preclinical models of peripheral nerve injury and neuroinflammation have used etifoxine to examine TSPO-mediated effects
• Pain and neurosteroid research — its capacity to raise neurosteroid levels has made it a reference ligand in studies of neurosteroid-mediated analgesia

Pharmaceutical and analytical use

• Drug substance for finished dosage forms — capsules containing etifoxine hydrochloride
• Reference and impurity standards — used in assay, related-substances, and stability-indicating method development and validation
• Comparator ligand in TSPO studies — used alongside other TSPO ligands in binding and functional assays

Impurities and Stability: What Quality Laboratories Monitor

Etifoxine hydrochloride's structure points to several categories of impurity that a well-designed control strategy must address.

Impurity Category	Likely Origin Why It Matters
Starting-material and intermediate carryover	Commonly reported routes start from 2-amino-5-chlorobenzophenone, which is converted to the tertiary carbinol 1-(2-amino-5-chlorophenyl)-1-phenylethanol and then cyclized with an ethylamine-derived reagent (e.g., ethyl isothiocyanate); carryover of these intermediates and reagents must be controlled Process-related; controlled through route design and purification
Ring-opened or hydrolytic degradants	Hydrolysis of the cyclic isourea under moisture, acid, or base Degradation-related; central to stability-indicating method design
Positional or halogenation variants	Impurities in the chlorinated starting material Structurally similar and chromatographically challenging
Stereochemical impurities	Relevant if a single enantiomer is intended Requires chiral methods to quantify the undesired enantiomer
Residual solvents	Reaction and crystallization steps Controlled under ICH Q3C
Elemental impurities	Catalysts and equipment Assessed under ICH Q3D
Nitrosamine risk	Exocyclic secondary N–H of the amidine/isourea group (and ethylamine potentially released on degradation); the risk of forming N-nitroso-etifoxine (an NDSRI) should be assessed Requires a documented risk assessment per current regulatory guidance

Analytical approach in brief

HPLC with UV detection is the natural platform for assay and related substances, given the two aromatic chromophores. LC-MS supports impurity identification, with the chlorine isotope pattern providing a built-in diagnostic. Forced degradation (acid, base, oxidative, thermal, humidity, and light) establishes that the method is stability-indicating, and orthogonal methods are advisable for the chloride counter-ion and for chiral purity where relevant. Chemicea Pharmaceuticals supports this work with characterized reference standards and impurity materials, so identification and quantification rest on authentic standards rather than area-percent estimates alone.

Regulatory Framework — What Specifically Applies

Regulatory Document / Action	Governs Relevance to Etifoxine
EMA Article 31 referral (CHMP opinion December 2021; EMA communication January 2022)	EU-level benefit–risk review of etifoxine-containing medicines Concluded the medicine could remain available for anxiety, with strengthened contraindications and warnings for severe skin and liver reactions
ANSM (France) product information update, June 2022	National safety communication Added contraindication in patients with prior severe skin reactions or severe hepatitis/cytolytic hepatitis on etifoxine
HAS (France) 2020 reimbursement opinion	Reimbursement assessment Concluded the medical benefit was insufficient to justify continued reimbursement
Product information in other jurisdictions (e.g., South Africa)	Local registration and labeling Reflects similar contraindications and warnings
ICH Q3A / Q3B	Impurities in drug substances and products Framework for reporting, identification, qualification
ICH Q3C / Q3D	Residual solvents / elemental impurities Process-related control
ICH Q1A(R2) / Q1B	Stability and photostability testing Required given hydrolytic and photolytic considerations
ICH Q2(R2)	Analytical procedure validation Assay and related-substances methods
ICH Q7	GMP for APIs Manufacturing controls
FDA / EMA nitrosamine guidance	Nitrosamine risk assessment Applies to any drug substance with vulnerable amine functionality

One practical point: because regulatory status differs between countries and has changed over time, product labeling and any regulatory submission should be built on the current local product information rather than on older literature.

Safety Precautions for Etifoxine Hydrochloride

Safety considerations divide into clinical (patient) safety and occupational (handling) safety.

Clinical safety considerations

Risk	Description Key Precaution
Severe cutaneous reactions	Very rare cases of DRESS syndrome, Stevens-Johnson syndrome, and generalized exfoliative dermatitis have been reported Stop treatment and seek immediate medical advice for any severe skin or allergic reaction; contraindicated in patients who previously had such reactions on etifoxine
Hepatic reactions	Cases of cytolytic hepatitis, some severe, have been reported Be alert to jaundice, vomiting, fatigue, or abdominal pain; contraindicated after previous severe hepatitis on etifoxine; take extra care in those with liver disease
Allergic reactions	Urticaria, facial swelling, and other hypersensitivity reactions Discontinue and seek care if swelling or hives develop
Gastrointestinal	Watery diarrhea has been highlighted in safety communications as a reason to stop and consult Report persistent watery diarrhea promptly
Drowsiness	Can impair driving and operating machinery Take care until individual response is known
Pregnancy and breastfeeding	Etifoxine crosses the placental barrier, and data in pregnancy and breastfeeding are insufficient Not recommended during pregnancy or breastfeeding; use only on a clinician's advice
Contraindications	Shock, severe hepatic or renal impairment, myasthenia gravis, severe respiratory failure, galactosaemia or lactose-related intolerance (lactose excipient), and a previous severe skin or liver reaction to etifoxine Check the current local product information before prescribing or dispensing
Children and adolescents	Safety and efficacy not established below 18 years Not recommended under 18 years
Drug interactions	Additive effects with other CNS depressants and alcohol are plausible Review all medicines with a pharmacist or physician

These points are general educational information and not individual medical advice. Anyone taking or considering etifoxine should follow their prescriber's and the local product information's guidance, and should seek urgent care for serious symptoms.

Occupational and laboratory handling safety

Etifoxine hydrochloride is a pharmacologically active compound, and powder handling should reflect that:

• Dust control — weigh and transfer in a ventilated enclosure or fume hood, and avoid generating airborne powder
• Personal protective equipment — gloves, eye protection, and a lab coat; use respiratory protection when dust cannot be avoided
• Avoid ingestion and skin contact — prohibit eating and drinking in handling areas and wash hands after use
• Spill management — collect powder without dry sweeping, using damp methods or HEPA-filtered vacuum, and dispose of waste according to institutional rules
• Sensitizer awareness — because hypersensitivity is a recognized clinical effect, personnel with known drug allergies should be advised accordingly
• Documentation — always consult the current Safety Data Sheet for hazard classification and exposure controls specific to the lot and supplier

Storage and stability precautions

Storage Consideration	Recommended Practice
Temperature	Follow the certificate of analysis; supplier data for research material commonly specify refrigerated storage
Moisture	Keep tightly closed and protected from humidity, given the hydrolytic sensitivity of the isourea linkage
Light	Store protected from light until photostability is established
Segregation	Store away from strong acids, bases, and oxidizers
Labeling and dating	Record date received and opened, and observe retest or expiry dates
Solutions	Prepare solutions fresh and verify solution stability before using them as analytical standards

A Worked Example: Developing a Stability-Indicating Method for Etifoxine Hydrochloride

Consider an analytical team asked to develop a stability-indicating HPLC method for etifoxine hydrochloride capsules.

Step 1: Start from the structure. Two aromatic chromophores make UV detection the obvious choice. The basic isourea-type nitrogen suggests a buffered, slightly acidic mobile phase to control ionization and peak shape, and a base-deactivated reversed-phase column to limit tailing.

Step 2: Force the molecule to degrade. The team stresses the drug substance with acid, base, peroxide, heat, humidity, and light. The expectation, based on the structure, is that the cyclic isourea is the most vulnerable site, so ring-opened degradants are anticipated under hydrolytic conditions; under acidic stress, the tertiary C4–O bond is a second possible weak point. The method must resolve these from the main peak and from one another, and peak purity (PDA or MS) confirms that no degradant co-elutes.

Step 3: Identify what appears. LC-MS gives molecular weights of the new peaks, and the chlorine isotope pattern confirms whether a degradant retains the chlorinated ring. Authentic impurity reference standards, rather than relative-retention assumptions, are used to assign structures where possible.

Step 4: Quantify correctly. Relative response factors are determined for impurities whose chromophores differ from the parent, so that area percent does not misstate real impurity levels.

Step 5: Cover the attributes UV cannot. The chloride counter-ion is assayed by titration, residual solvents by headspace GC, and, if a single enantiomer is ever in scope, a chiral method is validated.

Step 6: Add the risk assessments. An ICH M7 evaluation of potentially mutagenic impurities and a nitrosamine risk assessment complete the control strategy, since the structure contains a secondary amine-type N–H.

Notice how each decision traced back to a structural feature. This is also where well-characterized reference materials from a supplier such as Chemicea Pharmaceuticals make the difference between a defensible validation package and a guessing exercise.

Common Pitfalls in Handling and Quality Control of Etifoxine Hydrochloride

Confusing the free base and the hydrochloride. The two have different molar masses (300.78 versus 337.24 g/mol) and different CAS numbers, and mixing them up leads directly to potency calculation errors when preparing standards.

Assuming etifoxine behaves like a benzodiazepine because it acts on GABA-A. Its binding site, second target (TSPO), structural class, and safety profile all differ, and analogies from benzodiazepine literature can mislead.

Ignoring hydrolytic sensitivity during analysis. Standards and samples left in aqueous diluents may degrade in the vial, producing artifactual impurity peaks and unnecessary out-of-specification investigations.

Overlooking stereochemistry. The C4 stereocenter means that chirality should be addressed explicitly in specifications, even when the material is a racemate.

Skipping the nitrosamine risk assessment. A secondary amine-type N–H in the structure means the assessment should be documented rather than assumed unnecessary.

Relying on outdated regulatory information. Contraindications and warnings have changed over time, and any labeling or dossier should reflect the current local product information.

Underestimating powder-handling risk because the dose is small. A pharmacologically active, potentially sensitizing compound still warrants containment, even at laboratory scale.

Conclusion

Etifoxine hydrochloride shows how much of a drug's character is written into its structure. A chlorinated benzoxazine core, a cyclic isourea carrying an ethylamino group, and a stereogenic quaternary carbon together explain its salt-forming basicity, its UV-friendly analytical profile, its potential hydrolytic weak point, and its distinctive pharmacology: GABA-A modulation through β2 and β3 subunits at a site apart from the benzodiazepine site, plus TSPO-mediated neurosteroid synthesis. Its regulatory history adds a second lesson, that a molecule can remain useful while its risks, in this case rare but serious skin and liver reactions, are managed through clear contraindications and vigilant communication.

For laboratories and manufacturers, the practical takeaway is the same as for any well-understood molecule: read the structure, anticipate its liabilities, build forced degradation and impurity control around those liabilities, and use authentic reference standards to confirm what the data show. Respecting etifoxine hydrochloride means holding its therapeutic value and its safety considerations in view together.

Frequently Asked Questions

Q1: What is the chemical structure of etifoxine hydrochloride?

Etifoxine is a 4H-3,1-benzoxazine: a benzene ring fused to a ring containing one nitrogen and one oxygen, with an ethylamino group at position 2, a methyl and a phenyl group on the quaternary carbon at position 4, and a chlorine at position 6. The free base has the formula C₁₇H₁₇ClN₂O (300.78 g/mol), and the hydrochloride salt, C₁₇H₁₇ClN₂O·HCl, has a molar mass of 337.24 g/mol.

Q2: What is the structural formula for etifoxine hydrochloride, and what is its CAS number?

The salt is written C₁₇H₁₇ClN₂O·HCl (or C₁₇H₁₈Cl₂N₂O), and its CAS number is 56776-32-0. The free base, etifoxine, has CAS number 21715-46-8. Confirming which form you are working with matters because the molar masses differ.

Q3: How does etifoxine work, and how is it different from a benzodiazepine?

Etifoxine enhances GABA-A receptor function through β2 and β3 subunits at a site distinct from the benzodiazepine site, and it also binds TSPO, which promotes neurosteroid synthesis, including steroids that further potentiate GABA-A receptors. Its chemical class (benzoxazine), binding site, and second target all differ from those of benzodiazepines.

Q4: What is etifoxine hydrochloride used for?

Where authorized, it is used for psychosomatic manifestations of anxiety, generally as 50 mg capsules at 150–200 mg per day for no longer than 12 weeks. It is also used in research as a tool compound for GABA-A β-subunit pharmacology and TSPO biology, including preclinical studies of neuroprotection and nerve regeneration. Its regulatory status varies by country.

Q5: What are the main safety concerns with etifoxine?

Rare but serious skin reactions (including DRESS syndrome and Stevens-Johnson syndrome) and cytolytic hepatitis have been reported, and regulators contraindicate its use in people who had such reactions on a previous course. Drowsiness, allergic reactions, and watery diarrhea are also flagged, and it crosses the placental barrier, so it is not recommended in pregnancy. Anyone with symptoms such as severe rash, jaundice, or persistent diarrhea should stop and seek medical advice promptly.

Q6: Why does the quaternary carbon at position 4 matter?

It bears four different groups, so it is a stereocenter, meaning etifoxine can exist as two enantiomers. The drug is generally described as a racemate, and any specification or analytical method should state the stereochemical status explicitly, with chiral methods used where a single enantiomer is intended.

Q7: What impurity and stability issues should laboratories plan for?

Plan for process-related impurities from the precursor and cyclization chemistry, hydrolytic degradants arising at the cyclic isourea linkage, stereochemical purity where relevant, residual solvents and elemental impurities, and a documented nitrosamine risk assessment. A forced degradation study with LC-MS identification and authentic impurity standards is the most reliable way to build a stability-indicating method.