Bupropione SR 150 mg

Descrizione Del Prodotto

1. Classification and Chemical Overview

Bupropion SR 150mg is a licensed oral sustained-release pharmaceutical preparation of bupropion hydrochloride (originally developed by Burroughs Wellcome and marketed under proprietary brand names including Zyban for smoking cessation and Wellbutrin SR for major depressive disorder). Chemically designated as $(\pm)\text{-1-(3-chlorophenyl)-2-[(1,1-dimethylethyl)amino]propan-1-one hydrochloride}$, bupropion is a synthetic monocyclic aminoketone belonging to the substituted cathinone and phenethylamine chemical classes. Structurally, it features a core phenyl ring with a meta-chloro substitution, an aliphatic ketone moiety at the beta position, and a bulky tertiary-butyl (tert-butyl) group on the terminal amino nitrogen. This sterically hindered tert-butyl substitution prevents rapid Phase I oxidative deamination (which typically inactivates unsubstituted phenethylamines) and substantially blunts classical sympathomimetic abuse liability compared to street cathinones (e.g., mephedrone) or amphetamines. Its empirical molecular formula is $\text{C}_{13}\text{H}_{18}\text{ClNO}\cdot\text{HCl}$, with a molecular weight of $276.2\text{ g/mol}$.

Standard solid oral presentations of Bupropion SR 150mg in the United Kingdom appear as round, biconvex, film-coated sustained-release tablets (typically purple or white, imprinted with brand identifiers such as “Zyb 150” or “Wellbutrin SR 150”). The sustained-release (SR) formulation utilizes a hydrophilic polymer matrix (commonly hydroxypropyl methylcellulose / hypromellose) combined with microcrystalline cellulose, cysteine hydrochloride (as a chemical stabilizer), and magnesium stearate. This matrix hydrates upon contact with gastrointestinal fluid to form a viscous gel layer, moderating drug diffusion over an 8- to 12-hour period. This sustained release attenuates sharp peak plasma concentrations ($C_{max}$) that would otherwise breach the cortical seizure threshold.

Within the United Kingdom regulatory framework:

• Medicinal Classification: Bupropion SR 150mg is categorized as a Prescription Only Medicine (POM) governed by the Human Medicines Regulations 2012.

• Controlled Drug Status: Bupropion is not a controlled substance under the Misuse of Drugs Act 1971 or the Misuse of Drugs Regulations 2001.

• UK Licensed Indications: Under the Medicines and Healthcare products Regulatory Agency (MHRA), bupropion is licensed in the UK exclusively under the trade name Zyban SR 150mg as an adjunct to smoking cessation alongside motivational support.

• Depression Status: Unlike in North America (where Wellbutrin SR/XL holds first-line approval for Major Depressive Disorder and Seasonal Affective Disorder), bupropion is not licensed for depression in the UK. However, it is recognized in the British National Formulary (BNF) and frequently prescribed “off-label” by NHS and private consultant psychiatrists for treatment-resistant major depression, bipolar depression (due to low rates of manic switching), adult ADHD, and to counteract SSRI-induced sexual dysfunction.

While bupropion possesses low intrinsic recreational value when ingested orally, it is encountered in clinical toxicology in cases of crushing/snorting (nasal insufflation) or intravenous injection by individuals seeking a psychostimulant-like rush. These non-approved routes bypass the sustained-release matrix, dumping the full dose into circulation and precipitating intractable status epilepticus, severe cardiovascular collapse, and extensive localized tissue necrosis.

2. Mechanism of Action and Pharmacodynamics

The pharmacodynamic profile of bupropion and its primary active circulating metabolites involves dual neurochemical modulation: weak dual monoamine reuptake inhibition and non-competitive nicotinic acetylcholine receptor antagonism:

• Noradrenaline and Dopamine Reuptake Inhibition (NDRI):

  • Bupropion functions as a competitive inhibitor of the human noradrenaline transporter (NET / SLC6A2) and dopamine active transporter (DAT / SLC6A3).

  • Relative Potencies: In vitro assays demonstrate that bupropion is roughly twice as potent at inhibiting NET as it is at DAT ($IC_{50} \approx 1.4\text{ to }2.8\text{ }\mu\text{M}$ for NET vs. $IC_{50} \approx 2.0\text{ to }5.6\text{ }\mu\text{M}$ for DAT). It displays negligible affinity for the serotonin transporter (SERT) ($IC_{50} > 45\text{ }\mu\text{M}$), completely setting it apart from SSRIs and SNRIs.

  • In Vivo Occupancy: At clinical therapeutic doses ($150\text{ to }300\text{ mg/day}$), striatal DAT occupancy measured by positron emission tomography (PET) ranges between $14\%$ e $26\%$. While this level of DAT occupancy is sufficient to provide antidepressant, anti-craving, and pro-motivational effects in prefrontal and mesolimbic circuits, it remains far below the threshold ($\sim 50\text{ to }60\%$) typically required to drive compulsive behavioral reinforcement or euphoric psychostimulant misuse.

• Non-Competitive Nicotinic Acetylcholine Receptor (nAChR) Antagonism:

  • Bupropion acts as a high-affinity, non-competitive, open-channel antagonist at central human neuronal nicotinic acetylcholine receptors, specifically the $\alpha_3\beta_4$, $\alpha_4\beta_2$, E $\alpha_7$ subtypes.

  • By blocking these receptors, bupropion attenuates the reinforcing, dopamine-releasing cascades initiated by inhaled nicotine within the ventral tegmental area (VTA) and nucleus accumbens. It mitigates both the somatic symptoms of nicotine withdrawal and the psychological reward associated with tobacco consumption.

• Contributions of Active Metabolites (The “Prodrug” Paradigm):

  • Unchanged parent bupropion accounts for only a fraction of its total clinical pharmacodynamics. Its primary hepatic metabolite, hydroxybupropion, circulates at plasma concentrations 10 to 20 times higher than parent bupropion.

  • Hydroxybupropion is an equally potent inhibitor of NET and an effective antagonist at nAChRs, meaning that in clinical steady-state conditions, therapeutic efficacy is driven predominantly by its active circulating metabolites rather than parent bupropion.

• Receptor Non-Interactions: Bupropion exhibits no direct affinity for postsynaptic $\alpha_1$-, $\alpha_2$-, or $\beta$-adrenoceptors, $5\text{-HT}_1$ o $5\text{-HT}_2$ receptors, dopamine $D_1$ o $D_2$ receptors, histamine $H_1$ receptors, or muscarinic acetylcholine receptors, explaining its general absence of typical antihistaminic daytime sedation, anticholinergic drying, or metabolic weight gain.

3. Approved UK Clinical Indications and Therapeutic Scope

Bupropion SR 150mg occupies a clearly defined position within UK clinical and psychiatric practice:

• Smoking Cessation (Licensed UK Indication – Zyban):

  • Indicated as a short-term pharmacological aid to smoking cessation in combination with motivational behavioral support in nicotine-dependent individuals aged 18 years and over.

  • Evaluated within NICE Technology Appraisal TA123 e NICE Guideline NG209 (Tobacco: preventing uptake, promoting quitting and treating dependence), where it is recommended as an effective pharmacotherapy alongside nicotine replacement therapy (NRT) and varenicline.

  • Standard Dosing Protocol:

    • Days 1 to 6: $150\text{ mg}$ taken once daily in the morning.

    • Day 7 onwards: $150\text{ mg}$ taken twice daily, spaced by an absolute minimum interval of 8 hours (with the second dose taken late afternoon, avoiding bedtime to minimize severe insomnia).

    • Target Stop Date: The patient sets a target quit date during the second week of treatment (typically between Day 8 and Day 14), when steady-state plasma concentrations have been attained.

    • Duration: Treatment continues for 7 to 9 weeks. If no progress toward smoking cessation has been achieved by Week 7, treatment should be discontinued.

• Major Depressive Disorder (Off-Label UK Specialist Practice):

  • Prescribed off-label in NHS secondary care psychiatric services for moderate-to-severe treatment-resistant depression:

    • Used as an augmenting agent alongside SSRIs/SNRIs to alleviate residual fatigue, apathy, executive dysfunction, and anhedonia.

    • Used as monotherapy to avoid SSRI-induced adverse reactions: bupropion causes virtually zero sexual dysfunction (does not inhibit orgasm or libido) and does not cause weight gain (frequently promoting mild weight loss).

    • Dosing: $150\text{ mg}$ once daily, titrating to $150\text{ mg}$ twice daily (maximum $300\text{ mg/day}$) after several weeks if clinically indicated.

• Adult Attention-Deficit/Hyperactivity Disorder (ADHD – Off-Label):

  • Utilized off-label under specialist neurodevelopmental guidance as a non-stimulant alternative for adult ADHD when methylphenidate, lisdexamfetamine, or atomoxetine are contraindicated, ineffective, or poorly tolerated ($150\text{ to }300\text{ mg/day}$).

4. Pharmacokinetic Profile and Metabolic Fate

The pharmacokinetic profile of Bupropion SR 150mg is defined by matrix-controlled gastrointestinal absorption, extensive and complex hepatic biotransformation, and strong cytochrome P450 2B6 (CYP2B6) dependence:

• Absorption: Following oral ingestion of the SR tablet, bupropion is absorbed across the intestinal mucosa. Peak plasma concentrations ($C_{max}$) of parent bupropion are achieved within 2.5 to 3 hours (median $T_{max} \approx 3\text{ hours}$). Absolute systemic bioavailability is estimated between $5\text{ and }20\%$ due to extensive, saturable first-pass hepatic extraction. Ingestion alongside food slightly increases peak concentrations and overall exposure ($AUC$ increases by $\sim 11\text{ to }17\%$), but these variations are not clinically significant; tablets may be taken with or without food.

• Distribution: Bupropion and its primary metabolites are moderately lipophilic. Bupropion crosses the blood-brain barrier rapidly, displaying an apparent volume of distribution ($V_d$) of approximately $20\text{ to }47\text{ L/kg}$ (reflecting extensive cellular uptake and tissue binding). In circulating human plasma, parent bupropion is approximately $84\%$ bound to plasma proteins (predominantly human serum albumin). Its active metabolite hydroxybupropion is approximately $77\%$ bound. Bupropion and its metabolites freely cross the placenta and are excreted into maternal breast milk.

• Biotransformation (The CYP2B6 / Carbonyl Reductase Axis): Bupropion undergoes rapid and extensive hepatic metabolism through two primary parallel pathways, generating three major active circulating metabolites:

  • Hydroxylation Pathway (CYP2B6): Cytochrome P450 2B6 catalyzes the aliphatic hydroxylation of the tert-butyl side chain to form hydroxybupropion. Hydroxybupropion is pharmacologically active and reaches peak plasma concentrations ($C_{max}$) at approximately 6 to 7 hours post-dose. At steady state, its circulating systemic exposure ($AUC$) is approximately 10- to 20-fold higher than that of parent bupropion.

  • Carbonyl Reduction Pathway (Non-CYP): Cytosolic carbonyl reductases reduce the beta-ketone moiety to form two diastereomeric amino-alcohol metabolites: threohydrobupropion e erythrohydrobupropion. Threohydrobupropion accounts for an exposure roughly 5 times higher than parent bupropion.

• Elimination: Systemic elimination is predominantly renal. Following oral administration, approximately $87\%$ of the dose is recovered in the urine, primarily as polar Phase II conjugated and cleaved metabolites (less than $0.5\%$ is excreted as unchanged parent bupropion). Approximately $10\%$ is eliminated via the faeces.

  • The terminal elimination plasma half-life ($t_{1/2}$) of parent bupropion is approximately 14 to 20 hours (mean $\sim 20\text{ hours}$).

  • The terminal elimination half-lives of its active metabolites are substantially longer: hydroxybupropion exhibits a $t_{1/2}$ of 20 to 24 hours, while threohydrobupropion exhibits a $t_{1/2}$ of 30 to 37 hours.

  • Due to these extended half-lives, steady-state concentrations of bupropion and its active metabolites require 5 to 8 days of continuous dosing to be fully achieved.

5. Physiological Effects and Adverse Event Spectrum

The primary therapeutic physiological effect in clinical practice is the dampening of nicotine craving and withdrawal symptoms, accompanied by mild psychomotor activation and mood elevation. However, central catecholaminergic augmentation generates a distinct spectrum of multi-system adverse effects:

• Neuropsychiatric Disturbances (Very Common to Common, $\ge 1/100$ to $\ge 1/10$):

  • Severe Insomnia: Reported in up to $30\text{ to }40\%$ of patients. Driven by central noradrenergic vigilance stimulation. Patients must be explicitly instructed never to take the second daily dose at bedtime (the second dose should be taken late afternoon, at least 4 to 6 hours before planned sleep).

  • Agitation, restlessness, anxiety, irritability, and fine resting tremors.

  • Headache (very common, up to $20\%$), dizziness, and concentration difficulties.

  • Neuropsychiatric Warnings (MHRA Yellow Card): Depressive symptom exacerbation, paranoia, delusions, visual/auditory hallucinations, and emerging suicidal ideation. While nicotine withdrawal itself causes dysphoria and agitation, patients must be monitored for treatment-emergent psychiatric decompensation.

• Autonomic and Sympathomimetic Dysregulation (Common, $1/100$ to $<1/10$):

  • Xerostomia (Dry Mouth): Very common, occurring in over $10\%$ of individuals.

  • Marked diaphoresis (profuse sweating), hot flushes, and cutaneous flushing.

  • Nausea, vomiting, abdominal pain, constipation, and anorexia/taste perversion (frequently producing modest weight loss).

• Cardiovascular Effects:

  • Dose-dependent elevations in resting blood pressure (systolic increases of $5\text{ to }15\text{ mmHg}$) and resting heart rate (sinus tachycardia).

  • Palpitations, postural dizziness, and precordial discomfort.

  • Hypertensive crises (rare, but documented even in patients without pre-existing hypertension, especially when combined with transdermal nicotine patches).

• Severe, Emergent and Life-Threatening Hazards (Critical Safety Alerts):

  • Dose-Dependent Epileptic Seizures (The Defining Hazard):

    • Bupropion lowers the central seizure threshold in a direct, dose-dependent manner.

    • Incidence Rate: At standard therapeutic sustained-release dosing ($\le 300\text{ mg/day}$), the clinical seizure incidence is approximately $0.1\%$ ($1\text{ in }1,000\text{ patients}$). However, if the dose exceeds $300\text{ mg/day}$ or if the immediate-release form is used at $>450\text{ mg/day}$, the seizure rate escalates non-linearly to $0.4\text{ to }4.0\%$.

    • Mechanisms: Precipitated by sharp plasma spikes, underlying cortical hyperexcitability, or drug-drug interactions. Seizures can present as generalized tonic-clonic convulsions or progress to refractory status epilepticus.

  • Severe Hypersensitivity Reactions: Delayed-onset type I/III hypersensitivity reactions typically manifesting 10 to 20 days post-initiation. Characterized by generalized pruritic urticaria, angioedema, arthralgia, myalgia, fever, and serum sickness-like syndromes. Rarely, progression to Stevens-Johnson Syndrome (SJS) or anaphylaxis occurs.

  • Precipitation of Mania: In patients with unrecognized bipolar affective disorder, bupropion can trigger switch events into acute mania or mixed states.

  • Narrow-Angle Glaucoma: Mydriasis secondary to noradrenergic tone can precipitate acute pupillary block in patients with anatomically narrow anterior chamber angles.

6. Contraindications, Drug Interactions, and Clinical Precautions

Safe prescribing, dispensing, and toxicological evaluation of Bupropion SR 150mg requires strict adherence to absolute seizure-related contraindications, cytochrome enzyme interactions, and mandatory administration intervals:

• Contraindications:

  • Current Seizure Disorder or History of Seizures: Absolute Contraindication. Any prior history of unprovoked epilepsy, febrile seizures in childhood with persistent EEG changes, or post-traumatic seizures.

  • Central Nervous System Tumour or Structural Brain Lesion: Absolute contraindication due to high baseline epileptogenic focus.

  • Current or Previous Eating Disorder (Bulimia Nervosa or Anorexia Nervosa): Absolute Contraindication. Clinical trials revealed a markedly elevated incidence of generalized tonic-clonic seizures in patients with active or past bulimia or anorexia receiving bupropion, driven by baseline electrolyte shifts and rapid metabolic swings.

  • Abrupt Withdrawal from Alcohol, Benzodiazepines, or Barbiturates: Absolute contraindication. Halting central $GABA_A$ agonism lowers the seizure threshold, while bupropion concurrently elevates cortical excitability, precipitating severe status epilepticus.

  • Concurrent Monoamine Oxidase Inhibitors (MAOIs): Absolute contraindication during or within 14 days of discontinuing irreversible MAOIs (e.g., phenelzine, tranylcypromine, moclobemide, linezolid) due to the risk of hyperadrenergic crises and malignant hypertension.

  • Severe Hepatic Cirrhosis (Child-Pugh Class C): Absolute contraindication. Elimination is impaired, causing massive accumulation of active metabolites; in milder hepatic impairment, the maximum daily dose is strictly capped at $150\text{ mg}$ on alternate days.

  • Bipolar Affective Disorder: Contraindicated as monotherapy due to risk of inducing manic switching.

  • Pregnancy and Lactation: Not recommended during pregnancy unless clinically unavoidable (epidemiological signals suggest a modest increase in congenital cardiovascular malformations, specifically ventricular septal defects); bupropion and its metabolites distribute into breast milk and carry seizure risks for nursing infants.

  • Paediatric Population: Contraindicated in children and adolescents under 18 years of age.

• Drug Interactions:

  • Drugs That Lower the Seizure Threshold (High Clinical Danger): Extreme caution is mandated when co-prescribing bupropion with antipsychotics (e.g., clozapine, chlorpromazine), antidepressants (e.g., tricyclics, SSRIs), systemic corticosteroids, theophylline, fluoroquinolone antibiotics, or tramadol.

  • Potent CYP2D6 Inhibition by Bupropion: While bupropion is metabolized by CYP2B6, bupropion and hydroxybupropion are potent, clinical inhibitors of the cytochrome P450 2D6 (CYP2D6) pathway.

    • Co-administration significantly increases plasma concentrations of CYP2D6 substrates: tricyclic antidepressants (e.g., amitriptyline, desipramine), beta-blockers (e.g., metoprolol), selective serotonin reuptake inhibitors (e.g., paroxetine, venlafaxine), and class 1C antiarrhythmics (e.g., flecainide, propafenone), requiring marked dose reductions of the co-prescribed drug.

    • Tamoxifen Failure: Bupropion must never be prescribed to women taking tamoxifen for breast cancer. Tamoxifen requires CYP2D6 to bioactivate into its active anti-tumour metabolite, endoxifen. Bupropion-mediated CYP2D6 shutdown blocks endoxifen formation, resulting in documented increases in breast cancer recurrence and mortality.

  • CYP2B6 Inhibitors and Inducers:

    • Inducers (e.g., carbamazepine, phenytoin, rifampicin, ritonavir, efavirenz) significantly lower bupropion and hydroxybupropion concentrations, blunting therapeutic response.

    • Inhibitors (e.g., ticlopidine, clopidogrel, valproate) elevate bupropion exposure while lowering hydroxybupropion levels.

  • Nicotine Replacement Therapy (NRT Transdermal Patches): Combination therapy is utilized in smoking cessation, but requires regular blood pressure monitoring due to an increased risk of treatment-emergent hypertension.

• Clinical Precautions and Harm Minimisation:

  • Strict Administration Guidance (Seizure Prevention):

    • Do Not Crush, Chew, or Split: Tablets must be swallowed completely whole. Crushing, chewing, or breaking sustained-release tablets destroys the polymer matrix, resulting in rapid dose-dumping, immediate absorption, and high risk of precipitating generalized seizures.

    • Minimum 8-Hour Inter-Dose Interval: When escalating to the twice-daily regimen ($150\text{ mg}$ BD), individual doses must be separated by at least 8 hours.

    • Ceiling Dose Cap: The total daily dose must never exceed $300\text{ mg/day}$ in any patient.

  • Baseline and Ongoing Cardiovascular Monitoring:

    • Baseline blood pressure and resting heart rate must be measured prior to initiating therapy and monitored periodically throughout treatment, particularly in patients with pre-existing mild-to-moderate hypertension.

  • Acute Overdose Management (“Red Flags”):

    • Acute bupropion overdose ($>1.5\text{ to }3\text{ g}$) represents a toxicological emergency characterized by severe sinus tachycardia, QRS widening and QTc prolongation, hallucinations, agitation, stupor, and sudden, recurring generalized seizures or status epilepticus (which can occur up to 12 to 24 hours post-ingestion due to delayed tablet release):

      • Airway Protection: Early intubation must be anticipated; seizures can occur precipitously without warning auras.

      • Aggressive Benzodiazepine Titration: First-line management for agitation, tremors, and seizures is high-dose intravenous benzodiazepines (e.g., IV diazepam $10\text{ to }20\text{ mg}$ or lorazepam $2\text{ to }4\text{ mg}$ titrated rapidly). Phenytoin is ineffective and contraindicated.

      • Cardiac Arrhythmias: QRS prolongation ($>100\text{ ms}$) secondary to cardiac gap junction or sodium channel blockade should be treated emergently with intravenous sodium bicarbonate ($8.4\%$, $100\text{ to }200\text{ mL}$) boluses.

      • Cardiovascular Collapse: Severe, refractory shock and cardiogenic collapse can occur; preparations for mechanical circulatory support (Veno-Arterial Extracorporeal Membrane Oxygenation – VA-ECMO) or lipid emulsion therapy (Intralipid 20%) should be considered in critical care settings for refractory arrest.

      • Dialysis Ineffectiveness: Due to its massive volume of distribution ($>20\text{ L/kg}$), hemodialysis and hemoperfusion are completely ineffective for accelerating elimination.