Clinical Monograph: Amfexa (Dexamphetamine Sulfate)

1. Classification and Chemical Overview

Dexamphetamine is the dextrorotatory enantiomer of amphetamine belonging to the phenethylamine derivative class. Chemically designated as $(S)$-(+)-alpha-methylphenethylamine sulfate, it represents the more pharmacologically active stereoisomer of racemic amphetamine. Under the Anatomical Therapeutic Chemical (ATC) system, dexamphetamine is indexed under N06BA02.

• Product Context: Amfexa is widely distributed in the UK and European markets as immediate-release tablets (commonly available in strengths such as 5 mg).

• Controlled Status: Classified strictly as a Controlled Substance (Schedule 2 / Class B Controlled Drug in the UK; Schedule II under the US CSA) due to high potential for abuse, psychological dependence, and misuse. It is regulated as a Prescription Only Medicine (POM).

2. Mechanism of Action and Pharmacodynamics

Dexamphetamine exerts its primary central nervous system effects through direct interaction with presynaptic monoamine nerve terminals:

• Presynaptic Catecholamine Release: Enters presynaptic neurons via norepinephrine (NET) and dopamine (DAT) transporters, inducing the reversal of transporter direction and provoking the massive non-exocytotic release of dopamine (DA) and norepinephrine (NE) into the synaptic cleft.

• Vesicular Transporter Inhibition: Competes with monoamines at VMAT2, disrupting vesicular storage and increasing cytosolic neurotransmitter levels available for release.

• Pharmacodynamic Spectrum: Enhances sustained attention, working memory, and executive function while mitigating motor hyperactivity and impulsivity in ADHD. It also promotes wakefulness in narcolepsy.

3. Approved Clinical Indications and Dosing Scope

Licensing for Amfexa includes:

• Attention Deficit Hyperactivity Disorder (ADHD): As part of a comprehensive treatment program (including psychological, educational, and social measures) for ADHD in children aged 6 years and over and adolescents, where other measures alone are insufficient.

• Narcolepsy: Treatment of excessive daytime sleepiness associated with narcolepsy in adults.

Dosing & Administration Regimen:

• Individualized Titration: Initiated at low doses (e.g., 5 mg once or twice daily) and titrated gradually at weekly intervals until optimal clinical response is achieved.

• Immediate-Release Profile: Because Amfexa is an immediate-release preparation, total daily requirements are typically divided into 2 to 3 doses administered at 4- to 6-hour intervals during daytime hours to prevent insomnia.

4. Pharmacokinetic Profile and Metabolic Fate

• Absorption: Rapidly and completely absorbed from the gastrointestinal tract following oral administration. Peak plasma concentrations (Cmax​) occur within 1.5 to 3 hours (Tmax​).

• Distribution: Rapidly distributes into body tissues, with high concentrations found in the brain and cerebrospinal fluid. Plasma protein binding is low (~15% to 20%).

• Biotransformation: Metabolized in the liver primarily via aromatic hydroxylation, aliphatic hydroxylation, and $N$-dealkylation to minor active metabolites (such as 4-hydroxyamphetamine and norephedrine).

• Elimination: Excreted predominantly via the kidneys in urine. Urinary excretion rate is heavily dependent on urinary pH; acidic urine accelerates renal clearance, whereas alkaline urine decreases elimination and increases circulating half-life. The elimination half-life (t1/2​) averages 10 to 12 hours.

5. Physiological Effects and Adverse Event Spectrum

Dexamphetamine stimulates the sympathetic nervous system, accelerates heart rate, raises blood pressure, suppresses appetite, and alters sleep architecture.

Adverse Drug Reaction Spectrum

• Very Common ($\ge 1/10$): Insomnia, decreased appetite, weight loss, dry mouth, headache, nervousness, emotional lability.

• Common ($\ge 1/100$ to $<1/10$): Tachycardia, palpitations, dizziness, abdominal pain, nausea, vomiting, hyperhidrosis, restlessness, bruxism.

• Uncommon ($\ge 1/1,000$ to $<1/100$): Hypertension, cardiomyopathy (with chronic high-dose misuse), dysgeusia, rash, mydriasis, libido changes.

• Rare / Severe (<1/1,000): Psychotic episodes, hallucinations, aggressive behavior, seizures, cerebrovascular events, myocardial infarction, serotonin syndrome (when combined with serotonergic agents).

6. Contraindications, Drug Interactions, and Clinical Precautions

Contraindications

• Known hypersensitivity to amphetamines or formulation excipients.

• Advanced arteriosclerosis, symptomatic cardiovascular disease, moderate-to-severe hypertension, or structural cardiac abnormalities.

• Hyperthyroidism.

• Glaucoma.

• History of drug abuse or active substance use disorder.

• Concomitant administration with Monoamine Oxidase Inhibitors (MAOIs) or use within 14 days of MAOI cessation (risk of hypertensive crisis).

Key Drug Interactions

• Monoamine Oxidase Inhibitors (MAOIs): Concurrent use triggers life-threatening hypertensive crises and severe hyperthermia.

• Serotonergic Agents (SSRIs, SNRIs): Co-administration increases the risk of serotonin syndrome.

• Acidifying & Alkalinizing Agents: Gastrointestinal and urinary acidifiers (e.g., ascorbic acid, ammonium chloride) accelerate clearance and decrease efficacy. Conversely, urinary alkalinizers (e.g., sodium bicarbonate) increase reabsorption and prolong half-life.

• Antihypertensives: Amphetamines can antagonize the hypotensive effects of blood pressure medications.

Clinical Precautions and Monitoring

• Boxed / Safety Warning (Abuse, Misuse, and Dependence): Stimulants have a high potential for abuse and dependence. Assess clinical risk prior to prescribing and monitor patients for signs of misuse, diversion, or escalating doses.

• Cardiovascular Monitoring: Monitor blood pressure and heart rate regularly. Use with extreme caution in patients with underlying cardiac vulnerabilities.

• Growth Suppression in Children: Chronic use in pediatric populations has been associated with temporary suppression of weight gain and linear growth velocity; monitor height and weight curves during long-term treatment.