Amphetamine And Dextroamphetamine

Introduction Overview

Generic Name

amphetamine and dextroamphetamine

Brand Name

Adderall XR

Dosage Form

extended-release capsule

Drug Class

CNS stimulants

Pharmacokinetics

Absorption

Adderall XR capsules are designed for extended-release absorption, providing a biphasic release pattern that delivers both immediate and sustained plasma concentrations of amphetamine and dextroamphetamine. After oral administration, absorption begins in the gastrointestinal tract, with peak plasma concentrations typically reached between 7 to 10 hours, reflecting the extended-release characteristics. The formulation allows for steady systemic exposure over approximately 12 hours, improving symptom control throughout the day. Food has minimal impact on the extent of absorption but may slightly delay the time to peak concentration. Absorption can be affected by the concomitant intake of acidic substances such as citrus fruits or vitamin C, which can reduce amphetamine absorption by lowering urinary pH and promoting renal elimination.

Distribution

Amphetamine and dextroamphetamine are widely distributed throughout body tissues, crossing the blood-brain barrier efficiently due to their lipophilicity, which is critical for central nervous system stimulation. The apparent volume of distribution ranges from approximately 3 to 4 L/kg, indicative of extensive tissue distribution. Approximately 20-25% of the drug is bound to plasma proteins, allowing for significant free drug to exert pharmacological effects. The distribution in the CNS correlates with clinical efficacy in treating ADHD symptoms. The drugs also cross the placenta and are detectable in breast milk, necessitating caution in pregnancy and lactation.

Metabolism

Amphetamine and dextroamphetamine undergo hepatic metabolism primarily via oxidative deamination, aromatic hydroxylation, and other phase I processes mediated by cytochrome P450 enzymes such as CYP2D6 and flavin-containing monooxygenases. Metabolic pathways yield active and inactive metabolites, including para-hydroxyamphetamine and norephedrine, which may contribute to pharmacodynamic effects. The metabolic rate can vary significantly due to genetic polymorphisms and co-administered drugs that induce or inhibit metabolizing enzymes, influencing individual response and potential for toxicity.

Elimination

Excretion of amphetamine and dextroamphetamine occurs predominantly through the kidneys, with approximately 30-40% of the administered dose excreted unchanged in urine. The urinary elimination half-life ranges from 9 to 14 hours, which supports once-daily dosing of the extended-release formulation. Renal clearance is influenced considerably by urinary pH; acidic urine enhances elimination via ion trapping, while alkaline urine decreases clearance, prolonging drug action. Dose adjustments may be necessary in patients with renal impairment to prevent accumulation and toxicity.

Dosage Forms

Adults

Adderall XR is available in extended-release capsules with dosages of 5 mg, 10 mg, 15 mg, 20 mg, 25 mg, and 30 mg. The recommended initial dose is usually 20 mg once daily in the morning for adults with ADHD, with dosage adjustments based on clinical response and tolerability. Maximum daily doses typically should not exceed 60 mg.

Adolescents

The dosing for adolescents aged 6 years and older generally starts at 10 mg once daily in the morning, with gradual titration in increments of 10 mg weekly. The maximum recommended dose is 30 mg per day for children under 12, but may be higher based on clinical judgment in older adolescents. Dosage is weight-based and adjusted to minimize adverse effects while optimizing symptom control.

Special Populations

In patients with renal impairment, doses should be adjusted cautiously as decreased clearance can lead to increased drug exposure and toxicity. Use is contraindicated or requires caution in those with severe hypertension, heart disease, or history of drug abuse. The drug is not recommended during pregnancy unless the potential benefits justify the risks to the fetus, and breastfeeding is discouraged during treatment. Elderly patients should be monitored closely due to a higher risk of cardiovascular side effects.

Mechanism Of Action

Adderall XR contains a mixture of amphetamine salts: racemic amphetamine (equal parts dextro- and levo-amphetamine) and dextroamphetamine. These compounds exert their primary pharmacological effects by increasing the synaptic concentration of monoamine neurotransmitters, predominantly dopamine and norepinephrine, in the central nervous system. This is achieved mainly through promoting the release of these neurotransmitters from presynaptic vesicles and inhibiting their reuptake by transporters, leading to enhanced dopaminergic and noradrenergic neurotransmission in the prefrontal cortex and other brain regions involved in attention and impulse control.

At a molecular level, amphetamines enter presynaptic neurons via plasma membrane transporters such as the dopamine transporter (DAT) and norepinephrine transporter (NET), and facilitate release by interacting with vesicular monoamine transporter 2 (VMAT2) to promote neurotransmitter displacement from vesicles into the cytosol. Subsequently, reversed transport through DAT and NET leads to increased extracellular monoamine levels. Elevated dopamine and norepinephrine enhance signaling pathways involved in executive function, working memory, and behavioral inhibition, which are impaired in ADHD.

Clinically, this mechanism accounts for improved attention span, decreased distractibility, and reduced hyperactivity and impulsiveness seen in patients with ADHD treated with Adderall XR. Additionally, increased catecholamine signaling contributes to the mild sympathomimetic effects observed with the drug. Compared to other stimulants like methylphenidate, amphetamines primarily increase neurotransmitter release, while methylphenidate mainly blocks reuptake, conferring differences in pharmacokinetics and clinical effects.

Research studies have demonstrated that extended-release formulations such as Adderall XR provide sustained neurotransmitter modulation, resulting in prolonged therapeutic effects throughout the day with minimized peaks and troughs, improving adherence and reducing rebound symptoms compared to immediate-release formulations.

Pharmacological Actions

Enhancement of central nervous system dopaminergic and noradrenergic neurotransmission
Increased release of dopamine and norepinephrine from presynaptic neurons
Inhibition of monoamine reuptake transporters (dopamine and norepinephrine transporters)
Stimulation of central nervous system activity resulting in increased alertness and attention
Reduction of hyperactivity and impulsivity symptoms in ADHD
Sympathomimetic effects including increased heart rate and blood pressure
Elevated mood and increased concentration due to enhanced monoamine activity
Suppression of appetite due to hypothalamic stimulation

Therapeutic Clinical Uses

Treatment of attention deficit hyperactivity disorder (ADHD) in children aged 6 years and older, adolescents, and adults
Improvement of attention span, impulse control, and hyperactivity symptoms associated with ADHD
Management of narcolepsy (off-label with other amphetamine formulations)
Potential off-label use for treatment-resistant depression augmentation (limited evidence)

Side Or Adverse Effects

Insomnia and sleep disturbances due to stimulant effects
Loss of appetite and subsequent weight loss
Increased heart rate (tachycardia) and elevated blood pressure
Nervousness, anxiety, and irritability
Gastrointestinal upset including stomach pain and nausea
Headache and dizziness
Dry mouth
Potential for development of tolerance and dependence with misuse
Psychiatric symptoms including new or worsening psychosis, hallucinations, paranoia, and aggression
Circulation problems such as numbness, pain, or discoloration of fingers or toes
Tics and muscle twitches
Risk of seizures in predisposed individuals
Rare allergic reactions including rash and angioedema

Precautions

Use cautiously in patients with history of cardiac disease, hypertension, or arrhythmias
Monitor growth rates in children during prolonged therapy due to potential growth suppression
Avoid use in patients with severe anxiety, agitation, or psychosis
Assess for history of substance use disorder given potential for abuse and dependence
Regular monitoring of blood pressure and heart rate during treatment
Use caution in patients with thyroid disease or overactive thyroid
Careful dosage titration in patients with renal impairment
Avoid abrupt discontinuation to reduce withdrawal symptoms
Inform patients about potential for serotonin syndrome if used with serotonergic medications
Pregnancy and breastfeeding should be avoided unless benefits outweigh risks

Warnings

This medication is a Schedule II controlled substance with high potential for abuse and dependence
Increased risk of serious cardiovascular events including stroke, myocardial infarction, and sudden death in patients with underlying heart disease or hypertension
Do not use concomitantly with monoamine oxidase inhibitors (MAOIs) or within 14 days of MAOI therapy due to risk of hypertensive crisis
May precipitate or worsen psychotic or manic symptoms in patients with bipolar disorder or history of psychosis
Risk of serotonin syndrome when combined with other serotonergic agents
Potential for peripheral vasculopathy including Raynaud's phenomenon
Patients should not share medication due to abuse potential
May impair ability to operate machinery or drive until effects are known
Risk of growth suppression in pediatric patients with chronic use
Use cautiously in patients with history of seizures or predisposing conditions

Contraindications

Hypersensitivity to amphetamine, dextroamphetamine, or any component of the formulation
Use with monoamine oxidase inhibitors (MAOIs) or within 14 days of discontinuation of MAOIs
Patients with advanced arteriosclerosis, symptomatic cardiovascular disease, or moderate to severe hypertension
Known or suspected glaucoma
Hyperthyroidism or thyrotoxicosis
Agitated states or severe anxiety
History of drug abuse or dependence
Pregnancy unless benefits outweigh risks (relative contraindication)
Concurrent use with other stimulant medications

Drug Interactions

Concurrent use with MAO inhibitors can cause hypertensive crisis and is contraindicated
CYP2D6 inhibitors may increase plasma concentrations of amphetamines, enhancing toxic effects
Alkalinizing agents (e.g., sodium bicarbonate) can reduce renal clearance, increasing amphetamine levels
Acidifying agents (e.g., vitamin C, cranberry juice) can increase renal excretion, reducing efficacy
Additive cardiovascular effects with other stimulants, sympathomimetics, or antihypertensive drugs may reduce efficacy
Increased risk of serotonin syndrome when combined with serotonergic drugs such as SSRIs, SNRIs, or triptans
Concurrent use with antipsychotics may antagonize amphetamine effects
May decrease effectiveness of antihypertensive agents due to stimulant-induced elevated blood pressure
Combination with warfarin can alter coagulation parameters
Opioid analgesics may have altered effects when used with amphetamines

Preparations Of The Drug

Extended-release capsules containing mixed amphetamine salts
Available dosages: 5 mg, 10 mg, 15 mg, 20 mg, 25 mg, and 30 mg capsules
Capsules designed to be swallowed whole or opened and sprinkled on applesauce for patients with difficulty swallowing
Available as both brand name (Adderall XR) and generic formulations