L 21-22 Psychomotor Stimulants

Clinical Pharmacology MCQs: Psychomotor Stimulants

• Question 1: Synthetic Cathinone Toxicity     * Scenario: A $19$-year-old college student presents to the ED with severe agitation, hyperthermia, hypertension, and tachycardia after using a “bath salt” product purchased online. He is paranoid and combative.     * Question: Which mechanism most likely explains his symptoms?     * Correct Answer: C. Increased catecholamine release and monoamine reuptake blockade.     * Explanation: Synthetic cathinones (“bath salts”) act by increasing the release of dopamine, norepinephrine, and serotonin while simultaneously blocking their reuptake transporters. This leads to severe sympathomimetic toxicity, encompassing agitation, psychosis, hyperthermia, hypertension, and arrhythmias.     * Incorrect Options Analysis:         * A. Adenosine receptor antagonism: Refers specifically to the mechanism of caffeine.         * B. Weak dopamine transporter inhibition only: Consistent with the profile of modafinil.         * D. Irreversible MAO inhibition: Characteristic of Monoamine Oxidase Inhibitors (MAOIs), not cathinones.         * E. Muscarinic receptor blockade: Would result in anticholinergic toxicity symptoms such as dry skin and urinary retention.

• Question 2: ADHD Management and Side Effects     * Scenario: A $10$-year-old boy with ADHD and severe anxiety develops worsening insomnia and appetite suppression after starting mixed amphetamine salts.     * Question: Which medication would most likely provide symptom control with lower abuse potential and less sympathomimetic activity?     * Correct Answer: C. Modafinil.     * Explanation: Modafinil is classified as an atypical stimulant characterized by weak DAT (Dopamine Transporter) inhibition. It results in less euphoria, lower abuse potential, and reduced autonomic stimulation compared to traditional amphetamines.     * Incorrect Options Analysis:         * A. Methamphetamine: Notable for greater CNS penetration and significant neurotoxicity.         * B. Cocaine: Highly addictive with substantial cardiovascular risks.         * D. Ephedrine: Possesses strong sympathomimetic effects.         * E. Cathinone: A potent stimulant associated with high toxicity risk.

• Question 3: Lisdexamfetamine Pharmacokinetics     * Scenario: A patient with ADHD is switched from dextroamphetamine/amphetamine to lisdexamfetamine.     * Question: Which feature best explains why lisdexamfetamine produces less euphoria?     * Correct Answer: C. It is a prodrug requiring enzymatic hydrolysis in RBCs.     * Explanation: Lisdexamfetamine is converted to its active form, dextroamphetamine, via hydrolysis in Red Blood Cells (RBCs). This process ensures a slower onset and smoother plasma concentrations, thereby reducing the reinforcement and euphoria associated with rapid spikes.     * Incorrect Options Analysis:         * A. It does not cross the blood-brain barrier: Incorrect; it crossed the BBB once converted into the active metabolite.         * B. It is eliminated before CNS penetration: Incorrect; the active metabolite successfully reaches the CNS.         * D. It blocks serotonin reuptake only: Incorrect; amphetamine effects primarily involve dopamine and norepinephrine pathways.         * E. It is metabolized by CYP3A4: Lisdexamfetamine is notably less dependent on CYP metabolism, relying instead on RBC hydrolysis.

• Question 4: Cocaine Toxicity and Examination Findings     * Scenario: A $34$-year-old man presents with chest pain, severe hypertension, and tachyarrhythmia after recreational drug use. Physical exam reveals nasal septal perforation.     * Question: Which drug is most likely responsible?     * Correct Answer: B. Cocaine.     * Explanation: Cocaine induces intense sympathetic activation through the blockade of DAT, NET, and SERT. Chronic intranasal use is specifically associated with nasal septal perforation due to localized vasoconstriction.     * Incorrect Options Analysis:         * A. Modafinil: Less sympathomimetic and not linked to septal perforation.         * C. Methylphenidate: Results in less severe vasoconstrictive toxicity.         * D. Pseudoephedrine: Only a mild sympathomimetic.         * E. Caffeine: Does not cause destruction of the nasal septum.

• Question 5: Caffeine Mechanism of Action     * Scenario: A patient using high doses of caffeine reports insomnia, frequent awakenings, anxiety, and palpitations.     * Question: Which receptor blockade primarily contributes to increased alertness and wakefulness?     * Correct Answer: D. Adenosine A1 receptor.     * Explanation: Caffeine acts as an antagonist at adenosine receptors, particularly the A1 receptors in the brain, which leads to promoted wakefulness and heightened alertness.     * Incorrect Options Analysis:         * A/B. Adrenergic receptors: These are not the primary direct mechanism for caffeine's CNS effects.         * C. Dopamine D2 receptor: Caffeine affects dopamine signaling indirectly, not through direct blockade of D2.         * E. Muscarinic M2 receptor: Unrelated to the clinical effects of caffeine.

• Question 6: Caffeine and Drug-Drug Interactions     * Scenario: A $42$-year-old woman taking oral contraceptives (OCPs) notices increased jitteriness and insomnia after drinking her usual amount of coffee.     * Question: Why did this occur?     * Correct Answer: B. OCPs inhibit CYP1A2 metabolism of caffeine.     * Explanation: Caffeine is primarily metabolized by the enzyme CYP1A2. Oral contraceptives inhibit this enzyme, thereby slowing the metabolism of caffeine and increasing its plasma concentration.     * Incorrect Options Analysis:         * A. OCPs increase renal caffeine excretion: This is the opposite of the actual clinical effect.         * C. OCPs activate adenosine receptors: There is no evidence for this interaction.         * D. OCPs increase dopamine breakdown: Not a clinically relevant mechanism in this context.         * E. OCPs inhibit pseudocholinesterase: Pseudocholinesterase is involved in the metabolism of drugs like cocaine, not caffeine.

• Question 7: Preferred ADHD Treatment in Comorbidity     * Scenario: A teenager with ADHD and Tourette syndrome requires stimulant therapy.     * Question: Which stimulant is generally preferred due to its lower risk of worsening anxiety and lower abuse potential?     * Correct Answer: C. Methylphenidate.     * Explanation: Methylphenidate blocks the reuptake of dopamine and norepinephrine but does not induce the strong neurotransmitter release characteristic of amphetamines, leading to lower abuse and cardiovascular risk.

• Question 8: Methamphetamine Neurotoxicity     * Scenario: A patient using methamphetamine develops severe dental decay, weight loss, paranoia, and hallucinations.     * Question: Which property contributes most to its high neurotoxicity?     * Correct Answer: C. Greater CNS penetration and prolonged monoamine release.     * Explanation: Methamphetamine's high lipid solubility allows for massive CNS penetration, causing profound release of dopamine and norepinephrine, leading to addiction and neurotoxicity.

• Question 9: Amphetamine Withdrawal Symptoms     * Scenario: A patient presents after abrupt discontinuation of chronic amphetamine use.     * Question: Which symptom constellation is most likely?     * Correct Answer: B. Depression, hypersomnia, increased appetite.     * Explanation: The "crash" associated with amphetamine withdrawal includes fatigue, depression, hypersomnia, anhedonia, and intense cravings.

• Question 10: Cocaine as a Local Anesthetic     * Scenario: A surgeon applies a topical anesthetic during a nasal procedure that causes both vasoconstriction and voltage-gated sodium channel blockade.     * Question: Which drug was used?     * Correct Answer: C. Cocaine.     * Explanation: Cocaine is unique among stimulants for its ability to block voltage-gated sodium channels, acting as a potent topical anesthetic and vasoconstrictor.

• Question 11: Over-the-Counter Decongestants     * Scenario: A patient takes an OTC medication for nasal congestion and develops mild insomnia/nervousness. The drug has indirect $\alpha$-adrenergic activity and minimal BBB penetration.     * Question: Which medication is most likely responsible?     * Correct Answer: B. Pseudoephedrine.     * Explanation: Pseudoephedrine is an OTC nasal decongestant with limited CNS penetration compared to ephedrine.

• Question 12: Initial Management of Stimulant Overdose     * Scenario: A patient with stimulant overdose is severely agitated, hypertensive, hyperthermic, and tachycardic.     * Question: What is the best initial pharmacologic management?     * Correct Answer: C. Benzodiazepines.     * Explanation: Benzodiazepines are the first-line treatment for stimulant toxicity as they effectively reduce agitation, suppress seizures, and decrease sympathetic autonomic outflow.

Caffeine (Methylxanthines)

• Overview:     * The most widely used psychoactive stimulant globally.     * An alkaloid found in over $60$ different plants.     * Possesses high lipid solubility; readily crosses cell membranes and the blood-brain barrier (BBB).

• Mechanism of Action (MOA):     * Primary: Adenosine Receptor Antagonist.     * Secondary: Inhibits phosphodiesterase, which prevents the breakdown of cAMP.     * Tertiary: Causes intracellular $Ca^{2+}$ release, which can influence neurotransmitter (NT) release.

• Pharmacokinetics:     * Onset: $15-45$ minutes.     * Half-life ($T_{1/2}$): Approximately $5$ hours.     * Metabolism: Approximately $75-80\%$ is metabolized by the liver enzyme CYP1A2.     * Interactions: Oral contraceptive pills (OCPs) inhibit CYP1A2, slowing the metabolism rate and potentially increasing toxicity.     * Excretion: Via urine.

• Specific Adenosine Receptor Blockade Effects:     * A1 Receptors: Highly concentrated in the cortex, hippocampus, heart, and kidney. Blockade results in increased alertness, increased heart rate (HR), and diuretic effects.     * A2A Receptors: High levels in the basal ganglia, striatum, and cerebral blood vessels. Blockade leads to vasoconstriction (useful for headache relief) and increased dopamine signaling, which improves mood, attention, motivation, and cognitive function.     * A2B Receptors (High/Toxic Concentrations): Located in vasculature, lungs, and GI. Blockade causes bronchodilation. High doses lead to vasoconstriction and GI effects.     * A3 Receptors (Toxic Concentrations Only): Found on immune cells and the heart. Blockade results in neutrophil degranulation.

• Side Effects and Toxicity:     * Side Effects: Disrupted sleep architecture ($\uparrow$ sleep latency, more frequent awakenings), anxiety, and palpitations.     * Toxicity Threshold: Consumption exceeding $300\,mg/day$ may cause restlessness, nervousness, excitement, insomnia, facial flushing, increased urination, GI distress, tremors, chaotic speech/thought, irritability, arrhythmia, tachycardia, and psychomotor agitation.

• Withdrawal:     * Onset: Begins $12-24$ hours after discontinuation.     * Peak: Peaks on Day $2$.     * Duration: Can last up to $10$ days.     * Relief: Occurs within $30-60$ minutes upon caffeine reintroduction.     * Symptoms: Headache (HA), fatigue, nausea/vomiting (N/V), drowsiness, poor concentration, and depressed mood.     * Management: Gradual reduction, hydration, adequate sleep, and exercise.

Cocaine

• Origin and Status:     * Alkaloid from the leaves of Erythroxylum coca, a shrub indigenous to the Andes.     * Schedule II drug under the Controlled Substances Act.     * Historical uses: Fatigue, appetite suppression, and altitude tolerance.

• Mechanism of Action (MOA):     * Monoamine Reuptake Inhibition: Blocks the transporters for Dopamine (DAT), Norepinephrine (NET), and Serotonin (SERT).     * Voltage-Gated Sodium Channel (VGSC) Blockade: Acts as a local anesthetic by preventing nerve impulse generation.

• Systemic Effects by Transporter Blockade:     * DAT Blockade: Increases dopamine in the nucleus accumbens, leading to euphoria and activation of the reward circuit.     * NET Blockade: Increases norepinephrine, leading to increased arterial pressure, tachycardia, and ventricular arrhythmias (sympathetic activation).     * SERT Blockade: Increases serotonin, leading to elevated mood.

• Local Anesthetic Properties:     * Strongly binds to the open inactivated state of the sodium channel, not the resting state.     * Duration: $5-90$ minutes.     * Fiber Blocking Sequence: Small, numerous $A\delta$ pain fibers are blocked first.     * Order of Loss: $1^{st}$ Pain ($A\delta$) $\rightarrow$ Temperature ($A\delta$) $\rightarrow$ Touch ($A\beta$) $\rightarrow$ Pressure/Motor ($A\alpha$).     * Frequency-Dependent Block: Suppression of pain intensifies with continued neural activity.

• Pharmacokinetics and Use:     * Clinical Use: Topical anesthetic in ENT procedures (potent vasoconstrictor) and diagnosis of Horner’s syndrome.     * Absorption: Well absorbed through mucous membranes. Intranasal use causes local vasoconstriction, which actually delays systemic absorption, resulting in a longer local duration of action.     * Metabolism: >95\% hydrolyzed by carboxylesterases in the liver and pseudocholinesterase in the plasma. Less than $5\%$ is metabolized by CYP3A4, which produces a toxic metabolite.     * Excretion: Urine.

• Adverse Effects and Toxicity:     * CNS: Euphoria, anxiety, agitation, paranoia, insomnia.     * Autonomic: Tachycardia, hypertension, mydriasis.     * Serious Risks: Myocardial infarction (MI), stroke (ischemic or hemorrhagic), seizures, intracranial hemorrhage.     * Chronic Toxicity: Chronic paranoia, psychosis, orofacial perforations (nasal septum and palate), and pulmonary injury ("crack lung").     * Overdose Management: Benzodiazepines are used to control agitation and sympathetic outflow. Additional measures include IV fluids and active cooling.

• Withdrawal:     * Not medically dangerous but psychologically severe.     * Symptoms: Depression, fatigue, hypersomnia, anhedonia, increased appetite, and cravings.     * Timeline: Onset in hours/days, peak at $1-3$ days, cravings last months.

Cathinones (“Bath Salts”)

• Overview:     * Natural alkaloid found in the Khât plant (Catha edulis), indigenous to East Africa and the Middle East.     * Chemically similar to amphetamines.     * Schedule I: Illegal in the United States.     * Forms: Natural (chewed leaves) and Synthetic (sold as "bath salts," "fertilizers," or "air fresheners").

• Mechanism of Action:     * Increases catecholamine release and reduces catecholamine reuptake (DAT, NET, and SERT).     * Synthetic versions are more potent, long-acting, and toxic.

• Pharmacokinetics:     * Absorption: Rapid via oral, nasal, or IV routes.     * Half-life ($T_{1/2}$): Natural is short (approximately $90$ minutes); Synthetic is variable and often longer.     * Metabolism/Elimination: Liver metabolism; renal elimination.

• Toxicity and Overdose:     * CNS: Severe agitation, violent behavior, paranoia, hallucinations, and elevated sex drive.     * Autonomic: Severe hypertension, tachycardia, hyperthermia (up to $105^\circ F$), and serotonin syndrome.     * Serious Complications: Rhabdomyolysis, metabolic acidosis, and cardiovascular collapse.     * Management: Benzodiazepines, aggressive cooling, and monitoring for arrhythmias and rhabdomyolysis.

Ephedrine and Pseudoephedrine

• Origin: Derived from the genus Ephedra (Ma Huang). Used in Chinese medicine for over $2,000$ years.

• Ephedrine MOA (Mixed-acting Sympathomimetic):     * Indirect: Increases release of NE from presynaptic terminals.     * Direct: Stimulates $\alpha_1$, $\beta_1$, and $\beta_2$ adrenergic receptors.         * $\alpha_1$: Vasoconstriction ($\uparrow$ BP).         * $\beta_1$: $\uparrow$ Heart rate and contractility.         * $\beta_2$: Bronchodilation.     * Characteristics: Crosses the BBB (CNS stimulation).

• Pseudoephedrine (Ephedrine Enantiomer):     * Developed as a nasal decongestant with reduced CNS effects.     * MOA: Indirect $\alpha$-adrenergic activity; minimal BBB penetration.     * Status: "Behind the counter" because it is a chemical precursor for methamphetamine.

Amphetamines and Lisdexamfetamine

• Overview of Amphetamine/Adderall:     * Synthesized in the $1880s$. Schedule II drug.     * MOA: Enters neurons via DAT/NET, displaces DA/NE from vesicles via VMAT, and reverses DAT/NET to cause neurotransmitter release into the synapse.     * Dextroamphetamine: The more potent d-enantiomer.     * Half-life ($T_{1/2}$): Approximately $9-12$ hours (variable).     * Elimination: Renal excretion rate increases with acidic urine.

• Lisdexamfetamine (Vyvanse):     * Type: Prodrug; has no intrinsic stimulant activity.     * Activation: Hydrolyzed by enzymes in Red Blood Cells. This rate-limited step prevents rapid onset.     * Clinical Advantages: Slower onset, smoother plasma levels, less euphoria, and longer duration. Used for ADHD and binge-eating disorder.

Methamphetamine (Desoxyn)

• History: Synthesized in Japan in $1893$. Schedule II.

• MOA: Similar to amphetamine but with massive neurotransmitter release and significantly greater CNS penetration.

• Neurotoxicity: Highly neurotoxic compared to other stimulants. Chronic use leads to weight loss, malnutrition, and "meth mouth" (severe dental decay).

• Overdose: Managed with benzodiazepines for agitation, seizures, and sympathetic outflow reduction.

Methylphenidate (Ritalin)

• Overview: Therapeutic CNS stimulant developed in the $1950s$. Schedule II.

• MOA: Blocks the reuptake of dopamine and norepinephrine. Unlike amphetamines, it does not cause significant neurotransmitter release.

• Pharmacokinetics: Rapidly hydrolyzed by esterases; minimal CYP involvement.

• Clinical Use: Preferred in children and those sensitive to anxiety/insomnia due to a lower risk of physical dependence and cardiovascular side effects compared to amphetamines.

Modafinil and Armodafinil

• Overview: Developed in the $1970s$. Schedule IV drug (lower risk for abuse).

• Classification: Atypical psychostimulant.

• MOA: Weak inhibition of the dopamine transporter (DAT).

• Indications: Narcolepsy, shift-work sleep disorder, and Obstructive Sleep Apnea (OSA) related sleepiness.

• Pharmacokinetics: Modafinil $T_{1/2}$ is $12-15$ hours. It induces CYP3A4 and inhibits CYP2C19.

High-Yield Comparison Charts

• Mechanism Summary:     * Cocaine: Blocker (DAT, NET, SERT).     * Amphetamine: Releaser (DA, NE).     * Methylphenidate: Blocker without release.     * Caffeine: Adenosine receptor antagonist.

• Abuse Potential Ladder:     * Extreme: Cocaine, Amphetamine, Methamphetamine.     * High: Lisdexamfetamine, Methylphenidate.     * Lower: Modafinil.     * Low: Pseudoephedrine, Caffeine.     * Illegal (No accepted use): Cathinones (Schedule I).

• Toxicology Pearl: The universal first-line treatment for sympathomimetic overdose (agitation, HTN, tachycardia) is Benzodiazepines.