Methylphenidate

Mechanism

Chemical and Commercial Profile

Common trade names: Concerta, Rubifen, Medikinet, Ritalin.
Group: CNS psychostimulant, derived from phenylethylamine/piperidine.

Mechanism of Action

Methylphenidate acts by indirectly increasing central catecholaminergic synaptic tone. It highly competitively and selectively blocks the presynaptic transporters responsible for the clearance of dopamine and norepinephrine:

Synaptic Accumulation Mechanism \text{Metilfenidato} \longrightarrow \text{DAT and NET blockade} \longrightarrow \text{Increase of Dopamine and Noradrenaline in Prefrontal Cortex}

By binding to DAT (SLC6A3) and NET (SLC6A2), it promotes sustained activation of postsynaptic receptors D₁ and α_2A in the prefrontal cortex, optimizing the neuronal signal-to-noise ratio and improving sustained attention, working memory and the control of psychomotor hyperactivity.

Unlike amphetamines, methylphenidate does not promote the massive presynaptic release of newly synthesized dopamine vesicles (it lacks agonist action on the vesicular transporter VMAT2), significantly reducing its potential for neurotoxicity and abuse.

Pharmacokinetics

Pharmacokinetics

• Absorption: Rapid and complete orally; bioavailability of 30% due to an important presystemic metabolism by the first hydrolytic step. Extended-release formulations (such as Concerta's OROS systems or Medikinet microspheres) provide biphasic absorption that emulates conventional multiple daily dosing.
• Distribution: Crosses the BBB extremely quickly. Low binding to plasma proteins (15%).
• Metabolism: Mainly non-enzymatic by rapid hydrolysis catalyzed by carboxylesterase 1 (CES1), giving rise to the fast-clearing inactive metabolite ritalinic acid. It does not show significant interactions with the cytochrome P450 system.
• Excretion: Renal (90%) mainly as inactive hydrolyzed metabolites.
• Half-life (t_1/2): 2 to 3 hours, extending the biological therapeutic effect to 8-12 hours in prolonged release formulations.

Indicators and dose

Indications

• Treatment of Attention Deficit Hyperactivity Disorder (ADHD) in children over 6 years of age, adolescents and adults, as part of a comprehensive therapeutic program.
• Treatment of excessive daytime sleepiness associated with Diagnostic narcolepsy with or without cataplexy.
• Off-label: Adjuvant treatment of refractory fatigue in palliative care of terminal cancer, and apathetic depression in refractory geriatrics.

Dosage and Settings

• Pediatric population (>6 years): Start with 5 mg once or twice a day (rapid formulas) or 18 mg/day (extended-release formulas); optimal therapeutic range of 18 mg to 54 mg/day. Maximum authorized daily dose: 54 mg (children) or 72 mg (adults).
• Adult Population: Start with 18 mg/day; weekly slow titration to the usual clinical range of 36 mg to 72 mg/day.
• Renal / Hepatic Failure: It does not require special dose adjustments in nephropathy or liver disease due to its extrahepatic hydrolytic metabolism pathway mediated by plasma and tissue esterases.

Security

Contraindications

• Severe structural heart disease, hypertrophic obstructive cardiomyopathy, decompensated heart failure, complex arrhythmias or severe unstable arterial hypertension.
• Concurrent use of MAOIs (minimum safety interval of 14 days after suspension).
• Acute narrow angle glaucoma.
• Presence of severe motor tics or known diagnosis of Gilles de la Tourette syndrome (due to potential risk of dopamine-mediated motor worsening).

Adverse Effects (ADR)

Cardiovascular and developmental safety alert

Chronic treatment with methylphenidate requires strict monitoring of two major physiological parameters: blood pressure and heart rate (due to peripheral sympathetic adrenergic increase), and height-weight growth in the pediatric population. The drug can induce severe anorexia and a temporary delay in the rate of growth in height, with so-called "therapeutic vacation periods" being recommended during non-school periods.

• Very common (>10%): Persistent daytime-onset anorexia, sleep insomnia (especially if rapid formulations are administered after 4:00 p.m.), nausea, xerostomia and headache.
• Frequent (1%-10%): Sinus tachycardia, slight increase in blood pressure (3-5 mmHg), irritability, affective lability, transient motor tics.
• Rare or infrequent: Peripheral symmetrical Raynaud's phenomenon, worsening of motor tics and seizures in predisposed patients.

Clinical Interactions

• Antidepressants: Risk of hypertensive crisis in co-administration with tricyclics due to additive synaptic noradrenergic accumulation.
• Inhalation anesthetics: Increased risk of inducing acute ventricular arrhythmias during surgical procedures due to increased cardiac sensitivity to catecholamines.

Pregnancy and Breastfeeding

FDA Category: C. It may be associated with the risk of placental vasoconstriction, prematurity and low birth weight. Breastfeeding: Compatible with extreme caution. It is excreted in milk in extremely low proportions (Relative Infant Dose <0.5%), but requires control of irritability, insomnia and infant weight.