Dexmethylphenidate: Difference between revisions

| mechanism        = Norepinephrine–dopamine reuptake inhibition (DAT, NET) — d-threo enantiomer of methylphenidate

| intro            = '''Dexmethylphenidate''' — marketed as '''Focalin''' and '''Focalin XR''' — is the isolated d-threo enantiomer of [[Methylphenidate|methylphenidate]]. Racemic methylphenidate contains both d- and l-threo enantiomers in equal amounts, but essentially all pharmacologic activity resides in the d-isomer; the l-isomer is rapidly cleared on first-pass and contributes little beyond a possible minor role in peripheral side effects. By delivering only the active enantiomer, Focalin achieves equivalent therapeutic effect at approximately half the milligram dose of racemic methylphenidate, with some patients reporting a cleaner subjective profile (less jitteriness, more focused effect). Its clinical indications, mechanism, contraindications, and adverse-effect profile are otherwise nearly identical to racemic methylphenidate.

| pharmacokinetics  = '''Absorption:''' Rapid oral absorption; peak plasma in ~1–1.5 h (IR) or ~6.5 h (XR, which has a biphasic release profile with an initial peak around 1.5 h and a second peak ~6.5 h). Bioavailability ~22–25% due to extensive first-pass metabolism. '''Distribution:''' Volume of distribution similar to racemic methylphenidate (~13 L/kg); plasma protein binding ~15%. '''Metabolism:''' Like the racemate, dexmethylphenidate is metabolized primarily by '''carboxylesterase 1 (CES1)''' to ritalinic acid (inactive). This non-CYP metabolic pathway means dexmethylphenidate, like its racemate, has '''few clinically significant CYP-mediated medicine interactions'''. '''Elimination:''' Predominantly renal as ritalinic acid; parent half-life 2.2 h (IR) and ~3 h (XR parent). Multi-dose-daily IR dosing or once-daily XR is required for sustained therapeutic effect.

| pharmacodynamics  = Mechanistically identical to racemic methylphenidate at the molecular level — competitive inhibition of the dopamine and norepinephrine transporters (DAT, NET), increasing synaptic catecholamine concentrations without acting as a transporter substrate (i.e., no reverse transport or vesicular displacement; '''reuptake inhibition only'''). The functional difference is purely stereochemical: dexmethylphenidate provides the same effective amount of pharmacologically active material at half the milligram dose. Minimal effect on serotonin; no MAO inhibition; no significant binding at 5-HT, histamine, or muscarinic receptors.

* Not FDA-approved for narcolepsy (use racemic methylphenidate or an amphetamine for that indication)

| interactions      = * '''MAOIs''' — hypertensive crisis risk; contraindicated within 14 days

* '''Tricyclic antidepressants''' — possible elevation of TCA levels; additive cardiovascular effects

* '''Warfarin''' — possible elevation of INR

* '''Phenytoin, phenobarbital, primidone''' — possible elevation of anticonvulsant levels

* '''Antihypertensives''' — pressor effect of dexmethylphenidate may partially antagonize

* '''Other sympathomimetics''' (pseudoephedrine, phenylephrine, decongestants) — additive cardiovascular effects

* '''Antipsychotics''' — mutual pharmacologic antagonism

* '''Alcohol''' — may mask effects; possible increased exposure via altered metabolism

* '''Caffeine''' — additive stimulant and anxiogenic effects

* Periodically reassess continued need; consider medicine holidays in children

* Do not share or sell — Schedule II controlled substance.

* If switching from racemic methylphenidate, dose conversion is roughly half — confirm with prescriber.

'''Absorption:''' Rapid oral absorption; peak plasma in ~1–1.5 h (IR) or ~6.5 h (XR, which has a biphasic release profile with an initial peak around 1.5 h and a second peak ~6.5 h). Bioavailability ~22–25% due to extensive first-pass metabolism. '''Distribution:''' Volume of distribution similar to racemic methylphenidate (~13 L/kg); plasma protein binding ~15%. '''Metabolism:''' Like the racemate, dexmethylphenidate is metabolized primarily by '''carboxylesterase 1 (CES1)''' to ritalinic acid (inactive). This non-CYP metabolic pathway means dexmethylphenidate, like its racemate, has '''few clinically significant CYP-mediated medicine interactions'''. '''Elimination:''' Predominantly renal as ritalinic acid; parent half-life 2.2 h (IR) and ~3 h (XR parent). Multi-dose-daily IR dosing or once-daily XR is required for sustained therapeutic effect.<h2 id="Pharmacodynamics">Pharmacodynamics</h2>

Mechanistically identical to racemic methylphenidate at the molecular level — competitive inhibition of the dopamine and norepinephrine transporters (DAT, NET), increasing synaptic catecholamine concentrations without acting as a transporter substrate (i.e., no reverse transport or vesicular displacement; '''reuptake inhibition only'''). The functional difference is purely stereochemical: dexmethylphenidate provides the same effective amount of pharmacologically active material at half the milligram dose. Minimal effect on serotonin; no MAO inhibition; no significant binding at 5-HT, histamine, or muscarinic receptors.<h2 id="Indications">Indications</h2>

*Not FDA-approved for narcolepsy (use racemic methylphenidate or an amphetamine for that indication)<h2 id="Dosing">Dosing and titration</h2>

*'''MAOIs''' — hypertensive crisis risk; contraindicated within 14 days

*'''Tricyclic antidepressants''' — possible elevation of TCA levels; additive cardiovascular effects

*'''Warfarin''' — possible elevation of INR

*'''Phenytoin, phenobarbital, primidone''' — possible elevation of anticonvulsant levels

*'''Antihypertensives''' — pressor effect of dexmethylphenidate may partially antagonize

*'''Other sympathomimetics''' (pseudoephedrine, phenylephrine, decongestants) — additive cardiovascular effects

*'''Antipsychotics''' — mutual pharmacologic antagonism

*'''Alcohol''' — may mask effects; possible increased exposure via altered metabolism

*'''Caffeine''' — additive stimulant and anxiogenic effects

*Periodically reassess continued need; consider medicine holidays in children<h2 id="Counseling">Patient counseling</h2>

*Do not share or sell — Schedule II controlled substance.

*If switching from racemic methylphenidate, dose conversion is roughly half — confirm with prescriber.

[[Category:Stimulants & Wake-Promoting Agents]]