Dexmethylphenidate

Dexmethylphenidate hydrochloride is a CNS stimulant. The mode of therapeutic action in ADHD is not known.

Dexmethylphenidate is the more pharmacologically active d-enantiomer of racemic methylphenidate. Methylphenidate blocks the reuptake of norepinephrine and dopamine into the presynaptic neuron and increase the release of these monoamines into the extraneuronal space.

Cardiac Electrophysiology

A formal QT study has not been conducted in patients taking dexmethylphenidate; however, a large QT effect is not expected. At the recommended maximum total daily dosage of 40 mg, dexmethylphenidate extended-release capsule does not prolong the QTc interval to any clinically relevant extent.

Absorption

Dexmethylphenidate hydrochloride is readily absorbed following oral administration of dexmethylphenidate. In patients with ADHD, plasma dexmethylphenidate concentrations increase rapidly, reaching a maximum in the fasted state at about 1 to 1.5 hours postdose. No differences in the pharmacokinetics of dexmethylphenidate were noted following single and repeated twice daily dosing, thus indicating no significant drug accumulation in children with ADHD.

After single dose administration of dexmethylphenidate to pediatric patients, dexmethylphenidate exposure (C_max and AUC_0-inf) showed dose-proportional increase in the range of 2.5 mg to 10 mg. Comparable plasma dexmethylphenidate levels were achieved following single dl-threo-methylphenidate HCl doses given as capsules in twice the total mg amount (equimolar with respect to dexmethylphenidate).

Approximately 90% of the dose is absorbed after oral administration of radiolabeled racemic methylphenidate. However, due to first pass metabolism the mean absolute bioavailability of dexmethylphenidate when administered in various formulations was 22% to 25%.

Effect of Food

High fat breakfast did not significantly affect C_max or AUC_0-inf of dexmethylphenidate when two 10 mg dexmethylphenidate tablets were administered, but delayed T_max from 1.5 hours post dose to 2.9 hours post dose.

Distribution

The plasma protein binding of dexmethylphenidate is not known; racemic methylphenidate is bound to plasma proteins by 12% to 15%, independent of concentration. Dexmethylphenidate shows a volume of distribution of 2.65 ± 1.11 L/kg.

Elimination

Plasma dexmethylphenidate concentrations declined exponentially following oral administration of dexmethylphenidate. Intravenous dexmethylphenidate was eliminated with a mean clearance of 0.40 ± 0.12 L/hr/kg. The mean terminal elimination half-life of dexmethylphenidate was approximately 2.2 hours.

Metabolism

In humans, dexmethylphenidate is metabolized primarily via de-esterification to d-α-phenyl-piperidine acetic acid (also known as d-ritalinic acid). This metabolite has little or no pharmacological activity. There is little or no in vivo interconversion to the l-threo-enantiomer.

Excretion

After oral dosing of radiolabeled racemic methylphenidate in humans, about 90% of the radioactivity was recovered in urine. The main urinary metabolite of racemic dl-methylphenidate was dl-ritalinic acid, accountable for approximately 80% of the dose. Urinary excretion of parent compound accounted for 0.5% of an intravenous dose.

Studies in Special Populations

Male and Female Patients

Pharmacokinetic parameters were similar for boys and girls (mean age 10 years).

In a single dose study conducted in adults, the mean dexmethylphenidate AUC_0-inf values (adjusted for body weight) following single two 10 mg doses of dexmethylphenidate were 25% to 35% higher in adult female volunteers (n=6) compared to male volunteers (n=9). Both T_max and t_1/2 were comparable for males and females.

Racial or Ethnic Groups

There is insufficient experience with the use of dexmethylphenidate to detect ethnic variations in pharmacokinetics.

Pediatric Patients

The pharmacokinetics of dexmethylphenidate after dexmethylphenidate administration have not been studied in children less than 6 years of age. When single doses of dexmethylphenidate were given to children between the ages of 6 to 12 years and healthy adult volunteers, C_max of dexmethylphenidate was similar, however, pediatric patients showed somewhat lower AUCs compared to the adults.

Patients with Renal Impairment

There is no experience with the use of dexmethylphenidate in patients with renal impairment. Since renal clearance is not an important route of methylphenidate clearance, renal impairment is expected to have little effect on the pharmacokinetics of dexmethylphenidate.

Patients with Hepatic Impairment

There is no experience with the use of dexmethylphenidate in patients with hepatic impairment.

Drug Interaction Studies

Methylphenidate is not metabolized by cytochrome P450 (CYP) isoenzymes to a clinically relevant extent. Inducers or inhibitors of CYPs are not expected to have any relevant impact on methylphenidate pharmacokinetics. Conversely, the d- and l-enantiomers of methylphenidate did not relevantly inhibit CYP1A2, 2C8, 2C9, 2C19, 2D6, 2E1 or 3A. Clinically, methylphenidate coadministration did not increase plasma concentrations of the CYP2D6 substrate desipramine.