Wakefulness promoting agent for oral administration.
Pharmacology. Pharmacodynamics. The precise mechanism(s) through which modafinil promotes wakefulness is unknown. Modafinil has wake promoting actions but a pharmacological profile that is distinct from sympathomimetic amines, which increase wakefulness by other mechanisms.
Modafinil does not bind to most of the potentially relevant receptors for sleep/ wake regulation, including those for noradrenaline, serotonin, dopamine, GABA, adenosine, histamine-3 and benzodiazepines. Modafinil is not a direct or indirect acting dopamine receptor agonist and is inactive in several in vivo preclinical models capable of detecting enhanced dopaminergic activity. In vitro, modafinil binds to the dopamine reuptake site with low affinity and causes an increase in extracellular dopamine, but no increase in dopamine release. Modafinil does not appear to be a direct or indirect alpha1-adrenergic agonist. Although modafinil induced wakefulness can be attenuated by the alpha1-adrenergic receptor antagonist prazosin, modafinil has no activity in assay systems known to be responsive to the alpha-adrenergic agonists.
In rats, the wakefulness induced by amphetamine, but not modafinil, was antagonised by the dopamine receptor antagonist haloperidol. In cats, modafinil evoked neuronal activation in brain regions different from methylphenidate and amphetamine. Modafinil served as a positive reinforcer for cocaine in monkeys and was partially discriminated as stimulant-like in rats (see Precautions, Dependence potential).
The optical enantiomers of modafinil have similar pharmacological actions in mice, but have not been studied individually in humans. The two major metabolites of modafinil, modafinil acid and modafinil sulfone, showed little CNS activating activity in animal studies.
Modafinil in humans restores and/or improves the level of wakefulness. Changes are found in electrophysiological parameters reflecting alertness (ratio of power of alpha rhythm to power of theta rhythm), starting from a dose of 100 mg in the morning. An increase is seen in latency periods in the multiple sleep latency test, starting from 200 mg in the morning. Modafinil opposes the impairment of cognitive (in particular memory), psychomotor and neurosensory performance induced by sleep deprivation. This activity is observed in the absence of any modifications of appetite or behaviour.
Morning administration of 200 mg does not appear to affect nocturnal sleep. Administration of 100 mg morning and noon may prolong the subjective time taken to fall asleep. Evening administration may disturb sleep. This pharmacodynamic activity does not appear to affect the autonomic nervous system.
Pharmacokinetics. Modafinil is a racemic compound, whose enantiomers have different pharmacokinetics (e.g. the half-life of the L-isomer is approximately three times that of the D-isomer in humans). The enantiomers do not interconvert. At steady state, total exposure to the L-isomer is approximately three times that of the D-isomer. The trough concentration (Cminss) of circulating modafinil after once daily dosing consists of 90% of the L-isomer and 10% of the D-isomer.
Absorption and distribution. Modafinil is slowly absorbed with an absorption half-life of approximately one hour. Peak plasma concentrations (Cmax) of approximately 3.3 mg/L are reached three hours (Tmax) after administration of a 200 mg dose. Both the area under the plasma concentration curve (AUC) and the peak plasma concentration show dose proportionality in the 50 to 400 mg range. The absolute oral bioavailability could not be determined due to the aqueous insolubility (< 1 mg/mL) of modafinil, which precluded intravenous administration. Food has no effect on the overall bioavailability of modafinil, however, its absorption (Tmax) may be delayed by approximately one hour if taken with food.
Modafinil is well distributed in body tissue with an apparent volume of distribution (approx. 0.9 L/kg) larger than the volume of total body water (0.6 L/kg). Modafinil is weakly bound to plasma proteins (62%), mainly to albumin. At serum concentrations obtained at steady state after doses of 200 mg/day, modafinil exhibits no displacement of protein binding of warfarin, diazepam or propranolol.