Modafinil : A Review of its Pharmacology and Clinical Efficacy in the Management of Narcolepsy.

SYNOPSIS: Modafinil promotes wakefulness through an as yet unknown mechanism of action. It significantly increases daytime sleep latency and reduces excessive daytime sleepiness (EDS) compared with placebo in patients with narcolepsy. However, the drug does not suppress cataplexy. Although direct comparative data are lacking, modafinil offers advantages over amphetamines and methylphenidate in patients with narcolepsy because of its lack of rebound phenomena after treatment withdrawal and its low abuse potential. Clinical trials have shown modafinil to be well tolerated in patients with narcolepsy. Except for headache, which was reported with a significantly greater frequency in modafinil recipients, the tolerability profile of modafinil 200 to 400 mg/day was similar to that of placebo in patients treated for 9 weeks. Preliminary data suggest that the tolerability of modafinil is maintained long term (40 weeks). Thus, modafinil is effective in the treatment of EDS in patients with narcolepsy, although it is not effective against cataplexy. Preliminary findings indicate that, unlike other psychostimulants, the drug is unlikely to be abused and is not associated with withdrawal phenomena. Therefore, modafinil is likely to be an effective therapeutic option for the treatment of EDS in patients with narcolepsy.

PHARMACODYNAMIC PROPERTIES: The mechanism of action of modafinil has not been clearly established. However, it may indirectly increase wakefulness, at least in part, through inhibition of cortical γ-aminobutyric acid (GABA) release via serotonergic mechanisms. Modafinil induces wakefulness and increases locomotor activity in a variety of animal species without causing stereotyped behaviour. In rhesus monkeys, the effects of oral modafinil were not associated with changes in blood pressure or heart rate. In contrast to dexamphetamine 20mg, single night-time doses of modafinil 100 or 200mg had no significant effects on objective sleep variables or sleep structure in young or elderly healthy volunteers. Modafinil reduced the impairment of cognitive performance induced by sleep deprivation in volunteers, suppressed microsleeps and reduced the length of recovery sleep required. However, volunteers treated with modafinil reported recovery sleep patterns which were similar to those of placebo recipients. Studies in volunteers indicate that modafinil has less abuse potential than dexamphetamine or methylphenidate.

PHARMACOKINETIC PROPERTIES: Peak plasma concentrations of modafinil (mean 4.82 mg/L) were reached 2.3 hours after a single 200mg oral dose in healthy volunteers. Over the dose range 200 to 600mg, the pharmacokinetics of modafinil were linear and dose dependent. Orally administered modafinil is extensively biotransformed in the liver to the inactive metabolites modafinil acid and modafinil sulphone, before being eliminated primarily in the urine (elimination half-life 9 to 14 hours). The pharmacokinetics of modafinil are not affected to a clinically significant extent by volunteer age or food intake, but both the maximum plasma concentration and the elimination half-life of the drug are increased in patients with hepatic or renal impairment.

CLINICAL EFFICACY: The efficacy of modafinil 200 or 400 mg/day was rated as good or excellent (using a 4-point scale) in 84 of 131 evaluable patients (64%) with excessive daytime sleepiness treated for 1 to 114 months in a noncomparative study. 53 patients withdrew from the study (43 because of loss of efficacy). In another study, 1 or 2 months' treatment with modafinil reduced or eliminated symptoms of excessive daytime sleepiness in 17 of 24 patients with narcolepsy. Modafinil 200 or 400 mg/day for 9 weeks significantly increased daytime sleep latency and reduced daytime sleepiness compared with placebo in patients with narcolepsy, but did not suppress cataplexy. It tended to improve psychomotor performance, although this effect was variable. Importantly, modafinil did not affect patients' nocturnal sleep parameters, ability to nap when necessary or feelings on awakening. There was no evidence of a withdrawal phenomenon (e.g. fatigue, vivid or unpleasant dreams, insomnia or hypersomnia) after treatment cessation. Clinical data comparing the efficacy of modafinil with that of other stimulants in patients with narcolepsy are limited.

TOLERABILITY: Although tolerability data for modafinil are limited, the drug appears to be well tolerated in patients with narcolepsy. A study in 283 evaluable patients showed that 9 weeks' treatment with modafinil 200 or 400 mg/day was as well tolerated as placebo, with only headache reported with significantly greater frequency (52 and 51 vs 36% of patients; p < 0.05). Nausea and nervousness also tended to occur more frequently in modafinil than in placebo recipients, but this difference did not reach statistical significance. The incidence or severity of adverse events associated with the use of modafinil did not appear to be dose related. Preliminary data showed that open-label modafinil was well tolerated for up to 40 weeks.

DOSAGE AND ADMINISTRATION: The recommended dosage of modafinil is 200 or 400 mg/day, given once or twice daily (morning and midday) to patients with narcolepsy. Modafinil is contraindicated in patients with moderate to severe hypertension and may reduce the efficacy of low dose oral contraceptives via enzymatic induction.