The mechanism(s) through which modafinil promotes wakefulness is unknown. Modafinil has wake-promoting actions similar to sympathomimetic agents including amphetamine and methylphenidate, although the pharmacologic profile is not identical to that of the sympathomimetic amines.
Modafinil-induced wakefulness can be attenuated by the α1-adrenergic receptor antagonist, prazosin; however, modafinil is inactive in other in vitro assay systems known to be responsive to α-adrenergic agonists such as the rat vas deferens preparation.
Modafinil is not a direct- or indirect-acting dopamine receptor agonist. However, in vitro , modafinil binds to the dopamine transporter and inhibits dopamine reuptake. This activity has been associated in vivo with increased extracellular dopamine levels in some brain regions of animals. In genetically engineered mice lacking the dopamine transporter (DAT), modafinil lacked wake-promoting activity, suggesting that this activity was DAT-dependent. However, the wake-promoting effects of modafinil, unlike those of amphetamine, were not antagonized by the dopamine receptor antagonist haloperidol in rats. In addition, alpha-methyl-p-tyrosine, a dopamine synthesis inhibitor, blocks the action of amphetamine, but does not block locomotor activity induced by modafinil.
In the cat, equal wakefulness-promoting doses of methylphenidate and amphetamine increased neuronal activation throughout the brain. Modafinil at an equivalent wakefulness-promoting dose selectively and prominently increased neuronal activation in more discrete regions of the brain. The relationship of this finding in cats to the effects of modafinil in humans is unknown.
In addition to its wake-promoting effects and ability to increase locomotor activity in animals, modafinil produces psychoactive and euphoric effects, alterations in mood, perception, thinking, and feelings typical of other CNS stimulants in humans. Modafinil has reinforcing properties, as evidenced by its self-administration in monkeys previously trained to self-administer cocaine; modafinil was also partially discriminated as stimulant-like.
The optical enantiomers of modafinil have similar pharmacological actions in animals. Two major metabolites of modafinil, modafinil acid and modafinil sulfone, do not appear to contribute to the CNS-activating properties of modafinil.
Modafinil is a 1:1 racemic compound, whose enantiomers have different pharmacokinetics (e.g., the half-life of R-modafinil is approximately three times that of S-modafinil in adult humans). The enantiomers do not interconvert. At steady state, total exposure to R-modafinil is approximately three times that for S-modafinil. The trough concentration (Cmin,ss ) of circulating modafinil after once daily dosing consists of 90% of R-modafinil and 10% of S-modafinil. The effective elimination half-life of modafinil after multiple doses is about 15 hours. The enantiomers of modafinil exhibit linear kinetics upon multiple dosing of 200 to 600 mg/day once daily in healthy volunteers. Apparent steady states of total modafinil and R-modafinil are reached after 2 to 4 days of dosing.
Modafinil is readily absorbed after oral administration, with peak plasma concentrations occurring at 2 to 4 hours. The bioavailability of modafinil tablets is approximately equal to that of an aqueous suspension. The absolute oral bioavailability was not determined due to the aqueous insolubility (<1 mg/mL) of modafinil, which precluded intravenous administration. Food has no effect on overall modafinil bioavailability; however, time to reach peak concentration (tmax ) may be delayed by approximately one hour if taken with food.
Modafinil has an apparent volume of distribution of approximately 0.9 L/kg. In human plasma, in vitro , modafinil is moderately bound to plasma protein (approximately 60%), mainly to albumin. The potential for interactions of modafinil with highly protein-bound drugs is considered to be minimal.
Metabolism and Elimination
The major route of elimination is metabolism (approximately 90%), primarily by the liver, with subsequent renal elimination of the metabolites. Urine alkalinization has no effect on the elimination of modafinil.
Metabolism occurs through hydrolytic deamidation, S-oxidation, aromatic ring hydroxylation, and glucuronide conjugation. Less than 10% of an administered dose is excreted as the parent compound. In a clinical study using radiolabeled modafinil, a total of 81% of the administered radioactivity was recovered in 11 days post-dose, predominantly in the urine (80% vs. 1% in the feces). The largest fraction of the drug in urine was modafinil acid, but at least six other metabolites were present in lower concentrations. Only two metabolites reach appreciable concentrations in plasma, i.e., modafinil acid and modafinil sulfone. In preclinical models, modafinil acid, modafinil sulfone, 2-[(diphenylmethyl)sulfonyl]acetic acid and 4-hydroxy modafinil, were inactive or did not appear to mediate the arousal effects of modafinil.
In adults, decreases in trough levels of modafinil have sometimes been observed after multiple weeks of dosing, suggesting auto-induction, but the magnitude of the decreases and the inconsistency of their occurrence suggest that their clinical significance is minimal. Significant accumulation of modafinil sulfone has been observed after multiple doses due to its long elimination half-life of 40 hours. Auto-induction of metabolizing enzymes, most importantly cytochrome P-450 CYP3A4, has also been observed in vitro after incubation of primary cultures of human hepatocytes with modafinil and in vivo after extended administration of modafinil at 400 mg/day.
A slight decrease (approximately 20%) in the oral clearance (CL/F) of modafinil was observed in a single dose study at 200 mg in 12 subjects with a mean age of 63 years (range 53 to 72 years), but the change was considered not likely to be clinically significant. In a multiple dose study (300 mg/day) in 12 patients with a mean age of 82 years (range 67 to 87 years), the mean levels of modafinil in plasma were approximately two times those historically obtained in matched younger subjects. Due to potential effects from the multiple concomitant medications with which most of the patients were being treated, the apparent difference in modafinil pharmacokinetics may not be attributable solely to the effects of aging. However, the results suggest that the clearance of modafinil may be reduced in the elderly [see Dosage and Administration ( 2.4) and Use in Specific Populations (8.5)].
The pharmacokinetics of modafinil are not affected by gender.
The influence of race on the pharmacokinetics of modafinil has not been studied.
In a single dose 200 mg modafinil study, severe chronic renal failure (creatinine clearance ≤20 mL/min) did not significantly influence the pharmacokinetics of modafinil, but exposure to modafinil acid (an inactive metabolite) was increased 9 fold.
The pharmacokinetics and metabolism of modafinil were examined in patients with cirrhosis of the liver (6 men and 3 women). Three patients had stage B or B+ cirrhosis and 6 patients had stage C or C+ cirrhosis (per the Child-Pugh score criteria). Clinically 8 of 9 patients were icteric and all had ascites. In these patients, the oral clearance of modafinil was decreased by about 60% and the steady state concentration was doubled compared to normal patients [see Dosage and Administration (2.3) and Use in Specific Populations (8.6)].
In vitro data demonstrated that modafinil weakly induces CYP1A2, CYP2B6, and possibly CYP3A activities in a concentration-related manner and that CYP2C19 activity is reversibly inhibited by modafinil. In vitro data also demonstrated that modafinil produced an apparent concentration-related suppression of expression of CYP2C9 activity. Other CYP activities did not appear to be affected by modafinil.
Potential Interactions with Drugs That Inhibit, Induce, or Are Metabolized by Cytochrome P450 Isoenzymes and Other Hepatic Enzymes
The existence of multiple pathways for modafinil metabolism, as well as the fact that a non-CYP-related pathway is the most rapid in metabolizing modafinil, suggest that there is a low probability of substantive effects on the overall pharmacokinetic profile of modafinil due to CYP inhibition by concomitant medications. However, due to the partial involvement of CYP3A enzymes in the metabolic elimination of modafinil, coadministration of potent inducers of CYP3A4/5 (e.g., carbamazepine, phenobarbital, rifampin) or inhibitors of CYP3A4/5 (e.g., ketoconazole, erythromycin) could alter the plasma concentrations of modafinil.
The Potential of Modafinil to Alter the Metabolism of Other Drugs by Enzyme Induction or Inhibition
- Drugs Metabolized by CYP3A4/5
- In vitro data demonstrated that modafinil is a weak inducer of CYP3A activity in a concentration-related manner. Therefore, the blood levels and effectiveness of drugs that are substrates for CYP3A enzymes (e.g., steroidal contraceptives, cyclosporine, midazolam, and triazolam) may be reduced after initiation of concomitant treatment with modafinil [see Drug Interactions (7)].
- Ethinyl Estradiol — Administration of modafinil to female volunteers once daily at 200 mg/day for 7 days followed by 400 mg/day for 21 days resulted in a mean 11% decrease in mean Cmax and 18% decrease in mean AUC0-24 of ethinyl estradiol (EE2; 0.035 mg; administered orally with norgestimate). There was no apparent change in the elimination rate of ethinyl estradiol.
- Triazolam — In the drug interaction study between modafinil and ethinyl estradiol (EE2 ), on the same days as those for the plasma sampling for EE2 pharmacokinetics, a single dose of triazolam (0.125 mg) was also administered. Mean Cmax and AUC0-∞ of triazolam were decreased by 42% and 59%, respectively, and its elimination half-life was decreased by approximately an hour after the modafinil treatment.
- Cyclosporine — One case of an interaction between modafinil and cyclosporine, a substrate of CYP3A4, has been reported in a 41 year old woman who had undergone an organ transplant. After one month of administration of 200 mg/day of modafinil, cyclosporine blood levels were decreased by 50%. The interaction was postulated to be due to the increased metabolism of cyclosporine, since no other factor expected to affect the disposition of the drug had changed.
- Midazolam — In a clinical study, concomitant administration of armodafinil 250 mg resulted in a reduction in systemic exposure to midazolam by 32% after a single oral dose (5 mg) and 17% after a single intravenous dose (2 mg).
- Quetiapine — In a separate clinical study, concomitant administration of armodafinil 250 mg with quetiapine (300 mg to 600 mg daily doses) resulted in a reduction in the mean systemic exposure of quetiapine by approximately 29%.
- Drugs Metabolized by CYP1A2
- In vitro data demonstrated that modafinil is a weak inducer of CYP1A2 in a concentration-related manner. However, in a clinical study with armodafinil using caffeine as a probe substrate, no significant effect on CYP1A2 activity was observed.
- Drugs Metabolized by CYP2B6
- In vitro data demonstrated that modafinil is a weak inducer of CYP2B6 activity in a concentration-related manner.
- Drugs Metabolized by CYP2C9
- In vitro data demonstrated that modafinil produced an apparent concentration-related suppression of expression of CYP2C9 activity suggesting that there is a potential for a metabolic interaction between modafinil and the substrates of this enzyme (e.g., S-warfarin and phenytoin) [see Drug Interactions (7)].
- Warfarin: Concomitant administration of modafinil with warfarin did not produce significant changes in the pharmacokinetic profiles of R- and S-warfarin. However, since only a single dose of warfarin was tested in this study, an interaction cannot be ruled out [see Drug Interactions (7)].
- Drugs Metabolized by CYP2C19
- In vitro data demonstrated that modafinil is a reversible inhibitor of CYP2C19 activity. CYP2C19 is also reversibly inhibited, with similar potency, by a circulating metabolite, modafinil sulfone. Although the maximum plasma concentrations of modafinil sulfone are much lower than those of parent modafinil, the combined effect of both compounds could produce sustained partial inhibition of the enzyme. Therefore, exposure to some drugs that are substrates for CYP2C19 (e.g., phenytoin, diazepam, propranolol, omeprazole, and clomipramine) may be increased when used concomitantly with modafinil [see Drug Interactions (7)].
- In a clinical study, concomitant administration of armodafinil 400 mg resulted in a 40% increase in exposure to omeprazole after a single oral dose (40 mg), as a result of moderate inhibition of CYP2C19 activity.
- Interactions with CNS Active Drugs
- Concomitant administration of modafinil with methylphenidate or dextroamphetamine produced no significant alterations on the pharmacokinetic profile of modafinil or either stimulant, even though the absorption of modafinil was delayed for approximately one hour.
- Concomitant modafinil or clomipramine did not alter the pharmacokinetic profile of either drug; however, one incident of increased levels of clomipramine and its active metabolite desmethylclomipramine was reported in a patient with narcolepsy during treatment with modafinil.
- CYP2C19 also provides an ancillary pathway for the metabolism of certain tricyclic antidepressants (e.g., clomipramine and desipramine) and selective serotonin reuptake inhibitors that are primarily metabolized by CYP2D6. In tricyclic-treated patients deficient in CYP2D6 (i.e., those who are poor metabolizers of debrisoquine; 7 to 10% of the Caucasian population; similar or lower in other populations), the amount of metabolism by CYP2C19 may be substantially increased. Modafinil may cause elevation of the levels of the tricyclics in this subset of patients [see Drug Interactions (7)].
- Concomitant administration of armodafinil with quetiapine reduced the systemic exposure of quetiapine.
- Interaction with P-Glycoprotein
- An in vitro study demonstrated that armodafinil is a substrate of P-glycoprotein. The impact of inhibition of P-glycoprotein is not known.
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