Structure and Properties
Origin
Modafinil is a pharmaceutical designed for usage in sleep-related disorders, and is related to improved mental function in a sleep deprived state.[2]
Forms of Modafinil
The term 'modafinil' refers to a racemic mixture of two isomers, R-modafinil and S-modafinil. The R-isomer by itself is referred to as armodafinil.[3]
Modafinil is a racemic mixture of S-modafinil and R-modafinil, whereas Armodafinil is just the R-modafinil by itself
Pharmacology
Serum
The half-life of modafinil appears to be 13-15 hours, and steady state concentrations in serum are reached 2 days following supplementation.[3] Although the S-isomer by itself seems to have a short elimination half-life (4-5 hours) whereas the R-isomer is more prolonged (15 hours)[4][5] ingesting armodafinil does not appear to significantly differ from modafinil.[3][6][7]
An analysis of studies has noted, however, that armodafinil has an 18% higher Cmax value (5.44+/-1.64µg/mL relative to 4.61+/-0.73µg/mL) with a more rapid Tmax (1.8 hours relative to 2.5)[3] and that the overall AUC of armodafinil appears to be 32-40% greater than modafinil.[3]
Although the two variants of modafinil (modafinil and armodafinil) appear to have similar half-lives, armodafinil has a higher overall exposure to the body (assessed by AUC) as well as a higher and more rapid peak in the blood
Localization
According to c-Fos immunocytochemistry (c-Fos being a gene that rapidly activates in response to stimuli[8][9] and can be detected following neuronal stimulation[10][11] or sleep deprivation[12]), modafinil administration to cats is associated with strong c-Fos activation in the anterior hypothalamic nucleus and surrounding areas with weaker activation in the dorsal portion of the suprachiasmatic nuclei (SCN) with minimal activation in other brain regions such as the cortex or striatum.[13] This selective activation of the hypothalamus has been noted elsewhere, and the amygdala also implicated.[14][15] Modafinil has been confirmed in humans to have a different profile than does amphetamine.[16]
In contrast to amphetamine or methylphenidate induced wakefulness (characterized by widespread neuronal activation), modafinil appears to be somewhat selective for the hypothalamus and amygdala with some influence on the SCN
Neurology
Dopaminergic interactions
Modafinil seems to be able to increase extracellular levels of dopamine in the rat nucelar accumbens[17] and prefrontal cortex[18] and the dog caudate nucleus.[19] It has been shown to occupy both the dopamine and noradrenaline receptors (in the striatum)[20] and prevention of dopamine receptor occupancy abolishes the sleep-promoting effects in mice[21], suggesting this mechanism of action is crucial to the sleep-promoting effects.[22]
Past studies have noted a lack of potency of modafinil on dopaminergic systems[23][24][25] which may be due to the lower dosages they used in those studies relative to more recent ones.
Adrenergic interactions
The wakefulness effects of modafinil are significantly attenuated by antagonists of adrenergic receptors (both alpha and beta subunits)[26] although inhibiting catecholamine synthesis via α-methylparatyrosine does not appear to impair these effects.[26][27][28]
Serotonergic interactions
Orexinergic interactions
In those with faulty orexin levels (narcoleptics)[29], modafinil shows benefit possibly by acting on orexic neurons directly.[30][31] This effect is more potent in orexin-knockout mice than in normal mice[32], and the effects of modafinil on the orexin system of healthy persons is unknown.[2]
Sedation and Alertness
Some studies assess the effects of modafinil on intentional sleep deprivation, and modafinil (300mg) appears to be effective in reducing the disturbed mood and cognition seen during sleep deprivation with a potency comparable to 20mg D-amphetamine.[33][1] Elsewhere, it has been noted that the impairment of self-monitoring (ability to accurately assess oneself and their environment) is effectively reversed to a degree where an overconfidence effect (higher percieved assessment of skills relative to actual skills) is seen.[34]
These studies have extended up to 64 hours (two nights sleep deprivation) with a single dose of modafinil every 15 hours.[33][1]
Modafinil intervention prior to sleep can highly disrupt the sleep cycle and accompanying sedation, and usage of modafinil in this manner can attenuate the side-effects of sleep deprivation (cognitive and mood impairment)
Rebound hypersomnia is the phenomena where an anti-sleep agent is successful in reducing sleep, but after the effects of the drug wear off the user is significantly sleepier than before. Unlike amphetamine based drugs, modafinil does not appear to be associated with rebound hypersomnia in the cat[26][35] nor rats[36][37][38] or mice.[39] In humans who miss two nights sleep due to modafinil intervention (64 continuous hours awake), there does not appear to be any rebound hypersomnia like is seen with D-amphetamine.[1]
Modafinil does not appear to be associated with rebound hypersomnia
The increase in alertness during sleep deprivation seen with 300mg modafinil appears to be comparable to 20mg D-amphetamine over the course of 10-12 hours following one-time administration.[33][16][1][34]
The anti-sleep efficacy of modafinil (300mg) appears to be comparable to 20mg D-amphetamine
The sleep wake cycle of the brain is a balance of the 'ascending arousal system' consisting of arousing neurotransmitters (catecholamines, acetylcholine, orexin, etc.) and the neurotransmitters (GABA, Galanin) which act to suppress stimulation and promote sleep.[40][41] Varying levels of arousing and depressing neurotransmitters form an 'on-off' switch for arousal and sleep.[2]
Overall regulation of the wakefulness and rest cycle seems to be in part due to the circadian rhythm, mediated by the Suprachiasmatic Nuclei (SCN), and in part due to homeostatic needs for sleep that are gained during wakefulness.[40][42]
Modafinil seems to be able to interact with a multitude of stimulatory systems including serotonergic, noradrenergic, dopaminergic, glutaminergic, histaminergic, and orexinergic pathways; and also influences GABAergic pathways.[43]
Some studies that use modafinil for the treatment of some other states note that the side-effect of insomnia persists more than placebo[44] and modafinil is able to prevent participants from voluntarily falling asleep when taken prior to sleep.[45]
Supplementation of modafinil is able to preserve cognition in fatiged states.[45]
Memory and Cognition
In otherwise healthy persons, 100mg or 200mg of modafinil taken two hours prior to cognitive testing is able to improve working memory (digit span tests), visuospatial planning, and reaction time[46] whereas elsewhere it was found to improve cognition (task enjoyment, planning, and working memory) in completely normal and non-sleep deprived persons at 200mg.[47] Working memory and processing accuracy have been found to be improved with 200mg elsewhere.[48]
Alongside the improvements in working memory are improvements in the actual performance of the task, most notably an increase in motivation and enjoyment for undergoing the task.[47]
There is some cognitive enhancing properties of modafinil acute usage in otherwise healthy persons, and may increase motivation and enjoyment from doing a cognitive task
In methamphtamine dependent persons (known to have a degree of cognitive impairment[49][50]), modafinil at 400mg for three days is able to improve working memory and trended to improve fatigue in those with worse baseline scores yet was unable to improve performance in those with higher baseline scores[51] and 200mg for a single dose does not appear to be cognitive enhancing in this same population.[52]
In persons with possible cognitive decline associated with methamphetamine usage, modafinil has the potential to improve cognition although acute usage doesn't appear to be effective and the overall cognitive enhancement is relatively minor
Appetite
Studies that assess the effects of modafinil sometimes report a reduction in appetite as a side-effect, measured at 16% (164 persons[44])
Studies that note reductions in appetite sometimes note trends towards weight loss over a period of weeks, but usually do not reach statistical significance.[44]
Attention
Supplementation of modafinil in the range of 170–425mg for six weeks (dose titrated up from 170mg to 425mg unless tolerance was compromised) was able to reduce symptoms of ADHD as assessed by both CGI-I and ADHD-RS-IV in youth.[44]
Addiction
Due to the inability for modafinil to activate neuronal pathways involved in addiction, it is thought to have a low abuse potential relative to other drugs.[53][54]
It is thought that modafinil has a low potential for wide-spread drug abuse
Response inhibition (the ability to inhibit a prepotent response,[55] and thought to be indicative of impulsivity in persons undergoing drug abuse[56]) is improved in rats[57] and humans (alcohol dependence,[58] methamphetamine dependence,[52] and gambling[59]) with modafinil, but only in those with worse baseline scores rather than the whole group.
It is possible for modafinil to reduce impulsivity in persons addicted to drugs or some addictive behaviours, although this only appears to be significantly effective for persons with worse impuslivity prior to testing
Interaction with Medical Conditions
Multiple Sclerosis
Acute supplementation of armodafinil (250mg) in persons with multiple sclerosis has noted improvements in memory recall relative to placebo, although other measured parameters (fatigue, executive function, processing speed) were unaffected by supplementation.[60] This inefficacy on fatigue and arousal has not been noted elsewhere, where modafinil is attributed to possessing an antifatigue effect in MS.[61]
Safety and Toxicology
General
Adverse effects usually reported in trials are predominantly headache, dizziness, increased diuresis, palpitations and tachycardia, restlessness, nervousness, gastrointestinal complains such as nausea, dry mouth and abdominal pain. Despite these isolated adverse events, they usually do not differ significantly from placebo and thus modafinil is seen as well tolerated.
Reported side-effects that sometimes occur significantly more than placebo include insomnia and sleeplessness as well as appetite reduction.[44]
General side-effects of modafinil include insomnia and/or sleeplessness as well as a reduction in appetite
