Modafinil: past, present and future

Modafinil as a catecholaminergic agent: empirical evidence and unanswered questions

Modafinil, in its two clinical formulations (Provigil^® and Nuvigil^®), is a widely prescribed wake-promoting therapeutic agent. It binds competitively to the cell-membrane dopamine (DA) transporter and is dependent on catecholaminergic (dopaminergic and adrenergic) signaling for its wake-promoting effects. The clinical spectrum of effects for modafinil is distinct from the effects seen with other catecholaminergic agents. Relative to other commonly used agents that act through catecholaminergic mechanisms, modafinil has a relatively low abuse potential, produces wakefulness with an attenuated compensatory sleep recovery thereafter, and does not ameliorate cataplexy in narcolepsy. These clinically relevant phenomenological differences between modafinil and agents such as amphetamines and cocaine do not eliminate catecholaminergic effects as a possible mediator of its wake-promoting action; they merely reflect its unique pharmacological profile. Modafinil is an exceptionally weak, but apparently very selective, DA transporter inhibitor. The pharmacodynamic response to modafinil, as measured by DA levels in brain microdialyzate, is protracted relative to other agents that act via catecholaminergic mechanisms. The conformational constraints on the interaction of modafinil with the DA transporter – and probably, as a consequence, its effects on trace amine receptor signaling in the catecholaminergic cell – are unique among catecholaminergic agents. These unique pharmacological properties of modafinil should be considered both in seeking to thoroughly understand its putatively elusive mechanism of action and in the design of novel therapeutic agents.

The principles of agonist pharmacotherapy for psychostimulant dependence

ISSUES

Psychostimulant dependence is a chronic, relapsing condition which is highly treatment refractory. No medications to date have been any more successful than placebo in reducing psychostimulant use in dependent patients. Agonist strategies have attracted limited attention.

APPROACH

Successful examples of agonist pharmacotherapy in the treatment of heroin and nicotine dependence are first considered. Agonist pharmacological approaches to the treatment of psychostimulant dependence are then examined, based on the dopamine receptor agonist and indirect dopamine agonist strategies. Finally, the potential extension of the concept of agonist pharmacotherapy to include the novel non-amphetamine-type stimulant, modafinil, is discussed.

CONCLUSIONS

Agonist approaches appear to be viable with risks outweighed by benefits in carefully selected, monitored and motivated patients. On the other hand, the effectiveness of indirect agonists such as dexamphetamine and methylphenidate are not established. Further research is required to determine optimal treatment models (whether maintenance or withdrawal), effective safe dosages and duration (short or long term).