Preclinical Drug Evaluations
New Opportunities for the Pharmaceutical Industry
About 100 million Americans each year complain of disturbed sleep. Many different pathophysiologiocal/etiologoical mechanisms for these sleep disorders have been recognized, and the International Classification of Sleep Disorders (ICSD) lists 81 different types of disorders under 8 main categories. These sleep-related problems are often chronic and negatively affect the subject's quality of life. In a 24-hour society that encourages sleep deprivation, daytime sleepiness is also an emerging issue even in healthy subjects. Accidents (e.g., Chernobyl nuclear accident, Three Mile Island accident, Bophal Chemical Spill, Exxon Valdes accident) due to sleepiness are now well recognized as a major public hazard. There has never been a greater need for the development of pharmacological therapeutics for sleep-wake disorders. Indeed, the affected population that would benefit from sleep-wake therapeutics is dramatically increasing world-wide.
At Stanford University, we have established a team of research scientists and technical specialists who are dedicated to fostering joint academic-industrial research ventures in pharmaceutical research and development. We have also assembled the most technologically advanced research equipment and computer system to assay drug effects on sleep-wakefulness in laboratory animals. The large scale of our drug screening facility greatly accelerates the process of pre-clinical drug evaluation and provides simultaneous quantitative measures of sleep and wakefulness, body temperature, motor activity and other behavioral variables useful in selecting compounds for clinical trial.
Advantages of using the SCN lab sleep assay to screen novel compounds:
- High capacity: 64 rodents, surgically prepared for sleep-wake recordings, are continuously on-line.
- High technology: The SCN lab sleep assay constitutes the most advanced hardware and software application of its kind for sleep research
- Unsurpassed expertise: An existing team of experienced scientists and highly trained technical specialists operate and manage the SCN lab sleep bioassay facility, in addition to a Sleep Research Center with associate experts in many aspects of pharmacology.
- Various animal models for sleep disorders and appropriate controls
- Existing database of standard compounds
Situations ideal for using the SCN lab sleep assay to record EEG sleep:
- Comparing novel compounds to standards in our database.
- Evaluating animal models (e.g., rodent narcolepsy models) to determine if the compounds normalize sleep abnormalities.
- Selecting one of several similar compounds to go to clinical trials
- Distinguishing sleep from physical side effects in a novel compound
- Selecting a compound that has least sedative side effects
- Multiple physiological characterizations - EEG, locomotor activity, body temperature, consumatory behavior, and video behavior monitoring
- Experiments requiring more long-term data than in-house bioassays can provide: the SCN lab's sleep bioassay routinely examines after-effects for 24 hours and easily provides additional after-effect data for weeks.
- Characterizing arousal states during repeated - dosing trials with side effects monitoring
- Our advanced technology allows the fastest, most cost-effective approach to recording sleep
Major uses for hypnotics include:
- Sleep disorders associated with aging
- "Situational" transient insomnia (new environment, noise)
- Short-term insomnia due to stress, grief, pain, illness
- Intermittent treatment of chronic insomnia
- About 100 million Americans each year complain of disturbed sleep
The SCN lab sleep bioassay system is also routinely used to assess:
- Stimulants, especially in seeking non-euphoriant, non-anxiogenic compounds
- Phase-shifting agents, with potential use for shift work and jet lag
- Sedative side effects in anxiolytic compounds
Some Major Classes of Hypnotics
The following receptor-systems are thought to be involved in promoting sleep: The SCORE/SCN lab sleep assay system has been used to assess a very wide variety of "standard" hypnotics and stimulants, which are readily available for comparison with new compounds.
- GABA-A modulator: Triazolam (Halcion), flurazepam (Dalmane), zopiclone (Imovane) and many other compounds have been examined using the SCORE/SCN bioassay. Although these compounds are widely used, effects related to potency, interaction with alcohol, and "rebound insomnia" (compensatory wakefulness colored by anxiety) can limit their use in humans.
- Gaboxadol: GABA-A agonists acts as a full or partial agonist depending on the subunit composition of the GABA-A receptors that it binds to) is in Phase III.
- Gamma-hydroxybutyrate (Xyrem): Short acting hypnotic approved for the treatment of excessive daytime and cataplexy associated with narcolepsy. Night time administration improves these symptoms during day. Mode of action is unknown. The compound is classified as Schedule III, but illicit use of Gamma-hydroxybutyrate is subject to Schedule I penalties.
- Alpha 2-adrenergic agonists (e.g., clonidine, dexmedetomidine) induce high, sustained levels of NREM sleep for 2-4 hours, but also cause profound REM sleep inhibition. Cardiovascular effects and high potency currently limit hypnotic use in humans.
- ss-adrenergic antagonists (e.g., propranolol) promote NREM sleep but reduce REM sleep. Cardiovascular effects currently limit hypnotic use in humans.
- Dopamine antagonists (e.g., haloperidol, phenothiazines) increase NREM sleep with variable reductions of REM sleep. Pyramidal and other side effects limit usefulness as hypnotics.
- 5HT2-anatagonists (e.g., ritanserin) show no initial effect, but after 1-2 hours specifically increase slow-wave sleep ("deep" NREM sleep) with only slightly reduced REM sleep. Tested in humans but not yet marketed, these compounds may be useful for some sleep problems related to aging.
- H1-antihistamines slightly elevate NREM with variable reduction of REM sleep. Hypnotic effect is desirable when used at bedtime, but highly undesirable during the day. Relatively low potency and cholinergic side-effects limit hypnotic use in humans.
- H3- Histamine agonists slightly elevate NREM and possibly REM. Not yet tested in humans.
- Melatonin MT1, MT2 receptor agonist, ramelteon (Rozerem), recently approved for the treatment of insomnia. No abuse and dependence potency is claimed in the clinical studies.
- Tricyclic antidepressants (e.g., imipramine) vary widely in their potency to promote NREM sleep, but all potently suppress REM sleep. Hypnotic effect is desirable when used at bedtime, but highly undesirable during the day.
- Atypical antidepressants (e.g., fluoxetine, nefazodone) initially interfere with sleep, followed by increased NREM (particularly nefazodone). REM sleep is strongly reduced. Hypnotic effect is desirable when used h.s., but highly undesirable during the day.
- Compounds which increase the availability of adenosine (e.g., the adenosine agonist l-PIA, and the nucleoside-transport inhibitor, mioflazine) specifically promote slow-wave sleep ("deep" NREM sleep) with only slightly reduced REM sleep. Not yet tested in humans.
- Process"S" substances (muramyl dipeptide, uridine, DSIP, usually peptides or nucleotides derived from the CSF of sleep-deprived animals, tend to specifically increase slow-wave sleep ("deep" NREM sleep) with slight effects on REM sleep. Presently these compounds cannot be orally delivered to humans.
- Endogenous indoleamines (melatonin and tryptophan) may weakly promote sleep. The utility of these compounds as sedative hypnotics in humans has not been clearly established.
- Orexin/hypocretin antagonist. May be useful for inducing sleep and is in late stage- preclinical evaluation.
Stimulants and Novel Wake-promoting Therapeutics
With growing interest in developing pharmacological treatments for inappropriately timed excessive daytime sleepiness (e.g., narcolepsy, shift-work, jet-lag). SCORE has been used to assess a number of psychomotor stimulants as well as novel non-stimulant wake-promoting therapeutics. Some of these include:
- Adenosine receptor antagonists. Caffeine, cyclopental theophylline (CPT) and other AI adenosine receptor ligands can promote waking. Caffeine is widely used throughout the world and accepted as safe despite well known side effects that can include anxiety, irritability, and tremor. Caffeine dependency and tolerance is also rapid. Side effects and limited range efficacy limit the therapeutic utility of caffeine for major disorder of excessive sleepiness.
- Dopaminergic autoreceptor and dopamine transporter antagonists. Drugs that facilitate dopaminergic transmission (pemoline, AJ-76, UH-232) and dopamine transporter blockers (amineptine, GBR- 12909) potently facilitate wakefulness. Dependency and euphoria are the major limitations for most dopaminergic stimulants.
- Psychomotor stimulants (d-amphetamine, methamphetamine, methylphenidate) have mixed pharmacological action, increasing catecholaminergic transmission. These compounds are used to treat severe excessive daytime sleepiness (narcolepsy). And low-dose methylphenidate Ritalin) is used for attention deficit disorder in children. Beyond these indications, euphoria, dependency, tolerance, anxiety, and motor side effects limit the utility of these compounds to manage excessive sleepiness in the broader segment of the population with excessive sleepiness (e.g., shift-workers).
- H3-histamine antagonists (thioperamide) decrease NREM without the disproportionate motor effects characteristic of psychomotor stimulants. Although this class of compounds can stimulate thirst, new non-thiourea H3 ligands may have therapeutic utility for treating certain forms of excessive sleepiness. Histamine and classical H3 antagonists posses imidazole ring, and also bind to imdazoline binding sites, and so evaluation of non-imidazole H3 antagonists is required.
- Novel wake-promoting therapeutics: Modafinil, a non-amphetamine wake-promoting compounds, was developed in France and was recently approved in the USA for the treatment of narcolepsy and other forms of excessive sleepiness. Modafinil promotes waking without the undesirable side effects of psychomotor stimulants. Modes of action of modafinil are still controversial, but modafinil increases dopamine efflux in vivo, and wake-promoting effects of modafinil were abolished in mice lacking dopamine transporter, suggesting the dopaminergic mediation of these effects.
- Hypocretin/orexin agonists are likely to be used for EDS and cataplexy associated with the hypocretin-ligand deficit narcolepsy, but no synthetic agonists have yet been developed.
Seiji Nishino, M.D., Ph.D. and Stanford University, 2007
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