++
The sedative-hypnotics belong to a chemically heterogeneous class of drugs, almost all of which produce dose-dependent CNS depressant effects that cause sedation (with relief of anxiety) or hypnosis (encourage sleep). The major subgroup is the benzodiazepines, but representatives of other subgroups, including barbiturates, and miscellaneous agents (carbamates, alcohols, and cyclic ethers) are still in use. Atypical drugs with distinctive characteristics include the anxiolytic buspirone, several widely used hypnotics (zolpidem, zaleplon, eszopiclone), and melatonin agonists and orexin antagonists, novel drugs used in sleep disorders.
+++
A. Absorption and Distribution
++
Most sedative-hypnotic drugs are lipid-soluble and are absorbed well from the gastrointestinal tract, with good distribution to the brain. Drugs with the highest lipid solubility (eg, thiopental) enter the CNS rapidly and can be used as induction agents in anesthesia. The CNS effects of thiopental are terminated by rapid redistribution of the drug from brain to other highly perfused tissues, including skeletal muscle. Other drugs with a rapid onset of CNS action include eszopiclone, zaleplon, and zolpidem.
+++
B. Metabolism and Excretion
++
Sedative-hypnotics are metabolized before elimination from the body, mainly by hepatic enzymes. Metabolic rates and pathways vary among different drugs. Many benzodiazepines are converted initially to active metabolites with long half-lives. After several days of therapy with some drugs (eg, diazepam, flurazepam), accumulation of active metabolites can lead to excessive sedation. Lorazepam and oxazepam undergo extrahepatic conjugation and do not form active metabolites. With the exception of phenobarbital, a weak acid that is excreted partly unchanged in the urine, the barbiturates are extensively metabolized. Chloral hydrate is oxidized to trichloroethanol, an active metabolite. Rapid metabolism by liver enzymes is responsible for the short duration of action of zolpidem. A biphasic release form of zolpidem extends its plasma half-life. Zaleplon undergoes even more rapid hepatic metabolism by aldehyde oxidase and cytochrome P450. Eszopiclone is also metabolized by cytochrome P450 with a half-life of 6 h. The duration of CNS actions of sedative-hypnotic drugs ranges from a few hours (eg, zaleplon < zolpidem = triazolam = eszopiclone < chloral hydrate) to more than 30 h (eg, chlordiazepoxide, clorazepate, diazepam, phenobarbital).
++