Pharmacokinetics
In a single and multiple dose absorption, distribution,
metabolism, and excretion (ADME) study, using 3H labeled
drug, temazepam was well absorbed and found to have minimal (8%) first pass
metabolism. There were no active metabolites formed and the only significant
metabolite present in blood was the O-conjugate. The unchanged drug was 96%
bound to plasma proteins. The blood level decline of the parent drug was
biphasic with the short half-life ranging from 0.4 to 0.6 hours and the terminal
half-life from 3.5 to 18.4 hours (mean 8.8 hours), depending on the study
population and method of determination. Metabolites were formed with a half-life
of 10 hours and excreted with a half-life of approximately 2 hours. Thus,
formation of the major metabolite is the rate limiting step in the
biodisposition of temazepam. There is no accumulation of metabolites. A dose
proportional relationship has been established for the area under the plasma
concentration/time curve over the 15 mg to 30 mg dose range.
Temazepam was completely metabolized through conjugation prior to excretion;
80% to 90% of the dose appeared in the urine. The major metabolite was the
O-conjugate of temazepam (90%); the O-conjugate of N-desmethyl temazepam was a
minor metabolite (7%).
Bioavailability, Induction, and Plasma Levels
Following ingestion of a 30 mg temazepam capsule, measurable
plasma concentrations were achieved 10 to 20 minutes after dosing with peak
plasma levels ranging from 666 to 982 ng/mL (mean 865 ng/mL) occurring
approximately 1.2 to 1.6 hours (mean 1.5 hours) after dosing.
In a 7-day study, in which subjects were given a 30 mg capsule of a marketed
bioequivalent temazepam product one hour before retiring, steady-state (as
measured by the attainment of maximal trough concentrations) was achieved by the
third dose. Mean plasma levels of temazepam (for days 2 to 7) were 260 ± 210
ng/mL at 9 hours and 75 ± 80 ng/mL at 24 hours after dosing. A slight trend
toward declining 24 hour plasma levels was seen after day 4 in the study,
however, the 24 hour plasma levels were quite variable.
At a dose of 30 mg once a day for 8 weeks, no evidence of enzyme induction
was found in man.
Elimination Rate of Benzodiazepine Hypnotics and
Profile of Common Untoward Effects
The type and duration of hypnotic effects and the profile of
unwanted effects during administration of benzodiazepine hypnotics may be
influenced by the biologic half-life of the administered drug and for some
hypnotics, the half-life of any active metabolites formed. Benzodiazepine
hypnotics have a spectrum of half-lives from short (less than 4 hours) to long (greater than
20 hours). When half-lives are long, drug (and for some drugs their active
metabolites) may accumulate during periods of nightly administration and be
associated with impairments of cognitive and/or motor performance during waking
hours; the possibility of interaction with other psychoactive drugs or alcohol
will be enhanced. In contrast, if half-lives are shorter, drug (and, where
appropriate, its active metabolites) will be cleared before the next dose is
ingested, and carry over effects related to excessive sedation or CNS depression
should be minimal or absent. However, during nightly use for an extended period,
pharmacodynamic tolerance or adaptation to some effects of benzodiazepine
hypnotics may develop. If the drug has a short elimination half-life, it is
possible that a relative deficiency of the drug, or, if appropriate, its active
metabolites (i.e., in relationship to the receptor site) may occur at some point
in the interval between each night’s use. This sequence of events may account
for two clinical findings reported to occur after several weeks of nightly use
of rapidly eliminated benzodiazepine hypnotics, namely, increased wakefulness
during the last third of the night, and the appearance of increased signs of
daytime anxiety.
Controlled Trials Supporting Efficacy
Temazepam improved sleep parameters in clinical studies. Residual
medication effects (“hangover”) were essentially absent. Early morning
awakening, a particular problem in the geriatric patient, was significantly
reduced.
Patients with chronic insomnia were evaluated in 2-week, placebo controlled
sleep laboratory studies with temazepam at doses of 7.5 mg, 15 mg and 30 mg,
given 30 minutes prior to bedtime. There was a linear dose-response improvement
in total sleep time and sleep latency, with significant drug-placebo differences
at 2 weeks occurring only for total sleep time at the two higher doses, and for
sleep latency only at the highest dose.
In these sleep laboratory studies, REM sleep was essentially unchanged and
slow wave sleep was decreased. No measurable effects on daytime alertness or
performance occurred following temazepam treatment or during the withdrawal
period, even though a transient sleep disturbance in some sleep parameters was
observed following withdrawal of the higher doses. There was no evidence of
tolerance development in the sleep laboratory parameters when patients were
given temazepam nightly for at least 2 weeks.
In addition, normal subjects with transient insomnia associated with first
night adaptation to the sleep laboratory were evaluated in 24-hour, placebo
controlled sleep laboratory studies with temazepam at doses of 7.5 mg, 15 mg and
30 mg, given 30 minutes prior to bedtime. There was a linear dose-response
improvement in total sleep time, sleep latency and number of awakenings, with
significant drug-placebo differences occurring for sleep latency at all doses,
for total sleep time at the two higher doses and for number of awakenings only
at the 30 mg dose.