The pharmacokinetics and biotransformation of the new benzodiazepine lormetazepam in humans. I. Absorption, distribution, elimination and metabolism of lormetazepam-5-14C.

The pharmacokinetics and metabolism of the new benzodiazepine lormetazepam were investigated in five male volunteers using the 14C-labelled drug (position 5). Lormetazepam was administered intravenously and orally, at a dose of 0.2 and 2 mg respectively, to each of the test subjects. Measurements of total radioactivity showed that the drug was absorbed completely and eliminated almost exclusively by the renal route. Maximum plasma level of active ingredient and total radioactivity were observed about 2 hours and 5 hours following oral administration. As early as 30 min following oral administration, concentration of active ingredient amounted to 80% of the maximum values. After both treatments the terminal half-life of total radioactivity and lormetazepam glucuronide in plasma corresponded to the half-life of elimination in urine of about 13 hours. After enzymatic hydrolysis with beta-glucuronidase/arylsulphatase, an average of 90% of total radioactivity from various urine and plasma samples was extractable with ether. Extracts from plasma contained only unchanged drug, indicating free and conjugated lormetazepam as ingredients of total radioactivity. Extracts from urine could be separated into lormetazepam and its N-demethylation derivative lorazepam. The relative amount of excreted lorazepam conjugate was demonstrated to be time-dependent, probably due to enterohepatic circulation. Since less than 6% of the total dose was demethylated by both routes of administration, it can be assumed that lormetazepam is the active product.