Comparative metabolic study of nimetazepam and its desmethyl derivative (nitrazepam) in rats.

1. Nimetazepam distributed more rapidly in the brain than its desmethyl derivative (nitrazepam). The brain concentration of the active metabolites of the former was about twice that of the latter at 1h after oral administration. 2. At least four kinds of reactions were involved in the biotransformation of nimetazepam and its desmethyl derivative (nitrazepam) : (i) demethylation at N-1, (ii) hydroxylation at C-3, (iii) reduction of the nitro group at C-7 to the amino group and (iv) subsequent acetylation of the amino group. 3. The 1-N-demethylation of nimetazepam was slow compared with the other three reactions. 4. Nimetazepam was rapidly hydroxylated at C-3, while the 3-hydroxylation of its desmethyl derivative (nitrazepam) was very slow. 5. The reduction of the nitro group at C-7 and subsequent acetylation were important routes for the excretion of these drugs.