METABOLISM OF DIAZEPAM IN RAT, DOG, AND MAN

¹ Department of Pharmacology, Hoffmann-La Roche Inc., Nutley, New Jersey

Diazepam, 7-chloro-1,3-dihydro-1-methyl-5-phenyl-2H-1,4-benzodiazepin-2-one, labeled with H³ in the 5-phenyl ring was synthesized and administered to rats, dogs and 2 human subjects. One hour after a rat received a 0.6-mg/kg i.p. dose the highest levels of radioactivity were in the liver and perirenal fat, while the lowest concentration of H³ was in the brain. After 48 hours relatively low levels of radioactivity were detectable in the liver, kidney, testes, and carcass, and the rat had excreted 22% of the dose in the urine and 57% in the feces. Oral administration of the same dose also resulted in the fecal excretion of over half of the administered radioactivity.

In 2 dog experiments (VaD-1 and VaD-2) 1 mg/kg of H³-diazepam was given i.v. and an average of 61% of the dose was excreted in the urine and 34% in the feces. When the i.v. dose was increased to 10 mg/kg (VaD-3 and VaD-4) the average urinary excretion decreased to 40% of the dose and the average fecal excretion remained at 35% of the dose.

Ether-extracted metabolites of dog (VaD-2) blood which fell off with a half-life of 2 hours consisted of a mixture of intact diazepam and the N-desmethyl analog, 7-chloro-1,3-dihydro- 5-phenyl-2H-1,4-benzodiazepin-2-one (compound IV). At the 10-mg/kg dose (VaD-3) this fraction of the blood contained diazepam, which declined rapidly from 1 µg/ml of blood at 1 hour to undetectable levels at 6 hours, and compound IV, which persisted for 12 hours at levels above 1 µg/ml. Also detected in VaD-3 and VaD-4 blood extracts were small amounts of 7-chloro-1,3-dihydro-3-hydroxy-5-phenyl-2H-1,4-benzodiazepin-2-one (compound VI, oxazepam).

The 2 men (VaH-1 and VaH-2) given 10 mg of H³-diazepam orally excreted an average of 71% of the dose in urine and 10% in the feces. It was estimated that the peak levels of diazepam in blood at 2 to 4 hours averaged 0.07 µg/ml. The fall-off of ether-extractable blood radioactivity in VaH-1 was characterized by a fast component (half-life of 10 hr), a plateau from 1 to 4 days, and then a slow component (half-life of 2.7 days). In VaH-2 the fast component (half-life of 7 hr) was followed immediately by a slow component (half-life of 2.6 days) which was comprised of diazeparn and compound IV.

The urine of each of the 3 species contained polar metabolites which, to a very large extent, were hydrolyzed by Glusulase. The deconjugated urinary metabolites of the rat were not identical to diazepam or to 7 analogs and remained unknown. In the dog approximately 80% of this metabolite fraction was identified as compound VI (oxazepam). In man oxazepam accounted for a third of these metabolites while compound IV and the hydroxylated analog of diazepam, 7-chloro-1,3-dihydro-3-hydroxy-1-methyl-5-phenyl-2H-1,4-benzodiazepin-2-one, each accounted for over 10%.

Diazepam metabolism was compared with that of 2 other benzodiazepines, oxazepam and chlordiazepoxide.