Correlation between the duration of the anticonvulsant activity of diazepam and its physiological disposition in mice

Abstract A correlation between the duration of the anticonvulsant activity of diaze- pam [7-chloro-1,3-dihydro-1-methyl-5-phenyl-2H-1,4-benzodiazepin-2-one (Ro 5-2807)] and the physiological disposition of the intact drug and three metabolites—(1) the N -demethylated derivative [7-chloro-1,3-dihydro-5-phenyl-2H-1,4-benzodiazepin-2-one (Ro 5-2180)], (2) the hydroxylated derivative [7-chloro-1,3-dihydro-3-hydroxy-1-methyl-5-phenyl-2H-1, 4-benzodiazepin-2-one (Ro 5-5345)], and (3) oxazepam [7-chloro-1 ,3-dihydro-3-hydroxy-5-phenyl-2H-1, 4-benzodiazepin-2-one (Ro 5-6789)]—in mice is presented. Maximal protection for 6 hr against a standard 125 mg/kg s.c. convulsant dose of metrazol was afforded by a single 2.5 mg/kg oral dose of diazepam. Quantitative 3 H distribution after the oral administration of a single 2.5 mg/kg dose of diazepam- 3 H shows rapid absorption and a rapid increase over 1–30 min in the tissue-to-blood 3 H ratios of the brain, muscle, heart, fat and carcass. These ratios thereafter increase gradually or remain constant to 4 hr after drug administration. A 125 mg/kg s.c. injection of metrazol given 30 min after the oral administration of diazepam does not significantly affect the tissue-to-blood 3 H ratios nor does it alter the 3 H disposition, except between 4 and 6 hr, during which time a definite increase in the total 3 H concentration of the blood, brain and muscle is observed. This increase is reflected in an increase in the concentration of the parent compound and of the products of its hydroxylation and desmethylation in blood, brain and muscle. Differential analyses of blood, brain and muscle tissue samples for diazepam and three of its major metabolites (Ro 5-2180, Ro 5-5345 and Ro 5-6789) from 30 min to 24 hr show that, although the concentration of each component at 30 min is similar both in the absence and presence of metrazol, a definite shift in the slope of the fall-off patterns toward a slower rate of disappearance of the parent compound and of its hydroxylated and desmethylated derivatives is evident when the administration of diazepam is followed by a subcutaneous injection of metrazol. The major constituent in all three tissues, both in the absence and presence of metrazol, is the end product of metabolism, namely oxazepam (Ro 5-6789). Besides being present in much higher concentrations, unlike the parent compound and the other metabolites, Ro 5-6789 maintains a fairly constant level in all three tissues from 30 min to 12 hr and then gradually falls to 24 hr. Although the contribution of the parent compound and its hydroxylated derivative to the duration and degree of protection cannot be accurately defined, it appears that the degree of protection to the convulsant action of metrazol is most closely related to the rate of disappearance of the N -desmethyl derivative.