Abstract
3-Acyloxy-, 3-methoxy-, and 3-alkyl-substituted derivatives of the benzodiazepine (BZ) agonist desmethyl-diazepam (DMD) were resolved, and the stereochemical properties of binding to central BZ receptors were investigated in synaptosomal membrane preparations of rat brain. Decreasing potency and stereoselectivity of 3-methyl, 3-ethyl, and 3-isopropyl derivatives in displacement of [3H]diazepam binding can be attributed to differential susceptibilities for steric hindrance of 3-axial versus 3-equatorial substituents of the binding conformation M. Chirality in the alpha-methyl-beta-phenyl-propionic acyl moiety of oxazepam, the 3-OH-derivative of DMD, was noncritical in binding, whereas the beta-phenyl substituent selectively increased the binding of the 3S-stereoisomer. Changing the pH from 7.4 to 5.6 significantly increased the IC50 of (3R)-oxazepam acetate but not those of (3R)-methyl-DMD and diazepam. Binding data led to a steric model of the BZ binding site with the postulation of an additional hydrogen-bond-donating moiety, probably histidine in the "ceiling" of the receptor cavity, that binds the 3-carbonyloxy groups and hinders the 3-alkyl ones. In vitro efficacies of 3-substituted BZs were estimated by allosteric binding interactions within the gamma-aminobutyric acidA (GABAA) receptor-ionophore complex. Non-equilibrium enhancement of t-butyl-bicyclophosphoro[35S]thionate binding by the BZ agonist oxazepam was stereoselectively antagonized by (3S)-oxazepam-(S)-alpha-methyl-beta-phenyl-propionate, suggesting a mixed agonist-antagonist character. GABA enhanced the [3H]diazepam-displacing potencies of the 3S-enantiomers of the acetate, hemisuccinate, and (S)-alpha-methyl-beta-phenyl-propionate esters of oxazepam by a factor of about 1.5-1.6, whereas the GABA shifts for 3R-esters were about 1.2. UV affinity labeling with flunitrazepam resulted in a significantly smaller decrease in the displacing potency of (3R)-oxazepam acetate than in that of the 3S-enantiomer. GABA shifts of successively 3-methylated DMD derivatives were also compared. The GABA shifts of DMD and its (3S)-methyl and 3,3-dimethyl derivatives were all characteristic of full agonists (2.4-2.7), whereas that of (3R)-methyl-DMD was 1.5. The 3-methoxy enantiomers of DMD displayed stereoselectivity and GABA shift values intermediate between those of 3-methyl and 3-acetoxy derivatives. These allosteric interactions suggest that 3-carbonyloxy derivatives in general, as well as (3R)-BZ enantiomers bound with axial 3-alkyl and 3-alkyloxy groups, decrease the agonist efficacies of 1,4-BZs to modulate the GABAA receptor complex.
MolPharm articles become freely available 12 months after publication, and remain freely available for 5 years.Non-open access articles that fall outside this five year window are available only to institutional subscribers and current ASPET members, or through the article purchase feature at the bottom of the page.
|