Syntheses of 5-Thienyl and 5-Furyl-substituted Benzodiazepines: Probes of the Pharmacophore for Benzodiazepine Receptor Agonists

The synthesis of 5-thienyl- and 5-furyl-substituted benzodiazepines is described. These compounds were employed to probe the lipophilic pocket (L(3)) of the benzodiazepine receptor (BzR) and to determine the effect of occupation of L(3) on biological activity. Of the new analogs synthesized, the 5-(2-thienyl)-benzodiazepines 6a and 7a displayed high affinity for the BzR (IC50 28 and 18 nM, respectively) and exhibited both anticonvulsant (ED(50) approximate to 9 and 3 mg/kg) and muscle relaxant (ED(50) approximate to 10 and 7 mg/kg) activity. The 5-(3-thienyl)benzodiazepines 6d and 7d displayed only moderate affinity for the BzR (IC50 140 and 110 nM) and exhibited no biological activity (no anticonvulsant or muscle relaxant activity) at doses up to 40 mg/kg. The 5-(2-furyl)benzodiazepines (6b, 7b, 19b and 20b) exhibit low affinities for the BzR. These in vitro and in vivo findings are consistent with our model suggesting that pocket L(3) is very sensitive to lipophilic effects. Thus, decreasing the lipophilicity of functional groups which occupy this region decreases ligand affinity at BzR. The 2'-halogen (F or Cl) substituent of the 5-phenylbenzodiazepines increases ligand affinity in vitro because the active conformation of the phenyl N(4)=C(5)-C(1')=C(2') moiety is syn rather than anti. The syn conformation permits the 2'-halogen (F or Cl) atom to interact at the hydrogen bonding site H-2 and form a stable three-centered hydrogen bond in the proposed ligand binding cleft. The 3-thienyl and 2-furyl groups decrease the lipophilicity of the substituent which occupies L(3) but do not form a hydrogen bond at H-2, thus resulting in a diminished affinity at BzR.