Optimum Conditions of Radioligand Receptor Binding Assay of Ligands of Benzodiazepine Receptors

To obtain drugs which are more selective at benzodiazepine (BZD) receptors, design and synthesis of functionally selective ligands for BZD receptors is the current strategy of our pharmaceutical chemistry department. The affinity of newly synthesized ligands is assessed by radioligand receptor binding assays. Based on our previous studies, 2-phenyl-5-oxo-7-methyl-1,3,4-oxadiazolo[a,2,3]-pyrimidine (compound A) was chosen for design and synthesis of new triazole derivatives as GABAA BZD receptor agonist. The cortical membrane of male Sprague-Dawley rats was prepared as the source of the BZD receptors. Different concentrations of membrane protein and [3H]-flumazenil were incubated at room temperature at different time periods to reach the steady-state. To saturate the receptors, increased amounts of radioligand were incubated with membrane protein. The bound and un-bound ligands were separated by centrifugation. The affinity of compound A was measured in competition studies at optimum conditions by displacement of [3H]-Flumazenil from rat cortical membrane. Based on results, the optimum conditions of radioligand receptor binding assay of benzodiazepines were 35 min incubation of ligands with 100 g cortical membrane protein and 6×10-5 nmole 3H-flumazenil in a final volume of 0.5 mL Tris-HCl buffer (50 mM, pH 7.4) at 30°C. The binding parameters of [3H]-flumazenil, Bmax and Kd were determined through saturation studies as 0.638±0.099 pmol/mg and 1.35±0.316 nM respectively. The affinity of compound A was 1.9 nM comparable with diazepam (1.53nM). This finding makes the compound an interesting lead for further optimization. Starting from this compound, new ligands were synthesized and screened in vitro by competitive binding assays.