Peripheral benzodiazepine receptor binding properties and effects on steroid synthesis of two new phenoxyphenyl-acetamide derivatives, DAA1097 and DAA1106

The peripheral benzodiazepine receptor (PBR) is a key element of the steroidogenic pathway in peripheral tissues and the CNS. Two phenoxyphenyl-acetamide derivatives, DAA1097 and DAA1106, exert anxiolytic effects in mice and displace PER ligands binding in rat brain. Here we examined their effect on steroidogenesis and performed their pharmacological characterization using MA-10 Leydig tumor and C6-2B glioma cells, rat brain mitochondria, and recombinant PER. DAA1097 activated steroidogenesis similarly to the well-described PER ligand PK11195, more efficiently on brain than Leydig cells. By contrast, DAA1106 did not activate steroidogenesis, but partially inhibited the hCG-induced steroidogenesis. The affinity of [H-3]DAA1106 for PER was similar to that of [H-3]PK11195 in MA-10, C6-2B cells, and for recombinant PER, but was 10 times higher in rat brain mitochondria. Competition studies revealed that DAA1097 and DAA1106 displaced [H-3]PK11195 binding at nano- and picamolar concentrations, respectively, while the IC50 Of PK11195 against [H-3]DAA1106 was in the micromolar range. These results suggest that: 1) DAA1097 and DAA1106 binding sites on PER share common domain(s) with that of PK11195, but also contain motif(s) that do not interact efficiently with PK11195; 2) these additional sites are part of the PER molecule, since similar results are found using cells or recombinant PER; 3) the binding of DAA1097 to PBR induce changes in the receptor similar to that triggered by PK11195, allowing steroidogenesis activation; 4) the fact that DAA1106 does not activate steroidogenesis despite its high affinity for PER suggests that its binding on PER leads to conformational changes that do not permit or antagonize PER steroidogenic function. In conclusion, DAA1097 and DAA1106 represent new, sensitive probes for PER detection, similar in structure but with opposite effects on steroidogenesis. Thus, an extensive study of their respective binding sites should shed some new light on the structure/function relationship of PER. (C) 2001 Wiley-Liss, Inc.