Synthesis, biological activity and conformational analysis of cyclic GRF analogs

Abstract

A novel cyclic GRF analog, cyclo(Asp 8‐Lys 12)‐[Asp 8,Ala 15]‐GRF(1‐29)‐NH2, i.e. cyclo 8.12[Asp 8,Ala 15]‐GRF(1‐29)‐NH2, was synthesized by the solid phase procedure and found to retain significant biological activity. Solid phase cyclization of Asp 8 to Lys 12 proceeded rapidly (∼2h) using the BOP reagent. Substitution of Ala 12 with d‐Ala 2 and/or NH2‐terminal replacement (des NH2‐Tyr 1 or N‐MeTyr 1) in the cyclo 8.12[Asp 8,Ala 15]‐GRF(1‐29)‐NH2 system resulted in highly potent analogs that were also active in vivo. Conformational analysis (circular dichroism and molecular dynamics calculations based on NOE‐derived distance constraints) demonstrated that cyclo 8.12[Asp 8,Ala 15]‐GRF(1‐29)‐NH2 contains a long α‐helical segment even in aqueous solution. A series of cyclo 8.12 stereoisomers containing d‐Asp 8 and/or d‐Lys 12 were prepared and also found to be highly potent and to retain significant α‐helical conformation. The high biological activity of cyclo 8.12[N‐MeTyr 1,d‐Ala 2,Asp 8,Ala 15]‐GRF(1‐29)‐NH2 may be explained on the basis of retention of a preferred bioactive conformation.