New analogs of human growth hormone-releasing hormone (1-29) with high and prolonged antagonistic activity

Based on our previous results, in conjunction with various structural considerations, 19 new analogs of the GHRH antagonist [PhAc-Tyr¹,D-Arg²,Phe(pCl)⁶,Abu¹⁵,Nle²⁷,Agm²⁹]hGHRH(1-29) (MZ-5-156) were synthesized by the solid-phase method. These compounds were designed to develop further analogs of this class with increased receptor-binding affinity. All analogs had Abu¹⁵ and Nle²⁷ modifications and were acylated with phenylacetic acid at the N-terminus. Most of the analogs had d -Arg² and Phe(pCl)⁶ substituents and Agm²⁹ or Arg²⁹-NH₂ at the C-terminus. Additional single substitutions consisted of the incorporation of d - or l -Tic¹, d -Tic², Tic⁶ or Phe(pNO₂)⁶ and Arg²⁹-NH₂. The Arg²⁹-NH₂ analog of MZ-5-156 (KT-48) was further modified by single substitutions using Pal¹; d -Tpi²; d - or l -Phe⁴; Phe(pX)⁶×= F, Cl, I; Tyr⁷; Aib⁸; Tyr(Me)¹⁰ or Phe(pCl)¹⁰. Four peptides had multiple substitutions. All the analogs were evaluated for their ability to inhibit GH release induced by hGHRH(1-29)NH₂in vitro and some were also tested in vivo. Peptides [PhAc-Tyr¹,D-Arg²,Phe(pI)⁶,Abu¹⁵,Nle²⁷]hGHRH(1-29)NH₂ (KT-30), [PhAc-Tyr¹,D-Arg²,Phe(pCl)⁶,Aib⁸,Abu¹⁵,Nle²⁷]hGHRH(1-29)NH₂ (KT-50) and [PhAc-Tyr¹,D-Arg²,Phe(pCl)⁶,Tyr(Me)¹⁰,Abu¹⁵,Nle²⁷]hGHRH(1-29)NH₂ (KT-40) with Phe(pI)⁶, Aib⁸ or Tyr(Me)¹⁰modifications, respectively, showed high and prolonged inhibitory effect in superfused rat pituitary system. Analog KT-50 also exhibited a strong and long-term inhibitory activity in vivo in rats. Most of the new analogs showed high binding affinities to rat pituitary GHRH receptors.