NEW GABA-CONTAINING ANALOGS OF HUMAN GROWTH HORMONE-RELEASING HORMONE(1-30)-AMIDE .1. SYNTHESIS AND IN-VITRO BIOLOGICAL-ACTIVITY

Analogues of human growth hormone-releasing hormone (1-30)-amide have been developed. All analogues have been modified in position 27 with N le and with Gaba in position 30. Additional D-amino-acids have been in serted in the GHRH(1-3O)-NH2 sequence: A-1741: Nle(27), Gaba(30)-GH-RH (1-30)-NH2 A-495: D-Ala(2), Nle(27), Gaba(30)-GH-RH (1-30)-NH2 A-515: D Ala(2), Leu(15), Nle(27), Gaba(30)-GH-RH (1-30)-NH2 A-527: D-Ala(2) , D-Arg(11), Leu(15), Nle(27), Gaba(30)-GH-RH (1-30)-NH2. Our analogue s were synthesized by solid phase peptide synthesis and were tested is two different in vitro systems and in rat pituitary cell cultures. A- 495 and A-1741 were found to be the most active in releasing GH, howev er they showed different activities in the two different test systems. A-495 exhibited higher potency in the superfusion system (1.63 fold p otency of the GHRH (1-29)-amide), while A-1741 evoked higher GH releas e from cultured pituitary cells (1.5-2.5 times higher than the GH-RH(1 -44)-amide). The other analogues (A-515 and A-527) were found to be eq uipotent to the standard molecule. We can conclude that Nle(27) and Ga ba(30) substitutions appeared to be a good modification in in vitro te st systems, and Gaba(30) substitution served as a good spacer during t he synthesis, since it made the coupling of the C-terminal amino acids easier and produced quantitative coupling. In addition to the advanta geous properties in the synthesis these modifications with or without D-Ala at the N-terminus increased the in vitro biological activity to 1.5-2.5 fold of the GHRH molecule. The additional substitution of Gly( 15) with Leu and Arg(11) with D-Arg did not improve the in vitro GH-re leasing activity of our analogues. A detailed in vivo investigation, w hich is essential for the future clinical use, has been performed and written in Part II of this paper.