New Gaba-containing analogues of human Growth Hormone Releasing Hormene (1–30)-amide: II. Detailed in vivo biological examinations

Abstract

Analogues of human growth hormone-releasing hormone-(1–30)-amide [GH-RH(1–30)- amide] were tested for their ability to stimulate GH release in vivo by injecting the peptides intravenously (iv), subcutaneously (sc), and intramuscularly (im). The analogues involved derivatization with Nle²⁷ and Gaba substituents at the C-terminus with or without D-amino acid(s) in the peptide chain. The potency of the analogues was compared to that of GH-RH(1–29)-amid testing their ability to release GH at 5, 15 and 30 min after the administration. In iv test the potency of the analogues was 1.2–2 times higher than that of the GH-RH(1–29)- amide, and no significant differences were detected between the potencies of the analogues with or without D-amino acid. In the sc test the analogue with D-Ala², Nle²⁷, and Gaba³⁰ substitutions expressed 8.0–51.7 times higher potency than the GH-RH(1–29)-amide, however, the analogue with similar modifications but with L-Ala² showed the same low potency (1.2–2.1) as in the iv test. Results from the im experiments were similar to those of SC test. The most potent analogues were those which had D-Ala², Nle²⁷, and Gaba³⁰ substitutions with Gly¹⁵ or Leu¹⁵. Circular dichroism (CD) spectra of the analogues showed that Leu in position 15 increased the stability of the predominant α-helix conformation, which improved the absorption of the molecule. The introduction of D-Arg¹¹ into the D- Ala², Leul5, Nle²⁷ Gaba³⁰ GH-RH(1–30)-amide molecule decreased the sc and im potencies, without altering the iv activities. Our data indicate that in vivo GH-releasing potency of the GH-RH(1–30)- amide analogues highly depends on the route of administration, and that D-amino acid(s) are responsible for the increased sc and im potency. Parenteral absorption of the analogue appears to be related to its α-helical conformation.