NEW GABA-CONTAINING ANALOGS OF HUMAN GROWTH HORMONE-RELEASING HORMONE(1-30)-AMIDE .2. DETAILED IN-VIVO BIOLOGICAL EXAMINATIONS

Analogues of human growth hormone-releasing hormone-(1-30)-amide [GH-R H(1-30)-amide] were tested for their ability to stimulate GH release i n vivo by injecting the peptides intravenously (iv), subcutaneously (s c), and intramuscularly (im). The analogues involved derivatization wi th Nle(27) and Gaba substituents at the C-terminus with or without D-a mino acid(s) in the peptide chain. The potency of the analogues was co mpared 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)am ide, and no significant differences were detected between the potencie s of the analogues with or without D-amino acid. In the sc test the an alogue with D-Ala(2), Nle(27), and Gaba(30) substitutions expressed 8. 0-51.7 times higher potency than the GH-RH(1-29)-amide, however, the a nalogue with similar modifications but with L-Ala(2) showed the same l ow potency (1.2-2.1) as in the iv test. Results from the im experiment s were similar to those of SC test. The most potent analogues were tho se which had D-Ala(2) Nle(27), and Gaba(30) substitutions with Gly(15) or Leu(15). Circular dichroism (CD) spectra of the analogues showed t hat Leu in position 15 increased the stability of the predominant alph a-helix conformation, which improved the absorption of the molecule. T he introduction of D-Arg(11) into the D-Ala(2), Leul5, Nle(27) Gaba(30 ) GH-RH(1-30)-amide molecule decreased the sc and im potencies, withou t altering the iv activities. Our data indicate that in vivo GH-releas ing potency of the GH-RH(1-30)-amide analogues highly depends on the r oute of administration, and that D-amino acid(s) are responsible for t he increased sc and im potency. Parenteral absorption of the analogue appears to be related to its alpha-helical conformation.