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Amidated fragments 1 to 29 of human growth hormone releasing factor, (GRF1-29NH2), were designed to encompass modifications that could prevent degradation by exopeptidases. The NH2-terminus was blocked by either N-methylation of Tyr1 or by introduction of NMeDAla2 while the COOH-terminus was blocked by an N-ethylamide. Other substitutions such as Nle in position 27 for methionine conferred stability toward air oxidation while Asn28, an amino acid substitution found in rat GRF, seemed to confer stronger binding affinity to the GRF receptor. Potency in vitro and duration of action in vivo of [NMeTyr1,Nle27,Asn28]hGRF1-29NHEt (4SG-29) were compared to those of hGRF1-40OH. 4SG-29 was found to be both ten times more potent than hGRF1-40)OH and exhibit significantly extended duration of action.