ABSTRACT
The tirst growth hormone-releasing peptides (GHRPs) were invented, rather than isolated, in 1977 (1) and GHRP-6, a hexapeptide, was the first one active in vivo (2-4). The dose-dependent GH-releasing activity of GHRP-6 has been demonstrated in several species and in humans after intravenous, subcutaneous, intranasal, and even oral administration (2-4). Now the GHRP family includes more potent analoges of GHRP-6, such as GHRP-1, a heptapeptide, and GHRP2 and hexarelin*, two hexapeptides the activity of which has been extensively studied in animals and in humans (2-6). Other GHRP mimetics, having structures more amenable to chemical modifications and optimization of oral bioavailability, have been synthetized. Among them, nonpeptidyl GHRPs (L692,429, L-692,585, L-700,653, L-163, 191 or MK-0677) have been studied in animals and even in humans (7-12); more recently, some cyclic peptides as well as penta-, tetra-, and pseudotripeptides have also been synthesized and studied in animals ( 13-15).
