The development of 'impervious peptides' as growth hormone secretagogues.

The discovery and development of growth hormone (GH) secretagogues is briefly reviewed. GH-releasing peptide-6 (GHRP-6) was the first GHRP to be developed that was active in vivo. Smaller peptides were found to have no GH-releasing activity in vivo, even though they were potent releasers of GH in vitro. Substituting Trp with 2-Me-Trp led to the development of orally active hexarelin and other, smaller, peptides. Although they showed oral activity, absorption rarely exceeded 1% of the administered dose. Nonpeptide GH secretagogues have now been developed that combine reasonable oral absorption with high potency. The next development will be to produce GH secretagogues that overcome the lack of specificity shown by the present molecules.