GHRH and GH secretagogues: clinical perspectives and safety

The diagnosis and treatment of growth hormone deficiency (GHD), as well as the possibility of counteracting somatopause and age-related changes in body composition, structural functions, and metabolism, prompted interest in potential clinical uses of GH-releasing hormone (GHRH) and GH secretagogues (GHS). GHD often reflects hypothalamic GHRH deficiency and it has been clearly demonstrated that the age-related decline in the function of the GH/IGF-I axis reflects a reduction in hypothalamic function as evidenced by the preservation of the releasable pool of pituitary GH in aged subjects. The effectiveness of recombinant human GH (rhGH) is well established, but it is also recognized that GH replacement does not mimic physiological GH secretion which theoretically would be restored by GHRH and/or GHS. At present, it has been clearly demonstrated that GHRH and/or GHS represent reliable tools for the diagnosis of GHD. On the other hand, neither GHRH nor GHS has been shown to provide effective alternatives to rhGH for the treatment of GHD. Although GHRH and/or GHS represent the most logical approaches for the restoration of the GH/IGF-I axis to a youthful level of activity and for counteracting the somatopause, this hypothesis has never been proven definitively. Conceptually, GHRH replacement would be the most physiological approach and its safety is guaranteed, provided an appropriate dose is used, in order to avoid hyperactivity of the GH/IGF-I axis. However, a long-acting preparation is needed. On the other hand, GHS, e.g., ghrelin analogues, could be considered as a function of their selectivity of action. However, ghrelin has a wide spectrum of endocrine and non-endocrine actions at both central and peripheral levels. Thus, non-selective GHS, although available in orally active forms, could elicit unforeseen side effects. Previous studies with GHRH and/or GHS in aging patients provided encouraging results. However, it still remains to be definitively demonstrated that aged subjects would benefit from chronic treatment with these molecules.