Identity a synthetic pentapeptide (5 amino acids, < 40 aa → peptide), a selective agonist of the ghrelin / growth-hormone-secretagogue receptor (GHS-R1a). Derived from GHRP-1 by removing its central Ala-Trp dipeptide — the change that preserved GH-releasing potency while sharply improving selectivity. MW ≈ 712 Da. ## Development & history - Developed by Novo Nordisk A/S (Denmark), mid–late 1990s, out of a medicinal-chemistry programme seeking a GH secretagogue with fewer off-target effects.
- First characterised by Raun et al. 1998 (European Journal of Endocrinology) as "the first selective growth-hormone secretagogue" — releasing GH without the ACTH/cortisol/prolactin/FSH/LH/TSH elevations seen with older GHRPs (GHRP-6, GHRP-2, hexarelin), even far above the GH-release dose.
- Later licensed by Helsinn Therapeutics and developed for a specific medical use: postoperative ileus (gut-motility recovery after abdominal surgery).
- Phase 2 RCT (Beck et al. 2014, Int J Colorectal Dis, n=87) — the most rigorous published human study — did not meet its primary endpoint; not advanced to Phase 3.
- Never given a brand name, never approved for any indication; migrated to functional-medicine / gray-market use. ## Mechanism (as proposed) binds GHS-R1a (the ghrelin receptor) on pituitary somatotrophs → Gq/phospholipase-C → IP₃/calcium release → pulsatile GH secretion, mimicking ghrelin — but, distinctively, without meaningfully activating the ACTH/cortisol or prolactin axes.