Ipamorelin peptide: the selective secretagogue that reached phase 2

Designed for selectivity

The growth-hormone-releasing peptides of the 1980s worked, but not cleanly. GHRP-6 and GHRP-2 released GH, and with it ACTH, cortisol and prolactin. Novo Nordisk's chemistry programme in Måløv set out to keep the first effect and lose the others.

Raun and colleagues describe the outcome in 1998: among a series of compounds that lacked the central Ala-Trp dipeptide of GHRP-1, a five-residue peptide, Aib-His-D-2-Nal-D-Phe-Lys-NH2, released GH from rat pituitary cells with a potency and efficacy close to GHRP-6 (EC50 1.3 against 2.2 nmol/l; Emax 85 % against 100 %), released GH in anaesthetised rats and in conscious swine at doses comparable to GHRP-6, and did something GHRP-6 and GHRP-2 did not: at doses more than 200 times its ED50 for GH release, it left ACTH and cortisol at the levels seen after GHRH. None of the secretagogues tested changed FSH, LH, prolactin or TSH. The authors' title made the claim plainly: "the first selective growth hormone secretagogue" [Raun et al., 1998].

Two unnatural features explain the molecule's survival in blood. Aib (alpha-aminoisobutyric acid) at the N-terminus and the two D-amino acids in the middle are not substrates for the peptidases that dismantle a natural pentapeptide. That is why a 712-dalton peptide, which would otherwise be gone in minutes, lasts a couple of hours.

The receptor it binds, found before its hormone

When ipamorelin was designed, its receptor had a name but no natural ligand. Howard and colleagues had cloned the growth hormone secretagogue receptor from pituitary and hypothalamus in 1996 and shown that the synthetic GHRPs acted through it [Howard et al., 1996]. Only in 1999 did Kojima and colleagues find the endogenous ligand, ghrelin, an acylated 28-residue peptide from the stomach [Kojima et al., 1999].

Ipamorelin is therefore a ghrelin mimetic that predates the discovery of ghrelin, and the receptor is now called GHS-R1a or the ghrelin receptor interchangeably. The functional consequence is that ipamorelin and a GHRH analogue such as CJC-1295 act on two different receptors of the same somatotroph, which is the whole rationale for pairing them, explained in what is CJC-1295 ipamorelin.

Two hours in the body, one pulse of GH

The one human pharmacokinetic study was published in 1999 by Gobburu, Agersø, Jusko and Ynddal. Eight healthy men per level received a 15-minute intravenous infusion at 4.21, 14.02, 42.13, 84.27 or 140.45 nmol/kg. Exposure was dose-proportional; the terminal half-life was 2 hours, clearance 0.078 l/h/kg, steady-state volume 0.22 l/kg. At every dose, GH rose in a single episode, peaked at 0.67 hours and declined exponentially to negligible levels. The modelling put the concentration for half-maximal GH stimulation at 214 nmol/l and the maximal GH production rate at 694 mIU/l/h, and noted that subjects varied more in their GH response than in their handling of the drug [Gobburu et al., 1999].

Two points from that study frame everything else on this site. First, a 2-hour half-life means ipamorelin is a pulse generator, not a background signal; whatever it is paired with should act on the same time scale, which is why the research-market blend uses the linker-free CJC-1295 rather than the albumin-bound one (DAC versus no DAC). Second, all the human dosing data are intravenous; no subcutaneous dose-finding study in people has ever been published.

Bone growth in rats, without IGF-1

Novo Nordisk's pharmacologists followed the GH signal into bone. In adult female rats given 18, 90 or 450 µg/day subcutaneously in three injections for 15 days, the longitudinal growth rate of the tibia, read from tetracycline labels, rose from 42 µm/day with vehicle to 44, 50 and 52 µm/day (P < 0.0001), with a parallel gain in body weight.

Total IGF-1, its binding proteins and serum bone markers did not change, and the pituitary's GH response to a test dose of ipamorelin was marginally reduced afterwards while its response to GHRH was not [Johansen et al., 1999]. Two years later, in 8-month-old rats given methylprednisolone for three months, 100 µg/kg of ipamorelin three times daily prevented the loss of calf-muscle tetanic tension and quadrupled the periosteal bone formation rate relative to steroid alone [Andersen et al., 2001].

Those are rat data at rat doses. They are quoted here because they are the only published evidence of a tissue effect for ipamorelin on its own, and because the 1999 study contains the first hint of receptor desensitisation with repeated dosing, taken up in ipamorelin side effects.

Two phase 2 trials, then silence

Novo Nordisk did not take ipamorelin into the clinic for growth hormone deficiency. The molecule was licensed to Helsinn, and the indication chosen was the one where a ghrelin agonist had the most obvious rationale: postoperative ileus, the paralysis of the gut after abdominal surgery, since ghrelin-receptor stimulation promotes motility. The proof-of-concept trial (NCT00672074) enrolled 117 adults undergoing bowel resection and analysed 114; they received 0.03 mg/kg intravenously twice daily from day 1 to day 7 or discharge, or placebo.

The primary endpoint, time from first dose to tolerance of a standard solid meal, was 25.3 hours with ipamorelin and 32.6 with placebo, a difference that did not reach significance (p = 0.15). Adverse events were reported in 87.5 % of the treated group and 94.8 % of the placebo group, and the authors called the drug "well tolerated" [Beck et al., 2014].

A larger dose-finding trial (NCT01280344) followed, with 320 patients across 0.03 mg/kg twice daily, 0.06 mg/kg twice daily, 0.06 mg/kg three times daily and placebo. The registry marks it completed; no results were ever posted and no paper followed [Helsinn et al., 2011]. That is where the clinical development of ipamorelin ends. Neither trial measured growth hormone, IGF-1, body composition or sleep; the human evidence for those outcomes with ipamorelin is nil.

Where it stands in 2026

Ipamorelin is not an approved drug anywhere. In the United States it circulated for years through compounding pharmacies, which is what the FDA addressed on September 29, 2023, when it placed ipamorelin acetate in category 2 of the bulk substances nominated for compounding under sections 503A and 503B: substances that "may present significant safety risks", with the stated concern that compounded drugs containing it may pose a risk of immunogenicity for certain routes of administration [U.S., 2023]. In sport it is listed by name among the growth hormone secretagogues in class S2 of the WADA Prohibited List [World, 2026].

What remains is a well-characterised research reagent: a defined sequence, a defined mass, a measured receptor selectivity and a measured 2-hour half-life. That is the frame in which this site discusses it.

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