Ipamorelin: What Made the Pentapeptide "Selective," Read Through Its Source Papers

Ipamorelin came out of a Novo Nordisk medicinal chemistry programme in the late 1990s, and its developers described it as the first growth hormone secretagogue with a selectivity for GH release similar to that of GHRH (Raun 1998). This Vial Post explainer looks at what that "selective" label rested on, what a human pharmacokinetic paper measured, how the molecule was used as a template for later compounds, and where it sits with FDA and WADA.

Nothing below is guidance on use in people or animals. The Vial Post covers ipamorelin strictly as a subject of laboratory and literature research.

Quick Facts

• Structure: a pentapeptide, Aib-His-D-2-Nal-D-Phe-Lys-NH2 (Raun 1998).
• Formula and mass: C38H49N9O5, molecular weight 711.9 g/mol, CAS 170851-70-4; the development code was NNC 26-0161 (PubChem CID 9831659).
• Unusual residues: Aib (PubChem's IUPAC name renders it as a 2-amino-2-methylpropanoyl group), two D-amino acids (D-2-Nal, a naphthylalanine, and D-Phe) and a C-terminal amide (Raun 1998; PubChem).
• Receptor class: pharmacological profiling showed it releases GH through a GHRP-like receptor, as GHRP-6 does (Raun 1998).
• Regulatory note: FDA lists ipamorelin acetate in 503B category 2, added September 29, 2023 (FDA bulk substances page).
• Sport: WADA's 2026 list names ipamorelin among growth hormone secretagogues under S2.2.4 (WADA 2026).

Where the Molecule Came From

Raun and colleagues report that ipamorelin was identified within a series of compounds lacking the central dipeptide Ala-Trp of GHRP-1 (Raun 1998).

What "Selective" Meant in the 1998 Paper

The selectivity claim came from animal and cell experiments, and it is narrower than marketing copy tends to suggest (Raun 1998):

• Cells: in primary rat pituitary cells, ipamorelin released GH with potency and efficacy similar to GHRP-6 (EC50 1.3 nmol/L vs 2.2 nmol/L).
• Other pituitary hormones in swine: none of the secretagogues tested affected FSH, LH, prolactin or TSH plasma levels.
• The distinguishing result: GHRP-6 and GHRP-2 both raised plasma ACTH and cortisol. Ipamorelin did not release ACTH or cortisol at levels significantly different from those seen after GHRH stimulation.

Together, these swine results underpin the paper's conclusion that ipamorelin's selectivity for GH release resembles that of GHRH.

The Human Pharmacokinetic Study

A 1999 Pharmaceutical Research paper modeled ipamorelin's pharmacokinetics and GH response in healthy male volunteers in a dose-escalation design (Gobburu 1999). Its reported parameters:

• Terminal half-life: short, about 2 hours, with pharmacokinetics that were dose-proportional.
• Clearance and distribution: clearance 0.078 L/h/kg; volume of distribution at steady state 0.22 L/kg.
• GH time course: a single episode of GH release peaking at 0.67 hours, followed by an exponential decline.
• Variability: inter-individual variability was larger for the pharmacodynamic parameters than for the pharmacokinetic ones.

The Vial Post does not report the infusion rates used.

A Template for Later Compounds

Ipamorelin also served as a scaffold. A companion 1998 Journal of Medicinal Chemistry paper from the same company described a new series derived from it, aiming at oral bioavailability by shrinking the molecule and reducing hydrogen-bonding sites with the peptidomimetic fragment 3-(aminomethyl)benzoic acid and backbone N-methylation (Ankersen 1998). One product of that work was the tetrapeptide NNC 26-0235.

FDA and WADA Status

• FDA. Ipamorelin acetate is listed in category 2 under the 503B interim policy, and also in the page's withdrawn-nominations table. FDA cites possible immunogenicity for certain routes due to aggregation or peptide-related impurities, and notes that its unnatural amino acids add to the complexity of peptide characterization. It also states that a published study identified serious adverse events, including death, when ipamorelin was given intravenously for gastric motility (FDA bulk substances page).
• WADA. The 2026 Prohibited List, in effect from 1 January 2026, lists ipamorelin under growth hormone secretagogues and their mimetics in S2.2.4 (WADA 2026).

Our report on the interim compounding policy covers the policy context.

Questions Readers Ask

Is ipamorelin a GHRH analog? No. It is a pentapeptide secretagogue that acts through a GHRP-like receptor, the same route as GHRP-6 (Raun 1998), and WADA lists it with growth hormone secretagogues rather than GHRH analogues (WADA 2026).

What is ipamorelin's half-life? About 2 hours in the 1999 healthy-volunteer modeling study (Gobburu 1999).

Why is ipamorelin called selective? In swine, unlike GHRP-6 and GHRP-2, it did not raise ACTH or cortisol beyond levels seen with GHRH (Raun 1998).

A Note for the Lab

Because ipamorelin contains D-amino acids and Aib, identity confirmation by mass spectrometry and an HPLC purity trace on the batch Certificate of Analysis are worth checking before any experiment. For laboratory work, Ipamorelin research material from Dynamite Research Peptides is supplied with a batch Certificate of Analysis.

For research and educational use only. This article is not medical advice and does not describe dosing or human use.