GHRP-2 (Pralmorelin): Molecular Profile, GHS-R1a Signaling, and the Preclinical Record — A 2026 Research Guide

GHRP-2 is the compound that made the ghrelin receptor findable. Synthesized in the Bowers laboratory as a second-generation refinement of GHRP-6, it was potent enough to serve as the functional benchmark that let pharmacologists characterize a binding site nobody had yet cloned — a site that turned out to have an endogenous ligand made in the stomach. That history is why GHRP-2 still appears in the methods sections of GPCR papers three decades on: it is a well-behaved, protease-resistant tool compound for a receptor whose signaling is unusually complex. This guide summarizes the molecular profile, receptor- and pathway-level mechanism, and the published preclinical record as it stands in 2026.

The information below describes in-vitro and preclinical laboratory research only. It is not medical guidance, and none of the findings described here establish safety or efficacy in humans.

Section 1 — Molecular Profile

GHRP-2 is a synthetic hexapeptide built deliberately outside the boundaries of natural protein chemistry. Three of its six residues are D-enantiomers, and one is a non-proteinogenic aromatic amino acid. It is not a fragment of any endogenous hormone; it is a small-molecule-like peptidomimetic that happens to be assembled from amino acids.

Key identifiers used in the literature and on analytical documentation:

• Compound name: GHRP-2; pralmorelin; growth hormone-releasing peptide-2; KP-102; GPA-748
• CAS number: 158861-67-7 (free base)
• Molecular formula: C45H55N9O6
• Molecular weight: approximately 817.98 Da (free base)
• Length: 6 amino acid residues, single linear chain, C-terminal amide
• Sequence: D-Ala-D-2-Nal-Ala-Trp-D-Phe-Lys-NH2
• Class: growth hormone secretagogue; GHS-R1a (ghrelin receptor) agonist
• Relationship to GHRP-6: GHRP-6 is His-D-Trp-Ala-Trp-D-Phe-Lys-NH2; GHRP-2 replaces His1 with D-Ala and D-Trp2 with D-2-naphthylalanine

Several structural features determine how the molecule behaves on a bench. The D-configuration at positions 1, 2, and 5 makes the peptide substantially resistant to the aminopeptidases and endopeptidases that degrade L-peptides in serum-containing culture medium — the most practical difference between working with GHRP-2 and working with ghrelin itself, which is both shorter-lived and dependent on a labile acyl modification. The bulky D-2-naphthylalanine at position 2 distinguishes GHRP-2 from GHRP-6 and is generally credited with the increase in receptor potency reported across comparative studies.

The C-terminal amide is not decorative. The free-acid form is a pharmacologically distinct molecule, and incomplete amidation is a genuine synthesis failure mode that a purity percentage alone will not surface, because the two differ by roughly 1 Da and can co-elute under an undemanding HPLC method.

Two further points matter for analytical interpretation. First, the tryptophan at position 4 is photolabile and oxidation-sensitive; ambient light exposure of reconstituted stock is a real degradation pathway and shows up on LC-MS as oxidized species. Second, most commercial material is supplied as an acetate or trifluoroacetate salt rather than free base, so vial mass and peptide mass are not the same number. Any laboratory calculating molar concentrations from vial weight without a net-peptide figure is introducing a systematic error into its own dose-response curves.

Section 2 — Mechanism

All activity described here is at the receptor and pathway level, in defined experimental systems.

Receptor engagement. GHRP-2 is an agonist at the growth hormone secretagogue receptor type 1a (GHS-R1a, gene GHSR), a class A G protein-coupled receptor expressed on anterior pituitary somatotrophs and in the hypothalamic arcuate and ventromedial nuclei, with additional expression reported in peripheral tissues. GHS-R1a was cloned in 1996 by Howard and colleagues using precisely this class of synthetic secretagogue as the pharmacological handle — the receptor was identified by its ligand before its natural agonist was known. In 1999, Kojima and colleagues reported ghrelin, an octanoylated 28-residue gastric peptide, as the endogenous ligand for that orphan receptor, retrospectively reclassifying the entire GHRP series as ghrelin-receptor agonists.

Canonical signaling. GHS-R1a couples principally to Gαq/11. Receptor activation stimulates phospholipase C-β, generating inositol 1,4,5-trisphosphate and diacylglycerol, mobilizing calcium from intracellular stores, and activating protein kinase C. In somatotrophs, the resulting rise in intracellular calcium is the proximal trigger studied for exocytosis of GH-containing secretory granules. IP3-linked calcium mobilization, not cAMP accumulation, is the primary readout in GHS-R1a functional assays, which is a meaningful distinction from GHRH receptor pharmacology in the same cell type.

The cAMP question and species dependence. The relationship between GHRP signaling and the cAMP arm is genuinely species-dependent, and the literature is explicit about it. Work published in the Journal of Endocrinology in 1996 reported that GHRP-6 and GHRP-2 increased intracellular cAMP in partially purified ovine somatotrophs but did not do so in rat pituitary cells, despite producing GH release in both. That result is a standing caution against generalizing a signaling mechanism across preparations, and it is one reason ovine and rat primary pituitary systems are not interchangeable models in this field.

Synergy with the GHRH axis. GHRP-2 and GHRH act through separate receptors and are consistently reported to act synergistically rather than additively on GH release in vitro. Work published in 1994 reported that the GH-releasing effect of GHRP-2 on primary cultured ovine pituitary cells could be abolished by a specific GRF (GHRH) receptor antagonist — evidence that at least part of the secretagogue response in that system is permissive on intact GHRH receptor signaling rather than fully independent of it. This interdependence is the mechanistic basis for the GHRH-analog/secretagogue pairing seen throughout the in-vitro GH-axis literature.

Functional opposition to somatostatin. GHS-R1a signaling is studied for functional antagonism of somatostatin tone at the somatotroph, and hypothalamic GHS-R1a expression places part of the response upstream of the pituitary — which is why both isolated-cell and intact-tissue preparations persist in the literature.

A second, non-GHS-R1a binding site. GHRP-family peptides also bind CD36, a scavenger-receptor glycoprotein expressed in cardiomyocytes and microvascular endothelium. Bodart and colleagues reported in Circulation Research in 2002 that a photoactivatable hexarelin derivative labeled an 84 kDa cardiac membrane protein identical to rat CD36, and that hexarelin-induced changes in coronary perfusion pressure in perfused hearts were absent in CD36-null mice and in CD36-deficient spontaneously hypertensive rats — the structural basis for the observation that some GHRP effects in cardiac preparations are dissociable from GH release entirely.

Constitutive activity and biased signaling. GHS-R1a is notable for high ligand-independent constitutive activity, which complicates any antagonist study at this receptor and has driven a dedicated inverse-agonist literature. The receptor also recruits β-arrestin and undergoes agonist-driven internalization, and Gq-versus-arrestin bias at GHS-R1a remains an active research question in 2026. Throughout, these are mechanisms GHRP-2 has been studied for — GHS-R1a agonism, Gq/PLC/IP3 calcium mobilization, GHRH-axis synergy in cultured pituitary cells, and CD36 binding in cardiac membranes — not demonstrated clinical effects in humans.

Section 3 — Preclinical and In-Vitro Research Data

The core in-vitro record for GHRP-2 is primary pituitary cell culture. A study published in Life Sciences in 1997 evaluated the GH-releasing activities of GHRP-2 and GHRP-6 side by side in rat primary pituitary cells and reported that GHRP-2, like GHRP-6, acted synergistically with GRF to release GH, with the two differing in potency in that system. Because the peptides are near-identical apart from positions 1 and 2, this pairing remains the cleanest structure-activity comparison in the series.

The broader pharmacological characterization of the compound under its KP-102 designation was published in Arzneimittel-Forschung in 2004. A separate 2004 profile in Drugs in R&D catalogues its development history under the pralmorelin, GPA-748, and KP-102 designations.

Structural biology has advanced considerably since those functional studies. Shiimura and colleagues reported an antagonist-bound ghrelin receptor structure in Nature Communications in 2020, resolving the ligand-binding cavity and proposing a recognition mode for ghrelin’s acyl chain. In 2021, a companion Nature Communications paper presented cryo-EM structures of the Gq-coupled ghrelin receptor bound to ghrelin and to the synthetic agonist GHRP-6, identifying a hydrophobic pocket accommodating the octanoyl group and describing how peptide positioning initiates activation. Because GHRP-2 differs from GHRP-6 at two positions, that structure is the closest available structural context for the GHRP-2 binding mode — though it is not a GHRP-2 co-structure and should not be described as one.

The cytoprotection literature sits alongside, and partly apart from, the GH-release literature. Reviews of the GHRP family catalogue cardioprotective and anti-inflammatory findings in preclinical models attributed in part to CD36 binding and reported as dissociable from GH secretion. That body of work is preclinical in its entirety, and the mechanistic attribution across GHS-R1a and CD36 remains under investigation rather than settled.

What the record does not contain is human outcome evidence generated in this compound’s research-chemical form. Clinical-stage investigation of pralmorelin proceeded through a specific formulated product under regulatory oversight in a diagnostic context, and none of that transfers to a lyophilized research vial.

Section 4 — Published Literature

The following are real, published papers anchoring the GHRP-2 record. Researchers should consult the primary sources rather than relying on summaries.

• Bowers CY, Momany FA, Reynolds GA, Hong A (1984). “On the in vitro and in vivo activity of a new synthetic hexapeptide that acts on the pituitary to specifically release growth hormone.” Endocrinology 114(5):1537–1545. The foundational GHRP structure-activity work.
• Wu D, Chen C, Katoh K, Zhang J, Clarke IJ (1994). “The effect of GH-releasing peptide-2 (GHRP-2 or KP 102) on GH secretion from primary cultured ovine pituitary cells can be abolished by a specific GH-releasing factor (GRF) receptor antagonist.” Journal of Endocrinology 140(2):R9–R13. PMID 8169551.
• Howard AD, et al. (1996). “A receptor in pituitary and hypothalamus that functions in growth hormone release.” Science 273(5277):974–977. PMID 8688086. The GHS-R cloning paper.
• Wu D, Chen C, Zhang J, Bowers CY, Clarke IJ (1996). “The effects of GH-releasing peptide-6 (GHRP-6) and GHRP-2 on intracellular adenosine 3′,5′-monophosphate (cAMP) levels and GH secretion in ovine and rat somatotrophs.” Journal of Endocrinology 148(2):197–205. PMID 8699133.
• Chen C, Wu D, Clarke IJ (1997). “Growth hormone releasing peptides: a comparison of the growth hormone releasing activities of GHRP-2 and GHRP-6 in rat primary pituitary cells.” Life Sciences 60(16):1327–1336. PMID 9096259.
• Kojima M, Hosoda H, Date Y, Nakazato M, Matsuo H, Kangawa K (1999). “Ghrelin is a growth-hormone-releasing acylated peptide from stomach.” Nature 402(6762):656–660. PMID 10604470.
• Bodart V, et al. (2002). “CD36 mediates the cardiovascular action of growth hormone-releasing peptides in the heart.” Circulation Research 90(8):844–849. PMID 11988484.
• Pralmorelin: GHRP 2, GPA 748, growth hormone-releasing peptide 2, KP-102 D, KP-102 LN (2004). Drugs in R&D 5(4):236–239. PMID 15230633.
• Pharmacological characteristics of KP-102 (GHRP-2), a potent growth hormone-releasing peptide (2004). Arzneimittel-Forschung 54(11):761–772. PMID 15646370.
• Shiimura Y, et al. (2020). “Structure of an antagonist-bound ghrelin receptor reveals possible ghrelin recognition mode.” Nature Communications 11:4160.
• Molecular recognition of an acyl-peptide hormone and activation of ghrelin receptor (2021). Nature Communications 12:5064. Cryo-EM structures of Gq-coupled GHSR bound to ghrelin and GHRP-6.

Together these span the original structure-activity work, the receptor cloning that GHRPs enabled, the ovine and rat primary-cell pharmacology, the endogenous ligand, the CD36 secondary site, and the receptor’s structural biology.

Section 5 — Research Applications

In laboratory settings, GHRP-2 appears principally as a reference agonist and pathway probe rather than as an experimental subject in its own right. Reported in-vitro applications include use as a positive-control agonist in GHS-R1a calcium-mobilization and IP1/IP3 accumulation assays; as a comparator ligand in screening for ghrelin-receptor agonists, antagonists, and inverse agonists; as a stimulus in primary pituitary cell culture studying somatotroph secretory mechanics; as a probe in β-arrestin recruitment and receptor-internalization assays; and as a ligand in competitive radioligand binding studies against membrane preparations expressing GHS-R1a or CD36.

Several handling variables affect reproducibility. The tryptophan at position 4 makes reconstituted stock light- and oxidation-sensitive, so amber tubes and immediate aliquoting are standard practice; lyophilized material is generally stored desiccated at −20°C or below and protected from light. The peptide is relatively hydrophobic for its size due to the naphthylalanine and phenylalanine content, and incomplete dissolution — rather than degradation — is a common cause of apparent potency loss in newly opened vials.

The salt form deserves separate emphasis. Acetate and TFA counterions plus residual water account for a meaningful fraction of vial mass on a hexapeptide, and residual trifluoroacetate is itself a documented confound in sensitive cell-based assays. A preparation intended for cell culture should state which counterion it carries.

Endotoxin is the other variable that determines whether a vial is usable. GHS-R1a is expressed on immune cell populations and its signaling intersects inflammatory pathways, so endotoxin contamination can independently drive the exact readouts a GHRP-2 experiment is measuring.

Section 6 — How to Evaluate a Source

For a short, salt-form, amidated peptide, the certificate of analysis carries more weight than any other purchasing signal. When evaluating any GHRP-2 source, confirm:

• Identity by LC-MS, with an observed mass consistent with roughly 817.98 Da for the amidated free-base hexapeptide. Verify the mass corresponds to the amide rather than the free acid, and look for oxidation species on the tryptophan.
• Purity by HPLC, with a stated percentage and a visible chromatogram rather than a bare number. On a hexapeptide, deletion sequences elute close to the parent and a competent method should resolve them.
• Net peptide content and counterion, both disclosed. Gross vial weight is not molar input, and acetate versus TFA matters for cell-based work.
• Endotoxin by USP <85> LAL, with an actual figure.
• Heavy metals by ICP-MS.
• Lot-specificity: the COA must correspond to the exact batch shipped, be dated, and name the accredited laboratory that performed the testing.

Market pricing context: research-grade GHRP-2 is commonly listed in 5 mg and 10 mg lyophilized vials, and per-milligram pricing sits toward the low end of the peptide market because a six-residue sequence is inexpensive to synthesize at scale. That low cost floor is exactly why analytical documentation matters more here, not less — price offers almost no signal about quality on a compound this cheap to make. The COA is where the difference becomes visible.

Section 7 — The PYXAX Verification Standard

PYXAX uses accredited independent laboratories in its verification network, including ILS Labs, Krause Analytical, and Janoshik. The current per-batch panel covers purity by HPLC, potency against label claim, and identity by LC-MS. A lot-specific COA is published in the COA Library before dispatch and names the laboratory that tested that batch. Endotoxin, heavy-metals, and final-vial sterility screening are not part of the current panel.

Founding batches are documented end to end, and full analytical data files are available per lot. You can review current documentation in the COA library, read the underlying methodology on the verification standard page, or browse the full catalog in the shop.

All PYXAX compounds are supplied strictly for in-vitro and preclinical laboratory research use only. Not for human consumption. Not for veterinary use. Not for diagnostic procedures. These statements have not been evaluated by the FDA. Researchers are responsible for compliance with all applicable laws and regulations governing the use of research compounds in their jurisdiction.