Growth Hormone Secretagogues Compared

Growth Hormone Secretagogues Compared: Ipamorelin, CJC-1295, Tesamorelin and GHRP-6

US research-peptide catalogs almost always list these four molecules under a single heading — “GH secretagogues” — as if they were four strengths of the same reagent. They are not. Two act on the growth hormone secretagogue receptor; two act on the growth hormone-releasing hormone receptor. Separate receptors, separate second messengers, separate somatotroph responses — and a study design that treats them as interchangeable will produce results that do not mean what the design assumed.

This comparison is written for people planning in vitro pituitary work or rodent neuroendocrine studies. Everything below concerns cultured cells and animal models. None of it describes use in people.

The split that organizes everything: two receptors

GHS-R1a was cloned by Howard and colleagues and reported in Science in 1996 (PMID 8688086) — a receptor in pituitary and hypothalamus that mediates GH release in response to a class of synthetic peptides. Its endogenous ligand, ghrelin, was identified three years later by Kojima and colleagues in Nature (PMID 10604470). Signaling is predominantly Gq/phospholipase C, raising intracellular calcium. Ipamorelin and GHRP-6 are GHS-R1a agonists.

GHRH-R is the receptor for hypothalamic growth hormone-releasing hormone, signaling through Gs and cAMP. CJC-1295 and tesamorelin are both modified GHRH analogs acting here.

Cheng and colleagues demonstrated the separation experimentally in rat primary pituitary cell culture as early as 1989 (Endocrinology, PMID 2541999). GHRP-6 stimulated GH release with half-maximal effect at 7 × 10⁻⁹ M, and that release was not blocked by the GRF antagonist [N-Ac-Tyr¹,D-Arg²]GRF-(1-29)-NH₂, which did shift the GRF dose-response curve rightward. GHRP-6 had no effect on intracellular cAMP; GRF raised it three-fold. Two receptors, two mechanisms, one shared output. That paper is still the cleanest demonstration of the point.

The GHS-R1a arm

GHRP-6

The original hexapeptide, His-D-Trp-Ala-Trp-D-Phe-Lys-NH₂. Sethumadhavan and colleagues characterized specific binding to rat anterior pituitary and hypothalamic membranes (BBRC, PMID 1712588), and Bowers reviewed its actions in Endocrinology in 1991 (PMID 2004615). Smith and colleagues covered the peptidomimetic program that followed in two Endocrine Reviews pieces (PMID 9331545, PMID 15814848).

Its research limitation is selectivity. Raun and colleagues, comparing secretagogues in conscious swine (Eur J Endocrinol, PMID 9849822), found GHRP-6 and GHRP-2 both raised plasma ACTH and cortisol. In a study whose readout is downstream of GH, a corticotroph-axis confound is not a footnote — it is an alternative explanation for the result.

Bhatti and colleagues extended the comparison across the adenohypophysis in young and old Beagle dogs (Vet J, PMID 15951209), measuring GH, ACTH, TSH, LH, prolactin and cortisol responses to ghrelin, GHRP-6 and GHRH. Ghrelin was the most potent secretagogue in young dogs; in old dogs GHRH produced the highest plasma GH. Age of the animal changed which receptor arm dominated — a design variable rarely stated in supplier copy.

Ipamorelin

Ipamorelin (Aib-His-D-2-Nal-D-Phe-Lys-NH₂) came out of a Novo Nordisk program that removed the central Ala-Trp dipeptide of GHRP-1. Raun’s 1998 paper is the load-bearing citation and the numbers are worth quoting precisely.

In primary rat pituitary cells, ipamorelin released GH with potency and efficacy similar to GHRP-6 (EC₅₀ 1.3 ± 0.4 nmol/l, Emax 85 ± 5% versus 2.2 ± 0.3 nmol/l and 100%). Antagonist profiling confirmed it works through the GHRP-like receptor, not GHRH-R. In pentobarbital-anaesthetized rats, ED₅₀ was 80 ± 42 nmol/kg; in conscious swine, ED₅₀ was 2.3 ± 0.03 nmol/kg. All comparable to GHRP-6.

The difference is what it did not do. In swine, none of the secretagogues tested altered FSH, LH, prolactin or TSH — but where GHRP-6 and GHRP-2 raised ACTH and cortisol, ipamorelin did not, at concentrations more than 200-fold above its ED₅₀ for GH release. That is the entire reason ipamorelin exists as a distinct research tool, and the paper’s title states the claim plainly: the first selective growth hormone secretagogue.

For any protocol where an unintended corticotroph response would corrupt the readout, that 200-fold margin is the specification that matters — not relative GH potency, which is roughly equivalent.

The GHRH-R arm

CJC-1295

Jetté and colleagues reported the identification of CJC-1295 in Endocrinology in 2005 (PMID 15817669). Three maleimido derivatives of hGRF(1-29) were bioconjugated ex vivo to the free thiol at Cys34 of human serum albumin. All three showed enhanced in vitro stability against dipeptidyl peptidase-IV and remained bioactive in a GH secretion assay in cultured rat anterior pituitary cells.

CJC-1295 — a tetrasubstituted hGRF(1-29) bearing an N-ε-3-maleimidopropionamide lysine derivative at the C-terminus — was the best of the three, producing a four-fold increase in GH area under the curve over two hours versus unmodified hGRF(1-29) in Sprague-Dawley rats. It was detectable in plasma beyond 72 hours, and Western blot of plasma from an injected rat showed CJC-1295 immunoreactivity on the albumin band from 15 minutes and persisting past 24 hours. That in vivo albumin conjugation — the “DAC” modification — is the mechanism of the long half-life, and it is the axis on which DAC and non-DAC preparations differ.

Teichman and colleagues published human pharmacokinetics in JCEM (PMID 16352683), reporting prolonged GH and IGF-I elevation in healthy adults. Cited here as published clinical pharmacology literature, and for no other purpose.

Tesamorelin

Tesamorelin (TH9507) is a different chemistry solving the same DPP-IV problem. Ferdinandi and colleagues described the non-clinical package in Basic Clin Pharmacol Toxicol (PMID 17214611): hGRF(1-44)NH₂ minimally modified by a trans-3-hexenoyl moiety on Tyr¹. Note the length — a 44-residue analog, where CJC-1295 is built on the 29-residue fragment.

The modification slowed in vitro degradation in rat, dog and human plasma and prolonged in vivo elimination kinetics. Plasma GH and IGF-1 rose markedly in pigs, rats and dogs after repeat intravenous or subcutaneous administration at up to 600 µg/kg. Subchronic studies to four months showed increased body-weight gain that was significant but not dose-related, and dogs — with apparent elimination half-life of 21–45 minutes — showed more pronounced anabolic effects and reversible liver, kidney, hematological and organ-weight findings, attributed by the authors to sustained supraphysiological GH and IGF-1 exposure. Falutz and colleagues published the clinical program (NEJM, PMID 18057338; JCEM, PMID 20554713).

Species-dependent tolerability of that magnitude is a design fact. A rodent protocol and a canine protocol are not the same experiment with a different animal in it.

Why the two arms get combined

Cheng’s 1989 result answers this. GHRP-6 and GRF at individually maximal concentrations produced a synergistic effect on GH release, and combined treatment potentiated the GRF-induced cAMP increase even though GHRP-6 alone did nothing to cAMP. Bowers reported the same in vivo. Two receptors converging on one cell type, with the GHS-R1a arm amplifying the Gs/cAMP arm rather than duplicating it.

Tannenbaum and colleagues placed this in the context of endogenous pulsatile regulation (Endocrine, PMID 11322498; Endocrinology, PMID 12586774), where somatostatin withdrawal and GHRH pulses set the rhythm and secretagogues modulate it. Wagner and colleagues proposed and tested a model of the ghrelin–GH axis interaction (J Mol Endocrinol, PMID 19433492). A GHRH-R analog plus a selective GHS-R1a agonist is the combination that engages both limbs while contributing the fewest off-target endocrine signals — which is why that pairing, rather than any GHRH analog with any GHRP, recurs in the literature.

Cheng also reported that GHRP-6 pretreatment for one hour reduced basal GH release by 30% and blunted the subsequent GHRP-6 response while leaving the GRF response intact, and that this desensitization reversed completely within an hour of washout. Receptor-arm specific desensitization is a real variable in repeat-administration designs.

Downstream endpoints: read the method, not the headline

Svensson and colleagues gave ipamorelin or GHRP-6 (0.5 mg/kg per day), GH (3.5 mg/kg per day) or vehicle to 13-week-old female Sprague-Dawley rats by continuous subcutaneous osmotic minipump for 12 weeks, with DXA every four weeks and post-mortem pQCT, Archimedes displacement and ash-weight analysis (J Endocrinol, PMID 10828840).

All treatments raised body weight and total tibial and vertebral bone mineral content. But total BMC corrected for body weight was unaffected; total and vertebral areal BMD were unchanged; cortical volumetric BMD was unchanged; and the pQCT data showed the cortical BMC increase came from increased cross-sectional bone area. The bones got larger, not denser — a real finding about skeletal growth, routinely miscited as evidence of increased bone density, which the authors did not observe.

What this means for selecting a research compound

Isolating GHS-R1a signaling calls for ipamorelin, on the selectivity margin in Raun’s swine data. Isolating GHRH-R calls for a GHRH analog, with the choice between the 29-residue CJC-1295 scaffold and the 44-residue tesamorelin scaffold driven by which fragment your assay and antibodies are validated against. Studying the interaction of the two limbs calls for both, with single-agent arms included — Cheng’s synergy is only interpretable against them. GHRP-6 remains the reference GHS-R1a compound for comparative work, provided the corticotroph axis is measured rather than assumed away.

US sourcing and lot traceability

None of the above survives an identity problem. These are short synthetic peptides with non-standard residues — Aib, D-2-Nal, D-Trp, D-Phe — and a synthesis that drops or epimerizes one of them yields a compound that binds poorly and reads as a weak biological effect rather than as a failed reagent. For the albumin-conjugating CJC-1295 chemistry, the reactive maleimide is itself a stability-sensitive feature, so lot age and storage history matter more than they would for a plain amide-capped peptide.

Maple Research Labs supplies domestically: a Wyoming entity, in-house pick, pack and label from Santa Barbara with same-day dispatch before cut-off, and no customs step between the shelf and the laboratory bench. Every lot is traceable to its supplier and its batch certificate of analysis, and the COA shown on the product page is the document for the vials in the box — not a representative example from an earlier run. For work engaging both receptor arms, our CJC-1295/Ipamorelin Blend 10MG, batch-COA tested and shipped from the USA is a single co-formulated vial with one lot number and one batch document, which removes one source of cross-lot variability from a two-agent design. Where a protocol needs the two agents attributed separately, single-agent vials are the correct choice and the blend is not a substitute.

Ask any US supplier for the batch document matching the lot they will actually ship. A COA offered “on request” after purchase asks you to accept identity on faith, in an experiment where identity is the independent variable.

---

All products supplied by Maple Research Labs are for research use only. These materials are laboratory reagents intended for in vitro and animal-model investigation by qualified researchers. They are not for human use or consumption, are not approved for any diagnostic or clinical application, and must not be administered to people. Every study cited above was conducted in cultured cells, rats, swine or dogs, or by investigators working under regulatory oversight.

References

1. Howard AD, et al. A receptor in pituitary and hypothalamus that functions in growth hormone release. Science. 1996;273(5277):974–7. PMID 8688086.
2. Kojima M, et al. Ghrelin is a growth-hormone-releasing acylated peptide from stomach. Nature. 1999;402(6762):656–60. PMID 10604470.
3. Cheng K, et al. Synergistic effects of His-D-Trp-Ala-Trp-D-Phe-Lys-NH₂ on GRF-stimulated GH release and intracellular cAMP accumulation in rat primary pituitary cell culture. Endocrinology. 1989;124(6):2791–8. PMID 2541999.
4. Bowers CY. On the actions of the growth hormone-releasing hexapeptide, GHRP. Endocrinology. 1991;128(4):2027–35. PMID 2004615.
5. Sethumadhavan K, et al. Specific binding of growth hormone-releasing peptide to rat anterior pituitary and hypothalamic membranes. Biochem Biophys Res Commun. 1991;178(1):31–7. PMID 1712588.
6. Raun K, et al. Ipamorelin, the first selective growth hormone secretagogue. Eur J Endocrinol. 1998;139(5):552–61. PMID 9849822.
7. Jetté L, et al. hGRF(1-29)-albumin bioconjugates activate the GRF receptor on the anterior pituitary in rats: identification of CJC-1295 as a long-lasting GRF analog. Endocrinology. 2005;146(7):3052–8. PMID 15817669.
8. Ferdinandi ES, et al. Non-clinical pharmacology and safety evaluation of TH9507, a human growth hormone-releasing factor analogue. Basic Clin Pharmacol Toxicol. 2007;100(1):49–58. PMID 17214611.
9. Teichman SL, et al. Prolonged stimulation of GH and IGF-I secretion by CJC-1295 in healthy adults. J Clin Endocrinol Metab. 2006;91(3):799–805. PMID 16352683.
10. Falutz J, et al. Metabolic effects of a growth hormone-releasing factor in patients with HIV. N Engl J Med. 2007;357(23):2359–70. PMID 18057338.
11. Falutz J, et al. Tesamorelin (TH9507) in patients with excess abdominal fat: pooled analysis of two phase 3 trials with safety extension. J Clin Endocrinol Metab. 2010;95(9):4291–304. PMID 20554713.
12. Svensson J, et al. The GH secretagogues ipamorelin and GH-releasing peptide-6 increase bone mineral content in adult female rats. J Endocrinol. 2000;165(3):569–77. PMID 10828840.
13. Bhatti SF, et al. Effects of growth hormone secretagogues on the release of adenohypophyseal hormones in young and old healthy dogs. Vet J. 2006;172(3):515–25. PMID 15951209.
14. Tannenbaum GS, et al. Interactions of growth hormone secretagogues and GHRH/somatostatin. Endocrine. 2001;14(1):21–7. PMID 11322498.
15. Tannenbaum GS, et al. Interrelationship between ghrelin and somatostatin/GHRH in regulation of pulsatile GH secretion. Endocrinology. 2003;144(3):967–74. PMID 12586774.
16. Wagner C, et al. Interactions of ghrelin signaling pathways with the GH neuroendocrine axis: a new and experimentally tested model. J Mol Endocrinol. 2009;43(3):105–19. PMID 19433492.
17. Smith RG, et al. Peptidomimetic regulation of growth hormone secretion. Endocr Rev. 1997;18(5):621–45. PMID 9331545.
18. Smith RG, et al. Development of growth hormone secretagogues. Endocr Rev. 2005;26(3):346–60. PMID 15814848.