What Is Ipamorelin?
Ipamorelin (Aib-His-D-2-Nal-D-Phe-Lys-NH₂) is a synthetic pentapeptide growth hormone secretagogue developed by Novo Nordisk in the late 1990s. It was designed using structure-activity relationship studies to maximise GHS-R1a binding affinity while minimising off-target receptor activation — particularly the ACTH and prolactin responses seen with earlier GHRPs.
It was studied in Phase II clinical trials for post-operative ileus (gastrointestinal motility recovery) but was not advanced to Phase III for commercial reasons. Despite this, it remains the most pharmacologically characterised selective GHRP in preclinical research literature.
Mechanism of Action
Like all GHRPs, Ipamorelin binds and activates GHS-R1a on pituitary somatotrophs, triggering Gq/11 signalling → PLC → IP₃ → intracellular Ca²⁺ release → GH exocytosis. It also acts at hypothalamic GHS-R1a to increase GHRH and reduce somatostatin release, amplifying the pituitary response.
What distinguishes Ipamorelin is the structural modifications (Aib at position 1, D-2-naphthylalanine at position 3) that confer GHS-R1a selectivity. These substitutions reduce binding affinity for non-GHS-R1a receptors, preventing the "off-target" ACTH and prolactin release that limits the utility of GHRP-6 and GHRP-2 in clean GH-axis studies.
High affinity binding; no significant ACTH, prolactin, TSH, or FSH/LH stimulation at standard doses
Gq/11 → IP₃ → Ca²⁺ → GH granule exocytosis; pulse onset 15–30 min
↑ GHRH + ↓ Somatostatin at hypothalamic arcuate nuclei → amplified pituitary response
Unlike GHRP-6, minimal orexigenic activity — allows GH-axis research without confounding appetite variables
GH Release Effects
In rodent models, single SC injection of Ipamorelin (100–300 µg/kg) produces GH peaks 4–8× above baseline within 15–30 minutes, with return to baseline by 2–3 hours. This is comparable to GHRP-6 in terms of GH amplitude, but with substantially fewer side-effects.
Ipamorelin GH Response Summary
| Parameter | Ipamorelin | GHRP-6 (reference) |
|---|---|---|
| GH peak amplitude | 4–8× baseline | 4–8× baseline |
| Cortisol elevation | None / negligible | Moderate ↑ |
| Prolactin elevation | None / negligible | Moderate ↑ |
| Appetite stimulation | Minimal | Strong |
| GH pulse duration | 2–3 hours | 2–3 hours |
| IGF-1 (chronic 14d) | +25–45% | +30–50% |
Chronic administration studies (14–28 days) in aged rodents show Ipamorelin restores IGF-1 levels closer to younger animal values, with associated improvements in lean body mass, bone mineral density, and skin thickness — consistent with GH-axis restoration rather than supraphysiological stimulation.
The Selectivity Advantage
The selectivity of Ipamorelin matters practically for research design. When studying GH-axis effects in isolation, cortisol and prolactin co-elevation (as seen with GHRP-6/GHRP-2) introduces confounding variables:
- Cortisol confound: Cortisol is catabolic and immunosuppressive — elevation obscures anabolic readouts (lean mass, nitrogen balance) in GH research
- Prolactin confound: Prolactin modulates reproductive function, mammary gland biology, and immunomodulation — irrelevant co-activation confounds non-reproductive studies
- Hunger confound: GHRP-6's orexigenic effects change food intake, body weight, and metabolic parameters independently of GH, making GH-specific attribution difficult
Ipamorelin removes all three confounds, making it the preferred GHRP for clean, mechanistically interpretable GH-axis research.
CJC-1295 + Ipamorelin Stack Research
The combination of a GHRH analogue (CJC-1295 or Sermorelin) with Ipamorelin exploits the dual-axis architecture of GH secretion regulation. These peptides act on completely independent receptors:
- CJC-1295 activates GHRH-R → Gs/cAMP → PKA → GH transcription + exocytosis
- Ipamorelin activates GHS-R1a → Gq/IP₃ → Ca²⁺ → GH exocytosis
Because these signalling cascades are orthogonal, maximal activation of one does not reduce the capacity of the other — producing supra-additive GH responses. Studies in rodent models show co-administration produces GH peaks 3–5× higher than either peptide alone.
Ipamorelin was evaluated in a Phase II trial (Novo Nordisk, ~2000) for post-operative ileus — where GH axis stimulation was hypothesised to accelerate GI motility recovery. The trial was discontinued for commercial reasons. However, the human pharmacokinetic and pharmacodynamic data from this programme remains the basis for understanding Ipamorelin's dose-response and safety profile in human-relevant contexts.
GHRP Selectivity Comparison
| GHRP | GH ↑ | Cortisol ↑ | Prolactin ↑ | Hunger ↑ | Selectivity |
|---|---|---|---|---|---|
| GHRP-2 | ★★★★★ | ★★★★ | ★★★★ | ★★★ | Low |
| GHRP-6 | ★★★★ | ★★★ | ★★★ | ★★★★★ | Moderate |
| Hexarelin | ★★★★★ | ★★★★ | ★★★★ | ★★★ | Low |
| Ipamorelin | ★★★★ | ★ | ★ | ★ | Very High |
Research Protocols
GH Axis Research
- Dose: 100–300 µg/kg SC (rodent)
- Timing: Pre-active phase / fasted state
- Serial GH sampling: q15 min × 120 min (ELISA)
- Chronic: BID–TID × 14–28 days + serum IGF-1
GHRH + GHRP Stack
- Combine with Sermorelin (1–5 µg/kg) or CJC-1295 (10–50 µg/kg)
- Administer simultaneously SC
- Compare: Ipamorelin alone vs CJC-1295 alone vs combination
- Endpoint: GH AUC over 3h (area under curve)
Body Composition (Aged Models)
- Model: 18–22 month old rats (equivalent to ~60+ yo human)
- Dose: 200 µg/kg SC BID × 28 days
- Endpoints: DXA (lean/fat mass), bone density, IGF-1
- Controls: Young adult group, vehicle-treated aged group
FAQ
What makes Ipamorelin selective?
Structural modifications (Aib at position 1, D-2-naphthylalanine at position 3) give Ipamorelin high GHS-R1a affinity with minimal off-target receptor activity. Unlike GHRP-6/GHRP-2, it does not significantly stimulate ACTH or prolactin release.
Can Ipamorelin be stacked with CJC-1295?
Yes — Ipamorelin (GHS-R1a, Gq pathway) and CJC-1295 (GHRH-R, Gs/cAMP pathway) act on independent receptors and signalling cascades in the same pituitary somatotroph cell, producing supra-additive GH release when combined.
Is Ipamorelin stronger than GHRP-6?
GHRP-6 may produce marginally larger GH peaks, but Ipamorelin's selective profile makes it more useful for clean GH-axis research. GHRP-6's cortisol/prolactin/hunger side-effects introduce confounding variables.
How long does Ipamorelin last?
Ipamorelin's plasma half-life is ~2 hours — longer than GHRP-6 (~15–60 min). GH response onset is 15–30 min, peaking at ~30–60 min, returning to baseline by 2–3 hours.
Ipamorelin for Research
Lyophilised Ipamorelin ≥98% purity (HPLC) with mass spec and independent COA. Available individually or as part of a CJC-1295 stack.
View Ipamorelin →