Ipamorelin: Pioneering the Era of Selective Growth Hormone Secretagogues

Abstract:

This study delves into the unique properties of ipamorelin, a potent growth hormone (GH) secretagogue known for its pentapeptide structure. Ipamorelin demonstrates remarkable efficacy in releasing GH both in vitro and in vivo. Notably, ipamorelin showcases a distinct specificity for GH release, differing from other GH secretagogues as it does not affect plasma levels of ACTH or cortisol, even at high doses. This unparalleled selectivity, akin to GHRH, positions ipamorelin as an exciting candidate for clinical development, heralding a new era of selective GH secretagogues.

Introduction:

Since the introduction of recombinant GH in 1985, growth hormone (GH) has played a vital role in treating various growth disorders. Recent research suggests the potential benefits of GH treatment in conditions such as:

• Frailty in the elderly
• Osteoporosis
• Post-surgery catabolism
• And Obesity

The release of GH is regulated by hypothalamic peptides, including GH-releasing hormone (GHRH) and somatostatin. GH-releasing peptides (GHRPs), such as GHRP-6 and MK-677, have garnered attention for their ability to mimic natural GH pulsatility. These compounds effectively raise GH levels but also elevate levels of prolactin and cortisol. In this study, we introduce a novel series of GH secretagogues that activate the GHRP receptor, with ipamorelin as the prototype. Ipamorelin dosing stands out for its potent and selective GH-releasing properties, which have minimal impact on plasma ACTH and cortisol levels, making it a promising candidate in the field of GH modulation.

Methods:

Chemistry: The synthesis of secretagogues was carried out using either the Boc or Fmoc peptide strategy on peptide synthesizers, followed by purification via semi-preparative RP-HPLC. Ipamorelin synthesis specifically utilized various amino acid derivatives and purification methods, ensuring high purity.

Rat Pituitary Cell Assay: In vitro assays were conducted using pituitary cells obtained from Sprague-Dawley male albino rats to evaluate the GH-releasing effects of various compounds, including ipamorelin. Cells were cultured for three days, pre-incubated, and treated with GHRP ipamorelin test compounds (ipamorelin oral, GHRP-6, or GHRH) for GH release assessment.

Effects on Anaesthetized Rats: Anesthetized rats received intravenous test compounds (including ipamorelin), and blood samples were collected for GH analysis.

Effects on Conscious Swine: Female slaughter swine received test compounds, with blood samples collected at intervals for hormone analysis.

Hormone Assays: Various hormone levels were measured using different assays to assess the impact of the secretagogues.

Calculations: Parameters including Cmax, Tmax, and AUC were calculated to characterize hormone responses, and potency values were estimated using regression analysis.

Results:

• Rat Pituitary Cell Assay: all secretagogues triggered GH release in a dose-dependent manner. Ipamorelin, with Aib replacing L-Ala, demonstrated potency and efficacy levels similar to GHRP-1.

Detailed Result Analysis: All secretagogues elicited dose-dependent GH release from rat pituitary cells. GHRP-1, GHRP-2, and GHRP-6 demonstrated approximately equal potency. Deleting the central dipeptide Ala-Trp (NNC 26–0039) resulted in a minor EC50 decrease, with no Emax change compared to GHRP-1. Replacing N-terminal L-Ala with D-Ala (NNC 26–0133) decreased potency and slightly reduced Emax. Substituting L-Ala with Aib (ipamorelin) restored both EC50 and Emax to GHRP-1 levels.

• Pharmacological Specificity: Ipamorelin exhibited unique pharmacological specificity, acting as a GHRP receptor agonist but not affected by GHRH antagonists.

Detailed Result Analysis: The specificity of ipamorelin was evaluated using GHRP and GHRH antagonists. GHRP antagonists ((D-Lys3)-GHRP-6, L-692,400, and (D-Arg1,D-Phe5,D-Trp7,9, Leu11)-Substance P) and the GHRH antagonist (N-Acetyl-Tyr1,D-Arg2)-hGHRH(1–29)NH2 were tested. GHRP antagonists had no effect on GHRH-induced GH release, whereas both GHRP-6 and ipamorelin were inhibited by GHRP antagonists, indicating a GHRP receptor agonist profile. The GHRH antagonist potently inhibited GHRH-induced GH release but had no effect on GHRP-6 or ipamorelin-induced GH release.

• Anaesthetized Rats: In anaesthetized rats, GHRP ipamorelin outperformed other compounds in terms of potency and efficacy.

Detailed Result Analysis: In anaesthetized rats, GHRP-2 demonstrated over 100-fold greater potency than NNC 26–0039 and four times the potency of GHRP-6. Despite its high in vitro potency, NNC 26–0039 was inactive in vivo. NNC 26–0133, a derivative with L-Ala replaced by D-Ala, partially regained activity compared to GHRP-6 but with lower Emax. Ipamorelin, with Aib replacing L-Ala from NNC 26–0133, restored both potency and efficacy compared to GHRP-6.

• Conscious Swine: In conscious swine, ipamorelin dosing effectively induced GH release with minimal impact on stress hormones compared to other compounds.

Detailed Result Analysis: Regarding hormonal specificity, it's evident that among the tested compounds, GHRP-2 displayed the highest potency in stimulating GH release in conscious swine. While ipamorelin also effectively induced GH release, GHRP-6 demonstrated the most substantial increase in GH levels. In contrast, the GH-releasing effects of GHRH were notably lower compared to the GHRP ipamorelin compounds.

• Hormonal Specificity: These substances didn't seem to affect other hormones in the pituitary gland or cortisol, no matter how much we used. It didn't make a big difference whether the hormone levels were normal or at their highest point after stimulation.

Detailed Result Analysis: Other pituitary hormones (ACTH, FSH, LH, PRL, TSH) and cortisol remained unaffected by GH secretagogues across all administered doses. Basal levels did not differ significantly from Cmax values obtained after stimulation (P>0.05).

• Stress Hormone Response: When checking stress hormones like ACTH and cortisol, it is found that GHRP-6 and GHRP-2 made them go up a lot, but ipamorelin didn't increase them much, even when we used a lot of it. And there were no bad side effects with any of these substances.

Detailed Result Analysis: ACTH and cortisol levels increased significantly in response to GHRP-6 and GHRP-2 but were minimally affected by ipamorelin, even at doses over 200-fold higher than the GH release ED50. No adverse effects were observed with any secretagogue doses tested.

Conclusion:

Ipamorelin oral emerges as a GH secretagogue with remarkable selectivity for GH release, setting it apart from previous compounds. Unlike its predecessors, ipamorelin specifically targets GH without affecting other pituitary hormones. Pharmacological studies confirm its receptor mechanism, which is akin to GHRP ipamorelin rather than GHRH. In swine, ipamorelin induces GH release while maintaining minimal impact on stress hormones in comparison to other compounds. These findings underscore the clinical promise of ipamorelin as a selective GH secretagogue, opening new possibilities in the field of GH modulation.