Background

Ipamorelin is the first GHRP-receptor agonist with a selectivity for growth hormone release similar to that displayed by GHRH. The specificity of ipamorelin makes this compound a very interesting candidate for future clinical development. Ipamorelin is a white amorphous powder.(Figure 1) Its solubility in distilled water is>200 mg/ml. The pKas of the compound are 10.3,7.8, and 6.2.[1]

Synthesis of Ipamorelin

In particular, ipamorelin was prepared from a total of 4.53 g of the peptide resin Boc-Aib-His (Trt)-D-2Nal-DPhe-Lys (Boc)-NH-resin according to the Fmoc strategy on an Applied Biosystems 431A peptide synthesizer in two identical runs (to obtain sufficient material) in 1 mmol scale using the FastMoc UV protocols supplied by the manufacturer, that employ HBTU mediated couplings in NMP and UV monitoring of the deprotection of the Fmoc protection group.

The starting resin used for the synthesis was 2×1.75g (4-((2’,4’ -dimethoxyphenyl)-(Fmoc-amino)- methyl)-phenoxy resin (Rink resin) with a substitution capacity of 0.55 mmol/g. The protected amino acid derivatives used were Fmoc-Lys(Boc)-OH, Fmoc-D-Phe-OH, Boc-D-2Nal-OH, Boc-His(Trt)-OH and Boc-Aib-OH (Boc-Amino isobutyric acid). The peptide was cleaved from the 4.53 g peptide resin by treatment with 54 ml TFA/phenol/ethanedithiol/thioanisole/water 40:3:1:2:2 for 3 h at room temperature.

Purification was carried out by semi-preparative RP-HPLC and the final product was characterised by amino acid analysis, analytical RP-HPLC and by plasma desorption mass spectroscopy. Amino acid analysis and mass spectrometry were in agreement with the expected structure and within the experimental error of the method (mass spectrometry±0.9 amu, amino acidanalysis±10%). [1]

Application research

1. Ipamorelin induces longitudinal bone growth in rats

With the objective of investigating the effects on longitudinal bone growth rate (LGR), body weight(BW), and GH release, ipamorelin in different doses (0, 18, 90 and 450 μg/day) was injected s.c. three times daily for15 days to adult female rats. After intravital tetracycline labelling on days 0, 6, and 13, LGR was determined bymeasuring the distance between the respective fluorescent bands in the proximal tibia metaphysis. Ipamorelin dose dependently increased LGR from 42 μm/day in the vehicle group to 44, 50, and 52 μm/day in the treatment groups(P<0.0001). There was also a pronounced and dose-dependent effect on BW gain. The treatment did not affect total IGF-I levels, IGFBPs, or serum markers of bone formation and resorption. The number of tartrate-resistant acid phosphatase-positive multinuclear cells in the metaphysis of the tibia did not change significantly with treatment. The responsiveness of the pituitary to a provocative i.v. dose of ipamorelin or GHRH showed that the plasma GH response was marginally reduced (P<0.03) after ipamorelin, but unchanged after GHRH. The pituitary GH content was unchanged by ipamorelin treatment. Whether ipamorelin or other GH secretagogues may have a place in the treatment of children with growth retardation requires demonstration in future clinical studies.[2]

2. Efficacy of Ipamorelin in a Rodent Model of Postoperative Ileus

Ghrelin and ghrelin mimetics stimulate appetite and enhance gastric motility. The present study investigates whether ipamorelin, a selective growth hormone secretagogue and agonist of the ghrelin receptor, would accelerate gastrointestinal transitand ameliorate the symptoms in a rodent model of postoperative ileus (POI). Fasted male rats were subjected to laparotomy and intestinal manipulation. At the end of surgery, a dye marker was infused in the proximal colon to evaluate postsurgical colonic transit time, which was the time to the first bowel movement. In addition, fecal pellet output, food intake, and body weight were monitored regularly for 48 h. Ipamorelin(0.01–1 mg/kg), growth hormone-releasing peptide (GHRP)-6(20g/kg), or vehicle (saline) were administered via intravenous bolus infusion after a single dosing or a 2-day repetitive dosing regimen (four doses a day at 3-h intervals). Compared with the vehicle, a single dose of ipamorelin (1 mg/kg) or GHRP-6 (20
g/kg) decreased the time to the first bowel movement but had no effect on cumulative fecal output, food intake, or body weight gain measured 48 h after the surgery. In contrast, repetitive dosing of ipamorelin (0.1 or 1 mg/kg) significantly increased the cumulative fecal pellet output, food intake, and body weight gain. The results suggest that postsurgical intravenous infusions of ipamorelin may ameliorate the symptoms inpatients with POI.[3]

3. Efficacy of ipamorelin on gastric dysmotility in a rodent model of postoperative ileus

Delayed gastric emptying is a common disorder with few effective therapeutic options. The goal of this study was to investigate whether ipamorelin, a synthetic peptidomimetic that acts on the ghrelin receptor, accelerates gastric emptying in a rodent model of gastroparesis induced by abdominal surgery and intestinal manipulation. Fasted adult male rats were subjected to laparotomy and intestinal manipulation.Following the surgery rats received ipamorelin (0.014–0.14 μmol/kg) or vehicle control via intravenous administration. Gastric emptying was measured by the percent of total recovered radioactivity remaining in the stomach 15 minutes after intragastric gavage of 1.5 mL of 99mTc(technicium-99m) sulfur colloid in 0.5% methylcellulose. In a separate group of rats subjected to laparotomy and intestinal manipulation, the gastric fundus was isolated and tissue segments were suspended in an organ bath to assess the effect of ipamorelin (1 μM) on gastric smooth muscle contractility induced by acetylcholine and electrical field stimulation. Abdominal surgery caused a delay in gastric emptying with 78% ± 5% of the meal remaining in the stomach in vehicle controls. Ipamorelin (0.014 μmol/kg intravenous) resulted in a significant acceleration (P＜0.05 vs vehicle-treated rat) of gastric emptying with 52% ± 11% of the meal remaining in the stomach compared to nonsurgical control animals with 44% ± 6%. Following abdominal surgery and intestinal manipulation, isolated preparations of gastric smooth muscle exhibited a marked inhibition of acetylcholine and electrical field stimulation-induced contractile responses, which were reversed by ipamorelin and ghrelin.Conclusion: These results suggest that ipamorelin accelerates gastric emptying in a rodent model of postoperative ileus through the stimulation of gastric contractility by activating aghrelin receptor-mediated mechanism involving cholinergic excitatory neurons.

In conclusion, this study highlights the profound effects of ipamorelin on the upper GI tract and suggests that the compound may be of interest as a prokinetic agent to treat GI dysmotility, characterized by delayed gastric emptying and slow upper GI transit.[4]

References

[1]Raun K, Hansen BS, Johansen NL, et al. Ipamorelin, the first selective growth hormone secretagogue. Eur J Endocrinol. 1998;139(5):552-561. doi:10.1530/eje.0.1390552

[2]Johansen PB, Nowak J, Skjaerbaek C, et al. Ipamorelin, a new growth-hormone-releasing peptide, induces longitudinal bone growth in rats. Growth Horm IGF Res. 1999;9(2):106-113. doi:10.1054/ghir.1999.9998

[3]Venkova K, Mann W, Nelson R, Greenwood-Van Meerveld B. Efficacy of ipamorelin, a novel ghrelin mimetic, in a rodent model of postoperative ileus. J Pharmacol Exp Ther. 2009;329(3):1110-1116. doi:10.1124/jpet.108.149211

[4]Greenwood-Van Meerveld B, Tyler K, Mohammadi E, Pietra C. Efficacy of ipamorelin, a ghrelin mimetic, on gastric dysmotility in a rodent model of postoperative ileus. J Exp Pharmacol. 2012;4:149-155. Published 2012 Oct 19. doi:10.2147/JEP.S35396