Ipamorelin preserves GH-IGF-I axis function and body weight under methylprednisolone suppression in rats
Background
Glucocorticoid-induced growth suppression is a well-documented adverse effect of therapeutic corticosteroids, causing impaired body weight gain, muscle wasting, and blunted GH-IGF-I axis signalling. Patients on chronic synthetic glucocorticoids such as methylprednisolone frequently experience catabolism that current supportive care does not adequately address. Growth hormone secretagogues acting at the GHSR receptor represent a candidate class for countering this suppression by driving endogenous GH release. Ipamorelin, a selective pentapeptide GHSR agonist, was of particular interest because of its specificity for GH release with minimal off-target secretagogue activity. Whether glucocorticoid pre-treatment would blunt ipamorelin's ability to stimulate the GH-IGF-I axis — a prerequisite concern before any clinical co-administration — was unresolved.
Study Design
Two experiments were conducted in rats. In Experiment 1, animals received either saline or methylprednisolone (MP) 5.0 mg/kg for 8 days, then received an acute intravenous challenge of either ipamorelin (IPA) or growth hormone releasing hormone (GHRH); plasma GH was the primary endpoint. In Experiment 2, venous catheters were surgically implanted and rats were randomised to saline alone, MP 5.0 mg/kg alone, or MP plus IPA at 0.4 or 1.6 mg/kg/day intravenously four times daily for 10 days; endpoints were body weight and plasma IGF-I levels measured by ELISA or equivalent assay.
Results
In Experiment 1, MP 5.0 mg/kg for 8 days significantly reduced body weight gain (p < 0.001), yet the acute plasma GH response to both ipamorelin and GHRH was unchanged relative to saline-treated controls, demonstrating that glucocorticoid pre-treatment did not impair acute GHSR-mediated GH secretion. In Experiment 2, MP-treated animals lost 13.6 ± 2.9 g over the experimental period versus only 4.0 ± 3.5 g in saline controls (p < 0.05), confirming the catabolic burden of surgical recovery compounded by glucocorticoid treatment.
Co-administration of ipamorelin with MP reduced body weight losses to 2.3 ± 2.0 g (high dose, 1.6 mg/kg/day) and 1.6 ± 2.0 g (low dose, 0.4 mg/kg/day), both significantly better than MP alone (p < 0.05).
In parallel with body weight rescue, plasma IGF-I levels increased in IPA-treated groups, indicating downstream GH-IGF-I axis activation with repeated stimulation. Notably, the low dose produced numerically greater weight preservation than the high dose, though both were statistically superior to MP alone.
Key Findings
- MP 5.0 mg/kg for 8 days cut body weight gain significantly (p < 0.001) but did NOT blunt acute GH response to ipamorelin or GHRH.
- MP-treated rats lost 13.6 ± 2.9 g vs 4.0 ± 3.5 g in saline controls (p < 0.05) over 10 days post-surgery.
- IPA 1.6 mg/kg/day + MP reduced weight loss to 2.3 ± 2.0 g vs 13.6 ± 2.9 g with MP alone (p < 0.05).
- IPA 0.4 mg/kg/day + MP reduced weight loss to 1.6 ± 2.0 g, numerically superior to the high-dose IPA group.
- Repeated ipamorelin dosing raised plasma IGF-I levels in parallel with body weight recovery.
Why It Matters
Glucocorticoid users do not need to forgo ipamorelin co-administration out of concern that the steroid will block GH release — this study directly demonstrates the GH axis remains responsive. For clinicians managing patients on chronic corticosteroids who are experiencing catabolism, ipamorelin at doses as low as 0.4 mg/kg/day IV four times daily showed meaningful body weight protection and IGF-I elevation. For researchers and protocol designers, the preservation of the acute GHRH-stimulated GH response alongside ipamorelin's effect suggests the somatotroph axis is not globally suppressed by MP. The combination warrants further investigation as a formal anabolic support strategy during prolonged corticosteroid therapy, with translation studies in larger animals and humans needed before clinical implementation.
