SM-130686 orally increased fat-free mass and IGF-I in rats via GHS receptor
Background
Growth hormone deficiency and age-related GH decline drive reductions in lean mass, strength, and metabolic health, yet direct GH administration carries cost, inconvenience, and supraphysiological risks. GH secretagogues (GHSs) act at the GHS receptor 1a (GHSR1a) to stimulate endogenous pulsatile GH release, offering a more physiological alternative. MK-677, NN703, and hexarelin established the class but SM-130686 is structurally distinct from all of them, potentially offering a differentiated pharmacological profile. The endogenous GHSR1a ligand ghrelin — identified from stomach extract — provides the reference benchmark against which new GHSs are measured for partial versus full agonism. Understanding partial agonism at GHSR1a is critical because it may predict ceiling effects on GH, IGF-I, and anabolic endpoints.
Study Design
Researchers tested SM-130686 across in vitro and in vivo models. In vitro, cultured rat pituitary cells were treated with SM-130686 across a dose range to measure GH release; a GHS antagonist and a GH-releasing hormone antagonist were used as controls. Receptor binding was assessed using a radioligand displacement assay with (35)S-MK-677 against human GHSR1a. In vivo, pentobarbital-anesthetized rats received a single oral dose of 10 mg/kg SM-130686, with plasma GH measured by radioimmunoassay at intervals up to 60 min. For anabolic endpoints, rats received 10 mg/kg orally twice daily for 9 days, with daily body weights and fat-free mass assessed; serum IGF-I was measured 6 h after the final dose.
Results
SM-130686 stimulated GH release from rat pituitary cells in a dose-dependent manner, with half-maximum stimulation at 6.3 ± 3.4 nM. Displacement of (35)S-MK-677 from human GHSR1a gave an IC50 of 1.2 nM, confirming on-target receptor engagement; crucially, GH release was blocked by a GHS antagonist but not by a GHRH antagonist, confirming the GHSR1a-specific mechanism. Compared with ghrelin (set at 100%), SM-130686 reached approximately 52% of GH-releasing activity in vitro, classifying it as a partial GHSR1a agonist. After a single oral 10 mg/kg dose in anesthetized rats, plasma GH peaked between 20–45 min and remained above baseline through the full 60-min observation window.
After 9 days of twice-daily oral 10 mg/kg SM-130686, rats gained 19.5 ± 2.1 g in body weight and 18.1 ± 7.5 g in fat-free mass — a gain in lean tissue nearly equal to the total body-weight gain — alongside a significant elevation in serum IGF-I at 6 h post-final dose.
Key Findings
- In vitro GH release half-maximum stimulation: 6.3 ± 3.4 nM in rat pituitary cells
- Human GHSR1a binding IC50: 1.2 nM via (35)S-MK-677 displacement assay
- GH-releasing activity ~52% of ghrelin, indicating partial GHSR1a agonism
- 9-day oral dosing (10 mg/kg BID) increased body weight by 19.5 ± 2.1 g and fat-free mass by 18.1 ± 7.5 g
- Serum IGF-I significantly elevated 6 h after last dose; plasma GH peaked at 20–45 min post oral dose
Why It Matters
Oral bioavailability combined with significant anabolic output positions SM-130686 in the same practical niche as MK-677 but with a structurally novel scaffold — potentially relevant for researchers seeking alternatives with different tolerability or partial-agonist ceilings. The near-complete overlap between body-weight gain and fat-free mass gain over 9 days closely mirrors outcomes seen with exogenous GH, suggesting a clinically meaningful body-composition shift without significant fat accumulation at this dose. For protocol designers, the twice-daily oral dosing window and IGF-I elevation at 6 h post-dose offer concrete timing anchors. The partial-agonism profile (~52% vs. ghrelin) implies a built-in ceiling that may reduce oversaturation risk compared to full agonists, though this remains to be validated in humans.
