MK-677, GHRP-6, and L-692,585 Act as Orthosteric Super-Agonists, Not Allosteric Modulators, at the Ghrelin Receptor
Background
The ghrelin receptor (GHSR1a) is a constitutively active G protein-coupled receptor that drives growth hormone secretion via activation of Gαo1 and other G proteins. Several synthetic growth hormone secretagogues (GHS) — including MK-677 and GHRP-6 — were previously characterized not just as agonists but as 'ago-allosteric' modulators capable of simultaneously binding the receptor and modulating the affinity and maximum efficacy of the endogenous ligand ghrelin. This ago-allosteric model, proposed by Holst et al. (2005), implied these compounds operate through a distinct, non-competing binding site — a claim with major implications for drug design and combination dosing strategies targeting the GH/IGF-1 axis.
Study Design
Using membranes prepared from cells co-expressing the human ghrelin receptor and the G protein Gαo1, the study tested MK-677, GHRP-6, and L-692,585 as agonists in a functional assay system. Multiple fixed concentrations of each synthetic ligand were applied alongside concentration-response curves for ghrelin, and vice versa. Data were analyzed globally using a modified operational model of allosterism accounting for modulation of affinity, efficacy, and allosteric agonism. Radioligand binding kinetics were assessed using ¹²⁵I-[His]-ghrelin dissociation assays to test whether any ligand altered orthosteric binding.
Results
All three synthetic ligands — MK-677, GHRP-6, and L-692,585 — functioned as direct agonists at the GHSR/Gαo1 system and each displayed higher efficacy than ghrelin, making ghrelin a functional partial agonist relative to these compounds. Global data analysis found that each full dataset was best fit by a model of simple competition between a partial agonist (ghrelin) and a full agonist (the synthetic GHS), rather than any allosteric model.
None of the proposed ago-allosteric regulators had any effect on the dissociation kinetics of
¹²⁵I-[His]-ghrelin, and both GHRP-6 and MK-677 were able to fully displace¹²⁵I-[His]-ghrelinfrom the receptor — hallmark evidence of orthosteric, not allosteric, action.
Because true allosteric modulators — whether positive or negative — are expected to alter the binding kinetics of an orthosteric agonist, the absence of any kinetic effect on ¹²⁵I-[His]-ghrelin dissociation directly falsifies the ago-allosteric hypothesis for this system. The data reframe ghrelin as a partial agonist with respect to each of the three synthetic compounds tested, within this specific membrane/G protein preparation.
Key Findings
- MK-677, GHRP-6, and L-692,585 each showed higher efficacy than ghrelin at
GHSR/Gαo1 - All three GHS datasets were best fit by a simple competition model, not an allosteric model
- Neither GHRP-6 nor MK-677 altered dissociation kinetics of
¹²⁵I-[His]-ghrelin, ruling out allosterism - GHRP-6 and MK-677 fully displaced
¹²⁵I-[His]-ghrelin, confirming orthosteric binding - Ghrelin behaves as a partial agonist relative to each of the three synthetic secretagogues tested
Why It Matters
The ago-allosteric model for MK-677 and GHRP-6 is not supported in this assay system — meaning assumptions about synergistic or supra-additive effects when combining ghrelin-pathway compounds with these secretagogues need reassessment. For clinicians and researchers designing GH-axis protocols, stacking ghrelin-mimetic peptides with MK-677 or GHRP-6 may produce competitive rather than complementary receptor occupancy, reducing the expected additive benefit. The finding that ghrelin acts as a partial agonist relative to these synthetic ligands also suggests MK-677, GHRP-6, and L-692,585 could ceiling-cap or out-compete endogenous ghrelin signaling through Gαo1 at sufficient concentrations. Clinical translation of true allosteric GHS drugs targeting GHSR will require fresh mechanistic validation beyond prior ago-allosteric frameworks.
TitrateLab articles covering MK-677
Written by TitrateLab, separately from the study summarized above.
