Source comparison
MK-677 vs GHRP-2 and GHRP-6: Structural and Functional Differences
GHRP-2 and GHRP-6 are hexapeptide growth hormone secretagogues that also bind to GHSR1a, but their peptide structure makes oral administration impractical. They require subcutaneous injection to bypass gastrointestinal degradation. MK-677 was developed specifi
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- GHRP-2 and GHRP-6 are hexapeptide growth hormone secretagogues that also bind to GHSR1a, but their peptide structure makes oral administration impractical. They require subcutaneous injection to bypass gastrointestinal degradation. MK-677 was developed specifically to replace injectable GHRPs with an orally bioavailable alternative that retained receptor selectivity and agonist potency.
- At the receptor level, MK-677 binds to the same GHSR1a orthosteric site as GHRP-2, but with higher affinity (Ki = 0.7 nM for MK-677 vs 5–10 nM for GHRP-2). This higher binding affinity translates to more sustained receptor occupancy per dose, which is why MK-677 maintains efficacy with once-daily dosing while GHRP-2 protocols typically require 2–3 daily injections to maintain therapeutic plasma levels.
- One functional distinction: GHRP-6 stimulates appetite more aggressively than MK-677 or GHRP-2 due to secondary agonist activity at orexigenic pathways in the hypothalamus beyond GHSR1a. MK-677 increases appetite moderately. Reported in 20–30% of users. But the effect is less pronounced than GHRP-6. GHRP-2 shows the least appetite stimulation of the three compounds. This difference matters for researchers studying metabolic interventions where appetite modulation is an experimental variable.
- MK-677's plasma half-life (4–6 hours) is shorter than its functional duration (24 hours), which reflects persistent receptor occupancy even after plasma clearance. GHRP-2 has a half-life under 30 minutes, requiring repeated dosing to maintain receptor stimulation. From a protocol design perspective, MK-677's pharmacokinetic profile makes it the most practical oral GH secretagogue for sustained research applications.