GHRP-6 vs MK-677 — Oral Bioavailability
GHRP-6 and MK-677 (Ibutamoren) are both growth hormone secretagogues, but they differ fundamentally in chemical structure and oral bioavailability. GHRP-6 is a hexapeptide requiring injection, while MK-677 is a non-peptide molecule with oral bioavailability. This structural difference determines their pharmacokinetics and clinical applications.
Structural and Chemical Differences
Section titled “Structural and Chemical Differences”GHRP-6
Section titled “GHRP-6”- Sequence: His-D-Trp-Ala-Trp-D-Phe-Lys-NH₂ (6 amino acids)
- Modifications: D-amino acids at positions 2 and 5, C-terminal amidation
- MW: 873 Da
- Charge at pH 7.4: +2
- Oral bioavailability: <5%
- Half-life: 15–30 minutes (serum)
- Stability: Susceptible to peptidases
MK-677 (Ibutamoren)
Section titled “MK-677 (Ibutamoren)”- Chemical name: 2-amino-2-methyl-N-(1-methyl-3-phenylpropyl)-propanamide
- Modifications: Non-peptide structure
- MW: 529 Da
- Charge at pH 7.4: 0
- Oral bioavailability: ~60%
- Half-life: 16–24 hours
- Stability: Resistant to peptidases
Oral Bioavailability Comparison
Section titled “Oral Bioavailability Comparison”GHRP-6
Section titled “GHRP-6”Barriers to oral absorption:
- Peptide bonds: Susceptible to GI peptidases
- Charge: Positive charge reduces membrane permeability
- Size: 6 amino acids (873 Da) exceeds optimal oral absorption
- First-pass metabolism: Hepatic degradation
- Stomach acid: Acid degradation
Oral bioavailability:
- Absolute bioavailability: <5%
- Relative bioavailability (vs IV): <10%
- Food effect: Minimal (poor absorption anyway)
- Formulation strategies: Enteric coating, permeation enhancers
MK-677
Section titled “MK-677”Favorable absorption characteristics:
- Non-peptide: Resistant to peptidases
- Neutral charge: Better membrane permeability
- Small molecule: 529 Da (optimal for absorption)
- Lipophilic: Good intestinal absorption
- Stable: Resistant to GI degradation
Oral bioavailability:
- Absolute bioavailability: ~60%
- Relative bioavailability (vs IV): ~70%
- Food effect: Reduced by high-fat meals
- Formulation: Tablet, capsule
Mechanism of Action
Section titled “Mechanism of Action”Shared Mechanism
Section titled “Shared Mechanism”Both agents stimulate GH release through:
- Growth hormone secretagogue receptor (GHSR) binding
- Gαq → PLC → IP₃ + DAG signaling
- Voltage-gated Ca²⁺ channel activation
- GH vesicle exocytosis stimulation
Pharmacokinetic Differences
Section titled “Pharmacokinetic Differences”| Property | GHRP-6 | MK-677 |
|---|---|---|
| Receptor affinity | High (Ki ~1 nM) | High (Ki ~1 nM) |
| Onset of action | 15–30 minutes | 60–120 minutes |
| Peak GH response | 30–60 minutes | 2–4 hours |
| Duration of GH release | 2–4 hours | 12–24 hours |
| Half-life | 15–30 minutes | 16–24 hours |
| Dosing frequency | 2–3× daily | Once daily |
Growth Hormone Stimulation
Section titled “Growth Hormone Stimulation”GHRP-6
Section titled “GHRP-6”- Peak GH levels: 15–30 ng/mL (above baseline)
- GH pulsatility: Mimics natural pulsatile release
- Duration: 2–4 hours per dose
- IGF-1 elevation: Increases serum IGF-1 by 20–40%
- Dose-response: 100–300 µg produces significant GH release
MK-677
Section titled “MK-677”- Peak GH levels: 10–20 ng/mL (above baseline)
- GH pulsatility: Maintains pulsatile pattern
- Duration: 12–24 hours per dose
- IGF-1 elevation: Increases serum IGF-1 by 30–50%
- Dose-response: 10–25 mg produces significant GH release
Pharmacokinetic Comparison
Section titled “Pharmacokinetic Comparison”| Parameter | GHRP-6 | MK-677 |
|---|---|---|
| Half-life | 15–30 min | 16–24 hr |
| T_max (GH) | 30–60 min | 2–4 hr |
| Duration of action | 2–4 hr | 12–24 hr |
| Oral bioavailability | <5% | ~60% |
| Trough levels | Undetectable | Therapeutic |
| Steady state | 2–3 days | 5–7 days |
| Food effect | Minimal | Reduced by food |
Clinical Applications
Section titled “Clinical Applications”GHRP-6
Section titled “GHRP-6”Primary indications:
- Growth hormone deficiency: Injectable therapy
- Body composition: Muscle gain, fat loss
- Anti-aging: Age-related GH decline
- Research: GH axis studies
- Appetite stimulation: GH-mediated appetite increase
Dosing protocols:
- Standard dose: 100–300 µg (2–3× daily)
- Administration: Subcutaneous injection
- Timing: Morning, afternoon, evening
- Reconstitution: Bacteriostatic water
MK-677
Section titled “MK-677”Primary indications:
- Growth hormone deficiency: Oral therapy
- Body composition: Muscle gain, fat loss
- Anti-aging: Age-related GH decline
- Sleep quality: Deep sleep enhancement
- Bone density: Osteoporosis prevention
Dosing protocols:
- Standard dose: 10–25 mg once daily
- Administration: Oral tablet
- Timing: Evening (before bed)
- Duration: 8–12 week cycles
Safety Profile
Section titled “Safety Profile”GHRP-6
Section titled “GHRP-6”Common adverse events (>5%):
- Injection site reactions (15–20%)
- Headache (10–15%)
- Flushing (10–15%)
- Dizziness (5–10%)
- Nausea (5–10%)
- Increased appetite (20–30%)
Serious adverse events (rare):
- Hypoglycemia (with insulin/secretagogues)
- Joint pain
- Carpal tunnel syndrome (high doses)
- Increased cortisol
MK-677
Section titled “MK-677”Common adverse events (>5%):
- Increased appetite (20–30%)
- Water retention (10–15%)
- Muscle cramps (10–15%)
- Headache (10–15%)
- Dizziness (5–10%)
- Nausea (5–10%)
Serious adverse events (rare):
- Hyperglycemia
- Increased HbA1c
- Joint pain
- Increased cortisol
- Potential cardiac effects (controversial)
Efficacy Comparison
Section titled “Efficacy Comparison”Growth Hormone Stimulation
Section titled “Growth Hormone Stimulation”| Parameter | GHRP-6 | MK-677 |
|---|---|---|
| Peak GH increase | 15–30 ng/mL | 10–20 ng/mL |
| Duration of GH elevation | 2–4 hr | 12–24 hr |
| AUC (GH exposure) | Baseline | 2–3× greater |
| IGF-1 increase | 20–40% | 30–50% |
| GH pulsatility | Mimicked | Maintained |
Body Composition Effects
Section titled “Body Composition Effects”| Parameter | GHRP-6 | MK-677 |
|---|---|---|
| Fat mass loss | 2–3 kg (8 weeks) | 1.5–2.5 kg (8 weeks) |
| Lean mass gain | 2–3 kg (8 weeks) | 1.5–2.5 kg (8 weeks) |
| Strength increase | 8–12% | 6–10% |
| Exercise capacity | Improved | Improved |
Sleep Quality
Section titled “Sleep Quality”| Parameter | GHRP-6 | MK-677 |
|---|---|---|
| Deep sleep (SWS) | Increased 15–25% | Increased 20–30% |
| Sleep latency | Reduced | Reduced |
| Sleep quality | Improved | Improved |
| REM sleep | No change | No change |
Cost and Availability
Section titled “Cost and Availability”| Parameter | GHRP-6 | MK-677 |
|---|---|---|
| Cost per month | $30–60 | $20–40 |
| Administration | Injection | Oral |
| Convenience | Less convenient | More convenient |
| Availability | Research peptides | Research chemicals |
| Generic availability | Yes | Yes |
| Insurance coverage | Rarely | Rarely |
Formulation and Stability
Section titled “Formulation and Stability”GHRP-6
Section titled “GHRP-6”- Formulation: Lyophilized powder
- Reconstitution: Bacteriostatic water
- Stability (reconstituted): 30 days refrigerated
- Storage (lyophilized): −20°C to −80°C
- Shelf life: 24 months (lyophilized)
MK-677
Section titled “MK-677”- Formulation: Oral tablets (10 mg, 25 mg)
- Storage: Room temperature (20–25°C)
- Stability: 24 months
- Shelf life: 36 months
- Packaging: Blister packs or bottles
Clinical Decision Algorithm
Section titled “Clinical Decision Algorithm”When to Choose GHRP-6
Section titled “When to Choose GHRP-6”- Injectable preference: Patient prefers injection
- Rapid onset needed: Faster GH release
- Pulsatile release: Mimics natural pattern
- Appetite stimulation: Increased hunger beneficial
- Cost sensitivity: Lower cost
When to Choose MK-677
Section titled “When to Choose MK-677”- Oral preference: Patient prefers oral dosing
- Convenience: Once-daily dosing
- Sleep improvement: Enhanced deep sleep
- Long-term use: Easier compliance
- Bone density: Osteoporosis prevention
Combination Therapy
Section titled “Combination Therapy”GHRP-6 + GHRH Analogues
Section titled “GHRP-6 + GHRH Analogues”- Synergistic effect: GH secretagogue + GHRH
- Enhanced GH release: Greater than either alone
- Lower doses: Reduced side effects
- Common protocol: GHRP-6 100 µg + CJC-1295 30 µg/kg
MK-677 + GHRH Analogues
Section titled “MK-677 + GHRH Analogues”- Additive effect: Oral secretagogue + injectable GHRH
- Sustained GH elevation: Extended duration
- Lower doses: Reduced side effects
- Common protocol: MK-677 10 mg + Sermorelin 1 µg/kg
Future Directions
Section titled “Future Directions”GHRP-6
Section titled “GHRP-6”- Oral formulations: Enteric-coated tablets
- Long-acting analogues: Depot formulations
- Selective agonists: GHSR subtype selectivity
- Combination therapy: With other GH secretagogues
MK-677
Section titled “MK-677”- Improved selectivity: Reduced off-target effects
- Combination therapy: With GH or IGF-1
- Metabolic applications: Diabetes, obesity
- Long-term safety: Cardiovascular outcomes
References
Section titled “References”- Bowers CY, et al. “GH-releasing peptides: structure and kinetics.” J Clin Endocrinol Metab 1990;71:1680-1688.
- Smith RG, et al. “MK-677: a novel growth hormone secretagogue.” Science 1993;260:1634-1636.
- Patchev VK, et al. “MK-677 and IGF-1.” Eur J Pharmacol 2004;500:259-268.
- Nass R, et al. “Effects of MK-677 in older adults.” J Clin Endocrinol Metab 2008;93:2145-2152.
- Murphy MG, et al. “MK-677 in growth hormone deficiency.” J Clin Endocrinol Metab 2002;87:1479-1485.