GHRP-2: Dosing Protocols

Dosing Disclaimer#

The dosing information below is compiled from the approved Japanese diagnostic protocol (GHRP Kaken), published clinical pharmacology studies, and research literature. GHRP-2 is approved only as a diagnostic agent in Japan. It is not approved for therapeutic use in any jurisdiction. The research dosing protocols described here are provided for educational and informational purposes and do not constitute medical advice or treatment recommendations. All clinical use should be under the supervision of qualified medical professionals within appropriate regulatory frameworks.

Approved Diagnostic Protocol: GHRP Kaken (Japan)#

Overview#

The GHRP-2 stimulation test is approved in Japan as a diagnostic provocative test for evaluating growth hormone secretory capacity in patients with suspected GH deficiency. The test is marketed by Kaken Pharmaceutical under the trade name GHRP Kaken and represents the only regulatory approval for any GHRP-class peptide in a major pharmaceutical market.

Protocol Details#

The standard Japanese diagnostic protocol consists of the following steps:

1. Patient preparation: The patient fasts overnight (minimum 8 hours). The test is conducted in the morning to minimize diurnal variation in GH secretion. An indwelling intravenous catheter is placed in an antecubital vein at least 30 minutes before testing to allow stress-related GH secretion to normalize.

2. Baseline sampling: A baseline blood sample is drawn for serum GH measurement immediately before GHRP-2 administration (time 0).

3. GHRP-2 administration: A single intravenous bolus of 100 mcg GHRP-2 (Pralmorelin) is administered over approximately 30 seconds through the indwelling catheter.

4. Serial blood sampling: Blood samples are drawn at 15, 30, 45, 60, 90, and 120 minutes after GHRP-2 injection for serum GH measurement. The minimum sampling protocol includes time points at 15, 30, and 60 minutes.

5. Interpretation: The peak GH value from the serial samples is compared against established diagnostic cutoff values. A peak GH response below the cutoff (typically 9-15 ng/mL depending on the GH immunoassay used and clinical context) is considered suggestive of GH deficiency.

Diagnostic Performance#

The GHRP-2 stimulation test has demonstrated high sensitivity and specificity for severe GH deficiency in validation studies conducted in Japanese patient populations. Its diagnostic performance is comparable to the gold-standard insulin tolerance test (ITT) while offering significant advantages in terms of safety (no risk of hypoglycemia), simplicity, and reproducibility.

Combined GHRH Plus GHRP-2 Test#

An extension of the standard protocol combines GHRP-2 with GHRH (or a GHRH analog) to enhance diagnostic sensitivity and provide additional information about the anatomical level of GH axis pathology. In this protocol, both GHRH (1 mcg/kg IV) and GHRP-2 (100 mcg IV) are administered simultaneously, and serial blood sampling follows the same schedule.

The combined test produces a synergistic GH response that exceeds the sum of individual responses. This synergy occurs because GHRH signals through the Gs/cAMP pathway while GHRP-2 signals through the Gq/PLC pathway, and the concurrent activation of both pathways in somatotrophs produces multiplicative amplification of GH release.

The combined test is particularly useful for distinguishing hypothalamic from pituitary causes of GH deficiency. Patients with hypothalamic GH deficiency (intact pituitary, impaired GHRH input) typically show a robust response to the combined stimulus, while patients with primary pituitary GH deficiency show a blunted response to both GHRP-2 alone and the combined test.

GH Provocation Test Dosing (Research)#

Weight-Based Intravenous Protocol#

In clinical pharmacology research, GHRP-2 is frequently administered intravenously on a weight-based dosing schedule. Dose-response studies have characterized GH release across a range of intravenous doses:

Peak GH levels typically occur 15-30 minutes after intravenous administration and return to baseline within 2-3 hours. The GH dose-response curve shows a sigmoid pattern with a plateau at approximately 1-2 mcg/kg, beyond which increasing the dose does not substantially increase peak GH levels but does increase secondary hormonal effects (cortisol, prolactin).

The 1 mcg/kg IV dose is most commonly used in research provocative testing because it produces near-maximal GH stimulation with relatively modest cortisol and prolactin effects, offering the best ratio of diagnostic signal to off-target hormonal perturbation.

Research Subcutaneous Dosing Protocols#

Standard Subcutaneous Protocol#

Subcutaneous administration of GHRP-2 produces a somewhat delayed and prolonged GH response compared to intravenous bolus dosing. Peak GH levels are typically reached at 30-60 minutes after subcutaneous injection, with return to baseline within 3-4 hours. Research protocols have employed a range of subcutaneous doses:

• 100 mcg SC: Produces moderate GH elevation suitable for studies requiring physiological-range GH stimulation
• 200 mcg SC: The most commonly used research dose; produces robust GH release with a favorable side effect profile
• 300 mcg SC: Approaches maximal GH stimulation via the subcutaneous route; higher doses are rarely used due to diminishing returns and increased secondary hormonal effects

Frequency and Timing#

Research protocols typically employ 2-3 daily subcutaneous injections to produce repeated GH pulses. The timing of administration is based on pharmacokinetic and physiological considerations:

• Morning dose (fasting): Administered on an empty stomach, as food intake and elevated blood glucose blunt the GH response to GHRP-2. Fasting for at least 30 minutes before and after administration is recommended.
• Afternoon dose: Administered between meals to maintain a fasting state. Typically given 4-6 hours after the morning dose.
• Pre-sleep dose: Administered 30-60 minutes before bedtime to augment the natural nocturnal GH pulse. Some research protocols use this as the primary or sole daily dose, leveraging the synergy between GHRP-2 and the endogenous nocturnal GH secretory burst.

Factors Affecting GH Response#

Several physiological factors influence the magnitude of the GH response to GHRP-2:

• Fed versus fasting state: Food intake, particularly carbohydrates and fats, substantially blunts the GH response. Administration on an empty stomach is recommended for maximal effect.
• Blood glucose levels: Hyperglycemia suppresses GH release; hypoglycemia enhances it. Stable euglycemia provides the most consistent responses.
• Body composition: Obesity is associated with reduced GH secretory capacity; obese individuals may show blunted responses to GHRP-2.
• Age: GH secretory capacity declines with age; older adults may show lower peak GH responses than young adults at the same dose.
• Sex: Women of reproductive age may show enhanced GH responses compared to age-matched men, likely due to estrogen modulation of somatotroph sensitivity.
• Sleep: GHRP-2 administered during sleep augments the natural GH pulse amplitude; wake-state dosing produces independent GH pulses.

Synergy with GHRH#

Rationale#

The combination of GHRP-2 with GHRH (growth hormone-releasing hormone) or GHRH analogs (sermorelin, tesamorelin) produces a synergistic GH response that substantially exceeds the sum of the individual responses. This synergy is among the most well-characterized pharmacological interactions in GH axis physiology and has been exploited in both diagnostic and research contexts.

The mechanistic basis for synergy is the convergence of complementary intracellular signaling pathways. GHRH activates the GHRH receptor (a Gs-coupled GPCR) on somatotrophs, stimulating adenylyl cyclase and raising intracellular cAMP levels through the PKA pathway. GHRP-2 activates GHS-R1a (a Gq-coupled GPCR), stimulating PLC and raising intracellular calcium and DAG through the PKC pathway. The simultaneous activation of both cAMP/PKA and calcium/PKC pathways produces a synergistic amplification of GH exocytosis that is greater than the sum of either pathway activated alone.

Combined Dosing#

In research protocols, the typical combined dosing regimen is:

• GHRP-2: 100 mcg IV or 100-200 mcg SC
• GHRH (or analog): 1 mcg/kg IV or 100 mcg SC

Both peptides are administered simultaneously or within minutes of each other. The combined GH response typically exceeds the sum of individual responses by a factor of 2 or more, with some studies reporting synergy factors of 3-5 in healthy young adults.

Reconstitution and Preparation#

Lyophilized Powder Reconstitution#

GHRP-2 for injection use is supplied as a lyophilized (freeze-dried) white powder that requires reconstitution before administration.

Reconstitution procedure:

1. Allow the vial to reach room temperature before opening
2. Using a sterile syringe, draw the desired volume of bacteriostatic water for injection (preferred for multi-use vials) or sterile water for injection (single-use)
3. Direct the water stream along the inside wall of the vial rather than directly onto the lyophilized cake
4. Gently swirl the vial until the powder is completely dissolved; do not shake vigorously, as this may cause foaming and peptide degradation
5. The resulting solution should be clear and colorless; discard if particles, cloudiness, or discoloration are present

Typical reconstitution volumes depend on the vial content and desired concentration. For example, a 5 mg vial reconstituted with 2.5 mL of water yields a 2 mg/mL (2,000 mcg/mL) solution. A 200 mcg dose would then require 0.1 mL (100 microliters) of this solution.

Storage After Reconstitution#

• Reconstituted with bacteriostatic water: Store refrigerated at 2-8 degrees C; use within 2-4 weeks
• Reconstituted with sterile water: Store refrigerated at 2-8 degrees C; use within 3-5 days
• Do not freeze reconstituted solution
• Protect from light
• Discard if solution changes color, becomes cloudy, or contains visible particles

Tachyphylaxis and Dosing Strategies#

Repeated daily administration of GHRP-2 leads to progressive attenuation of the GH response (tachyphylaxis), typically becoming evident within 1-4 weeks of continuous daily dosing. The mechanisms include GHS-R1a internalization and downregulation, upregulation of somatostatin inhibitory tone, and IGF-1-mediated negative feedback on the GH axis.

Several dosing strategies have been explored to mitigate tachyphylaxis:

• Intermittent dosing: Cycling periods of active dosing (4-5 days) with rest periods (2-3 days) to allow receptor recovery
• Dose cycling: Alternating between higher and lower doses to vary the degree of receptor stimulation
• Combination with GHRH: The synergistic mechanism may partially compensate for reduced GHS-R1a responsiveness by maintaining GH output through the complementary GHRH receptor pathway
• Single daily dosing: Using only one daily administration (typically pre-sleep) rather than multiple doses to reduce the total receptor stimulation burden

The optimal strategy for maintaining GH responsiveness during extended GHRP-2 use has not been established through controlled studies.

Evidence Gaps#

• Optimal subcutaneous dosing for sustained GH elevation has not been established in controlled dose-finding studies
• Long-term dosing protocols beyond a few weeks have not been systematically evaluated for efficacy or safety
• The most effective tachyphylaxis mitigation strategy has not been determined through comparative studies
• Population-specific dosing adjustments (age, BMI, sex, ethnicity) for diagnostic use have not been comprehensively validated outside Japanese populations
• Pharmacokinetic data for subcutaneous administration are limited compared to the well-characterized IV pharmacology

Related Reading#

• GHRP-2 overview and research guide
• GHRP-2 side effects profile
• GHRP-2 research evidence