GHRP-2 masks rhGH doping by cutting Rec/Pit ratio 39.9–43.9% in males
Background
Recombinant human growth hormone (rhGH) doping is detected via the isoform differential immunoassay, which exploits the fact that exogenous 22 kDa rhGH suppresses endogenous non-22 kDa pituitary GH through negative feedback, raising the recombinant-to-pituitary GH ratio (Rec/Pit ratio). Athletes seeking to evade this test may co-administer secretagogues that stimulate endogenous GH release, potentially restoring the suppressed pituitary fraction and normalising the ratio. GHRP-2 is a synthetic hexapeptide GHSR agonist known to potently stimulate pituitary GH secretion, making it a candidate masking agent. Understanding whether GHRP-2 administration confounds the isoform test — and whether direct GHRP-2 detection can close that loophole — is critical to anti-doping surveillance integrity.
Study Design
Japanese male subjects were assigned to four arms: a reference population (n=100), 0.04 mg/kg rhGH subcutaneous (n=5), 100 µg GHRP-2 intravenous (n=10), and 0.04 mg/kg rhGH combined with 100 µg GHRP-2 (n=10). The primary endpoint was the Rec/Pit ratio measured by isoform differential immunoassay. Urine samples from GHRP-2-treated subjects were also analysed for GHRP-2 and its metabolite AA-3 by liquid chromatography/tandem mass spectrometry (LC-MS/MS). The Japanese reference population limit served as the threshold for doping positivity.
Results
Low-dose 0.04 mg/kg rhGH alone produced a significantly elevated Rec/Pit ratio versus the Japanese reference limit (P < 0.001), confirming the isoform assay's sensitivity at this dose. 100 µg GHRP-2 IV alone raised concentrations of both recombinant GH (recGH) and pituitary GH (pitGH) simultaneously, so no significant change in the Rec/Pit ratio occurred (P > 0.05). In the combined rhGH + GHRP-2 arm, GHRP-2 administration drove the Rec/Pit ratio down to 39.9–43.9% of the elevated value caused by rhGH alone, constituting clinically meaningful masking of the doping signal.
GHRP-2 co-administration reduced the elevated rhGH Rec/Pit ratio to just 39.9–43.9% of its rhGH-only level, demonstrating active masking of isoform-based doping detection.
Critically, LC-MS/MS analysis identified both GHRP-2 and its urinary metabolite AA-3 during the same windows in which the Rec/Pit ratio was being masked, providing a parallel detection strategy that directly compensates for the isoform assay's vulnerability.
Key Findings
- 0.04 mg/kg rhGH SC significantly elevated Rec/Pit ratio vs. Japanese reference limit (P < 0.001)
- 100 µg GHRP-2 IV alone caused no significant change in Rec/Pit ratio (P > 0.05)
- Combined rhGH + GHRP-2 reduced Rec/Pit ratio to 39.9–43.9% of rhGH-only elevated value
- GHRP-2 metabolite AA-3 detectable in urine by LC-MS/MS during the masking window
- Reference population n=100; GHRP-2 and combination arms each n=10
Why It Matters
GHRP-2 is an active masking agent for rhGH doping that can depress a positive isoform test result by more than half — anyone combining GHRP-2 with rhGH for performance enhancement should understand this interaction carries dual detection risk: both the rhGH isoform assay and direct urinary GHRP-2/AA-3 testing are in play. Anti-doping agencies and clinicians overseeing GH-based protocols should recognise that isoform assay negativity does not rule out co-administration masking. The detection window for GHRP-2 and AA-3 in urine, confirmed by LC-MS/MS, directly overlaps the masking window, making simultaneous urinary secretagogue screening the practical countermeasure. For therapeutic or research contexts, these data highlight that GHRP-2 co-administration meaningfully perturbs GH isoform ratios and complicates any biomarker-based GH monitoring.
