This study created a very sensitive test to measure hexarelin and measured how the peptide behaves in dogs after injection. It found that after an IV dose, hexarelin stays in the blood for about two hours and is cleared at a steady rate, while sub‑cutaneous injections show consistent clearance across a wide dose range. Although the work is in dogs, the clear pharmacokinetic numbers give hobbyists a rough idea of how long the drug lasts and how dosing might scale, but human data are still needed.

A radioimmunoassay (RIA) method for hexarelin, a peptidic growth hormone secretagogue, has been developed and applied to pharmacokinetic studies in dogs following an IV dose of 1 microgram/kg, and three SC doses of 1, 10, and 100 micrograms/kg. The sensitivity of the assay was determined to be 1.34 fmol/assay. Cross-reactivity of the antiserum with nine hexarelin analogues was less than 1% upon modification of positions 3, 4, or 5 of the peptide. No apparent cross-reaction with endogenous hexarelin metabolites were observed. Intra- and interassay coefficients of variation were less than 3% and 4%, respectively. Intravenous bolus pharmacokinetics of hexarelin displayed a high terminal half-life of 120 min, a fractional plasma clearance of 4.28 ml/min/kg, and a volume of distribution at steady state of 387.7 ml/kg. Following SC administration of hexarelin, despite the increase in dose administered, both clearance (3.93-5.17 ml/min/kg) and volume of distribution (316-544 ml/kg) parameters remained constant over the dose range studied.