Oral Presentation 24th International Conference of Racing Analysts and Veterinarians 2026

Administration study of somapacitan, a long-acting growth hormone derivative, in horses and its elimination in equine plasma for doping control purposes (129516)

Yoshibumi Shimizu 1 , Michiko Sugai-Bannai 1 , Haruka Tanabe 1 , Kazunobu Saito 1 , Hiroki Ito 2 , Hirotaka Tokushige 2 , Kazuhiro Kamiya 3 , Misato Hirano-Kodaira 1 , Masayuki Yamada 1 , Gary LEUNG 1
  1. Laboratory of Racing chemistry, Utsunomiya, Tochigi, Japan
  2. Equine Veterinary Clinic, Horse Racing School, Japan Racing Association, Chiba, Japan
  3. Anti-Doping Section, Equine Department, Japan Racing Association, Tokyo, Japan

Somapacitan is the second generation of recombinant human growth hormone (rhGH) medication that retains the pharmacological effects of rhGH but exhibits a longer duration of action due to its reversible albumin-binding in the body. In general, the use of all recombinant growth hormone (rGH) analogues is banned by the human and animal sports regulatory authorities due to their anabolic and lipolytic effects. However, little is known about the elimination kinetics and biological effects of the newly introduced long-acting rhGH, somapacitan, in horses. In this study, we developed a test method for the identification and differentiation of somapacitan and rhGH in equine plasma by using liquid chromatography high-resolution mass spectrometry (LC–HRMS). The method involved C4 solid-phase extraction after ammonium sulfate precipitation, followed by chloroform/methanol precipitation, trypsin digestion, and analysis by LC-HRMS. Although somapacitan is 99% identical to rhGH in terms of amino acid sequence, successful differentiation was achieved through the detection of their respective unique T10 peptide fragments. Our method could detect the somapacitan-specific T10 peptide fragment with its albumin binding moiety intact, while simultaneously monitoring the T1, T8, and T9 peptide fragments (shared between somapacitan and rhGH) as supplementary markers to demonstrate the exogenous nature of somapacitan in horses. All four target peptide fragments were detected for up to 14 days and identified for up to 10 days after a single subcutaneous administration of 90 mg somapacitan to Thoroughbred horses. Additionally, plasma IGF-1 concentrations were significantly increased in all horses after administration, indicating that an indirect biomarker of GH administration in horses previously reported for conventional rGH, is also applicable to this long-acting GH derivative. The results demonstrated, for the first time, the plasma kinetics and biological activity of somapacitan in horses and proved the method’s applicability for equine doping control.