Blood-to-muscle distribution and urinary excretion of higenamine in rats

Higenamine is a β₂-agonist that has been prohibited in sports by the World Anti-Doping Agency. Higenamine could potentially promote anabolism and lipolysis; however, its crucial pharmacokinetics data, particularly muscle distribution, remain unavailable. The present study aims to investigate the blood-to-muscle distribution as well as the urinary excretion of higenamine in laboratory rats. In the first experiment, the microdialysis technique was employed to continuously measure free, protein-unbound concentrations in blood and muscle for 90 min (sampling at a 5-min interval) after rats received IV infusion of higenamine. The mean half-lives of higenamine in blood and muscle were 17.9 and 19.0 min, respectively. The blood-to-muscle distribution ratio (AUC_muscle/AUC_blood) of higenamine was estimated to be 22%. In the second experiment, rats were orally administered with a single-dose higenamine, and their urine samples were profiled at a 12-h interval for up to 48 h. Results showed only a small portion of total consumption (1.44%, ranging 0.71%–2.50%) was excreted in the urine. Among these time points, about 43% cumulative amount of higenamine was eliminated within the first 12 h. Our data suggested that one-quarter of the unbound higenamine rapidly penetrates from the vessels into muscle, distributes to the interstitial fluid, then eliminates from the rat in a short span of time. The muscle tissue is likely to have a low binding affinity for higenamine, and renal excretion plays a minor role in its elimination. Together, our findings provide valuable pharmacokinetics data that may gain deeper insights into higenamine's role in skeletal muscle functions.