Beta₂‐adrenergic agonist salbutamol exhibits enantioselective disposition in skeletal muscle of lean young men following oral administration

Salbutamol is a common short-acting beta₂-adrenergic agonist used in treatment of asthma and exercise-induced bronchoconstriction but also possesses anabolic and metabolic actions in skeletal muscle. As a chiral compound, salbutamol is a racemic 1:1 mixture of two enantiomers, (R)-salbutamol and (S)-salbutamol, which exhibit divergent pharmacokinetic and pharmacodynamic actions. Despite salbutamol being available for decades, information on the enantioselective disposition of salbutamol enantiomers in human skeletal muscle is absent. In this study, we determined concentrations of (R)-salbutamol and (S)-salbutamol by UHPLC-MS/MS in arterial plasma and vastus lateralis muscle samples from 12 lean young men 2½ and 7 h following ingestion of 24 mg oral salbutamol. Mean (range) arterial plasma concentrations were 10-fold higher (p < 0.001) for (S)-salbutamol than (R)-salbutamol, being 33(9-62) and 49(30-84) ng·mL^-1 for (S)-salbutamol and 4 (1-6) and 4 (2-5) ng·mL^-1 for (R)-salbutamol 2½ and 7 h following administration, respectively, reflecting faster elimination of the (R)-enantiomer. Mean (range) muscle concentrations were higher (p < 0.001) for (S)-salbutamol than (R)-salbutamol 2½ h (0.17 [0.1-0.26] vs. 0.04 [0.02-0.06]) and 7 h (0.31 [0.21-0.46] vs. 0.06 [0.04-0.12] ng·mg_d.w. ^-1) after administration. However, muscle:plasma partition coefficient was two-fold higher (p < 0.001) for (R)-salbutamol than (S)-salbutamol 7 h following administration. These observations demonstrate that oral salbutamol exhibits enantioselective disposition in systemic circulation and muscle favoring the (S)-enantiomer but with higher relative partitioning of the (R)-enantiomer in skeletal muscle. Furthermore, the concentration-time profiles of salbutamol enantiomers are different in skeletal muscle and systemic circulation following oral ingestion. These findings have implications for the application of chiral switch (R)-salbutamol in doping control.